Caster

Caster angle

A short dissertation on CASTER, the most misunderstood of the three front wheel alignment angles, with regard to production based roadrace cars.

Of the three main front wheel alignment angles, camber, caster, and toe, it is certain that caster is the most confusing and most subtle, and therefore the most misunderstood. This subject requires a working knowledge of racecar front end geometry as found in one of the many books available, some of the best of which are listed below.

Camber is the angle at which the top of the tire is tilted in, as viewed from the front of the car. As the front tires are turned left and right, the camber changes slightly because the pivoting points for the tire are not vertical as viewed from the side. If the car has positive caster, as most every racecar does, the topmost pivot is behind the lower pivot and this causes the tire to tilt in more at the top as the tire is steered inward. This small amount of negative camber gain is the most common reason for the popular misconception that a lot of caster is a good thing.

While it is true that most cars DO handle better with more caster than the factory spec, and many handle well with as much caster as possible within the limitations of the body structure, it is definitely a fallacy that “more is better”.

The most confusing thing about too much caster is that the car can FEEL better, but actually be slower on the track. This is aggravated by the possibility that the car can feel so much better that the driver actually goes faster, even though the bumpsteer and corner weights are made worse. This occurence is the driver’s problem for not going fast enough to begin with and there are books for that too.

Changing caster primarily affects four things, high speed stabilty, camber gain, bump steer characteristics and relative corner weights (wedge). There is no disagreement that high speed stability is a good thing, so extra caster is a plus there. Camber gain with extra caster just happens to be in the direction we want, more negative, so that’s good too, however the amount is usually greatly overestimated as shown in the example below. Bump steer however is affected adversely, but this can be changed, however I have seen many racers get caught out by this one. Corner weights are the big problem with too much caster, as extra caster definitely affects an otherwise balanced racecar for the worse.

What occurs with extra (“too much”) caster is that more and more weight is transferred off of the outside front and inside rear tires, while this may at first sound good because taking the load off the outside front CAN be good, the reality is that the outside rear tire will be doing too much work in the middle of the turn, so steps then taken to alleviate this will cause a corner entry push. Additionally, on any rear wheel drive car the inside rear tire will be light and won’t come off the corners well. Remember we’re talking about a well balanced car here, not a car where this extra caster covers up a sway bar or spring problem.

There’s nothing like a good example so here are some actual numbers from the use of data acquisition, a set of electronic scales and an alignment machine. With caster + 3.5, camber -1.5,when the tires are turned 7 degrees as typical for sharp hairpin, the camber gain was 0.35 degrees (not much!) however the corner weights changed by 22 on just the outside rear tire. With caster +5.0, camber -1.5, and 7 degrees toe-in, the camber gain was 0.50 degrees (still not much), but the corner weight changed 35 lbs. on the outside rear tire, which is just too much. The most interesting item in these examples is actually how little the camber gain changed. This stuff applies equally to FWD and AWD cars, but for slightly different reasons.

Books: Race Car Engineering by Paul Van Valkenburg, any Carroll Smith book, How to Make Your Car Handle by Fred Puhn, and the new Don Alexander book is way better than the old one.