Roll Center Adjustment versus Turbo Geometry

[ian c2]

Originally Posted by winders

unless you really know what you are doing and why.

Scott , for this very reason I won’t be doing this work on the project

All this is iirc from what I was told by people far better at explaining what they mean than I am at understanding it ..
The top slots are for 930 aluminium and rsr or custom steel arms.
The bottom slots are for stock aluminium arms .
Use stock , as Stock arms have a better curve and are much cheaper and available if you need a replacement .
Use erp or similar 935 plates .
And something about the engine and transmission moving forward if that means anything ?
Maybe the guy that told me to use stock arms was thinking of cutting off what the car came with and making something else , maybe he planned on welding washers to the bottom slots like you guys advised or maybe I’m missing the adjusters for the bottom slots . I cannot remember as he touched on a lot of things as he looked over the car .

[winders]

Standard 911 arms do not have a better "curve" but they certainly are cheaper. I have a brand new set of spare 930 trailing arms just in case. Yes, use the ERP spring plates.

It scares me to ponder how the standards 911 trailing arm bearings would hold up with the 14" slicks I am running.

[ian c2]

Originally Posted by winders

Speaking of racing stresses, my race car has 12" slicks front and 14" slicks rear so the chassis is seeing forces it was never intended to endure. The chassis was cracking where the transmission tunnel met the rear fire wall and at the top of the pivot boxes. So:



You can see how high my inner pickup points have been raised.

This is how mine was tied together . (Pic below) you can see the remains of it in the bad pics I took from under the car earlier .
Yours is a much more elegant solution now it’s just going to be used as a hot rod street car , and I could still run vertical upto the cage if needed as it’ll never have rear seats but need the passenger seat back so the rather elaborate “selfie camera mount bar” has to go !!
but decisions like that I’ll leave to the experts .
Interesting talking to you Scott and dan , but I don’t want to take over toms thread .
I’ll keep watching this thread , and if he says ok to carry on I’ll be glad , but if not I might ask a you guys a question or two in my own thread later if you could please reply .
Thanks
Ian

[dannobee]

Do yourself a favor and get rid of that seat, or at the very least, add the shoulder supports. If you go into a wall, you're gonna break ribs. We quit using that style of seat about 20 years ago for just that reason. If you go bust a front door down, you use your shoulder, you don't lift up your arm and use your ribs. And broken ribs can easily puncture lungs. I've witnessed it first hand.

Are there grommets on the brake lines where they go through the fabricated rear firewall? You might want to put some there.

[ian c2]

Dan , that is a very old picture .
You made a comment earlier about taking the car on track , and that is when I realized I should probobly bug out of toms thread as I was causong c0nfusion .
Tom is building a street car that may/should evolve into a track car , mine is a track car that is evolving into a street car .

And I think you may be talking about the lines for the remote reservoirs for the rear suspension components ?

[dannobee]

I was talking about the tee near the rear firewall. Isn't that a brake bias knob for the rear brakes, where the handbrake normally goes?
I see the reservoirs for the shocks. Is that what the tee feeds?

No worries if it's a street car. Get some comfy seat without those rib breaking side wings.

[Mahler9th]

There is one company that still sells the pivot boxes, last time I checked. They sell two versions: one for 930 arms, and one for SC-style arms. Both require welding.

I have the latter.

Folks may want to be careful about the term "ERP." Eisenlohr Racing Products is the original "ERP." Eisenlohr is a former pro racer and suspension guru/consultant. Decades of serious pro racing experience. You can buy the ERP spring plates from Tarett Engineering. That is where I purchased mine.

In these forums, some folks use the term "ERP" to refer to Elephant Racing Products. They have some great products that we are all lucky to be able to buy. Their racing experience is, I believe extremely modest or perhaps non-existent.

Finally, folks should be aware that the geometric improvements and range of adjustment resulting from changes to the outer and inner pickup points shown on Winders' car and available with a combination of the 935-style plates and inner pivot boxes (whether custom or commercially available) can be limited if the factory rear cross member is left in place.

In Winders' case, you can see it has been removed.

My car (built by the same expert as Winders) uses the ERP plates and pivot boxes with SC arms. Same tires as Winders. In my case, the factory cross member is still in place. Therefore I have far less flexibility in moving the inner and outer pickup points.

The performance potential difference between what I have and what Winders' has is of course debatable, but my experience tells me that it isn't likely to be more than a second a lap at most tracks we race and with all other variables considered.

