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Caster/Castor on a FC

Old 10-26-03, 12:59 AM
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Caster/Castor on a FC

What Castor degree are you guys running on your FC? stock is 4 degrees i think.

Somone told me 6 degrees is good, i can't remember who though.
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Old 10-26-03, 02:07 PM
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I think stock is more like 6? Don't bother with it. You can't change it (fronts) unless you run plates, and even then most people won't know what exactly changed.


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Old 10-26-03, 02:25 PM
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what's castor degree??
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Old 10-26-03, 03:41 PM
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the FSM says 4 degrees. K2RD plates are set at 6 degrees... I'm planning on picking up a new coilovers system. Was looking at option

Caster...
taken from ground control website

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.

This page is for thinking purposes only, and of course you may find totally different results, but all engineering students who feel typically argumentative are cheerfully advised to get out of the computer lab and race something, instead of E-Mailing me about why their theories disagree.

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.
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Old 10-26-03, 09:14 PM
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In my experience with the FC and castor is that as long as both sides are equal I can't tell the difference and it made no impact on lap times, driver "feel", or tire temp/wear. now when the two side were not equal things got a little funny and unpredictable. I believe I have about 2* of castor in my car at the moment.
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Old 10-26-03, 10:24 PM
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by changing caster you increase your camber gain which is a function of your wheel travel. however not by much...

i wonder how you tune for castor exactly.

Last edited by Cheers!; 10-26-03 at 10:27 PM.
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Old 10-27-03, 07:10 AM
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Originally posted by Cheers!
by changing caster you increase your camber gain....
Only when the steering is turned of course.

Here's what goes wrong with too much caster, as stated in your quote: "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."


Originally posted by Cheers!

i wonder how you tune for castor exactly.
Caster is adjusted by changing the angle of the kingpin (steering axis). On a car with struts like the FC this is accomplished by leaning the strut top towards the rear of the car (increased castor). The FD acomplishes this by putting the lower wishbone on eccentrics. Picture the lower end of the kingpin moving slightly forward or aft while the top remains fixed; this adjusts the angle of the kingpin and thus the caster.

I run about 6 degrees of caster on my FD and like it except for really slow, tight autox corners. The wedge it creates in the chassis is just too much; you can feel the fornt end give up right after turn in. You have to be going slow and have lots of steering cranked in for this to happen though; it would never happen on something like a road course. Personally I am going back to less caster and more negative camber in the front compared to what I am doing now.
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Old 10-27-03, 07:13 AM
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Originally posted by tims
I believe I have about 2* of castor in my car at the moment.
2 degrees is not much, though I don't know what the FC runs stock. Generally more caster is most easily felt as increased steering effort because the front tires' centering force becomes stronger.
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Old 10-27-03, 08:37 AM
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I thought caster was different from kingpin angle.

I think caster is when you look at shopping carts, the front wheels. Basically the Shock is either infront or behind the wheel centre with respect to the longitundinal length of the car. (the axis if you were to draw a line connecting your front to your rear licence plate).

Kingpin is the angle at which your upright is with respect to the axis which is defined as the line drawn from your passenger side mirror to your driver side rear view mirror no?
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Old 10-27-03, 09:43 AM
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Originally posted by Cheers!
I thought caster was different from kingpin angle.
Caster is the angle of the kingpin meaasured in degrees from vertical when viewed from the side. 0 degrees (vertical) is no caster.

Originally posted by Cheers!
I think caster is when you look at shopping carts, the front wheels. Basically the Shock is either infront or behind the wheel centre with respect to the longitundinal length of the car.
Shock angle has nothing to do with caster except in a car with a strut type suspension (like an FC). Since the steering pivots around the strut in a strut type suspension then adjusting the angle of the strut adjusts the castor angle. Caster is like a shopping cart as you described. If you had no castor the steering axis is vertical when viewed from the side. As you tilt the kingpin back towards the rear of the car you are moving the tire contact patch behind the steering axis' ground intersect. The amount of difference between the center of the tire contact patch and the ground intersect of the caster angle when viewed from the side we call "trail". Increasing negative caster increases trail.

Originally posted by Cheers!
Kingpin is the angle at which your upright is with respect to the axis which is defined as the line drawn from your passenger side mirror to your driver side rear view mirror no?
You lost me here.

The kingpin is the physical hinge that the steered wheels revolve around when the steering wheel is turned. The kingpin angle is actually 3 dimensional; it is defined by the angles when viewed from the side (castor angle) and also by the angle when viewed from the front (steering axis inclination). Changing either the castor or the SAI changes the kingpin inclination.

Camber throws a curve into all of this because changing camber also changes the SAI which also changes the kingpin angle...That's why when you do alignment work you have to set everything in a specific order: First camber, then caster, then toe. If you don't do it in that order your subsequent adjustments will alter the previous ones.

Caster:


Steering Axis Inclination:


Scrub radius:


Check out this thread here

Last edited by DamonB; 10-27-03 at 09:53 AM.
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