Faiz One

Can someone please explain to me what caster is beyond the "the backward or forward tilt of the steering axis"? Moreover what does more or less caster do in terms of handling?

I have checked out http://www.ie.net.au/ausrotary/artic.../Alignment.htm but damned if I understand what they are saying. So please use terms a simpleton like me can understand.


Faizi

PS – Is it spelled castOR or castER?

Edited for caster spelling.

Kevin M

iTrader: (12)

Caster is the imaginary line that stands straight up and down that your wheel pivots on when you turn the steering wheel. It's not perfectly vertical or centered on the footprint of the car. Basically, caster is why your car will handle differently at different speeds. Going around a given corner much faster than normal causes the outside wheel's susension to compress. Since the caster line isn't perpendicular to the ground, it changes the shape of the contact patch slightly and changes the exact pressure on the tread at any given point where it meets the road. Basic concept, incredibly difficult to tune with modified suspension.

Imprezer

iTrader: (9)

Next time please use your head when posting subjects, ok?

Patrick Olsen

iTrader: (1)

What am I missing?

Anyway, caster (with an E) is very hard to describe, so I won't even bother trying. How does changing it affect things? Well, with a MacPherson strut suspension (which is what our cars have) positive caster is a good thing, the more the better. As you add positive caster (which is done by tilting the strut back farther at the top using adjustable top mounts, or by relocating the steering knuckle with an anti-lift kit) you change the dynamic camber curve so that there is less camber gain. That's a good thing. Why? Well, let me try to explain my understanding of it...

We start with some amount of static negative camber - as your car is sitting at rest, the tops of the front tires are tilted in some amount. As the car corners, the suspension compression combined with body roll causes the outside tire to gain camber (gain positive camber = lose negative camber = bad). Eventually the tire can't take any more and rolls over onto the outer shoulder, which causes it to lose grip.

By adding positive caster, the camber gain is reduced. You don't change the camber gain due to body roll - that stays constant unless you change spring rates or the sway bar (AFAIK). However, you do change the amount of camber gain due to suspension compression.

AFAIK (again ) the only downside to adding positive camber is that you increase steering effort because adding positive camber increases the self-centering effect (the desire of the wheels to always be pointing straight ahead). However, from what I understand you have to add a lot of positive camber - more than you could do without resorting to enlarging the hole in the top of the strut tower - to really feel an adverse effect.

My other car is a Mustang, which also uses a MacP strut setup (well, it's a modified MacP strut, but whatever...). From the factory it can run maybe +3deg caster, which is a bit less than our cars run from the factory. With a good set of C/C plates and an aftermarket front control arm setup, there are Mustang auto-x and road race types that run as much as +7 or +8deg caster and they have no problems with that. From what I understand the guys running the (relatively) new MRT C/C on their Subies are getting about +5deg of caster - clearly an improvement, but it shouldn't be anywhere near enough to cause problems with steering feel.

Hope that helps,
Pat Olsen
'97 Legacy 2.5GT sedan

John #555

good info guys!

I'll try to add visual (well, verbally)
if you lock your steering to one side you will notice the front wheels leaning in towards the inside the turn. eg. turn steering to right side, wheels tilt to right side. If you look at dune buggy's you can clearly see the tilting of front wheels when you lock the steering to one side. also, I think Mercedes Benz's add more caster than other cars.
more caster = more responsive, but darty & harder to keep it in straight line

chrisw

iTrader: (1)

The steering feel is excellent. The understeer is virtually gone although the swaybars helped alot too.