The most important thing and something that cannot be stressed enough is the fact that alignment settings are NOT an “absolute truth”. It is a dynamic process with the “correct value” varying from driver to driver, car to car not to mention the driving environment.
If you think of wheel alignment geometry as a means by which to pre-empt dynamic changes to the tyres attitude it will be a lot easier to understand what is appropriate. Though a very simplistic definition, it helps us understand that maximising tyre contact while driving and cornering is the object of the whole exercise. So, in deciding an optimum static alignment angle strive to make predictions as to what angle the tyre will take under various dynamic conditions so to pre-empt these changes.
For example, having lots of negative camber on a drag car would be fairly pointless as cornering is not an issue but traction is, hence the amount of negative camber on a general purpose vehicle is often a trade off between conflicting functions.
One of the only measures of success in evaluating the “correct” angles is to use a pyrometer to measure tyre temperatures across the face of the tyre after use. This and a good physical examination of the tyre will reveal many things and will ultimately determine the accuracy of your predictions. We don’t all have pyrometers so a good inspection of tyres after every track session will give an indication of how good the set up is.
Camber is the inclination of the wheel from the vertical when viewed from the front. When the top of the wheel leans out you have positive camber, lean in equal’s negative camber.
Static negative camber is used to compensate for body roll, body distortion and tyre roll under when cornering. Stiffer bodies and tyre sidewalls are in, and unnecessary suspension compliance is out. Therefore requirements for large amounts of static negative camber are disappearing.
Its important to highlight the fact that camber settings are ultimately a personal thing. The final number will depend on driving style, average driving conditions, tyre size and many other things. You need to start with a relevant number, then monitor tyre wear and compensate if necessary.
Dynamic & static
A vehicle will have different caster, camber and toe readings when it is moving compared to when the alignment was done in static form in the workshop.
In an ideal world, all wheel alignments would be done on a dynamic wheel aligner but these are expensive and quite rare. This concept is very important, as the only suspension angles that really matter are those present while the vehicle is moving (dynamic). What is done to the vehicle’s alignment while the vehicle is stationary, (static) is a process of trying to predict the levels of change while the vehicle is moving and setting the angles according to these predictions.
Toe-in or toe-out?
Toe is the difference in distance between the front and rear of the tyres. Historically front toe was the only thing that could be modified as rear wheel drive vehicles using a rigid axle had fixed rear toe, usually neutral. Obviously this doesn’t apply to Skylines and certainly not GT-Rs!
- Excessive amounts of either will adversely effect tyre wear.
- Excessive toe-in makes the car unresponsive and doughy.
- Excessive toe-out makes the car nervous and lacking in directional stability.
- Minor values are useful for improving turn-in, steering response and masking torque steer.
For a 4wd car rear toe setting are important – rather than just traditionally focusing on the front
Both front and rear will tend to toe-in marginally under acceleration (torque application), the front more so than the rear. Although fitting the anti-lift kit will reduce this. The body lift component of accelerative forces will have minimal effect on toe change, albeit towards toe-in. However, friction (resulting from grip under way) will force some toe-out which is cancelled by drive torque with the net result being close to neutral.
Setting up a toe-in front for turn in creates 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.
Negative scrub radius geometry (ed: I knew that!)) leads to toe-out off acceleration or when cruising with low torque. If we remove torque from the equation, there is now doubt that toe-in delivers more straight-line stability during steady cruise however at the expense of turn-in response. That is why race teams will change the toe values depending on track and race distance. A tight track will see higher values of toe (both in or out depending on driver preference), while endurance races on relatively fast open tracks will see smaller values.
You can conclude from this that you can either promote stability/instability at the front or the rear, using toe to deliver a desired result. It is also possible to conclude that in contemporary steering and chassis design, bump steer effect is negligible and only serves to confuse the argument.
Taken from an original article by Whiteline Automotive. Reproduced and edited with permission