A hollow tube is not as strong as a solid cylinder of the same diameter, but it is stronger than a solid cylinder of the same weight. This is why tube steel is used to build race car frames, roll structures, and high-stress parts such as sway bars and suspension components. Tube is also used for exhaust pipes and occasionally, intake manifolds as well. It’s a near-universal material in the automotive hobby.

Most tube-shaped mild steel used for automotive roll structures is between 1.25 and 2 inches in diameter and has a wall thickness of between .095 and .125 inch. Thinner tubing is used for exhaust systems or decorative light bars but for structural strength, .095- or .120-wall tube steel is generally required. Never use exhaust tubing or other inappropriate materials for structural work—it fails when you need it most. If your car is entered in any event that requires technical inspection, they’ll drill a small hole in the bar or use an ultrasonic tester and measure the wall thickness anyway.

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Russ works on a complex tube roll structure. It takes good materials, good design, and good welding to produce a cage good enough to protect the lives of the people inside the car. If you doubt your ability to deliver all three aspects, it’s best to take this project to a professional.

 

 The tube on the left is .120- wall 1.5-inch DOM suitable for a roll bar. The piece on the right is .063-wall 3-inch ERW exhaust tubing. When selecting a piece of tube for a project, you must consider the material, the manufacturing process, the diameter, and the wall thickness.

 

This Kakumei stainless steel exhaust system is precisely bent and carefully welded and polished to look great as well as flow well.

 

A roll cage has to be built to the specifications of the car’s racing organization. NHRA, SCCA, CORR, and NASCAR all have good, proven designs for you to use.

 

You can easily see the blue stripe on this piece of exhaust tube that marks it as ERW construction. Notice that the bender has placed the blue stripe to the inside of the bend.

 

Virtually every competition car in the world is required to run a roll bar or cage in the modern era, and countless convertibles and other street cars also use roll bars to protect their occupants in case of rollover. Roll bars also provide a sturdy mounting point for competition seat belts and fire extinguishers, and with a little extra work can include seat mounting rails as well.

In this chapter we’ll look at what it takes to put a roll bar into a car, how to make a custom exhaust pipe, and how to make a subframe brace out of tube metal. Other common projects include driving light bars, tube bumpers, radiator protection, and utility structures for pickup trucks. You can build an entire race car or dune buggy chassis or even an airplane out of tube steel, but such projects require an entire book to themselves. The skills and experience necessary to undertake more extensive projects are all contained in these basic exercises.

 

Working with Tube Steel

There are three kinds of tubing commonly used for structural automotive fabrication. The first is ERW, which stands for Electronic Resistance Welding. This is a flat piece of steel that has been wrapped around a buck and then welded to form a tube. The entire seam is welded along the length of the tube by a machine that is similar in operation to a spot welder.

Some organizations allow ERW tubing to be used for structures, and if you’re working with ERW in a structural context, remember that the weld (visible as a bluish line on the tube) is usually placed on the inside of a bend. Some builders place the weld at the side of the bend. But if you try to stretch the weld on the outside of a bend, the welded area will weaken and possibly crack or break during bending, or worse, after you have installed it in the car.

 

DOM tube lacks the blue stripe to tell you what it is, but there are usually markings on the tube. If you’re in doubt about the materials or construction of a tube, don’t use it for a critical application like a roll bar.

 

The second type of tubing is called DOM, which stands for Drawn Over Mandrel. DOM tubing is also welded from a flat sheet, but after welding, the tube is reheated and formed around a mandrel to precise specifications. DOM tubing is significantly more expensive than ERW, but also stronger. It does not have an obvious blue stripe.

Seamless tubing is extruded from a single mass of molten steel and formed on a mandrel, but this material has no welds. It’s very strong, but also very expensive.

 

Cutting Tubing

To join up roll cage bars, you also need a fishmouth tubing cutter. This is a hole saw connected to a mount that can hold a piece of tube at any angle. This allows you to measure and mark an intersecting piece and then cut the tube for a perfect fit. You can also cut and grind to get a proper fit, but this is far more labor intensive. The shop that does your bending probably has a fishmouth cutter, and if you have measured your angles, they can cut your support pieces as well.

 

A fishmouth cutter with a protractor to indicate the angle of the cut is essential for almost all tube structures. The alternative is hours and hours of difficult and exacting work with a grinder.

 

The result of a good fishmouth cut is a tube that’s ready to weld into place with no gaps. Just make sure that you get the length measured correctly. It’s easy to make a fishmouth a little too short.

 

This sand car chassis is made entirely of custom-bent and welded tube. The structure is both light and strong, and also inexpensive to produce.

 

For straight cuts in tube steel, a chop saw, metal-cutting band saw or a hacksaw works best. Don’t use a gas cutting torch if you can avoid it, as it burns the steel at the cut.

 

Bending Tubing

The first thing to say about bending and welding tube is that it’s as much an art as a science. You can calculate everything to the millimeter, but each bending machine and bending die is a little different from every other one. It’s important to know how your particular equipment works and how much fudge factor you have to allow. The only way to know that is through experience, and you’re likely to waste a lot of tubing on the learning curve.

 

On a rotary tube bender, the paint mark on the tube marks the end of the die on which this bend was made. The pen marks the point where the metal actually begins to bend. The difference is about 1/2 the tubing diameter.

 

One trick as you’re getting to know your bender is to make a few test bends to find out exactly where the bend curve begins. Then you can use a long protractor or angle finder to keep track of the bend progress.