For a street car, none of this may make any sense.

[winders]

I don’t know why you think the rear crossmember has something to do with pickup point location. It matters not....

[Mahler9th]

It does , sorry you are mistaken. It happens.

The angle of the rear shocks is limited by the cross member through which they have to pass.

So let's say, like me that you have that cross member in place. The coil over shocks (in my case Bilstein RSR with 2.25 springs with a rate and length that you already know, wink) travel through the cross member.

If I try to move the outer pickup point up using the built-in capability afforded by ERP's design (which I think is 1 inch or so) and the inner pickup point by a like wise amount using the pivot boxes (in my case obtained from Tarret), the angle of the coil overs changes and there isn't enough clearance for them to travel through the cross member. Again this is with SC trailing arms.

So my point is that with the cross member, what you can do is more limited than without.

There are a variety of variables at play (e.g, camber settings, et cetera), but the fact is that the cross member passageway through which the coil overs travel creates constraints not present in your car's set up.

And that these constraints may not be obvious.

The were not obvious to me until I installed and monkeyed around with everything. Then there was a "doh!" moment.

This "lessens" the benefits of the pivot boxes and pickup point change capabilities inherent in those product designs.

For me the big benefit of the plates is the ease with which I can adjust toe. And I can (but don't) use the pivot boxes to change the camber.

[winders]

You are talking about what some might call the shock tower bridge or shock tower crossmember. The rear crossmember is where the engine mounts are located. Mine is also cut out......

[tperazzo]

Good discussion guys and I like reading about others cars and experiences.

I feel like its good to know where the factory geometry is and what the range of adjustments are. Ie Turbo, RSR, 935, etc. Then it makes sense to compare with a lowered/modified track setup.

I measured:
1) the roll center of a stock 911 is 3.3" off the ground
2) A car lowered one inch moves the roll center down to 3" off the ground
3) A factory turbo roll center is 5.4" off the ground
4) Moving the inner pivot up about .7" raises the roll center 5.2"

I also checked the camber gain using stock aluminum arms at two inner pivot points and there isn't much change.

The ride height in the above graph is measured as in the factory manual between the torsion tube center and the hub center. When the hub goes above the torsion tube the number become negative. Kind of weird...

Then I looked at the Camber versus chassis roll and got this



This tells me that the factory was wise in going with the shorter trailing arm in the 930. The chart shows that the camber gain can't keep up with the chassis roll. This chart doesn't take into account anti-roll bars, so its not that bad.

Seems that most are in agreement that the inner boxes are set it once and done. I was amused that some said that my car should have slots and then end up welding washers to the slots, ha ha. So many ways to dig in to this.

Anyhow, these charts might keep the discussion going.
Tom

[winders]

You really should be looking at what happens to toe......

[Walt Fricke]

There are a number of ways to hold the pivot height on slotted pivot boxes. Some clever ones are shown or mentioned. My former SCCA GT3 or so car came with slotted pivot boxes (though camber boxes is also a recognized term, I think as long as you understand it just means dealing with the pivot height). These had welded to the stock boxes a pair of U shaped channel iron verticals, the U facing out with the inner slotted. The top piece had flat side up. Inside this was an upside down U, with a hole through it for the pivot bolt. When tightened, the bolt and its nut squeezed everything together, resisting up and down movement of the pivot. In addition, to aid both retention and adjustment was a threaded hole in the top center with, if you will, a push down bolt, which pushed against the inner U part, and had a lock nut. To pull the inner U up, a pair of 6mm studs were attached to the top of the inner U and went through holes toward the sides of the outer U's top, also with double nuts. To pull the pivot up, loosen the pivot bolt, loosen and screw upward the push down bolt, and tighten the two smaller nuts. When where you want to be, lock everything down. Lowering the pivot is just the reverse. The pivot point doesn't budge. Not an issue.

The force on everything is pretty significant, though. My setup wasn't reinforced against rotation. Suspension forces on the stock inner pivots try to twist the torsion tube. Raising the pivot increases this force by increasing the lever arm. I found after a while that my torsion tube had ripped. Winders' photo of his setup shows me one of various ways to reinforce the pivot boxes against torsional forces on the torsion tube.

[tperazzo]

Originally Posted by winders

You really should be looking at what happens to toe......

I did, it toes in during bump and isn't affected much by inner pivot point.

[winders]

What is “much”?? Any change in toe is big....