Roll Center Adjustment versus Turbo Geometry

[tperazzo]

Recently I added camber boxes to the back of my 911 to give a bigger range of adjustment and play with the roll centers a bit to mimic the 930 geometry.




That was a lot of work for two extra holes!

The holes were planned based on some comparisons with the shorter 930 trailing arm geometry.

Here's a 3D model.



The result is a bunch of bench racing theory! Please have a look at the video and let me know what you think...


Cheers,
Tom

[Fleming]

Tom,
like the way you are doing your car.

Have not heard as concise a explanation of the black art of rear suspension. Your animation of how it's relationship moves and the resultant changes it makes, sure is more meaningful than the line drawings from Fred Puhn's 1980's book "How To Make Your Car Handle". Same subject but far better. And all Porsche 911.

We keep learning stuff.

Thank you.

Bob

[ian c2]

Why didn’t you slot them for more adjustability ?
Here’s mine

[ian c2]

Just watched the first 5 minutes of your video .
You mention “last weeks video” , so I guess I need to catch up .
Not been on your YouTube channel for a while .

You’re progressing about a million times faster than I am , and I’m not slowing down to make videos !!

[Trackrash]

I thought that mod was for camber adjustment, especially in a very low car with RSR spring plates?

I imagine the roll center is affected and it will be interesting to see your results.

Be careful, a small change in the height of the center mount makes a noticeable difference in camber. Possibly making it hard to get the alignment setting you want, AHIK.

[Walt Fricke]

1) Agree that if you go to all this trouble, you should slot it so you can raise/lower the pivot as theory and testing and whatnot show will optimize things. Of course, if all you are doing is moving the outer pivot (spring plate) up so you can lower the car more without screwing up something, you'd increase the inner mount the same amount.
2) When I bought a car with adjustable inner mounts, I thought I was in alignment hog heaven. You could adjust camber without really affecting toe enough to worry about (assuming you didn't go from one end of the range to the other). But it seems you should set these so the roll center is where it is best, and adjust camber in the usual old way (which, with 935 spring plates, can be a lot easier than with the stock setup).

I used to lust for the double A arm type of suspension, thinking you could optimize things. Turns out there is no free lunch - Carroll Smith shows that even with this setup - the most used on full on race cars - there are trade offs.

[1QuickS]

If you read Mark Donohue's book The Unfair Advantage you will see he was testing at Paul Ricard in a 917/10 and was asked to try the RSR. He said thew rear needed to be modified to raise roll center to minimize roll couple and to transfer more roll resistance to the front which would naturally help overcome the front/rear weight imbalance. Front roll center was dropped also. The Turbo took the RSR geometry & shortened the trailing arms also which I think also raised roll center a bit but the real benefit is causing more camber change as a result of body roll which keeps tire contact pattern better planted on the outside tire.

[tperazzo]

Originally Posted by ian c2

Why didn’t you slot them for more adjustability ?
Here’s mine

Nice Ian! I didn't slot them because the fine camber adjustments can still be made at the spring plates. It's most likely a set it once and forget it type of thing. I did space the holes so that it is within the range of adjustment on the spring plates. So the inner is a coarse adjustment and the outer is a fine adjustment. One affects the roll center and one doesn't.

Also I got nervous about the arms slipping in the slots or not being even from side to side. Maybe I'm just paranoid???

BTW, with your shorter trailing arms and those raised slots your roll centers are very high. Did you also raise the outer pivots too?

[tperazzo]

Originally Posted by Trackrash

I thought that mod was for camber adjustment, especially in a very low car with RSR spring plates?

I imagine the roll center is affected and it will be interesting to see your results.

Be careful, a small change in the height of the center mount makes a noticeable difference in camber. Possibly making it hard to get the alignment setting you want, AHIK.

You are right, they are primarily for camber, then roll center, and then anti-squat. That's the order of influence in my opinion.

Yes, the inner holes I have drilled will make a large change in camber. I won't be able to use the upper hole unless the car is very low or I change the outer pivot too. I'm running coilovers, so this is a possibility down the road as they are bolt on parts.

Thanks for your comment, its always good to bounce ideas around....

[tperazzo]

Originally Posted by Walt Fricke

1) Agree that if you go to all this trouble, you should slot it so you can raise/lower the pivot as theory and testing and whatnot show will optimize things. Of course, if all you are doing is moving the outer pivot (spring plate) up so you can lower the car more without screwing up something, you'd increase the inner mount the same amount.
2) When I bought a car with adjustable inner mounts, I thought I was in alignment hog heaven. You could adjust camber without really affecting toe enough to worry about (assuming you didn't go from one end of the range to the other). But it seems you should set these so the roll center is where it is best, and adjust camber in the usual old way (which, with 935 spring plates, can be a lot easier than with the stock setup).