 

Getting the Right Bender

The shape of tube steel presents certain special challenges for the fabricator. While it is possible to make smooth bends in tube steel, it is not easy. Improper bending technique will crimp, kink, or collapse the tube, destroying its structural integrity. Special tube-bending equipment is required to bend tubing accurately without reducing its strength too much. The inexpensive kind of tube benders that press upward with a bottle jack are absolutely not appropriate for creating roll bars, tube-frame chassis, or other critical components. They stretch the metal far too much and will tend to crimp materials in significant bends.

For roll cage and chassis work, you need a mandrel or a rotary tube bender, and these are expensive tools. You can purchase hand-powered benders, but these require a very strong installation base because of the long levers required to put enough torque on the tube to bend it. Powered mandrel and rotary tube benders can be free-standing, but are even more expensive yet.

 

A powered free-standing rotary tube bender costs a few thousand dollars. Make sure it comes with a good supply of dies for different size tubes and different radius bends.

 

Don’t rely on your protractor to make identical bends in matched pieces. Hold the original part over the bender and confirm the bend with your own eyes. It’s far easier to get it right the first time than to have to put it back in the machine for a little more bend.

 

Whatever bender you choose, it’s critical that you make your bender installation absolutely level. Any errors in your bends because the system isn’t level will be doubled every time you flip the tube around to bend the other side! If you think you have an error factor in your bender, see if you can work the tube from one end to the other, as opposed to working outwards from the middle.

 

Bending is Forever

Tube steel hardens a little when you bend it. Because the steel on the outer side of the bend has been stretched, it becomes more brittle. Therefore, most organizations that specify roll structures have specific rules about the number of bends, the maximum number of degrees of each bend and total bend, and the inner radius of each bend in a piece of tubing. The purpose of these rules is to limit the amount of work hardening that can be introduced to a piece of tube through bending. 

Bending a piece of tubing properly requires a manual or hydraulic mandrel or rotary tube bending machine. In the past, people bent steel tube by filling the tube with dry sand and welding the ends shut, then heating the bend area in a forge and bending the tube around a buck of the desired radius. This worked well to avoid crimping, but is workintensive and much less accurate than using a bender. The most important fact to remember is that bending is forever once bent, you can’t unbend a piece of tubing. If you bend it wrong, it’s done.

 

Planning Your Roll Structure Project

A good tube steel project begins with careful planning. A roll bar or cage takes more material and more work than it looks like in the results. Moreover, each bend and cut must be precisely correct even a small gap is difficult to weld over and shows in the final product.

The first part of planning your project is to design the bar or cage you want. The design should fit well with your car and require a minimum of alteration to the surrounding interior panels and bodywork. But if you are building a roll structure to use in a racing or off-road vehicle, you must also plan to meet the requirements of at least one sanctioning body or insurer. 

 

The Sports Car Club of America website (www.scca.com) allows anyone to download their General Competition Rules. Section 9 of these rules includes detailed drawings of a variety of roll structures. Cages built to SCCA rules are accepted by most track racing organizations worldwide. For drag racing cage rules, you can consult the National Hot Rod Association (NHRA) Tech Faq on www.nhra.com. Performance rally has some of the most involved cage rules in motorsports, and the rules are standardized by the Federation Internationale de l’Automobile (FIA). Pictures of FIA-approved roll structures are available at www.rally-america.com.

In general, most sanctioning bodies for racing or other organized automotive activities have common or very similar standards. A good roll bar design works for drag racing as well as drifting or autocross. There are some variations, however, and not all organizations let you overbuild your roll structure because a roll structure also enhances your car’s frame stiffness. Vintage racing organizations in particular are picky about overbuilding a car’s roll structure.

 

Judd Weld of Grindesigns in Bend, Oregon, built this rally cage for a Subaru WRX. A rally cage is about as tough as they come, and welldesigned with plenty of supporting braces.

 

A basic main hoop, with four bends. The first shallow bends allow the hoop to follow the car’s body lines, then the main bends are slightly less than 90 degrees to create the top of the bar.

 

Here Russ has a nylon tie-down showing where the second crossbrace will go in a roll cage for a 1958 Chevy Bel Air. You can use string, tape, or anything that makes a straight line.

 

Always plan to make your tube welds all the way around the joint. If you cannot get in to weld all the way, ask and find out if a gusset is an acceptable substitute.

 

 

Removing the seats, carpet, sound deadening material, and maybe even the headliner is a good start to a roll bar project. You need space to work and you’ll be making a lot of heat in the car.

 

A good rollbar design generally starts with one main hoop made of one uninterrupted piece of DOM mild steel or chrome-moly tube. That tube will have a total of about 180 degrees (plus or minus 10 degrees) of bend along its length. It should not mushroom like a pickup truck light bar, which is to say that there should not be reverse bends in the main hoop.

A good roll bar design uses strong rear braces and cross braces. Generally this means that the rear braces attach along the top or within 6 inches of the top of the main hoop and proceed without bends back to a solid mounting point. These braces should be at least 30 degrees away from parallel to the main hoop for good fore and aft support.

Most rules for roll bars and cages require that your welds meet ASTM (American Society for Testing and Materials) standards. Those standards call for a 360-degree weld for the supporting bars that connect to your main hoop. If you cannot weld all the way around, you may be able to use supporting gussets to reinforce the joint. Check your rulebook before you begin!

