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Higher Castor= Cross Weighting a Chassis

Motorsport Wheel Alignment Fundamentals

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Ok Question.....I come from a karting back ground so this really hits home with me. I noticed that this wasn't brought up in the Castor section of the video series.

I read once that running a high castor in a road car/track car will cause a "jacking/ligting" effect to the inside rear tire, just like a racing kart. Unlike a racing kart, a road car will not lift its rear inside tire due to its weight but heavily unload it. Since then I have not heard much about this aspect of running a high castor.....

.....until

Motor Trend started testing the new C8 Corvette runs 8 degrees of Castor and noted:

"High caster will also cross-weight a chassis because the outside wheel swings in an arc upward as the inside wheel swings down. These will both typically work to reduce the understeer that we squawked about in earlier tests".

Just wanted to hear more about this. To be honest, when I read MT test driver Randy Pobst bring up cross jacking due to high castor, I was surprised because its just not something people mention when talking about castor

no takers yet...??

I agree that caster will cause a cross-weight change that will reduce understeer. You can only run 8 degrees of caster if you've got power steering to assist. On my sports racer's I would find it very difficult to turn the steering wheel in a high downforce corner.... We typically run 3-4 degrees of caster.

I was just curious why they never mention this in the Castor section? Ive read this before researching Suspension/steering geometry. When i read the Motor Trend article on the new C8 Corvette was actually surprise to see the "mainstream" talk about it. Just figured it deserved more attention.

RCVD says that the effect of caster on load shift is really quite minimal in all practicality. The reason being that the vast majority of race applications have very slight steering angles and thus very slight rise. When the steering angles are more substantial (e.g. hairpin), then the speed is dramatically lower and load transfer isn't worth worrying about.

Circle track cars are a different story since they have higher steering angles, caster split, and are exceptionally sensitive to crossweight changes.

hoping Andre Simons can chime in on this.....

I am curious on this as well, from a oval racing perspective. We are currently running a split of positive / negative castor and I didn't even realize I could be tweaking my cross weights with that.

OK, I'll hazard a comment on this.

Increasing positive caster has two primary, direct affects -

It loads the inner and unloads the outer front tyres - the greater the caster and/or wheel offset the greater the affect - and unloads the inner rear and loads the outer rear. Normally with tyres, the best grip is with both tyres on the axle evenly loaded, so understeer should be reduced by both aiding ront grip and reducing rear.

The second affect is increasing castor will move to increase the negative camber at the outer wheel, and take it away from the inner. This will reduce the net camber gain from roll at the more heavily loaded outer which will give a net grip gain. While the inner is lightly loaded (or lifted from the surface) and any gains will be reduced, it will gain positive camber, also aiding grip.

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