What Is Toe?
Toe is the angle between each wheel with relation to the longitudinal axis of the vehicle. It is usually measured when the car is static and is measured either in degrees or in millimetres. If it is measured by distance then it is usually the difference in distance between the two rear edges of the tyres and the two front edges of the tyres.
The measure of toe in degrees is usually the angle of the wheel away from the perpendicular longitudinal axis of the car where toe in is defined as positive and toe out is written as negative.
There are 3 types of toe setting:
Toe in is where the front edge of the front two tyres is closer together than the rear edge of the same tyres; giving the image of the wheels pointing towards the centre line of the car.
Toe out is where the back edge of the front two tyres is closer together than the front edge of the same tyres; giving the image of the wheels pointing out, away from the centre line of the car.
Zero or neutral toe is where the front edge of the tyres are the same distance apart as the rear edges of the same tyres, statically pointing both wheels directly ahead perfectly parallel with the longitudinal centre axis of the car.
The Effects Of Toe
Each toe setting has a separate purpose and an impact upon directional stability when used on a car. Toe has an effect on three major areas which are: corner entry handling, tyre wear and straight line stability.
On the front of the car, toe in increases straight line stability. This is because if one of the wheels is disturbed during straight line driving and is pulled rearward of the steering axis then the wheel will steer outwards.
This minimal outward steering would only straighten the wheel and develop zero toe so would help the car stay straight. However, upon turn in, the inside wheel is more hesitant to turn making for a less responsive turn in. This can be compensated for by using high Ackermann angle steering geometry.
Toe in on the rear also improves the straight line stability of the car for the same reasons as above. Toe in is also used on the rear to induce a slip angle on the tyre to increase the straight line grip and performance of the tyre to transmit as much torque as possible to the ground.
On the front of the car, toe out decreases straight line stability. When a wheel is disturbed in a straight line with toe out, pulling the wheel rearward of the steering axis, it increases the toe out and effectively turns the inside wheel more outward, pulling the car into yaw. This decreases straight line stability as it makes the car develop a twitchy nature which can be uncomfortable at high speeds.
However, running toe out makes for faster turn in on the front end.
Toe out on the rear will induce a slip angle on the tyre to increase rear end grip for acceleration. However, during cornering the toe out increases the tendency for the rear of the car to oversteer which is especially bad for rear wheel drive racing cars.
Running zero toe at the front and/or rear of the car results in minimum tyre wear and power loss. This is because if any toe is run, it develops scrub of the tyre in a straight line due to the incurred slip angle, decreasing the rolling efficiency of the tyre.