964-C2

What are the drawbacks with "to much" caster?
Thanks.

Voith

FullThrottle64

The above photo is generally a result of too much camber (though caster results in higher camber under steering). Caster is a different setting - basically kingpin angle.

Caster affects centering force, stability, camber gain, and ackerman. You use caster to get more stability at high speeds for track use; probably don't want that much steering force requirement on the streets, nor that much camber increase at lower cornering rates.

IanM

Max caster makes the car more stable in a straight line. You'll notice though, especially at parking lot speeds, that the steering wheel is less willing to go back to center on it's own after you've gone around a corner. I'm so used to it now I don't even notice.

Think of a motorbike with tons of front rake (i.e. chopper). The bike will track straight very well, but once you turn a corner the front wheel will flop over and the bike will want to keep turning. That's an extreme example of positive caster.

964-C2

I understand your first claim, but not your second.
I would think that more caster would make the steering more willing to go back to center (?).

Van

This is the accurate statement, our cars don't lean over like a motorcycle.

Caster is the angle that the strut is to the ground - and the strut (well, the upper strut mount to the control arm ball joint) is the pivot point of the wheel/hub assembly when you steer. If this strut angle was zero - or perpendicular with the ground - the wheel would have no propensity to try and "self straighten". The greater this angle - meaning the pivot axes extends to the ground in front of the tire's contact patch - the more the tire will self straighten so the the contact patch follows this axis/pivot point. Think about a "caster" wheel on a shopping cart...

As this distance between the axis point on the ground and the contact patch gets larger (e.g. more caster), there is a longer moment on the axis - meaning more force will be required to turn the wheel to overcome this "leverage".

Most racers I know run the minimum amount of caster so the car is easier to turn. In other words, the car is less likely to just want to go in a straight line - the geometry of the wheels allows for an easier change in direction.

On the highway, of course, where you want to just go straight with the car super stable so you don't have to think about course corrections much, max caster would be better.

There is also another reason for minimum caster on the track: if you have manual steering, it takes less force to turn the car, increasing steering feel/feedback and making it less tiring.

964-C2

Ok, thanks.
I thought all (most) 944-racers set their caster at maximum.

Is it correct that more caster will give you more camber on the outside wheel in a corner, and less camber on the inside wheel?

Van

That may be true... I don't really know - but it's really a moot point. You're envisioning if you crank the wheel all the way to one extreme, then it looks like there's more camber.

My data acquisition shows me that the tightest corner in the northeast - meaning the corner where I put in the most steering input - is corner 5b at Mosport. In this corner, I reach a maximum of about 5 degrees of front wheel angle. Most average race track corners require about 2-4 degrees.

At 5 degrees of steering input (again, this is measured at the front wheels, not the rotation of the steering wheel), any camber gain or loss from your caster setting is beyond negligible.

Van

p.s. if you're turning the front wheels more than that, you're exceeding your tires' slip angle to the point, that if you had an extra 0.125 degrees of negative camber, it wouldn't help.

Dubai944

I personally run as much caster as I can get. Better turn in response and more directional stability. But I do run power steering.

333pg333

I think of less castor to be like a shopping trolley.

PorscheDoc

Actually that is typical of a toe issue, not camber. While most people think camber eats the inside edges, it is more common for toe to do that.

333pg333

Yes, having a decent amount of positive toe is great for Auto-X or Motokhanas but will eat inside and really wriggle under heavy braking.

Cheburator

Most race cars with McPherson struts require more caster than Street cars. More caster gives you more dynamic camber, which on a McPherson strut is only a good thing. Given that most modern race tyres - even the street legal T1Rs, 888s, V70As, Nittos etc - don't work well at all below 3deg negative camber you need caster to help you in the corner, without compromising your straight line contact patch.

KevinGross

Van's description is correct. Caster essentially translates into steering self-centering force. The effect its adjustment has on other alignment values (camber and toe for most of us) depends on the suspension design but is not particularly relevant where all three are adjusted for during alignment.

Kevin
Catellus Engineering