Most rules also require that you place the main hoop as close to the inner roof as you can. But be aware that when you weld in the rearward supports, you might burn your headliner. Many roll bar builders make the top welds before moving the bar into its final mounting place. This also allows you to conveniently paint the cage. Depending on your car, you may have to pass bars through one or more bulkheads to meet the angle and attachment requirements in a given set of rules. Cars such as the Porsche 914 and Toyota MR2 almost always require some holes in bodywork and the rear windows to accommodate a roll structure. Figure this into your plans because you can’t change supporting bar angles or attachments too far and still expect to pass technical inspection.

If you are building a roll bar for your own use and have no need to meet any particular specification, you should still consult at least one reputable racing rule book because the designs they specify are proven, safe, and generally very well documented. You don’t want to get too creative when your safety is on the line.

While you design your cage, you should also be preparing your car. You need to remove carpet and seats and other material and objects in or around the mounting locations. Pick strong locations such as reinforced corners and add mounting plates there if your car is a unibody design, or cut through the floor and tie your roll bar to the ladder frame if your car has one. Once you have settled on a design, it’s time to figure out what supplies you need. Measure all the basic dimensions for your roll structure and add 10 to 20 percent for wastage, bends, and fitment. Some roll structure specifications allow you to use a lighter grade of tube for supporting bars, but in general it’s best to make the entire structure out of the same size and grade of material.

 

Calculating Bends in Tube Steel

Before you cut or weld any tubing, you have some homework to do. Measure the dimensions of your project carefully, and get a friend to repeat your measurements to be sure. As a rule of thumb, cut your tubing parts a little long, because you can always shave a little off, but adding a few inches is hard to do.

Calculating the length of tubing required for a bent roll hoop is more tricky than it looks. It’s not enough to add the vertical and horizontal requirements, and if you get it wrong your roll bar will be wrong in some dimension. Here is a basic equation to help you calculate the length of tubing (in inches) required to make a bend:

 

Total Length = (Center Line Radius x .01745) x (Bend in Degrees)

 

Below is a simple graphic representation of the relevant measurements. For the rest of the example, you can assume that the tubing being bent is .120 wall and 1.75 inches Outside Diameter (OD) and that the Center Line Radius (CLR) of the bending die being used is 6 inches.

 

Let’s break the equation down:

 

6.0 CLR x .01745 = .1047 inches per degree of bend .1047 x 90 degrees of bend = 9.423 inches total length

 

So with a CLR of 6 inches and a 90-degree bend, 9.423 inches of tube are used in the example bend. But there’s another calculation you need to perform, and that’s to find out how tall your roll hoop will be once you’ve made your bend. Again, it’s not obvious.

 

 

To find the total height of your 90-degree bend, take the CLR of the die and add half the OD of the tube. This is because the CLR takes you from the start of the die to the center of the tube, not the top! Then add another half of the tube’s OD to account for a little fudge factor caused by the tendency of steel to stay straight a little way past the starting point on the die. The math for the height calculation is shown here:

 

1.75/2 = .875 .875 + .875 + 6 = 7.75 inches from the start of the bend to the top of the tube

 

Of course, on the legs of your hoop it’s easier to just cut the tube a little long and measure from the center of the bar. Measure outwards from the center and make your bends, then trim the legs to the desired height, measured from the top of the mounting plate to the top of the hoop. Where these equations really come in handy is when you have to measure the distance along the top of your roll hoop between the two bends. So they’re really used more for calculating the total width of a roll hoop. For example, if we want our roll hoop to have a total distance of 36 inches from the outside edge to the outside edge, we must subtract the 7.75 inches of bend length from each end of the top of the hoop.

That makes the distance between bend start points along the top of the hoop just 20.5 inches for a 36-inch-wide 1.75- inch OD roll hoop with a 6-inch CLR.

But not all bends are an even 90 degrees, so you have to be able to calculate the length of tubing used in a bend of any number of degrees. The table below shows the length of tubing (in inches) used for a 1-degree bend at a number of common CLR measurements. To find the length of tubing used, simply multiply the relevant figure for your die’s CLR by the number of degrees in the bend you’re making. Using the figures in the table below, if your CLR is 6.5 and you’re making a 35-degree bend, you will use 3.969875 inches of tube in the bend.

 

Alternate Method: Working It Out on The Floor

With all the computer programs and equations, calculating your tubing bends can be an intimidating proposition. However, there is another way: Just draw your structure on the floor!

Actually, you can draw on anything big enough to make a 1:1 scale drawing of your roll bar, but a flat concrete floor works best, and most people have easy access to one of those. You still need a tubing bender, but for a simple roll bar, this procedure should yield good results. You need a carpenter’s square, a tape measure, and a felt-tip marker. Then get some chalk or paint markers and follow these steps.

 

 

1: Measure your car for the size of roll bar you need. That is, the overall height from the base plate or frame rail to your headliner, or however tall you want the bar to be. Then measure the width of the hoop at the top and between the base plates or frame rails.

2: Use carpentry tools and your chalk to draw a square or rectangle on the floor that is as large in every dimension as your roll bar. For example, if your bar is 36 inches tall and 40 inches wide, that’s the size of your rectangle. Measure it carefully and make sure it’s square by measuring along the diagonals. When the diagonal measurements are the same, the box is square. Make a vertical line down the exact center of the box so you know the location of your centerline. That’s 20 inches from either side in this case.

