POSITIVE CASTOR-are there no negatives?

[johnny yuma]

IMHO,positive castor is desirable to overcome positive camber which is induced by body roll,or tyre distortion induced by G force in cornering.The downsides are said to be heavy steering,directional instability as speeds rise[even in a straight line],unpredictable steering on higher speed corners[partly caused by unintended geometry alignments in road cars over worked],bad feedback in a lack of lightness and predictability if the limits are approached. Any comments? Is there any point in pos. caster beyond what is useful to control pos.camber in tight corners--and is there a risk of too much pos.castor leading to undesirable oversteer in sweepers?

[graham bahr]

on macphereson strut cars once you get to a certain point the instead of the suspension working in a up and down motion it would start to work more in a back and forth motion and your wheel base would start to change dramatically with bump and rebound, that would feel very strange to drive

[Al Weyman]

Do you think that is what may be causeing the trouble on a mate of mines car Graham? He has fitted a Rover V8 in a 2 door seirra shell and had to mess around bending the antiroll bar (front) which as you know also locates the TCA. Anhow the car is very unstable in a straight line when it hits a bump. I wonder if he may have overdone it on the castor.

[graham bahr]

how much is it running Al? 9 degrees on the beemer works ok.

does it do it when one or both wheels hit a bump?

if only when one wheel maybe, particulary with wide wheels lots of inset and lots of neg camber.

more likely i would of thought is bump steer

[breezeblock]

i agree with graham bahr sounds very much like bump steer. how much camber is he using?

[Al Weyman]

I thought bumpsteer but he recons he has not shifted the rack position, I will have to take my guages around to his place and have a look at ot because he does not even know what the settings are.

[GordonG]

If you have tilted the uprights rearwards significantly, the steering arm will have gone up (for a front mounted rack) or down (rear mounted rack). Moving the bottom of a McP strut will also alter the fore and aft relationship with the rack. That will alter the bump steer, but I'd be surprised if it would be *that* significant. duff bushes in teh Sierra suspension seem more likely - there's some HUUUUUGE chunks of rubber in there...

G

[rcr286]

I am not sure how he was bending the bar, but if it was not done correctly this could cause differences in castor from side to side. This will usually make the car want to pull side to side, feeling very similar to bumpsteer...especially when you hit a bump.

-Roger

[greenamex2]

Originally Posted by johnny yuma

IMHO,positive castor is desirable to overcome positive camber which is induced by body roll,or tyre distortion induced by G force in cornering.The downsides are said to be heavy steering,directional instability as speeds rise[even in a straight line],unpredictable steering on higher speed corners[partly caused by unintended geometry alignments in road cars over worked],bad feedback in a lack of lightness and predictability if the limits are approached. Any comments? Is there any point in pos. caster beyond what is useful to control pos.camber in tight corners--and is there a risk of too much pos.castor leading to undesirable oversteer in sweepers?

In the end you need good camber control and castor is ONE way of getting to that goal. Unless you have something clever like the DAX camber system then they all have negative effects like unwanted geometry changes.

In most saloon based race series you can't do much to correct the camber compensation curve on a car so castor is a useful tool.

On a 'custom built' vehicle then you have a lot more options and probably wouldn't have so much castor and it's associated negative effects.

One overlooked factor is if you have too much castor resulting in too much negative camber the correcting the resulting understeer becomes counter intuitive, you have to back off the steering to get the grip back. A lot of cars that rely on castor suffer from this in the wet.