Torque steering
Encyclopedia
Torque steering is the influence of engine torque on the steering
for some 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 patch
es of the left and right drive wheel
s. 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
.
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.
Steering
Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel or vehicle to follow the desired course...
for some front-wheel drive
Front-wheel drive
Front-wheel drive is a form of engine/transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement generally found in rear-wheel drive and...
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 patch
Contact patch
Contact patch is the portion of a vehicle's tire that is in actual contact with the road surface. It is most commonly used in the discussion of pneumatic tires, , where the term is strictly used to describe the portion of the tire’s tread that touches the road surface...
es of the left and right drive wheel
Drive wheel
A drive wheel is a roadwheel in an automotive vehicle that receives torque from the powertrain, and provides the final driving force for a vehicle. A two-wheel drive vehicle has two driven wheels, and a four-wheel drive has four, and so-on....
s. 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
Steering kickback
Steering kickback relates to the sharp and rapid movements of an automobile's steering wheel as the front wheels encounter a significant obstruction or imperfection in the road...
.
Causes
Root causes for torque steer are:- Incorrect sidewall ply design allowing deformation of the tire sidewall.
- Assymmetric driveshaftDriveshaftA drive shaft, driveshaft, driving shaft, propeller shaft, or Cardan shaft is a mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement...
angles due to any combination of- Unequal driveshaft length or diameter
- Transient movement of the engine
- Tolerances in engine mounts
- Body rollBody rollOn wheeled or tracked vehicles, body roll is a reference to the load transfer of a vehicle towards the outside of a turn. When a vehicle is fitted with a suspension package, it works to keep the wheels or tracks in contact with the road, providing grip for the driver of vehicle to control its...
- 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 (µ-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 shaftHalf shaftThe term half shaft is another name for a front-wheel drive shaft, which is also called the constant velocity axle or CV axle. There is one half shaft per wheel. Some may further to this part as a CV 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 usually result in negligible effect. 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 (king pin).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 radiusScrub radiusThe scrub radius is the distance in front view between the king pin axis and the center of the contact patch of the wheel, where both would theoretically touch the road....
an optimum solution is not generally possible with simple suspension configurations such as Macpherson strutMacPherson strutThe MacPherson strut is a type of car suspension system which uses the axis of a telescopic damper as the upper steering pivot. It is widely used in modern vehicles and named after Earle S. MacPherson, who developed the design.-History:...
. - 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 differentialLimited slip differentialA limited slip differential is a type of differential gear arrangement that allows for some difference in angular velocity of the output shafts, but imposes a mechanical bound on the disparity...
s, intended to increase power transfer, actually make torque steer much worse. For this reason, limited slip differentials by automobile transmission manufacturers like QuaifeQuaifeR.T. Quaife Engineering, Ltd. is a British manufacturer of automotive drivetrain products. It designs and manufactures replacement gearboxes and gear sets, some of which are for four-wheel drive systems....
, TorsenTorsenTorsen is a type of differential used in automobiles. It was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a contraction of Torque-Sensing...
, 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. AudiAudiAudi AG is a German automobile manufacturer, from supermini to crossover SUVs in various body styles and price ranges that are marketed under the Audi brand , positioned as the premium brand within the Volkswagen Group....
AWD cars equipped by TorsenTorsenTorsen is a type of differential used in automobiles. It was invented by American Vernon Gleasman and manufactured by the Gleason Corporation. Torsen is a contraction of Torque-Sensing...
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.