explain how caster affects camber?

[Andrew]

I have read repeatedly that more positive caster gives you more negative camber as the front wheels turn. Can someone explain how/why that works? I try to picture it in my head and can't. I'm an experienced Alfa mechanic and driver but not an engineer. Thanks
Andrew

[101 Alfa Mike]

When you turn the steering wheel, the tires rotate around the steering axis, which is a line drawn through the upper and lower ball joints. Castor is how much the steering axis leans back at the top (i.e. toward the rear of the car).

Say you have 3 degrees of positive castor. If you were able to turn the wheels 90 degrees, that would introduce 3 degrees of negative camber on the outside wheel and 3 degrees of positive camber on the inside wheel. Both are good for cornering, however, too much castor makes for heavy steering.

Hope that helps.

Mike R

[Andrew]

What I can't picture is this: with increased caster, the steering axis moves backward at the top, but does it more inward at the top relative to the bottom? I guess as the wheel turns it does, but that's what I have trouble picturing.
Andrew

[conedriver]

Andrew,

With incresed caster, the wheel on the outside of the turn tends to "flop over". The best example can be seen on a road grader or some Mercedes sedans. Your current Alfa will exhibit this; turn the wheel to full lock, then stand so you can look straight down on the outside wheel. The bottom of the tire will be some distance outward of the top of the tire. Very plain to see.

Quote:

Originally Posted by Andrew What I can't picture is this: with increased caster, the steering axis moves backward at the top, but does it more inward at the top relative to the bottom? I guess as the wheel turns it does, but that's what I have trouble picturing. Andrew

[Andrew]

Thanks both of you. I like minimal caster on the street for light steering, but will be dialing in some more on my track GTV, which is currently stock spec. Can you recommend a reasonable amount of caster for street/track 2000 GTV (relatively fast tracks, no autocross), 195/60 Hankook Ventus tires, Rugh springs, about 2" lowered?

Andrew

[geh458]

Andrew,

My suggestion is that if this car is to be dual purpose, keep the castor within factory specs. You can always adjust the camber provided you have the adjustable upper arms. When I had my GTV race car, I kept castor within spec and adjusted the camber to the tire manufactures suggestions to start with and then worked from there until it feels good.

[Andrew]

I do have adjusable upper arms and my front negative camber is up against the rules limit without accruing more class points on the car. Dialing in more caster draws no points penalty but gets you more camber for free.

The car feels good as is (-.5 degree camber) but I'd like to try more to see how it works.

Andrew

[conedriver]

Andrew,

While Gifford suggests leaving castor at stock and adjusting camber, you said the car is already at max allowable negative camber (It sounds like you are working with AROC Time Trial & Autocross rules). A benefit of more caster is that the tires stand more upright and have a better footprint for braking, then only adopt more negative camber (on the outside only) when turning. However, carried to an extreme, lots of caster results in heavier steering since you are "raising" the car when turning and the weight wants to center the wheels. This can also result in excessive negative camber on the outside tire, especially in tighter autocross turns, and result in low speed - tight corner- understeer. I experienced this when I went to much wider autocross tires a few years back. The excessive negative camber was verified with my pyrometer, and I went faster with less caster.

In my case, with 275/35x15 tires, I need the tire footprint to be relatively upright for straight travel. Hoosier recommends 3 degrees negative camber for their A6 Autocross tires, but I'm currently running around -2.

Quote:

Originally Posted by Andrew I do have adjusable upper arms and my front negative camber is up against the rules limit without accruing more class points on the car. Dialing in more caster draws no points penalty but gets you more camber for free. The car feels good as is (-.5 degree camber) but I'd like to try more to see how it works. Andrew

[Andrew]

Yes, AROSC time trial rules, running mostly Buttonwillow and Willow Springs. Some Streets of Willow, and some Thunderhill.

Thanks for the explanation; I hadn't thought about the tire-wear issue and straight-line vs. corning wear.

I'm going to stay in the range of -.5 camber, but I was specifically asking about what to adjust caster to, not camber. Stay at the high end of the stock range, or go beyond a bit? Centerline site shows 2 degrees stock caster. I don't want killer hard steering and I'm not a super-aggressive driver or in a super-fast car.

Andrew

Damn that's a big tire you're running on a 115 car.

[Alleggerita]

Hi Andrew,

Caster on a 105 chassis is limited by the room in the slots of the front cross member where the upper arms go into. You are unlikely to get more than 3 degrees of caster - and that usually involves grinding of the lip and the "right", e.g. no protruding hex nut - upper arms. 3 degrees is absolutely fine for street use, even with a small steering wheel.

I gather you use the -.5 camber to avoid points for negative camber according to time trial rules - I believe -.5 is within stock range. -.75 is absolutely fine for street driving and I personally favor -1.25. With stock spindles you'll still wear the outside of the front tires rather than the inside.

[conedriver]

Andrew,

I think the answer is to just try something to see how it works with your car, your tires, and your driving style. Also, a tire pryometer can be had for around $90, and is really valuable when experimenting with alignment setup.

Quote:

Originally Posted by Andrew Yes, AROSC time trial rules, running mostly Buttonwillow and Willow Springs. Some Streets of Willow, and some Thunderhill. Thanks for the explanation; I hadn't thought about the tire-wear issue and straight-line vs. corning wear. I'm going to stay in the range of -.5 camber, but I was specifically asking about what to adjust caster to, not camber. Stay at the high end of the stock range, or go beyond a bit? Centerline site shows 2 degrees stock caster. I don't want killer hard steering and I'm not a super-aggressive driver or in a super-fast car..

[Andrew]

Thanks again all. I will try 3 degrees caster if I can get that much within the upper arm box. I do have adjustable upper arms, though they're currently pretty well centered in the box now.

Andrew

[bshorey]

Quote:

Originally Posted by Andrew Thanks again all. I will try 3 degrees caster if I can get that much within the upper arm box. I do have adjustable upper arms, though they're currently pretty well centered in the box now. Andrew

[60sRacer]

Just thought I'd put in my 2 cents for the driver's role. Especially on AX courses, driving style has a very large effect on how well any alignment adjustment works.

For the "lashers" (turn, stomp, brake, turn ) heavy steering is not important (lots of caster is good) but available oversteer is, as is good engagement by the outer tire (lots of effective neg camber).

For the "floaters" (fingertips and soft touch on the pedals to keep speed up) even balance is important, and getting that right involves every parameter, but none of them gets too wild.

There is no "right" answer. Both styles win.

Robert

[gattia86]

I just put my GTV back together with adjustable upper arms and lower, stiffer springs. I adjusted caster to the maximum available within the "box" of the upper arm, since I had heard from multiple sources that the GTV handles better with more caster.

After a quick alignment and short drive, I'm disappointed. Too much caster kills the turn-in response. With my 185/70-14 tires, the steering effort is not really "hard", but more "rubbery" just off center, when initiating a turn. Feels nice and stable mid-corner once the weight has transfered and its taken a set.

I'm guessing the turn-in response would be less compromised with stiffer sidewall tires or 195/60-14 tires, since, of course, these contribute to the rubbery turn in feel. But the car handled crisply with these tires before and standard caster.

Maybe a bit of toe-out would help counteract the sluggish turn-in. I'll try this next.

Has anyone added toe-out to their track-day alignment?

George