IMCA Stock Car: November 2000 - The Rollcage

CSC recommends checking the squareness of the frame. I did that, finding that it was OK. “OK” means nothing was quite right but it couldn’t really be improved upon. One measurement traded off for another. However, when placed on a level surface it rocked like a teeter-totter. Two 8-foot-long pipes, a 200-pound gorilla on one and a pit puppy on the other removed the twist.

I spoke with de Winter and found him helpful and understanding. He was more than willing to work with me on the custom kit. Several items are nonstandard, such as the rearward-placed cage, the front downtubes, and the “third beam” internal bracing. CSC can make anything you want. Front weight is a problem on these cars, so I designed the front to be light. You can build a tank or a race car.

A concrete floor is seldom flat and level. Before chassis work started, I made four stands with similar height—about 22 inches. Each has a stud welded to the top so it can fit in a hole in the frame. The studs keep the car from slipping off the stand during construction. The frame was placed on the stands and then shimmed to level it. Leveling is necessary for the kit parts to fit properly.

If this is your first time to tackle a chassis/rollcage project, make sure you have the tools to do the job. A welder, preferably a MIG, commonly called a wire welder, of at least 130 amps. I have built six race cars with one of these. A reason to choose a larger welder is to get a longer duty cycle. The duty cycle determines how long one can weld until the machine heats up and turns off.

Larger industrial models have a 100 percent duty cycle, and smaller ones often have a 20 percent cycle.

You will need an oxygen/actylene torch. It should have included a cutting head. Besides a complement of hand tools, an abrasive wheel cutoff saw is a must. It will enable you to cut and notch tubing at various angles. A 4½-inch hand grinder goes a long way in fine-trimming tubing to fit. It will also grind off those tack-welds where you put a tube in the wrong place.

Before you start cutting and welding, though, refer to the IMCA rule book. It is very specific as to where body metal may be removed. In a broad sense, if it bolts on or is in the way of the roll- cage and supporting tubes, it may be removed. You are allowed “2 or 3 inches” clearance, and clearance to weld as necessary. There are some exceptions. In my case, I kept every piece of body metal removed from the car and tossed them all in a box, just in case I had to weld something back in. Start the assembly with the rear hoop of the cage. Positioning of this hoop is critical. The rest of the cage is built in relation to it.

Recall in the November 2000 issue, the body was mounted and spaced up 1½ inches with square tubing to raise it to stock height. This was necessary when the rubber biscuits were removed. The spacing lets the body shift ¾ inch rearward. The body bolts can still be screwed into their holes. This is required for legal location. Measure the rear hoop with it standing on the floor of the shop. From the floor to the top of the tubing, I cut it to 45½ inches. This gave me 3/8-inch clearance under the roof. Always check for proper measurements. Don’t cut it off twice, or it’ll be too short.

IMCA says the front edge of the rear hoop can be no more than 41 inches from the center of the windshield opening. I took advantage of this to shift weight to the rear. After the rear hoop was welded in place, I realized if the body took a hit and shifted forward, the car would be illegal. I could have moved the body forward an inch and it would have taken only 6 pounds off the rear. Leave yourself a little room.

The rearward legal location of the rear hoop means it will fall in behind where the frame tucks in. This requires building a box mount. You will need to have some 1/8x2-inch flat steel. Get about 8 feet, as you will need to make some other brackets before the car is finished. Cut eight pieces 4 inches long. Two of these must be notched to go halfway around the tubing. Use the torch if no other way is available; just be sure to grind off all the slag before welding it in place. These eight pieces will make both rear hoop brackets.

Set the height of the hoop by clamping plates to the bottom of the frame on each side. Let the hoop rest on them while you test-fit the roof. Using a level, make sure the hoop is perpendicular. Now assemble the parts of the box for both sides, and tack in place. Note, you will need a sizable hole in the body to allow access for welding. A reinforcing triangle from the hoop to the top of the chassis is good insurance. You should be able to follow the kit instructions for the rest of the main cage.

In the rear, attach the rear hoopsupport bars to the frame between the rear-spring center hole and the shock mount hole. Place the curved end up and tack it to the hoop plate. This close coupling adds stiffness to the chassis where the load is applied, at the springs. It also isolates rear damage from the main structure.

Next, install the X under the trunk floor. Then make two small hoops, 16 inches long and 10 inches tall. Cut the open ends at a 45-degree angle. These weld on top of the frame on each side. Let the top of the hoop rest against the rear fender sheetmetal and then weld.

A spreader bar across the rear ties them together. It will be braced to the bumper when the bumper is installed. This protects the 8-gallon ATL fuel cell, which will be mounted to the X.

At the front of the car, notice the downtubes. This is a departure for many stock cars. My work with balsa stick models has shown me that this is the stiffest and lightest way to make the front end. Double check for upper A-frame clearance when in the full bump position. This will be all the tubing up front except for that protecting the super-light C&R radiator and mounting the new QA-1 shocks.

There is one more area of construction to cover. For this we must revert to the balsa model. This car uses a “third beam”–type structure. If a stock car chassis was sliced vertically from end to end, you would have two beams or bridges. Each could support a considerable vertical load, but they would be flimsy when twisted. With the crossbars welded back together, there would still be a lot of torsional flex. The addition of a third beam running diagonally from the left rear to the right front adds tremendous stiffness to the car. It is a long triangle, and its lower tension member is 92 inches long. This bottom member and top tube attach to the same bracket at the left rear. The top tube runs forward to the front-cage crossbar. The third triangle member begins at the front-cage crossbar and stops at the lower right front behind the A-frame. It is the long lower tube that makes this concept work.

When building an IMCA Stock Car like this one, ask de Winter at CSC Racing Products for an IMCA Stock Car kit with front downtubes and short, bent rear kickers. He will also have materialfor the rear X, rear side braces, and the third beam. Altogether, the CSC Racing Products kit will net you a strong, stiff, safe, and well-handling race car.

With all the tack-welding done, welding begins in earnest. When I welded all I could comfortably, I let the car down and scooted it outside. With the help of the engine hoist, I rolled the car up on its side at about a 60-degree angle and blocked it securely. Remember, the roof has not yet been installed. It sure is nice when those overhead welds become right-side up. Be careful—the car weighs about 1,000 pounds at this point. Do not work under the car when it is suspended only from the engine hoist.

Before the next issue, your chassis should be welded and ready to take on some life with its suspension. Coming next month: springs, shocks, weight jacks, rearend, and so on.

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