3: Get a piece of tube that is somewhat longer than you really need. This is critical because you’re going to start in the middle and make the bar, then trim it to length by cutting off the ends. For example, in our project, we’ve got 40 inches of width and 36 inches on each leg. So you need at least 112 inches, plus your fudge factor. A 144-inch bar would be more than enough. Mark your piece of tube in the exact center, at 72 inches in this case.

4: Line up the centerlines of your tube and your box, so the tube is on top of the box and sticks out to either side.

5: Grab a piece of scrap tubing of the same size as your bar material and give it a 90-degree bend in your bender. Make sure you’ve got enough material to leave at least six inches hanging off of either end of your bend. Place the bent tube in one upper corner of your square, just alongside of the roll bar tube. You can see the point where the bend ends on your scrap tubing. Mark that same spot on the roll bar tube. Then repeat this step on the other side. Now you know where to start your 90-degree bends to maintain your 40 inch total width.

6: Bend one end of your hoop and then lay it down inside your diagram to make sure you got an exact 90-degree bend. You might have to overbend just slightly to achieve the perfect 90 when the bar is removed from the bender. Then make the second bend, keeping the piece perfectly level with the first bend so it lays flat.

7: When the second bend is done, lay the bar back onto your diagram. It should sit perfectly flat and contained within the width, with the centerlines lined up. The legs, however, will stick out beyond the height you need. Mark them according to the bottom of the diagram and cut them off with your chop saw or hacksaw.

By adding lines inside your basic box using a protractor, carpenter’s bevel, and a long carpenter’s level, you can figure arbitrary degree measurements with this method. For example, to make two 45-degree bends instead of a single 90 you can cut off the top corner of the box with the protractor and level and then bend your tube to follow that contour. The more complex the design, the harder it is to implement. But it can be done because all your measurements derive from that square box that matches the overall dimensions of the hoop. The trick is to center your piece of tubing on the centerline of the box, then work outward from the center.

 

Fishmouth Cuts

Fishmouth ends are among the most challenging joints to create. A good fishmouth tube cutter makes this job possible, if not easy. A fishmouth cutter is a modified hole saw and a vise that holds the tube at a desired angle while the hole saw cuts the shape of the receiving bar. You can also do this with a good vise mounted on a drill press, but a fishmouth cutter is easier. Without some kind of fishmouth cutter, be prepared to spend hours at your bench grinder.

 

 Getting a tight fishmouth may require a few tries, especially at first. But you need to get those angles and cuts just right for a good weld and a strong cage.

 

For a simple example, consider welding a straight horizontal bar between the vertical posts of a basic hoop 40 inches wide. Each end of the bar needs to wrap around the outside diameter of the vertical posts. At its shortest point, that bar must be 40 inches long, but the outer edges may be over an inch longer. To create this bar, calculate the length between the centerlines of the two posts and cut your bar to that length. The measurement will be the distance between the insides of the posts plus one tube diameter, or the outer distance minus one tube diameter. If you’re using 1.75-inch tube, that’s 38.25 inches in our 40- inch example.

To cut a 90-degree fishmouth, you can use a hole saw of the same outer diameter as the tubing and clamp the tube into your drill press. Make sure you’ve got some wood under the tube! Bring the hole saw down so that it cuts a semicircle out of the end of the tube. Then swap ends and make sure that you line up the second cut the same as the first! You may have to grind a little to get the length just right, but if your hole saw matches your receiving tube diameter, your fishmouth should line up cleanly.

Cutting a fishmouth at an arbitrary angle is harder. This is where a dedicated fishmouth cutter comes in handy. You can set an angle for the base plate on most drill presses, but more care is required to get the cut just right. Always measure the shortest and the longest lengths required in any bar. Some very complex bars may require the fishmouths to be cut at different angles or with the two ends cut along a different bias, and you must measure those individually.

 

Welding Tube Parts Into Your Car

When you go to weld tubes into your car, you typically have two types of weld to run. First are straight cuts where you mount the tube to a flat surface such as a mounting plate or frame rail. You also have fishmouth cuts where you mount the tube to the side of another tube and must cut the open end to wrap around the body of the second tube.

 

If possible, the best place to attach a roll bar is straight to the box frame of the car. If you have a unibody car, you need to make base plates of at least 36 square inches.

 

 

To weld flat ends (and angle-cut ends that are otherwise flat), make sure that your entire tube is positioned correctly and that both ends of the tube fit flush against their mounts. You can bevel the edges a little bit for a strong weld. Begin as always by tacking your work into place and rechecking all your measurements. When you’re satisfied that everything is in its correct place, go ahead and complete your welds. Make sure all your welds go a full 360 degrees around the tube and achieve good penetration all the way through the base plate. Unless you’re working with 4130 chrome-moly, MIG welding is generally the best way to attach tubes to base plates.

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Find more Tech Tips like this in the book, WELD LIKE A PRO: BEGINNING TO ADVANCED TECHNIQUES.

Learn everything you wanted to know about welding by getting your copy of this book here!

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More Projects

The first question to answer with any roll bar or chassis project is purpose of the project, and the rules (if there are such) that must be satisfied. Those factors generally dictate the design you must use. Typical rulebased roll bar design requirements are tubing size and thickness, base plate size or tie to the frame, and then the overall design—usually the number of supports, minimum distance above the driver’s head, seating the driver within or near to the plane of the main hoop, and the number, angle, and configuration of the supports. Also, the minimum and maximum radius of the bends is often specified.