I used to lust for the double A arm type of suspension, thinking you could optimize things. Turns out there is no free lunch - Carroll Smith shows that even with this setup - the most used on full on race cars - there are trade offs.

Hi Walt,
You summed up my method perfectly. I plan to move the inner mount up so the roll center is equivalent to the 930 roll center. Then use the spring plates to get the camber I want. Once the inner position is set, then I don't anticipate changing it again unless I change the outer pivots. Then I will go to the next highest hole.
Cheers,
Tom

[tperazzo]

Originally Posted by 1QuickS

The Turbo took the RSR geometry & shortened the trailing arms also which I think also raised roll center a bit but the real benefit is causing more camber change as a result of body roll which keeps tire contact pattern better planted on the outside tire.

I agree with this 100%. The roll center is raised with the shorter trailing arm. You are right that the camber gain curve is even more aggressive. I suppose that is a benefit, but I do know the rear already has way more camber gain then the front!

I will read Mark's book again as I really enjoy this kind of stuff.

[tperazzo]

Originally Posted by Fleming

Tom,
like the way you are doing your car.

Have not heard as concise a explanation of the black art of rear suspension. Your animation of how it's relationship moves and the resultant changes it makes, sure is more meaningful than the line drawings from Fred Puhn's 1980's book "How To Make Your Car Handle". Same subject but far better. And all Porsche 911.

We keep learning stuff.

Thank you.

Bob

Thank you for watching Bob. I also have Carroll Smith's old book, "Tune to Win". I will be out your way in a few days for vacation.

[winders]

What you are calling "camber boxes" are really called pivot boxes. Camber boxes are installed at the front of the car. The reason I make this distinction is that you don't want to use the pivot boxes to make rear camber adjustments. Their purpose is to relocate the inner pickup points based on particular geometry requirements/choices. The spring plates should be used to adjust both camber and toe. Yes, you can make camber adjustments with the pivot boxes, but you may also negatively affect the suspension geometry in the process.

Porsche created the shorter trailing arms for the mid-year 1973 RSR to better match camber gain to body roll. From what I have read here on Pelican, the RSR was not very stable in high speeds turns. So they raised the inner pickup on the Turbo to get some additional toe-in as the suspension compressed to gain some rear stability in those high speed turns. I do not believe the Turbo gained any anti-squat from this change. In fact, it may have lost some anti-squat.

My race car is built on an 1987 Turbo chassis using 930 trailing arms. The trailing arm inner and outer pickup points are raised. Here are some photos:





You don't want to raise the inner pickup point compared to the outer pickup point without a plan. Porsche did an excellent job with the 930 rear suspension geometry. The inner and outer pickup points on my car were raised about the same amount so we could lower the rear of the car a bunch, not have the roll center get too low, and keep the geometry in the proper operating range. We raised the spindles in the front for the same reason. You want the rear roll center higher than the front roll center and you want both of them above the ground.

Using the standard trailing arms, there may be something to be gained by raising the inner pickup point some on its own. If just raising the inner pickup point was such a great idea, Porsche would have just moved the pickup point up and not changed to a shorter arm for the RSR. There had to be a reason for the arm change. I would look for that reason with the modeling you are doing. I would look at toe change specifically. You want some toe in increase as the suspension compresses, but not too much. To see the differences, you will want to model the 930 trailing arm as well.

The bottom line is that the main purpose of the pivot boxes is to allow you to raise the inner pickup points of lowered cars to maintain appropriate roll centers and keep the suspension in the proper operating range.

[dannobee]

I've played with front and rear roll centers, anti-squat, and camber curves quite a lot on the race cars over the years (SCCA early on, then nascar for two decades). Here's what I know.

Shortening a control arm will give a more aggressive camber curve. Think of an SLA suspension and visualize a super short upper control arm length, then watch the camber change during jounce/compression. (Assuming of course that the instant center isn't outside the tire where the camber curve would be opposite of what we want.) We would generally want a longer instant center than a shorter one, all else being equal. It's more predictable for the driver.

Raising the rear pivot points will increase anti-squat. Often this is necessary to get back what lowering the car took away. IRS cars can never get as much as we'd like because of the force vector going through the center of the axle instead of the tire contact patch as is the case with a live rear axle. *BUT* the same forces that make the rear tires "hook" also REDUCE the rear grip when you get off the throttle. (Think of 930's and the "DON'T LIFT IN THE CORNER!" speech that the first time drivers get). With brake floaters you could eliminate this phenomenon entirely, and actually plant the rear tires harder on braking, but good luck fitting that in anything but a live axle car.

