Unequal length wishbones question

[Ant]

Regarding unequal length wishbones "Is there a ratio of length like 2/3rds for the top compared to the lower wishbone?

I appreciate roll centers etc. bump steer, Ackerman and the rest of it, which I dont know much about .........yet!
I have just shortened my wishbones 3/4" to accommodate Trigo wheels and the adapters for guard clearance, but have run into a problem(s) I want to use 26.5 diameter rubber, so I have set the chassis up with 4" front ground clearance and 5" for the rear, and what I have ended up with is the bottom arms angled up about 2.5" at the upright, and the top arm much greater angle - moves about 1.75" and binds in the balljoint on bounce.

Looking at making longer arms and relocating altering innner pickup points to solve this problem, and I see some good books advertised, Tune to win by Carroll Smith, and Chassis Engineering by Alan Staniworth etc may not have spelt that right. Also there is available computer programs that help with roll centers etc, but my computer is a Macintosh.

Anyone got any tips on any of the above, any recommendations or direction gratefully appreciated.

[strictlypersonl]

The shorter upper arm derives from the time when control arms were almost always parallel to the ground. At the time of the "birth" of independent front suspension, it was conventional wisdom that the roll center had to be at ground level. With narrow tires and high center of gravity, the angle of the contact patch wasn't that important with narrow tires anyway.

So... the short/long control arm design was created to keep the position of the contact patch in a straight vertical line under bump and rebound. The tire tilts in going up - and also tilts in going down.

Things have changed a bit since. If you want to see some "interesting" contact patch movement, analyze a McPherson/Chapman strut. But - most of them survive quite well by having very long LCAs.

At any rate, to your problem! It sounds like both of your arms are angled much too much. They will put the roll center below ground, and you want it slightly above ground. Since you already have ball joint bind, that's a good enough excuse to change pivot points already. A few more details here.

Is it necessary to have 1" chassis rake?

[Ant]

strictlypersonl

Thanks for your post, the chassis doesn't need 1" rake, it may help stability at high speeds, but when you take into consideration the shape of the body front of bonnet/windscreen etc that area should help to hold down the front!

Will check out your links, and I am going to have to do something with the pickup points. I see a lot of different Cobras etc apart from JBL have quite short top arms, there is opportunity with the original style chassis to have quite long arms. My chassis rails are 26" apart and my custom alloy radiator is about 1" away from the front/top pickup point, but I can change the shape of the bends on the arm and come in if necessary - more research required!Will check out your links