Caster gain

[pav8427]

What would be a reasonable amount of caster gain per inch of travel on a street driven car?
Any good reason to have any more or less than this amount?

Thanks, Doug

[astroracer]

Do you mean "camber" gain? Caster is used to generate self-centering of the steering wheel and can be used to change the way the steering feels by increasing or decreasing caster.
"Camber" gain is the angle change, from verticle, of the tire/wheel assembly as it articulates around the a-arm pivots. Ideally the suspension should have zero camber in all situations, whether cornering or on the straights. Excessive amounts of negative camber can make the car squirrley due to camber thrust or pushing and this will cause excessive tire wear also. A lot of camber change is not a good thing unless you are using it to correct the contact patch in a corner in a racing situation. Even then the additional negative caster will slow the car down in the straights and wear out the tires faster.
Too answer your question directly, "0" camber gain is ideal per inch of travel and there is no good reason to deviate from this number. Of course you will never have that because the suspensions ultimately induce some camber change, but you want to keep it as minimal as possible.
Mark

[High Plains Mopars]

While zero camber gain may be ideal, so long as a car has body roll, camber gain is necessary to maintian the contact patch of the tire.

The best camber gain for the street is tough to call. Some of the factors that can influence this curve are roll center height, spring stiffness, tire sidewall stiffness, track width, tread width versus wheel width, and any banking built in to the turn itself.

Back in my oval track days, we tended to want more camber gain on a flatter track and less on banked tracks. This is primarly due to the flatter track generating higher side loads with reduced downloads whereas the banked track produced high down loads with lesser side loads so the car rolled over less while turning.

In general, we wanted more camber gain with the flatter tracks. Since street driving has very little banking, I'd guess more gain to be better, but again, it depends on a lot of factors. If you have little body roll and short stiff sidewalls, then you won't need much. If you have lots of body roll and tall sidewalls, then you need more. Less being about 1 degree per inch and more being up to 2 degrees per inch.

Depending on the vehicle you are building and the availability of aftermarket upper control arms, the best camber gain may be just a phone call away. if you are fabbing your own uppers, then it is a whole 'nuther ball game as you will need to plot out your pivot points to determine what length of arm will give you the best gain.

[sinned]

Caster is also responsible for the stability affect at higher speeds, adding caster will to an extent help with the "speed wobbles" some vehicle experience. Again though, no "gain" per travel.

Camber gain is essential on any vehicle (maybe F1 and IRL excluded), no matter how well the chassis set up. Almost every radial tire has enough sidewall flex to require some level of negative gain to compensate even if the chassis is et with enough roll stiffness to eliminate body roll (gosh how would that ride though).

[pav8427]

Caster gain is what I am looking at. Mainly as a reference dimension to see if I have my anti-dive set anywhere close to something that is usable. I am laying this out in 2d, so if I had something of a number to shoot at, I can predict what degree to set my upper arms at front to back. Right now I have them set at 3*, but I am fine tuning this part of the front end and would change this if need be.

Thanks, Doug

[High Plains Mopars]

Ideally you should have zero caster gain. However, any time you have anti-dive, your going to have caster change. The only way to avoid it is to have zero anti-dive built in to your mounting points and control dive with springs and shock rates.

What amount of caster change do you have with the three degree mounting points?

[pav8427]

With 3" compression I have about 3/4* more caster than at ride height.

Doug

[astroracer]

3/4's of a degree is nothing to worry about Doug. Suspension deflection will probably cause more then that on a 3" hit. That much travel is only going to occur in an extreme situation, usually at very low speeds, entering or exiting an angled driveway. You will never see or feel that little bit of a change at low speeds. If it happens at high speed it is doubtful you will feel it because you will be worrying about things falling off the car after you hit that huge Michigan pothole... At least that's what I worry about.
Mark

[High Plains Mopars]

I would tend to agree. 3/4 of a degree is not enough to worry about.