Chassis requirements are even more complex. Most organizations have approved homologated designs that they prefer. These are proven designs and you should use them unless you’ve got a very good reason to try something else. Organizations such as SCCA require new tube chassis designs to be submitted to engineering analysis before they can be used in competition.

Usually there are chassis and roll structure diagrams in every organization’s rule book, and those designs must be followed. The insurance companies that cover motorsports in the modern age require these standards. Alternately, you can ask your local officials to suggest a good local car to use as a model. There are also cage kits you can purchase that meet common rule sets such as NASCAR, NHRA, or SCCA.

 

Making a Basic Drag Racing Roll Cage

This project involves creating a basic rear-supported roll bar, with forward braces and a driver’s side door bar, made to NHRA specifications for a car that runs 8.5 second or higher quarter-mile times. The rules and diagrams are downloadable from www.NHRA.com.

1: Remove the seats and carpet from the car. You need a completely accessible work area, and carpet or seats are in your way, plus they pose a fire hazard. If possible, you might want to remove the headliner, but it is difficult or impossible to replace the liner after you’ve completed your roll bar.

 

The best possible environment for installing a roll cage is a completely bare-metal chassis. There’s nothing to catch on fire or get in your way while you work.

 

2:  Measure the main hoop dimensions. If you are tying the plate to a mounting plate in a unibody car, measure from the floor (less the thickness of your plate), but if you are tying to a traditional ladder frame, go ahead and cut the holes in the body floor above the frame and measure down to the frame. Measure your dimensions so that the top of the roll bar is as close to the headliner or roof metal as you can make it.

You also need to measure the width of the roll hoop at the top of the hoop and at the bases. Most rules (and good structural design) want about 180 degrees (plus or minus 10 degrees) of total bend in your hoop. Mushroom-shaped designs such as those used in light bars for pickup trucks are not as sturdy, which is one reason those bars specifically disclaim protection in rollover accidents.

 

Use clamps to hold everything in place while you measure and make adjustments. You can also use racheting tie-downs as a low-cost come-along to move and hold pieces in place.

 

3: Take rough measurements for your supporting bars. In general, these are straight pieces. You’re just looking for approximate lengths at this time, and you’ll remeasure, cut, and install the supporting bars after the main hoop is in place.

 

Russ measures the distance for a supporting brace. You have to wait until you get the main hoop in place to get the exact lengths, but you can get a rough estimate of your total tubing needs before you bend or install any tube.

 

4: Calculate your bends using the formulas and procedures given above. Now you know the total length of tube you need for your main hoop and where the bends go, and about how much extra tube you need for supporting braces. Now you can buy your tube and have the main hoop bent to your specifications. As always, cutting the arms a little long is better than a little short. 

5: Purchase some steel plate and make your base plates if you’re installing your bar in any kind of unibody (stamped sheetmetal) car. You want to use large base plates to spread the load in a rollover. As a general rule, most sanctioning bodies demand plates of at least 6 inches square (36 square inches) and 1/8-inch (.125) thick. Weld the plates into the base area you measured for your roll bar. You may need to spend some time with the wire brush or other tools cleaning the base metal before you weld. As always, tack first, then form the metal to the mounting place, and finally fill in the welds all around your base plates.

In this installation, the main hoop mounts to a gusset-supported extension of the frame. This is almost as strong as attaching to the frame itself.

 

Here’s a base plate formed around a corner using two plates. It still meets the 36-square-inch rule and makes a very strong installation.

 

6: Make final adjustments to your main hoop and tack it to the base plates. Tack the hoop securely, but don’t weld it all the way around yet. You want to make sure everything fits right before you make that commitment.

7: Make and weld in your supporting tube mounting plates. If a mounting plate must fit around a curve, you can cold-forge it (beat it with a hammer) around an anvil horn of the same size or ask the same shop that’s bending your tube to bend the plate for you. Either way, you will likely have to do some hammer- adjusting when you install the plate. You want a nice flush fit to your mounting point, so you can securely tack the plate into place, then work it with the hammer to finish the fit before you weld.

 

Under the spreader bar holding this main hoop in place, you can see the hammer-formed base plate welded to an already strong point on this unibody chassis.

 

This base plate was formed around a front strut tower on a 1985 VW Golf rally car. This will really strengthen the chassis when this point is tied into the roll cage. It will also help keep the strut tower from failing under increased stress.

 

8: Remeasure your supporting braces from the main hoop to your mounting plates, and use a protractor or carpenter’s angle finder to get the correct angles. Remember to allow a little length for the fishmouth, because the ends of your supporting braces need to fit around the curve of the tubing. Mark the places where your braces attach.

Cutting and finishing your braces may be the most time-consuming part of the project. But it is absolutely critical to a quality result that these pieces fit perfectly with minimal gaps. You must bridge any gaps with weld, and the closer everything fits, the easier that will be.

 

Test-fitting a support brace to make sure the length and fishmouth angles are just right. This brace will go through that hole in the floor and mount straight up to the car’s frame.

 

Here Russ uses a piece of angle iron to hold the divided half of an X-shape in perfect alignment with its mate while he tacks the piece into place.

 

If you’re creating an X-shape in the braces, make one diagonal brace full-length, then make the second bar out of two pieces. Use a piece of angle iron with a notch cut out of the middle to keep the second bar of your X straight when you tack it into place.

9: Tack your support braces into place. You may have to make a few attempts at this before everything lines up just right. Once the supports are all tacked, you can start welding everything into place. If you have welds near the headliner or other interior panels, use an aluminum plate or other non-weldable, non-flammable material to protect the parts you don’t want burned. Note that you might not be entirely successful in that goal. If possible, you may want to pre-assemble the parts and then move them into place.

