|The Explanation of Toe Angle|
|The term tracking incorporates camber, caster and toe. Toe and camber (by the adjustment of ride height) are the only two components of tracking that can be adjusted on the MGF.|
|The toe angle is the angle between the long axis of
the car, and the long axis of the of the individual wheel.
Toe IN is where the front of the wheel is angled towards the centre of the car.
Toe OUT is where the front of the wheel is angled away from the centre of the vehicle.
|Figure 1. As set up as standard, the MGF has toe
out at the front, and toe in at the rear
So what does the toe-angle do?
Toe IN [as found on the rear axle on the MGF] provides directional stability- this provides a highly desirable trait, insofar as the car becomes less influenced by side winds, buffeting surface irregularities or other factors that may tend to throw the car off the desired line on fast roads.
Toe OUT [as found on the front axle on the MGF] provides the steering with improved feel. This is often varied by motor sports experts to increase steering response. Unfortunately, increasing the toe angle increases frictional loads upon the tyre, there by increasing the rate of wear on the inner edge of the tyre (which may already be heavily loaded if there is a significant amount of negative camber applied...
Incorrectly set toe angles in combination with an abnormal camber angle will dramatically increase rates of tyre wear- this, in large part, is the root cause of the infamous "tracking problem"!Tell me more!
Okay, read on!!!
- Excessive toe-in makes the car unresponsive and doughy to turn-in
- Excessive toe-out makes the car nervous and lacking in directional stability.
"Historically, static toe-in was used to pre-empt dynamic toe out or to deliver a certain "numbness" to the steering, hence the opposite to response. It does also deliver stability at cruise. Toe neutral is a natural state, where as toe-out is an unstable state. This can be harnessed to deliver exaggerated steering or directional change response to either front or rear. Whiteline recommend marginal toe-out on rear to allow the rear to respond quickly to directional change at the front."
If minimum tire wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other at a point along the central longitudinal axis of the car. Under this condition, the wheels are at odds with each other, and no turn results.
When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.
If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. 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.
With toe-in, a deflection of the suspension does not cause the wheels to initiate a turn as with toe-out.
The toe setting on a particular car becomes a trade-off between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-the never-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.
With four-wheel independent suspension, the toe must also be set at the rear of the car. Toe settings at the rear have essentially the same effect on wear, directional stability and turn-in as they do on the front. However, it is rare to set up a rear-drive race car toed out in the rear, since doing so causes excessive oversteer, particularly when power is applied.
Remember also that toe will change slightly from a static situation to a dynamic one. This is is noticeable on independently-suspended rear-drive cars. When driving torque is applied to the wheels, they pull themselves forward and try to create toe-in. Likewise, when pushed down the road, a non-driven wheel will tend to toe itself out. See figures on tracking page for further explanation...
The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition.
It should be noted that in recent years, designers have been using bushing compliance in street cars to their advantage. To maximize transient response, it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tires. By allowing a bit of compliance in the front lateral links of a wish-bone type suspension, the rear axle will toe-in when the car enters a hard corner; on a straightaway where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tire wear and stability characteristics. Such a design is a type of passive four-wheel steering system. This is how the MGF rear suspension is set up: see figures on tracking page for further explanation...