When a vehicle pulls to one side, many folks think it is time for a wheel alignment. Torque-steer often produces the same symptom, but requires a very different repair.
Where does a torque steer come from?
A torque steer is a tendency for the vehicle to pull to one side, as the engine drives the vehicle. This is common on acceleration, and results from an imbalance of power applied to the drive wheels. This may result from several causes and may be inherent in the design of some vehicles. If the torque steer is not severe, we may hardly notice it. A mild torque steer does not necessarily represent a problem. For instance, most people do not take their hands off the steering wheel when accelerating and may not notice the issue. When the torque steer is worse, it is noticeable most of the time and we need diagnosis to find the cause.
When we drive at highway speed, the engine is supplies constant power to the drive wheels. Though we may not consider this acceleration, the effect is on our vehicle is similar. If the engine transmits more torque to one side, the vehicle will roll in that direction. The driver experiences this as a pull, felt in the steering wheel. The pull feels similar to an alignment problem except it goes away when we release the throttle.
Front, rear and all-wheel drive
Depending on design, the front, rear or all wheels may supply power to the road. Front-wheel drive uses the forward wheels only. Rear-wheel drive vehicles are propelled by the rear wheels alone. Some vehicles use a four-wheel or all-wheel drive. The difference is in the way the vehicle was manufactured and a torque-steer is possible with all three types.
The two distinct types of torque steer
The first, and the most common type produces a noticeable pull to one side on initial acceleration. This is when the steering wheel turns to one side, when taking off. Releasing the throttle makes the problem far less noticeable. The vehicle may also drive straight once speed is obtained.
A torque steer can also produce a steady pull in the steering, while the throttle is applied. This may initially be a less severe pull, and is often misdiagnosed. Applying constant pressure to the steering wheel to maintain straight tracking may feel like a wheel alignment or tire issue. To diagnose the problem, the vehicle is driven at the speed, at which the pull occurs. The throttle is released, and the vehicle is shifted out of gear. A pull to one side, which goes away when in neutral, shows a torque steer rather than a wheel alignment issue.
What are the causes of a torque steer, type one?
With the first type, loose steering components are common, particularly on the front-wheel drive.
Any movement in suspension components, can allow the wheels to steer in one direction.
On vehicles designed with reaction bushings, slack can cause the wheel to steer on acceleration. Other common causes include a tire low on air pressure or a tire with conicity. To test for tire conicity, the tires can be temporarily moved from side to side. If the vehicle now pulls in the other direction, a tire issue is likely. Any tire problem should be resolved before attempting further diagnosis.
Rear-wheel drive vehicles may exhibit torque steer when the rear suspension shifts, due to worn components. Control-arm bushings and tie rods may allow the drive-wheels to shift under acceleration. Movement changes the thrust angle and can cause the vehicle to pull. A close examination of components in the rear should reveal slack.
What are the causes of a torque steer, type two?
Type-two diagnoses of a torque steer may be more involved. A front-wheel drive must divide the torque very equally between the wheels to avoid pulling under load. Engineers may use several things to help design vehicles that drive straight.
Front drive axle length is one factor in controlling torque-influence. Axles of near equal length tend to transmit power more consistently. This is why many vehicles use a carrier bearing to split axles of greater length. With a divided-axle, "pivot length" of the axles remains constant, though the overall shaft is not.
When a design has a natural tendency to pull on acceleration, axle length may be used to correct. Shorter axles transmit more torque and can counteract other issues. The diameter of the axle may also have an effect. Sometimes a hollow axle will be used on one side and a smaller diameter, solid axle on the other. Wheel camber can also help, and some specifications take this into account, when the wheel alignment is set.
The angles of the drive components are also critical. Equal angles better transmit the same amount of power. An engine or suspension, out of level will change the drive-shaft angle. With other factors equal, a vehicle will pull toward the side with the lower angle. Bad engine mounts, bent frame or body components are often a cause for this type of misalignment.
Changes to axle angle, in either plane, will affect the torque transmitted. The more of an angle, the less torque is transmitted. Such problems are often seen on vehicles that have sustained damage. Driving over curbs, large pot holes or into a ditch are a few causes of this type damage. A high-quality frame and alignment shop can correct this problem. Many vehicles use bolt-in engine cradles to support the drive-train. Loose or bent bolts, slipped adjustments and worn support cushions can allow movement.
Differential problems that cause a torque steer
Wear and damage within a differential assembly may also cause more torque to be applied to one wheel. This can occur with front, rear or all-wheel drive vehicles. In a front-wheel drive, the differential is normally part of the trans-axle assembly.
The outside wheel travels in a larger radius in a turn than the inside. This means the outer wheel must rotate faster to keep pace, and the differential allows this. When the gears of the differential become worn or damaged, they can influence the power applied to the axle. This can produce a torque steer, in front or rear-wheel drive.
Differentials are the lease likely cause for a torque-steer. Testing for differential-influence is complex and is usually left to a specialist. Indication of a problem may be inferred by noise, burned fluid or excess metal in the lubricant and case. With any of these conditions, repair is necessary.
A persistent pull in the steering can indicate a torque steer and not a wheel alignment issue. Proper diagnosis is the key to repair, and AGCO can provide solutions in either case.
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