 

A practice weld of a piece of tube to a base plate. You can see that Russ got good penetration into the base plate from the weld all the way around the tube. Chances are you won’t be able to check your work like this, but practice and check your results beforehand.

 

10: Note where your driver-side door bar is required to mount for your car and measure to that point. In a unibody car, you need to add a mounting plate of .125-inch mild steel just like you used for your main hoop and support braces. Cut the plate-mounted end at an angle and fishmouth the end that attaches to the main hoop. Be sure of your measurements because the bar is required to intersect the main hoop at a minimum distance from the floor. Technical inspectors are very strict, so don’t yield to the temptation to move that bar to a more convenient angle. You can buy kits that make the door bar removable, and these kits are worth the investment if you’re concerned about day-to-day use of your car.

 

This is the upper end of the removable door bar. We used a removable pin for this end because this bar is designed to swing out to allow easy entry to the car.

 

This little kit allows you to weld in a removable door bar. You weld two of these tabs to your main hoop and forward A-pillar brace, and then weld the clevis ends into a length of tube. The clevis ends can be pinned or bolted into place.

 

Mild steel cages start rusting as soon as you stop welding. So it’s best to get a coat of paint on them as soon as possible. The best way to do this is to put your cage in a fully stripped car and then send the whole thing off to the paint booth.

 

This is the bottom end of the door bar. It’s bolted, but will still function as a hinge. Make sure you get the tabs welded in at just the right angles or your bar won’t fit right!

 

Clean up your welds, mask off your car, and paint your roll bar. Mild steel rusts quickly and your project looks best with a fresh coat of paint applied as soon as you’re done.

 

Creating a Tubular Chassis Brace

All performance cars benefit from stiffening their chassis. This is equally true for modern unibody designs, older half-tub designs such as GM’s Camaro and Firebird, and full-frame designs on vintage and antique cars.

To begin with the oldest designs, early mass-produced cars such as the Ford Model A had frames made from stamped channel steel, welded and riveted together into a ladder structure. As we discussed in Chapter 6, enclosing the fourth side of the frame is known as boxing the frame and by itself improves chassis rigidity. This modification has been part of the hot rodder’s art since the beginning. Later cars went to a central unibody tub with bolt-on subframes at the front and rear. A roll cage further adds to the stiffness by connecting the sides and the diagonal corners under a modified dome.

Finally, modern unibody cars benefit from tying together the suspension mounting points in several places. This reduces chassis flex during cornering and during acceleration for high-powered cars.

Frame and subframe braces work best when welded into place, but many are designed to be bolted into place, and these offer a good fraction of the benefit of a weld-in brace, and are removable to return the car to stock configuration. The same design can often be used for both welded and bolted braces, and the only difference is the fastening method.

This project creates a bolt-in subframe brace for a unibody car. The goal is to reduce front-end flex in hard cornering. This design fits a 1990–1997 Mazda Miata, but you can adapt the particular design and dimensions to your own purpose. We made this part with about $5 worth of supplies from the metal scrapyard and about 3 hours of work with the plasma cutter, tubing bender, drill press, and TIG welder. 

1: The Miata has two box-section parts in its undercarriage that support the lower A-arms of the front suspension and two transmission support mount points. These form the rectangle of the frame brace. Other models of car have different undercarriage configurations but it is generally possible to find four (or more) points to conveniently bolt on some kind of brace. If your car has a ladder frame, you can weld tabs to the frame to mount a brace.

 

The mount point for one side of our Miata chassis brace. The box section with three holes holds the lower front control arm, and we’ll slip a mounting plate in there. The loosened bolt in the picture will be our other control point.

 

2: Measure the distances between each of the four holes and draw your rectangle on a piece of scrap cardboard, plywood, or on the floor of your shop. Check and recheck your measurements to make sure they’re accurate. But remember that depending on the car you’re working with, the convenient mount holes may not define a perfect rectangle! That’s OK, as long as your finished piece lines up with the mounting holes. 

3: Based on your rectangle, design a basic brace that relies on arches or triangles for support. We decided to create a lightweight brace with one arched and one straight beam, tied together with a lightened and stressed skin. Some cars with more places to anchor a brace can use more complicated bracing. Do some research for your particular make and model and see what others have done before you design your own part.

 

We laid out the jig for our chassis brace on this piece of plywood. We wanted a light, strong brace, so we borrowed a roll cage trick and brought a bent tube up next to a straight one, making three strong triangles. We’ll tie the pieces together with a stressed skin.

 

4: You may want to create a jig when you begin welding. Depending on the materials and method you choose, warping and shrinking may bend your brace out of true. One way to create a good brace jig is to draw your base rectangle on a sheet of thick plywood and drill holes at the locations of the mounting points. Then attach your first pieces to the plywood as you tack your work together. As you weld each part into place, the plywood template holds the dimensions for you. If you plan to produce several pieces, create your jig out of steel and you can weld right on it without fear of lighting it on fire. 

5: Start with large pieces for the mount points. We used 2-inch lengths of 1.75-inch .120 wall aluminum tube. Then we cut some discs with a 1.875-inch hole saw to make cups out of our short sections of tube. Weld the discs to the ends of the tubes and then bolt the resulting cups to your template. We used the TIG welder at 110 amps with 1/16-inch aluminum rod for this part, using the foot pedal to modulate welding power.

