Calculating expected camber gain due to caster.

[MazdaMatt]

I am trying to find a way to calculate a relation between the steering angle and the camber gain for a given caster angle. (Note: this has nothing to do with Dynamic Camber as it is caused by body roll. Consider these calculations to be on a car that is sitting still.)

S = Steering angle
Cm = Camber (negative)
Delta(Cm) = Camber Gain
Cs = Caster (positive)

Here are some givens:
If Cs=0 Then Delta(Cm)=0 for all S.
If Cs=90 Then Delta(Cm)=S
If S=90 Then Delta(Cm)=Cs
What happens in between 90 and 0 is what's eluding me.

What I would like to find is an equation relating these three variables. Given this, one could decide what level of caster they would like for their application (intended camber at expected steering angle). An autocrosser would want to use a higher steering angle than a road racer. If they wanted the same camber, they would want less caster.

Is it as simple as:

90*/Cs = S/Delta(Cm)? This would indicate a linear relationship between S and Delta(Cm) for a given Cs.

[Marsh]

I'm pretty sure the relationship is linear. This is fairly rudimentary dynamics. If you think of the steering axis as a line in 3 space then caster is the angle that line makes with the x-axis, camber is the angle it makes with the y-axis. You want the angle the steering axis line makes with the x-y plane when measured perpendicular to the wheel direction.

I'll give it some thought tonight, unless somebody else can chime in without having to dust off a text book?

[thgear]

if you are talking static camber why would camber change at all if the ONLY thing you are changing is caster...

[Greg F]

Try modelling it in SolidWorks. (You might need to model the tire as a motorcycle tire to keep the mating condition between the tire and the road as the wheel is turned.)

[ScotcH]

Originally Posted by thgear

if you are talking static camber why would camber change at all if the ONLY thing you are changing is caster...

I'm pretty sure he means dynamic camber gain during bump.

[thgear]

Originally Posted by ScotcH

I'm pretty sure he means dynamic camber gain during bump.

argument still stands, if you look at the car from the front, assuming the only thing that is changed is caster, then the camber gain/loss will still be the same, only with more caster you get some toe in there as well

the whole idea behind caster is due to dynamic camber change as a result of steering angle input...

yet he explicitly stated in his post that he does not care for any dynamic changes in alignment.


Matt.... can you clarify your post please?

[Marsh]

He's talking about zero change in bump, but camber change with STEERING ANGLE as a result of caster angle. Caster is what causes camber change with steering angle. He wants a way of quantifying this change so he can better evalluate what his caster angle should be.

Matt, your post was perfectly clear to me.

[ScotcH]

Originally Posted by thgear

argument still stands, if you look at the car from the front, assuming the only thing that is changed is caster, then the camber gain/loss will still be the same, only with more caster you get some toe in there as well

the whole idea behind caster is due to dynamic camber change as a result of steering angle input...

yet he explicitly stated in his post that he does not care for any dynamic changes in alignment.


Matt.... can you clarify your post please?

D'oh! I'm tired from all that winning

[thgear]

Originally Posted by Marsh

He's talking about zero change in bump, but camber change with STEERING ANGLE as a result of caster angle. Caster is what causes camber change with steering angle. He wants a way of quantifying this change so he can better evalluate what his caster angle should be.

Matt, your post was perfectly clear to me.

well in that case... all i know is that FWD benefit from reduced caster... upwards to 0, while RWD cars benefit from increased caster



ohh and answers, as a response to a question not understood, promote discussion

[MazdaMatt]

Yeah, Marsh is on the right track.

Visualize. The wheel, the steering axis is tilted (positive caster). Now turn the wheel. There will be both steering as well as camber. You can really see it if you picture the wheel at 0, 45* and 90*. At 90* the steering wheel ONLY causes camber change, not steering. At 0* it ONLY causes steering, not camber change. The idea is to find the relationship between the steering and the camber change for a given caster.

Where did you get the idea that FWD wants none and RWD wants some? I would think that in a straight line you'd basically always want your wheels upright for max accel and deccel, and when you're turning you want them to tilt into the turn for max cornering. I think those concepts apply to both front and rear wheel.

I didn't get a chance to sit down with a pencil and paper yesterday, I'll see if i can run a comparison tonight. I'd rather see it all derived from equations, but a set of pictures as a test condition might show me if its linear.

[BigPune]

Well, using a rough approximation of the angles and distances created from them, and my rusty trig skills, I have come up with this.

f(c,s) = asin(sin(c)*sin(s))

Where c is the caster angle, s is the steering angle, and f(c,s) is the camber.

I don't have a calculator with an asin function, but if you plug in say, c = 45*, and s = 90*, you would get a camber of 45*.... if someone has a calculator and want to try some random calculations this might at least give you an idea of what the camber gain would be.

Ryan

[MazdaMatt]

We're going to need some derivation.

[BigPune]

Using my formula above, 1.5* caster and 40* steering, there would be just under 1* of camber gain...


Seems reasonable.

Slightly more out of whack would be caster of 4.5, and steering of 40*. This gives 2.89* camber gain.

Thats a lot of camber gain, but that is also a lot of caster.

I feel like a blind man trying to teach about DaVinci's works.

[Marsh]

Originally Posted by thgear

... all i know is that FWD benefit from reduced caster... upwards to 0, while RWD cars benefit from increased caster

??? Since when did FWD want no caster?
For solo-2: Caster = good. Most cars don't have enough adjustment to put in too much caster. Especially heavier cars and cars on stock suspension.

[JGraves]

Originally Posted by ScotcH

D'oh! I'm tired from all that winning

I think you're tired from carrying around that swollen ego