![]() One reason that most trucks have live axle rear suspensions is because it completely avoids toe and camber changes with a load. This results in very heavy inner tire wear on the rear wheels of a car that is heavily loaded or towing. Negative Camber will often be greatly increased as well. This causes rapid tire wear, can cause the car to follow cracks in the road and can cause tire temperatures to rise higher than normal due to an increase in friction. When a car has been heavily loaded (if it does not have a live rear axle), the suspension will compress greatly in order to support the load resulting in an extreme amount of toe in on the rear wheels. Typically cars are loaded by having heavy loads in the trunk, many passengers or towing a trailer, thus mainly affecting the rear wheels. When a vehicle is heavily loaded, it lowers the ride height. Other vehicles after lowering will exhibit an increase in toe change compared to stock, this results in the car feeling very "twitchy" on straight, bumpy roads, and at the same time feeling unwilling to turn requiring more driver input than normal due to an increase in roll understeer. In some cases, the car will have less toe change, this can make the car exhibit less roll understeer and therefore feel more "twitchy" during a turn. After the tie rod lengths are changed, bump steer values will also change. This is accomplished through adjustment of the steering tie rod length. When a car is lowered or lifted, it will have to be re-aligned to avoid excessive tire wear. ![]() When a car is lowered or lifted, the wheels' toe setting will change. During a turn, if some or all of the bushings deflect then their pickup points have changed If any of the arms and tie rods bend then their effective length will change resulting in a change of toe.īump steer can become a problem when cars are modified by lowering or lifting, when a spring has become worn or broken causing a lower ride height, or if the vehicle is heavily loaded. Another factor that affects bump steer is bushing compliance and deflection and arm bending. This is the determining factor in designed bump steer. Whichever parts are longest tend to have less change in effective length because their arc radius is longer. As the suspension goes through bump and droop, each part follows an arc resulting in a change of effective length. The linearity of the bump steer curve is important and relies on the relationship of the control arms and tie rod pickup points, and the length of each part. If both wheels on a live axle move upwards by the same amount, they tend not to steer. Cars with rear live axles, also known as solid axles do not exhibit true bump steer, but can still cause some steering over one wheel bumps, see § difference between bump steer and roll steer. They can also be designed to have very little or no bump steer at all. Many modern vehicles have rear suspension designs which are opposite of the front suspension: Toe in under bump, and out under droop. Rear suspension can be designed a number of ways. Another example, is that when most vehicles become airborne their front wheels will noticeably toe in. ![]() For example, if the front left wheel rolls over a bump it will compress the suspension on that corner and automatically rotate to the left (toe out), causing the car to turn itself left momentarily without any input from the steering wheel. Excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads. Steering input caused by axle oscillation of a tractor īump steer is the term for the tendency of the wheel of a car to steer itself as it moves through the suspension stroke.īump steer causes a vehicle to turn itself when one wheel hits a bump or falls down into a hole or rut. ![]()
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