Torque steering is the influence of engine torque on the steering, especially in front-wheel drive vehicles. For example, during heavy acceleration the steering may pull to one side, which may be disturbing to the driver. The effect is manifested either as a tugging sensation in the steering wheel, or a veering of the vehicle from the intended path. Torque steer is directly related to differences in the forces in the contact patches of the left and right drive wheels. The effect becomes more evident when high torques are applied to the drive wheels either because of low transmission gearing, high engine torque, or some combination of the two. Torque steering is distinct from steering kickback.
Root causes for torque steer are:
- Incorrect sidewall ply design allowing deformation of the tire sidewall.
Asymmetric driveshaft angles due to any combination of
- Unequal driveshaft length or diameter
- Transient movement of the engine
- Tolerances in engine mounts
- Body roll
- Single wheel bump
- Different driveshaft torques left to right (due to wheel bearing or differential problems)
- Suspension geometry tolerances
- Unequal traction forces due to road surface ( -Split) in combination with kingpin offset
Ways to reduce the effect of torque steer
- Employ the use of a tire with proper sidewall ply design, mitigating the sidewall deformation.
- Have both driveshafts of the equal length by using an intermediate shaft (or "lay shaft") on one side of the transmission. This is already implemented on most modern cars. When the driveshafts have different length and excessive torque is applied, the longer half shaft flexes more than the shorter one. However, this is a short term transient effect. To avoid fatigue failure, the amount of drive shaft torsional deflection must necessarily be small. Effects due to one wheel spinning more slowly than the other are usually negligible. Equal lengths of the driveshafts, in the case of no asymmetric suspension deflection due to roll or bump, keep the drive shaft angles equal. The main component of torque steer occurs when the torques in the driveshaft and the hub are summed vectorially, giving a resultant torque vector around the steering pivot axis (kingpin). These torques can be substantial, and in the case of shafts making equal angles to the hub shafts, will oppose one another at the steering rack, and so will cancel. These torques are strongly influenced by the position of the driveshaft universal joint (CV joint) in relation to the steering axis, however due to other requirements such as achieving a small or negative scrub radius an optimum solution is not generally possible with simple suspension configurations such as Macpherson strut.
- Equalise the torque better between the driveshafts by using a low friction differential. The torque difference is zero if the differential is frictionless, and limited slip differentials, intended to increase power transfer, actually make torque steer much worse. For this reason, limited slip differentials by automobile transmission manufacturers like Quaife, Torsen, TrueTrac, Gold Trac have not been much used until recently, and require other measures to be implemented, such as careful positioning of suspension pivot points and driveshaft CV joints, in order to keep the resultant torque steer to a manageable amount. Limited slip differentials do not improve cornering, or steering feel, however they will improve power transfer in situations where one wheel experiences limited adhesion, and so may improve overall performance.
- Reduce the amount of torque from the front axle by passing part of torque to the rear axle. This is achieved on all-wheel-drive (AWD) vehicles with full-time AWD, e.g. with mechanical gear-based transaxle differential, e.g. Audi AWD cars equipped with Torsen transaxle differential.
Power assisted steering (set on most modern cars) make the torque steer effect less noticeable to the driver. Steer-by-wire also hides the effect of torque steer from the driver. EPAS can be calibrated to directly suppress the torque effect at the handwheel, and the steer effect on the vehicle.
Rear-wheel-drive vehicles still are affected by torque steer in the sense that any of the above situations will still apply a steering moment to the car (though from the rear wheels instead of the front). However, the torque-steer effect at the rear wheels will not send any torque response back through the steering column, so the driver will not have to fight the steering wheel. Additionally, almost all rear-drive vehicles designed with half shafts use equal lengths.