Question on caster.

rnln · 06-07-2007, 08:42 PM

I have always heard more caster means better handling. I didn't know our car can adjust caster. If so, why not adjust it to the max. What is the disadvantage? What will happen if I adjust it to the max?

Eagledriver · 06-07-2007, 08:51 PM

More caster means you will feel more centering force in the wheel when you are going around a corner. It also will increase the negative camber on the outside wheel in a turn. Both of these things are good in general. Most racers set up for maximum caster. You need to make sure both sides are the same however or you'll have a pull to one side.

-Andy

rnln · 06-07-2007, 09:24 PM

Quote:

Originally posted by Eagledriver
centering force in the wheel.

-Andy

If I understand it correctly, this is good. I would max it all the way. May I ask why normal street car doesn't set it up this way?
Thanks Andy.

jluetjen · jluetjen's Garage

More castor is not all good...

1) Some increase in the centering force is good, too much can make it difficult to hold the wheel at lock during sharp turns without having massively strong wrists and arms.

2) While castor increases the camber gain on outside tires during cornering, it also tends to transfer load off of the outside tire to the inside tire by virtue of the geometry moving the contact patch up. If you crank in full lock on a softly sprung car with a lot of castor while at rest, you'll find that "outside" corner of the car will settle while the "inside" corner of the car will come up a bit. If you sit on the ground in front of your car while someone turns the steering wheel from lock to lock, you'll actually be able to see the front of the chassis move as a result of this.

If you were to check the corner weights, you'd find that a hypothetical car with perfect corner weights:

Front: 25%-25%
Rear: 25%-25%

will look more like this if you have a lot castor dialed in and turned the front wheel to the left?

Front: 30%-20%
Rear: 20%-30%

The result will be an increase in oversteer as the outside rear tire (already somewhat overworked in our rear engined 911) gets even more load to handle -- which will result in an increase in oversteer.

So some is good, more may not be better. The most important thing is that it be even on both sides. If it's not you'll find that you'll have a car that pulls to one side or the other.

rfn026 · 06-08-2007, 04:54 AM

Even on both sides is more important than the total amount. btw - I absolutely love a lot of caster.

Richard

rnln · 07:52 PM

I don't think I have corner balance problem since I never reindex the torsion bars, and the ride is ok. If I have corner balance problem, the handling should be funny isn't it?
Could you read my problem and see if I have too much caster?
http://forums.pelicanparts.com/porsche-911-technical-forum/350335-alignment-pro-please-give-me-free-diagnostic-tire-worn-problem-post3315373.html#post3315373

Eagledriver · 06-08-2007, 06:28 PM

The tires have a little more wear on the outside than the inside but not enough to worry about IMHO. Your rears will still wear out first so just drive the car and forget about it.

-Andy

rnln · 06-08-2007, 07:47 PM

no no, my rear tires look perfect and not even 1/2 of life yet. The front tires are worn much faster. I guess rear tires on rear wheel drive car should be worn much faster. So worn uneven is onething but I also worry about them wearing out too fast.