What the heck is torque steer? (ABCs of Car Tech)

You may have heard the Car Tech guys mention "torque steer" in our reviews. Today, I explain just what that phenomenon is and how to deal with it.

Josh Miller/CNET

Torque steer is a phenomenon that mostly affects front-wheel-drive cars where heavy acceleration causes the vehicle to veer to the left or right.

Take a front-wheel-drive car to a wide, open space where you can test acceleration from about 0 to 15 mph without hitting something or someone (perhaps a big empty parking lot). Take both hands off of the steering wheel and give the accelerator a brief but firm squeeze. What did you notice? For most FWD cars, you likely saw the steering wheel twisting to one side and the vehicle lurching to either the left or to the right. Some cars pull more than others, but what you just witnessed is torque steer.

Torque steer can be scary because it can be unpredictable, particularly for novice drivers. However, because you have to be on the throttle to even experience torque steer, it's not something that most people ever have to deal with. From an enthusiast's standpoint, you want less torque steer simply so that you can go faster in the direction your car is actually pointed.

What causes it?

In a rear-drive vehicle, the half-shafts are equal in length and react identically to a load. Antuan Goodwin/CNET
Torque steer can be caused by a number of factors, such as a difference of traction available beneath the two drive wheels, or a difference in the inflation levels of the tires causing the two sides to grip unequally. However, the most common cause of torque steer is a compromise that is inherent to the front-engine, front-wheel-drive configuration: the transversely mounted engine.

In most rear-wheel-drive vehicles, the engine, transmission, driveshaft, and differential sit in a nice line that runs down the center of the vehicle. The central positioning of the differential means that the half-shafts that connect the wheels to the differential are of equal length. This means that the half-shafts should ideally react to forces and deform in a roughly identical manner to one another.

In most front-drive vehicles, there is no driveshaft. The engine, transmission, and differential are combined into a single package. Because this package has to still fit in an engine bay with its weight more or less centered between the wheels, the transmission and differential end up hanging off the end of the engine block to one side of the vehicle. This configuration results in half-shafts that are of unequal length.

In a front-drive vehicle, differences in half-shaft length cause the drive and steering wheels to react differently to the same load. Antuan Goodwin/CNET

Without getting into the physics of material deformation, suffice it to say that half-shafts of unequal lengths react to torque loads differently. This causes one drive wheel to put power to the road more efficiently and get slightly ahead of the other, and because the drive wheels are also the steering wheels, you'll feel the vehicle pull to one side and the steering-wheel twist.

The effect is compounded by the fact that sudden acceleration causes the weight of the vehicle to rock back to the rear axle, causing the front wheels to unload and, in the case of McPherson strut suspensions, gain a bit of positive camber and possibly a bit of toe out. Again, I'll leave the specifics of suspension alignment for a later discussion, but suffice it to say that this combination can multiply the twitchiness of the vehicle's steering.

Once the initial shock of the torque load has passed, the half-shafts snap back into sync and the vehicle settles back to more or less its static weight distribution, which is why you only really feel torque steer when you jump on the throttle and during gear changes and not so much at, for example, freeway speeds.

How do automakers deal with it?
For some of you, our little hands-free experiment may have been the first time that you ever recognized that your front-driver was capable of exhibiting torque steer. That's because automakers use a number of techniques to mitigate or eliminate torque steer.

The first and most common technique is to simply tune the power-steering program to compensate or hide the effect. When the electronic power-steering system detects torque steer, it can counter the effect automatically. Additionally, the power-steering boost can just be cranked up to the point where you'd barely notice it when holding the wheel straight with your hands, and since most drivers don't do hands-free zero-to-sixty runs, no one is the wiser.

Because torque steer is caused by, well, torque, limiting engine output until the accelerating vehicle gets its legs under itself also works. This is the technique used by Mazda in the Mazdaspeed3 . Boost from the 2.3-liter engine's turbocharger is limited in first and second gear, reducing the normally 280 available pound-feet of torque to a level that's a bit more manageable. Even with this "Advanced Torque Management" system doing its thing, the 'Speed3 still exhibited a noticeable level of torque steer during our testing.

Another solution is to use some sort of differential technology to control what wheel gets the torque. However, because the effects of torque steer happen downstream from the diff, there's little a traditional mechanical differential can do to help, and in some situations, the way they send power to the wheel with the most ability to use it can actually create even more torque steer. Electronic differentials that can be programmed to recognize torque steer and compensate or brake-biasing virtual differential systems typically fare better here.

The Fiat 500 Abarth uses equal-length half-shafts to mitigate the torque steer that's common to powerful front-drive vehicles. Wayne Cunningham/CNET

Finally, the best way to fix torque steer happens at the vehicle and suspension design level. Employing packaging tricks to mount the differential closer to the centerline of the vehicle can result in equal-lengh half shafts, which can cancel out torque steer. The Fiat 500 Abarth uses this system. Additionally, employing better suspension geometry than is afforded by the standard McPherson strut setup, such as the RevoKnuckle system made famous by the 300-horsepower Ford Focus RS, can help to keep the steering wheel straight.

What can I do to limit torque steer?
The best way for you to limit the torque steer in your current car without dramatically altering the engine bay and suspension geometry is to exercise restraint with your pedal foot for the first few feet of acceleration. The Internet is awash with accounts of backyard tuners experiencing success with everything from chassis braces and adjustments in alignment to stronger bushings for the engine mounts and various suspension components. However, there is comparatively little documented proof that any of these bolt-on solutions really work consistently.

 

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