The amount of toe is expressed in degrees, as the angle from parallel of the front wheels. Sometimes Toe is expressed as the difference between the track widths of the leading and trailing edges of the front wheels.
Toe adjustment can be used to overcome handling difficulties in the car. Rear toe-out (independent rear suspension) can be used to improve the turn−in. As the car turns in the load transfer adds more load to the outside wheel and the effect is in an over-steer direction. The amount of static toe in the front will depend on factors such as Ackermann steering geometry, ride and roll steer, compliance steer and camber.
For minimum tyre wear and power loss, the steering wheels should be parallel and point directly ahead when the car is running in a straight line. Toe settings have similar effects when applied to the rear wheels.
Excessive toe-in or toe-out causes the tyres to scrub, since they are always turned relative to the direction of travel.
Angles of Toe are set in conjunction with Ackermann geometry so set steering performance in bends.
Before making any Toe adjustments, make sure the steering is straight ahead. For a truely accurate method, measure the both steering arms to identical mirrored parts of the chassis.
Toe settings affect three major areas of performance:
- Tyre wear
- Straight-line stability
- Corner entry handling characteristics.
Toe Angles :
- For a vehicle to have Zero Toe both front wheels would be parallel and pointing directly forward.
- For a vehicle to have Toe IN the leading edges of both wheels are pointed slightly towards each other.
- For a vehicle to have Toe OUT the trailing edges of the front wheels are pointed slightly towards each other
Toe in / Toe Out
So why have Toe?
Minimum tyre wear and power loss are achieved with zero toe, so why have any toe angles at all? Toe settings have a major impact on directional stability.
Toe settings becomes a tradeoff between the straight−line stability (toe-in) and the quick steering response (toe-out) and tyre wear. Driving a passenger car with low straight line stability can be very tiresome, but racing cars sacrifice a bit of stability on the straightaway for a sharper turn−in to the corners. Therefore, passenger cars are generally set up with a small amount of toe-in, while race cars are often set up with toe−out.
For more information on Toe see Ackermann Geometry
Bump steer is the change in toe angle due to wheel travel.
With the steering wheel centered, toe−in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results. The vehicle wants to drive in a straight line and the steering would naturally try to self centre.
When the wheel on one side of the car encounters a bump, that wheel is pushed rearward about its steering axis. This action also pulls the other wheel in the same direction. If it’s a minor disturbance, the disturbed wheel will steer only a small amount, so that it’s rolling straight ahead instead of toed-in slightly.
NB. With slight steering inputs, the wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.
Passenger cars are usually equipped with soft rubber bushes. Distortion in these bushes actually allow the wheels to track parallel and reduce tyre wear. Racing cars on the other hand have joints with spherical bearings with optimum rigidity, thus a road car requires greater toe-in.
A small degree of toe−in on a car with positive camber will cancel the turning tendency caused by camber, reducing wear and rolling resistance.
- Toe In promotes straight line stability.
- Too much toe-in causes accelerated wear at the outboard edges of the tyres.
Toe out promotes a very quick and light feeling steering response.
The front wheels are aligned so that if a steering wheel hit a bump the vehicle will tend to turn towards it. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it’s clear that toe−out encourages the initiation of a turn, while toe-in discourages it.
Front wheel drive cars with independent suspension are often set up with a bit of toe-out on the rear wheels, as this induces a bit of over-steer to counteract the greater tendency of front-wheel−drive cars to under-steer. Doing this on a rear wheel drive car could cause excessive over-steer, particularly when power is applied.
A small degree of toe−out for car with negative camber will cancel the turning tendency caused by camber, reducing wear and rolling resistance.
- Toe out promotes a very quick and light feeling steering response.
- Too much toe-out causes wear at the inboard edges.