December 2000 - Adding the Suspension

In this third installment of building an IMCA Stock car, we will finally get to see some moving parts. The front suspension, the rear suspension, and the brakes will go on the car. In addition, you will learn how to make some fittings and install plastic brake lines.

QUITE A SHOCK

I like high-mounted front shocks. Different, perhaps, but they are up and out of the way. They mount on top of the A-frame. This may look unusual, but it works well. I have used this system on several cars in the past and liked the results.

The shocks themselves are prototype QA-1s. They should become available about the time you read this. One big advantage is their rebuild feature. Dealers will offer a rebuilding service, or a skillful racer can do his own. Also available will be a valving kit that can re-valve four units. This should cut a racer’s shock inventory somewhat and at the same time allow custom-shock rates.

QA-1 describes the shock as a deflective-disc valve design. This means it is more accurate on a repeating basis in rate than some others. Also, the gas-bag is of cellular construction and designed so it cannot fail all at once.

I mounted the front shocks on top of the A-frame for two reasons. First, the shock is out of the way and should survive more of those competition adjustments the front end will receive on the track. Second, according to QA-1, mounting the shock closer to the wheel means a lighter valve rate can be used. This makes the shock more responsive and less likely to fade.

When mounting the front shocks on top of the A-frame, the key is the homemade bracket. Start by slicing through an old ball joint with an abrasive wheel cutoff saw. Then punch or drill out the rivets or spot welds. Now you will leave two plates with bolt holes matching the A-frame holes. Weld a shock mount from Stock Car Products to each of these plates. This will form a right and a left unit.

To assemble, put the shock mount on top of the A-frame and the ball joint below. Run ¼-inch bolts through all three pieces. These mounts should outlast several A-frames and spindles. If you mount shocks to the lower A-frame, you must weld on new brackets each time the A-frame is replaced.

The top mount for the shock looks like a grasshopper leg. It is a piece of 1-inch tubing bent to an enclosed angle of 60 degrees. Each tube should be placed so its center is on a line from spindle to spindle. The rear leg is welded inside the main downtube, and the front one is outside the downtube. It welds both to the chassis and the downtube. Measure vertically at the center of the bend. The top of the tube should be no more than 18¼ inches from where the weight jack nut welds to the chassis. The hood will rest on the tubes.

Locate the top mount as follows. Attach the shock to the bottom mount. With the spring removed, raise the suspension until the lower A-frame contacts the chassis. Compress the shock, protect it from weld splatter, and bolt on the top mount. Clamp the shock mount to the tube and weld it in place. It should be nearly vertical when viewed from the side and tilted toward the center of the car at 20 degrees viewed from the front.

Since the QA-1 shocks are rebuildable, they are a bit longer and must mount to the inside of the grasshopper bar. Shocks that cannot be rebuilt mount to the outside.

To properly locate the yoke on the bar, jack the front suspension up until the A-frame contacts the chassis. With the bottom mount secured, compress the shock and bolt it to the yoke. Now you can tell where to weld the yoke to the grasshopper bar.

With the shocks mounted on top of the spindle, a No. 5 valve was deemed a good starting spot. Typically with a lower A-frame mounting position, a No. 7 or No. 8 valve would be required.

FRONT WEIGHT JACKS AND SPRINGS

While you have the welder at the front of the car, weld in the front weight-jack nuts, too. Be sure to enlarge the existing hole in the frame, and have the jack bolt in place when you weld the nut. My weight jacks and springs came from Stock Car Products. I first ordered 6-inch jack bolts for the front. These were too long and touched the A-frame. I traded them for the 4-inch length.

I spoke with Butch Zervakis and others at Stock Car Products about spring selection. I knew I wanted 5½x9½-inch front springs. In consultation with these folks, we chose to start with the following rates: right front, 1,100 pounds; left front, 950 pounds. Spring rates must be tailored to the track, the car, and the driver’s style, so don’t hesitate to make changes. Look for an upcoming article in Stock Car Racing on how to select spring rates.

REAR WEIGHT JACKS, SPRINGS, AND UNDERBARS

At the rear, I have added a new wrinkle. Perhaps I should call it a new curve. These pieces are called underbars. Seldom seen on a stock car, they add greatly to the strength of the rear area of the frame. I first used them 10 years ago on a stock car. Modifieds and sprint cars have been built like this for years. These bars do not limit rear-end movement. One nice touch is to make it easier and safer to install the rear end. Lay it up on the bars, then leisurely hook up the four trailing arms and other pieces.

These bars weld to the chassis and go under the axle while clearing the trailing arms. The guy running the tube bender doesn’t like them because of the goofy angles. Even so, CSC Racing, the rollcage supplier for this car, will make them for your cage kit, too—just ask. The front of the underbar fits into a body-mount hole in the bottom of the chassis. The rear welds to the frame where the chassis straightens out behind the axle. The bottom should be parallel to the ground. There needs to be 12¼ inches between the bar and the frame.

