Double A-Arms
 

Here is an interesting double a-arm conversion using existing LCA. It is a show/drag car but maybe something like this would work for autocross FP. It looks like the frame rails have been replaced with thicker rectangular tubing.

http://cgi.ebay.com/ebaymotors/ws/eB...&category=6187

If I were going to do this I wouldn't use the stock pieces. If you look at the pics it doesn't look like they did much to imporve scrub or steering axis inclination. Those are two biggies that need to be resolved or you won't end up with much imporovement over a strut. I drove a car that was modified like this and was expecting a lot of improvement and didn't see it.

It certainly is a pretty car. I'm guessing a live axle in the back with some form of 4-link connected to the subframe stiffeners.

Cary

Just as a real grassroots way of doing it, I had thoughts of cutting off the top of the strut, welding in an adapter, which would allow me to bolt in a rod end. That way I could keep the lower control arms, and make an A-arm for the top. The lower ball joint could then be moved closer to the centerline of the wheel therefore reducing scrub.

I also wanted to do the same on the rear, but possible fabricate dual pick up points for double h-arms of sorts. But maybe a single point like the front would be sufficient, and therefore making toe adjustments easier too.

Here's one that I druel at quite often. The upper A-arm would look something like this. But on the other hand, if you're going that far, maybe new uprights should be used all together.

Does anyone know where you'd get ones like these?

http://race-cars.com/carsales/other/...2560599py1.jpg

Take a look at the attached pics. This is basically what you're talking about. It will solve the scrub issue especially with the correct wheel offset. The only issue you still have is the steering axis, which remains close to that of the strut. It would be better to reduce this if you went to this much trouble.

Take a look at a coleman or afco catalog to see uprights like you pictured below. You can buy off the shelf pieces for dirt/asphalt stock cars. Or if you can fab you can get the pieces to make this a lot easier. My front TC arms are made from some afco pieces. No 30 year old Datsun stuff still in use.

Turn it upside down and it's very close to what you'd see on the back of a current racing car. You can cut the strut tube and attach an a-arm to it. Very much like the JGTC Z photos I posted in an earlier thread.

Thanks Cary. That's what I was talking about, but I was thinking of cutting it down lower in order to clear the wheel (with more offset like you mentioned)? But maybe that's not needed?

I'm at a loss with the steering axis problems. If bumpsteer is good, what else would you change? What's different on your custom arms? Like in the other post, one could move the tie rod on top of the arm.

I did find uprights in the Colman catalog (listed as spindles), but I wonder what the king pin inclination is, and if the pick up points are in the best spot. Maybe I'm just being lazy, but I don't really want to think about these because then there's everything else; bearings, hubs, calipers....

The upright in the picture will fit inside a 15 inch wheel. Since it has a 12 degree SIA (steering axis inclination) it doesn't need to be too far into the wheel to keep scrub to a mininum.

The SIA has a large influence on steering induced weight transfer. If you reduce the SIA you have less weight transfer. SIA also controls the amount of decambering you get when you turn. For small angles we use caster to offset this effect. The general rule of thumb being to use one half the SIA for your baseline caster. This is a large part of why cars have a larger front roll bar, which I haven't really seen talked about much in suspension books.

Most stock car spindles are listed as a 8 degree or X degree spindle. This is the SIA. It gets expensive quickly when you start thinking about needing all the other bits that make them up. I don't think it would be too hard to remove the tube from the strut and change it's angle, which would be what we are looking for.

Cary

I feel like I'm beating a dead horse here, but was brainstorming a little about this and had an idea. What if one was to machine an offset plug for the top of the strut tube where it is cut, where the upper a-arm attaches? You could then theoritcally adjust the amount of SIA. You'd have to cut pretty low to get the full effect, but I'm thinking it could give you a few degrees difference then.

EDIT: Another thought, this could be taken further, and an adapter could be made to effectively put the rod end outside the strut tube all together, sort of back tracking towards the hub.

The stock angle is 12 or 13 degrees if I remember correctly. For a double a-arm arrangement you could offset the top to lower this but I don't know how muh lower it can get. You have to watch out that you don't end up having the upper control arm hit the strut tube in this scenario. But if you take the tube out and change the angle you could effectively get this down to a more reasonable number, like 6 degrees.

The option of having a top mount that bolts on is good and would allow you to set things like trail. But for SIA you'll want as little as possible and would probably end up with a front drive type wheel to take care of scrub.