 

 A mounting cup made from larger aluminum tubing with a plug of aluminum plate welded in. This is large enough in every dimension to accommodate the tube and to allow us some fudge factor when we install the part.

 

6: Now measure your distances and cut your aluminum tubes. We used 7/8-inch .120-wall aluminum tube that we picked up from the scrap bin at the metal yard. Make one brace straight and bend the other one on your bender to make a pointed arch. The two pieces may touch in the middle, and you may choose to weld them together, but you don’t have to. Weld the tubes to the cups all the way around. Aluminum warps very easily, so keep the cups bolted to the template to hold everything in place. We left the TIG welder at 110 amps and continued to use 1/16-inch aluminum rod.

 

Keep the centerlines of your tubes lined up. This will help you keep the whole piece accurately placed on your template.

 

Keep your TIG heat low and you can get good penetration without burning through the aluminum.

 

7: When the tubes are welded to their cups all the way around, use some cardboard to make a template for the skin. You can cut one piece and form it around the straight tube, but it works just as well to cut two sides. We used .095 sheet aluminum for this material.

 

As always, our trusty cardboard makes a good template for the stressed skin of our chassis brace.

 

Russ uses a piece of angle iron to get a straight cut in the aluminum sheet with his plasma cutter.

 

8: If you choose, you can use your hole saw and lighten the skin by taking a few holes out of it. This is already a very lightweight piece, but the holes give it a very sporting look. We also recessed the holes with a special clamp made just for the purpose. These are available through metalworking supply houses.

 

 Taking out these holes doesn’t reduce the skin’s strength, but it makes it a little lighter. You can see the hole on the left has already been recessed, and we’re about to do the one on the right.

 

9: Weld the skins to each side of the brace using a stitch pattern. We switched to 1/8-inch rod for this part of the process, as it requires more filler material. The stitch pattern helps avoid warping the part and is almost as strong under tension as an uninterrupted bead. Keep the piece bolted to the template as you install the first skin, then unbolt the piece and clamp it to the table as you weld on the second skin. When you’re done, try to flex it—you’ll find that it’s strong!

 

First tack the skin into place, then work in short stitches with plenty of cooling time to keep the brace from warping. Keep the brace bolted to the jig as much as possible.

 

You can see where Russ has marked off regular intervals on the piece for his stitch welds. He’s stitching the skin to the tube in short increments. Aluminum sheet warps very easily, so take your time on this step.

 

10: Mounting your brace to the Miata’s A-arm support boxes requires backing plates and we decided to weld a nut to the backing plate with the MIG welder. For the Miata, the plates must be 2-inch square, 1/8-inch steel. If you can’t get a backing plate into the mounting area, you may have to weld a nut to a solid location (such as a frame rail) to bolt up your brace. Don’t weld a nut to thin sheetmetal floors, or you’ll just rip the sheetmetal as the brace takes up the chassis flex loads.

 

 Drill a 3/8-inch hole in a 2-inch square mild steel plate. Three good, solid tack welds are all it takes to affix this nut to the plate.

 

 Installing the brace takes a little more work with a drill to enlarge the holes in the cup. Despite our best efforts, our measurements were a little off. That’s why you allow a bit of fudge factor.

 

Detail shot of one corner of the brace. Our plate with the nut welded on is up in the box section behind this cup.

 

You may also want to create a strut tower brace. Look in the engine bay of most modern cars and at either side you’ll see the strut towers where the shock absorber and spring are bolted on. When the suspension compresses, these towers have a tendency to flex inwards toward the centerline of the car. A strut brace resists that tendency and keeps your chassis in its designed shape. A basic strut brace mounts to the top of each strut tower and places a firmly held bar between them. The brace may also connect to the firewall bulkhead for additional support. Commercial strut braces don’t cost much, but if you want to create your own or none are available for your make and model, they’re easy to design and build.

 

Making a Custom Exhaust Pipe

One of the most common tasks when creating a custom car is to create a modified exhaust pipe. Some of these are complex creations with 4-into-1 collectors and delicate routing around underbody components, and some are little more than straight pipes. Many builders also add features such as sidepipes and lake pipes or dump pipes that bypass the muffler when you remove a plate or even just pull a lever. It’s not hard to fabricate these add-ons and the results can be very effective.

 

 

One hard truth about exhausts is that you cannot make custom pipes using the inexpensive jack-based tubing benders. If you crimp an exhaust tube, it’s not the end of the world, but it’s the end of a good highperformance exhaust. An exhaust pipe only allows as much gas flow as at its narrowest point, and if you kink it you just made that part!

 

This V-8 header is tacked together and ready to weld. The mating surface with the cylinder head and the 4-into-1 collector at the other end are purchased pieces, but the bends are made up of short sections that can be adjusted before they’re welded.

 

The old exhaust is laid out next to the new components. We’ve got a stick of 2-inch tube, two 180-degree bends, and a resonator. We’ll reuse the muffler and the input Y-pipe from the old system.

 

To do a good job on exhaust pipe bends requires a very specialized mandrel-based tubing bender. Your best alternative is to buy pre-made bends, cut them as needed, and then weld them together. If you do this job well, you can get a smooth exhaust path and a good-looking part.

Exhaust tubing has very thin walls, and for this reason, low-power MIG or TIG welding is ideal. TIG is generally considered to be overkill for exhaust systems. You can also weld exhaust tubing with gas and flux-core wire, but that takes more finesse. Stick welding exhaust tube is very difficult because of the tendency to burn through the material.

