Rear Control Arm Pivot Box to reduce camber

[Jedi]

Hi All,

An old topic but I cant seem to find the answers via the search engine.....

I am running a track only '74 RSR with 13" rear bias slicks. With the lowered ride height we are battling to get less than 2.5 deg of camber on the rear and it is chewing up the tires. I need to get back to around 1 deg and believe that raising the inner mounting point of the rear control arm will solve the problem. I am running std 911 control arms and Rebel Racing 935 spring plates.

Questions:

1. Will raising the inner mounting point achieve what I want?
2. Apart from Tarrett where else can I source them?

Thanks,

[Mahler9th]

As you know, we have quite a few 911 racing cars running 16" diameter 13 inch wide bias ply slicks. Some are still using up their Goodyear inventories and a few are experimenting with Avons. I suspect that when everything is said and done, most will convert to 16" Hoosier radials, as there is no solid visibility on the ongoing availability of Goodyear bias plys, and the Hoosier bias plys are considered less desirable form a performance standpoint.

As to your question, I think the weld-on Hamlin Rear Control Arm Pivot Boxes may be what you are looking for. They used to be sold by SmartRacing Products (SRP), but that company is no longer in business. Many of the products from the old SRP catalog are available from Jerry Woods Enterprises-- I am not sure about the pivot boxes. I can check with JWE tomorrow at Sears Point and get back to you. The SRP brain trust will be there as well.

It looks like Tarrett sell similar/identical rear pivot boxes. Ira Ramin at Tarrett can help you understand whether they will work for what you intend. He knows his stuff for sure. I suspect they will.

On my car with SC trailing arms, I wanted to get ~0.75 degrees of negative camber for best performance versus wear. With stock spring plates and a super low ride height, this was not possible. So I had to modify the spring plates. As you may know, the stock plates and the 935 style plates have a slot for camber adjustment and a hole. I basically had the slot and hole on each plate welded up. Then cut a new slot and drilled a new hole on each plate. As I recall, these are about 10 mm lower than the normal street car slots/holes. I will check my records.

By doing this, I was able to get as little as 0.5 or so of negative camber with my low ride height without having to change the inner pickup points.

The geometry changes afforded by the Pivot Boxes and 935 style spring plates add performance potential and make it easier to set camber and toe. From a performance standpoint, I suspect it is less than one second per lap. I am currently not motivated to spend ~$2-3k for a second a lap.

Since you already have the spring plates, you can probably just modify them like I did to get the camber you want.

In my case, a change to radials will necessitate a further hardware change as I will want 2-3 degrees of negative camber.

[Mahler9th]

Here is a DWG from SRP showing the ERP spring plates. Note the location and shape of the holes for fixing to the trailing arm. Also note the location of the slot for camber.





If I were to purchase these, I don't think placing the small hole on top would allow me to get to 0.5 - 1 degree of negative camber at my low ride height. I would likely have to weld up the slots and cut new ones that were lower. Again, I have SC arms and I do not have the pivot boxes to raise the inner pickup points.

I am not sure whether the Rebel Racing Spring plates are dimensionally equivalent to those sold by ERP. I suspect they are close.

[Mahler9th]

I spent a few more minutes on this. The parts that SRP used to sell had two versions: One for 930 arms in either a 911 or 930 chassis, and one for SC arms. So for my car, I'd have to buy the latter. I suspect that would be true for you as well.

I am not sure whether the parts sold by Tarrett are designed for both types of arms. The instructions suggest that they are only for 930 arms. So you might want to check on that with Tarrett/Ira.

I will check with the JWE folks about the boxes they used to sell at SRP.

[Jedi]

Thanks guys, as always a wealth of knowledge. I look forward to your feedback. I received some excellent advice from Clint at Rebel Sport Racing and that was the same as yours to remove the concentric bolt and weld up the plates. He still recommends the raising of the inner mounting points as the best solution and has offered to get me in contact with somebody who makes these as well.

[kandhmfg]

Jim Hendrix at John James Racing in AZ makes the camber boxes. (for Tarett and others) or you can buy direct if you want to get in contact with him direct. He builds some of the best vintage RSR re-creations out there.

[Mahler9th]

You are welcome. I am off to Sears Point in a coupe of hours and I can speak with the JWE/SRP folks. I saw that John James Racing makes the boxes for Tarrett in the instructions. The documentation says they are for 930 arms. And my old SRP catalog says that the Hamlin version came in two styles... one for 911 and one for 930 arms. Perhaps the JJ Racing ones will work with either, but I would check.

When I started with my current car 12 years ago, I ran about 1. degrees of negative camber. Since then I increased the rear spring rate and lowered the rear at least 1/4 inch. And our local group think on wear versus performance for my car's configuration identified about 0.75 degrees as optimal.

Again this was not achievable for me with the camber slot in the factory location. It was easiest for me to remover the spring plates and have them welded up (Peter Weber did this for me... great guy.... makes Phase 9 mufflers.. Quiet HORSEPOWER). Then with guidance from my good friend and the car's builder, I made new slots located lower on the plates. Easiest and cheapest way to go. Cost was abut $25 if I recall correctly. Pete is awesome.

Those pivot boxes are nice, but it would cost me at least $1k to buy them and have them welded in, and I just cannot justify that kind of expense for less than a second a lap. I have bigger fish to fry for the $$$.

[Walt Fricke]

When first I acquired my track only 1968 911 race car, it had been converted to rear camber boxes. It isn't all that hard to fabricate these. These (which were fabricated, not an off anyone's shelf, item) had a large bolt in the top center of the pivot tower, which could be screwed in to push the pivot itself down. And it had two smaller studs on each side of the large one, which could be used to pull the pivot up. Each had a lock nut as well. Great, I can adjust camber at the track, and without really affecting toe.

So I did this (since I had the SRP camber tool) with some frequency.

Then I realized that the pivot height affects the roll center. And having the pivots at different heights to get equal camber settings maybe wasn't such a hot idea. So I started setting the pivots at equal heights, and doing my camber adjustment back at the spring plate, modifying it as needed. And with 935 style spring plates you can do stuff, like use smaller bolts through the toe and camber adjustment slots in the middle of the plate, to give you more range of motion there.

Seems to work better.

Anyway, that is a caveat about getting carried away here.

I run 0.5 negative as a target on my 10" bias ply slicks.

Like the rest of us bias ply fans, Goodyear pulled the rug out from under me with my 15" rims for rear use. And, blast it, Hoosier discontinued its 10 (or 11) x 15 for 10" or so rims at the same time. Leaving me with three sets of light weight 10x15 rims, and no clear path for tires for them. But I digress.

[Mahler9th]

JWE informed me that the Pivot Boxes formerly sold by SRP are not available from them. They still sell/support some of the SRP-designed products, but the Pivot Boxes are not amongst those items.

They do have a similar set up they use for in-house built cars only-- it is not a kit for re-sale.

If I recall correctly, many (most?) with these parts do not use them for major camber changes if at all. I am pretty sure that the geometric benefits are the major motivation for these parts.

If/when I change to radials and need another 2-3 degrees of negative camber, it is my hope that I can get what I want by flipping the spring plates over, or welding them up again and cutting new slots.