For the rear-axle assembly, I chose a 9-inch Ford floater from JR Motorsports. It was complete, even down to new pads to go on the rebuilt calipers. After speaking with both Brent and Steve at JR Motor-sports, we decided the 6.50:1 ratio would be best for the quarter-mile tracks where we will be racing. That will allow the Mike’s Transmissions Powerglide to run in top gear, which improves reliability.

I asked about several things in reference to selecting the rear. One was the location of the lower trailing-arm mounting holes on the housing. IMCA takes the stance that if the mounting holes on each side are the same distance away from the housing, it is OK. The JR Motorsports unit lowers those holes 15/16 inch, which is what I wanted.

Lowering these holes increases the anti-squat in the rear. Anti-squat concerns the tendency of the rear of the car to rise upon acceleration. Too much will allow the car to visibly rise, while too little will find the rear drooping out of a corner. When the car tends to rise (or at least not droop), the wheels are trying to drive under the car. This adds rear weight under acceleration. Too much of a good thing will make the car skittish or loose under braking and corner entry. So I believe 15/16-inch lower mounting is all that is necessary.

Another concern was whether I needed a mini- or a full spool. Steve at JR Motorsports answered my question. The full spool reduces weight. A rear assembled with a stock carrier and a mini-spool is 12.95 pounds heavier than one with the full spool. This is rotating weight as well as unsprung weight, both of which are detrimental to performance. The full spool wins.

With a choice of 31- or 28-spline axles, I choose the 31 spline. Steve told me they seldom ever see a broken 31-spline but sometimes see the 28 splines broken.

Floater assembly is rather straightforward. Clean the housing of the shipping grit that accumulates on the gears. The pumpkin should be installed first. Mine still had the tire white marking compound on the gears. This enabled me to see the tooth-contact pattern, and I was satisfied it was correct. Check the machined surfaces on the outer ends of the housing for burrs that might have been incurred in transit. Be sure to pre-lube bearings and seals. Don’t try to shorten the number of bolts holding the rotor to the hub. This can cause a warped rotor or worse.

The spring perches must be welded to the rear end housing. Note the welds are on the sides, not the top of the housing. This helps eliminate warpage. Before welding, I chopped ¾ inch off the height of the spring-locator tube. This was done to make spring removal easier.

As with the front, I used the original hole, enlarged for the weight-jack bolt. It would be best to install these before welding in the rear kicker bars or the cage. I didn’t, and the bolts hit the bars. A 1/8-inch shift is sufficient.

I used 11x5-inch springs in the rear. The starting rates are right rear at 200 pounds and left rear at 225 pounds. I change springs often to try and find the best combination in any given track condition.

Both rear shocks carry No. 5 valving and are 9-inch stroke.

GIMMIE A BREAK

All the calipers on this car are stock GM metric calipers. IMCA requires the calipers to be stock and similar front to back.

About 25 years ago I began using a plastic tubing for racing hydraulic disc-brake applications. It has a working pressure of about 2,400 pounds. I have used it on sprint cars, modifieds, and stock cars with never a problem. In all these applications I have attached the plastic tubing directly to the caliper with the right fittings. It is an inexpensive, light, and easy way to plumb in a brake system for a short track car. Do not substitute any other tubing for what comes in the kit. These brake-line kits are available from Speedway Motors. If you need additional fittings, check your local auto-parts store.

As a test, I lit the cutting torch and turned on only the acetylene to get a 12-inch flame. I applied this to a section of the tubing for 45 seconds. I determined that no damage had been done.

That said, for this car, here is the way to make fittings for the occasion. To do this, first purchase four 7/16-inch brass exhaust-manifold–to-head pipe nuts—the long ones, not the short ones.

When clamping the nut in a vise, clamp on the sides, not on the ends. This will prevent damage to the ends, which will be the sealing surfaces. Use a “Q” drill (.339 inch), to drill a hole in the center of one side of the nut. Thread this hole with a 1/8-inch pipe tap. Screw a straight fitting from the plastic brake-line kit tightly into this hole. Now drill out the original threaded hole with a 13/32-inch drill bit. Do this four times, and you have the fittings necessary for all four calipers. Use the original brake-line attachment bolt with no washers. Brass is softer than steel, so the parts will seal just like the originals.

The plastic brake line should be tie-wrapped to the axle housing or the A-frames to hold it in place. Then route it to the master cylinder. Try to keep the lines running uphill to the master cylinder; this will make brake bleeding easier. The translucent tubing also will allow you to see any bubbles in the lines. Be sure to protect the tubing where it passes through sheetmetal. Rubber fuel line, tie-wrapped in place, works well.

NOTES and UPDATES

Since November, I’ve found a better way to make the rear hoop of the rollcage. Don’t go to the trouble of building a box to the frame, as I did. Instead, the rear hoop can be bent in at the bottom to fit the narrowed part of the frame. Ralph de Winter at CSC Racing now has these dimensions and will bend yours accordingly.

Kirk Niehouse at IMCA had some concerns about the location of the third beam in the car, as described in the November’s “Building an IMCA Stock Car.” I checked; with our driver in place, there seems to he plenty of room. Still, check your clearance. If you need more room, bracket over, but keep the bracket stiff.

Click Here to Return to the Building an IMCA Stock Car home page