Ask Away with Jeff Smith: Why Does My Car Torque Steer So Badly?

I have an all-steel Willys coupe with an all-aluminum 498ci Keith Black HEMI with a Don Hampton 8-71 supercharger and an EFI system using electronic fuel injectors located under the hat. The engine makes killer power and it has a TH-400 trans behind it with a 9-inch rear and a spool. The car cruises down the highway very nicely. The problem is when I stand on it. The car torque steers badly to the right. The car has a complete tube chassis with a gasser style dropped front axle. The rear suspension is a parallel four-link with coil-overs and a track bar. Do you have any ideas on why the car moves to the right so hard under acceleration?

M.M.

Wow–that’s quite the combination!

Even with an underdriven supercharger and the fact that it probably only weighs about 2,800 pounds, my guess is that this is a handful to drive, even if it didn’t torque steer. Let’s start with the obvious and easy-to-solve ideas and then we can get into the more obscure stuff. This is important because at one time or another we’ve all been caught up in an issue with a car and most of the time, the solutions are relatively simple–once you know where the problem lies.

The challenge is the diagnosis.

I always attempt the simple and easy fixes first. I’ll admit that I’ve fallen victim to skipping over the obvious stuff because I was convinced some strange malady has infected my car, only to discover that the gas gauge failed and I merely ran the tank dry. I certainly felt stupid after we discovered the real problem, but the point is to always eliminate the obvious and easy fixes first.

So here are some possible causes to look for in order:

#1 Tires

My first thought was while you didn’t share tire size, with the gasser look you described, it’s obvious that you have very large tires in the back. My guess is also that these are bias belted tires. Let’s say for example that you are using a Mickey Thompson 31 x 16.5 x 15-inch ET Street tire that is DOT-legal. This is a bias belted tire that can be somewhat affected by air pressure that can change the overall diameter of the tire. I talked with tire engineer Jason Moulton with Mickey Thompson tires and he said that brand new tires can be managed with air pressure to equalize runout, but once the tire has been heat cycled, the nylon in the tire’s cords takes a set. The reason this is important is that you mentioned that your car is equipped with a spool. All street cars and most hot rods use a differential in the rear-end. The reason for this is that when you turn a corner, the outside rear tire travels a greater distance than the inside tire. The differential allows the two tires to scribe their different arcs with no problem. A spool, however, solidly ties both axles together, and does not allow for any difference. You have probably noticed that your car hops or squeals the tires when negotiating a corner. That’s because the spool does not allow the tires to turn at different speeds to compensate. This places a twisting load on the axles every time you turn a corner and can eventually cause an axle to break. This is why spools are not a good idea on the street.

Now, let’s apply this to your Willys. For argument’s sake, let’s assume that there is a difference in the overall diameter of the two rear tires. Because your tires have been used, using air pressure will probably not affect this change in circumference. Mickey Thompson also recommends looking at the date code on the side of the tire–they should be within six months of the same build date. If they are not, this mismatch may also affect circumference since tires age differently. Mickey Thompson’s tech bulletin on these tires states that a circumference difference of less than ½-inch isn’t a problem. But remember, we’re talking about the circumference of the tire at 99 inches. So a ¾-inch difference in circumference could be an issue with a spool–enough to push your car under acceleration to the right. This would mean that the tire on the driver (left) side is slightly taller than the right side tire. Circle track racers call this stagger. The classic visual representation for this is to roll a Dixie cup on its side. The tall side will push the cup a farther distance than the short side, moving the cup in a circle. Short track oval racers and Sprint Car racers use this concept to help the car turn left by placing the taller tire on the right rear. I would start by just switching the rear tires left to right. If the car now torque steers to the left, you’ve found your problem. It will probably necessitate new tires to alleviate the problem.

#2 Rear Axle

Assuming the tires are found not to be the problem, then the next suggestion would be to ensure the rear axle is square in the car. You mentioned that the car has a four-link rear suspension so the next place to look would be to either take the car to an alignment specialist to ensure the rear axle is square to the rest of the car. Or you can try stringing the car yourself. In the not-too-distant past, this is how circle track racers would ensure their car was “square”, which means the front and rear axles were set perpendicular to the chassis and the front and rear axles are parallel. Our description will just hit the high points–if you want more details, there are several sites on the web that go into more elaborate descriptions. Here’s how it works:

Set up four jack stands around the four corners of your car. Tie a string between the front and rear stands at the axle centerline height. Measure the distance from the centerline of the rear axle flange to the string and adjust the two rear stands until those distances are exactly the same. You can use a dial caliper if you like. Keep the distance short if possible–4 to 6 inches will work well. Do the same with the front stands and front axle with the exact same distance of 4 to 6 inches. With these four dimensions set, measure the distance from the leading and trailing edges of the left rear wheel. If the rear axle is square to the car, then these dimensions should be the same or very close. If the leading edge measurement is greater than the trailing edge distance on the left rear, then the rear axle is pointing to the right side of the car, creating a thrust angle that will drive the car in that direction under power. To verify your left (driver) side measurements, move to the right rear and measure the leading and trailing edge wheel distances. Here the leading distance will be less than the trailing distance. If the front is more, there’s an error somewhere in your setup and you will need to go back through the measurements again. Unless the rear axle is bent, in which case you have a whole different problem!

#3 Weight Distribution

Let’s assume that after stringing the car, the rear axle is square. Now we move to something that requires some expensive tools. If you know someone who has a set of electronic scales, you should then scale the car both with and without driver weight in the seat. We will be looking for excess weight on the left rear side that would cause the car under acceleration to push the car to the right under hard acceleration. This can be caused by a number of different variables. If you find that the two rear axle weights are more than 75 pounds different left to right, this might require looking for obvious reasons for this shift. Much of this can come from driver weight, so comparing the car with and without driver weight will help. I would also make sure there is no preload in the upper four-link arm on the passenger side. Some chassis tuners will do this to help counteract torque reaction of the chassis under load.

#4 Coil-Over Shocks

If you find nothing there, try counting the number of threads exposed on the bottom of your coil-over shocks. The numbers for both shocks should be the same. If they are not, it’s possible that the shocks have been installed incorrectly or the springs are different side-to-side. If you have adjustable rear shocks, make sure they are adjusted the same, especially with the compression settings. At least one of these issues will probably point you in a direction that will solve your torque steer problems. If not, then you may have to resort to taking the car to a professional chassis shop that can help diagnose your problem. But it’s more than likely that one or more of the above conditions are contributing to your problem. Once you find the issue and correct it, your next problem will probably be buying new rear tires all the time because that big Hemi will light ‘em up with ease!

But that’s a good problem to have!