For this project, we re-create an existing exhaust system that is rusted out. We’ll replace a muffler along the way, keep some good parts from the old unit, and smooth out the flow for better performance. When we’re done, the new product will fit just like the old one. This particular exhaust is made for a 1965 –1974 Alfa Romeo GTV coupe.

Before you begin, you should decide if you’re going to increase the size of the pipe. An exhaust system can only flow as much gas as its narrowest point allows, so look for restrictive areas in the original design. It does you no good to put a 3-inch tailpipe on a system that has a 2-inch segment somewhere along its line. You should also check with your engine builder or mechanic and find out if your engine requires a certain amount of resistance (called back pressure) to stay healthy and happy. Some engines also require a resonator or balance pipe to work properly.

1: Lay out the old pipe (or as much of it as you’ve got) on the floor and measure it to see how much tube you’re going to need. Buy some extra pipe in case you make some mistakes. Also, obtain the mufflers, catalytic converters, resonators, or other parts you need for the project. If you’re going to install any bungs for exhaust gas temperature or oxygen sensors, have those on hand as well. On our project, we smoothed out some unnecessary bends in the old system, but we still needed to build an arch over the rear axle.

 

Welding the new resonator onto the old Y-pipe. We had brushed off as much of the surface rust as possible and had dressed up the edges of the Y-pipe on the belt sander.

 

Cutting loose the Y-pipe from the rusted-out resonator chamber. We used a cutoff wheel for accuracy and found that the original Y-pipe piece was made of stainless steel and in very good condition under a light patina of rust. 

 

2: Look at the design of the original pipe to see if there are logical places to cut the pipe and create the new one in sections. This makes fabrication, installation, maintenance, and modification all easier. Look for welds on the old pipe where sections were joined, or just identify places where the pipe could be sectioned. An exhaust tube expander costs less than ten dollars and allows you to make slip-joint fittings with ease. If you’re concerned about keeping the pieces together, exhaust clamps are readily available at any auto parts store. Working in sections, build up the new pipe to match the original part.

3: We completed our project with just two 180-degree bend pieces and a 10-foot stick of straight tube. By measuring carefully and cutting the pre-bent pieces, we were able to use those 360 degrees of bend to create a couple of complex bends. You can tack the new parts right up to the old to make sure you’re getting the bends and the angles matched up exactly right.

 

If you get a supply of U-bends, you can cut as much or as little curve as you need out of them to match any existing bend. You might have to cut short sections, however.

 

Matching up sections of bend cut from our 180-degree bent pieces to the original muffler, which is fairly new and in good shape.

 

4: If you have identified places where you can smooth out the pipe and improve flow, make those sections as you designed them. You may also need to fabricate and weld on tabs or spears to suspend the new exhaust on the existing hangers. You may also need to weld or bolt the resonator, catalyst, or muffler into the system. If the part uses a mounting flange, you can purchase a matching flange and a gasket at a muffler shop and weld that flange onto your pipe. See the exercise in Chapter 3 about welding thick and thin parts together for the proper technique.

 

When you match a section of curve, tack weld it to the original in its desired location. You can build up the whole exhaust this way and be sure that it matches the old one.

 

Building up the precise curve that we want to clear the rear axle. We’ll tack the pieces into place and then complete our welds when we know this design is what we want.

 

5: Add in any new design features you planned to include in your project. If you need a 4-into-1 collector or other complex part, these can be purchased pre-made if they’re beyond your current skill level to make. Simple items like dump pipes are a good way to start practicing your custom fabrication skills, or they can be purchased in pre-made segments.

 

Measuring the overall length of the pipe. We’re going to eliminate that bend in the middle of the pipe. The bend was originally designed to skirt a bulge in the body pan, but since this exhaust will be installed on a race car, we don’t have to worry so much about it, and we’d rather have a straight pipe.

 

Welding the resonator to the new pipe. With this done, we’ll break our tack welds and commit to the rest of the welds in this new system.

 

6: We decided to keep the Y-pipe from the front of the old exhaust system and the muffler and tips from the rear. So we cut them off the old pipe when we had the basic parts measured and we cleaned up the mating surfaces to weld them to our new material.

 

7: Finally, assemble your new pipe and match it up against the original. Just tack-weld everything together at first. Your part needs to match critical dimensions, and it’s easy to get the length right,but lose accuracy on rotational orientation as you weld. When you’ve got everything close, go ahead and test-fit it to your car. You may need to adjust things a little bit to avoid rattles and knocks. That’s another reason to build your pipe in sections: you can make adjustments without removing (or rebuilding) the whole assembly. Then when it’s all just right, go ahead and weld it all into place.

 

The new aluminized steel of the resonator welds up nicely to the stainless of the Y-pipe. Most different steels will weld together quite well if you use the process approved for the more exotic alloy.

 

Here’s the new exhaust, ready for our final welds and then installation. It has the same overall dimensions as the old piece, a straight pipe in the center, and should flow better than the old unit. We spent $50 on the pre-made parts and this took us about 2 hours—a significant savings over a muffler shop!

 

As an extension of this exercise, you can spend some time grinding your welds smooth to give your new exhaust pipe a seamless, onepiece look. Even though most cars are never inspected that closely underneath, you’ll know it’s there.

Written by Russell Nyberg & Jeffery Zurschmeide and Posted with Permission of CarTechBooks

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