Raising the rear roll center is necessary as you increase the rear wheel width. FWIW, it roughly coincides with the width of the rim (i.e., start with the width of the rear wheel, and if the rules suddenly changed to allow you a 2" wider rear wheel, increase the rear roll center about that much).

Rear bump steer. Yup, just as important as front bump steer, only harder to easily change. Wanna turn a great driver into one who shakes and sweats every time he enters a corner? Put some wicked bump steer in the rear then check the stains on the driver seat after a main event.

When we'd build a car from scratch, we'd agree on F&R roll centers and alter the pickup points to maintain those numbers. You're limited by rule on things like frame height, suspension types, spindles, etc. Those became the parameters from which we worked.

Walt and Scott are right. There is no free lunch when it comes to suspensions. The factory did things the way they did for a reason. Have a good plan in place if you want to deviate widely from their knowledge. That said, we would NEVER construct a modern race car with a strut front end and banana arm rear end unless it was required by rule. I can see why someone advised "put the rear suspension off of a 996 in there."

I'm old school. Even though I've used many suspension analyzers, I like measuring stuff. Get down on the ground with a tape and measure everything, then see where your pickup points are at various body roll and ride heights. Transfer those to the garage floor by snapping chalk lines and measure the actual instant centers and roll centers. Make changes and take copious notes. Learn how to check and adjust bump steer, both front and rear. Test on the track and see how things work. Take tire temps and pressures every single time the car comes off the track. See what changes make YOU faster. Have fun.

[ian c2]

Originally Posted by tperazzo

Nice Ian! I didn't slot them because the fine camber adjustments can still be made at the spring plates. It's most likely a set it once and forget it type of thing. I did space the holes so that it is within the range of adjustment on the spring plates. So the inner is a coarse adjustment and the outer is a fine adjustment. One affects the roll center and one doesn't.

Also I got nervous about the arms slipping in the slots or not being even from side to side. Maybe I'm just paranoid???

BTW, with your shorter trailing arms and those raised slots your roll centers are very high. Did you also raise the outer pivots too?

Tom ,

Here’s a pic to show what’s needed and pics in place showing adjustment .
There is a hole in the top plate and the washer and nylok are tightened from inside the box , but leave a small amount of clearance so the top adjuster will turn freely which in turn raises or lowers .
My original holes are slotted too , gives options ....

As for the outers , I’ll be using 935 style .
But that’s way down the road so not decided on brand or purchased yet .
I think you said a while ago you were doing the reinforcement mods to run coil-overs , but not installing until into the future ?
This will limit your choices as 935s ran coilovers so the design was coil-over only , and most versions follow the design of the original .
But I have seen ones that will let you run bars then change to coil overs in the future , cannot remember where though .
I’ll be running 11” rears but will still be high enough to ride on the streets .
Will probably have to carefully choose which gas stations to use though .
For some reason in socal they build them all on top of a hill ....

[winders]

Originally Posted by ian c2

As for the outers , I’ll be using 935 style .
But that’s way down the road so not decided on brand or purchased yet .

The problem with the "935-style" spring plates is that let you the out pickup point only an inch or so and it is a fixed point. Yet the pivot boxes you have installed allow for much more.

Since it appears you are using 930 trailing arms, I would suggesting using the 930 geometry. So, if you are using stock spring plates, I would put the inner pickup points in the stock 930 location. If you use "935-style" spring plates and move the pivot point to the upper location, I would move the inner pickup points that same amount.

My car has the inner pivot bolt washers welded to the pivot box so the pivot point cannot move.

[dannobee]

Welded washers are exactly how it should be done. The rear suspension pieces are under tremendous forces under acceleration and braking. Don't just casually dismiss these forces. 11:35 1st gear and 7:31 R&P and a modest 200 lbft of engine torque =2800 lbft of torque acting on the pivot points. Slots might not be strong enough to resist these forces.

Set the rear roll center and anti-squat where you want it, then weld the washers in place. Be as accurate as you can with your measurements. Small changes make big differences.

I'm not trying to nitpick, just trying to help you address any issues before the car hits the track. And be careful to keep roll center centered on the chassis centerline.

[ian c2]

Scott , I thought you could go to 930 inner with stock outer .
Then an inch above that with 935 outer ?
I don’t think there’s anyway I’ll ever need that much on a street car on 15x11 ?

Dan , I could get in touch with the guy who raced this car originally .
He raced it with about 3-4 times the horsepower you’re quoting , but not sure if he ever used the bottom set of holes and if he did what was used to secure them .
The adjusting device for the top slots does not fit there in any way I can tell .
If I do end up using then , then I can find no other way Than welded once setting is right .
Before it comes to this though I’ll ask the guy who built the car what his plan was to hold the bottom slots .
Unlike me , he must have had a plan

I sold the 930 arms , and will probobly go with stock rear arms when the time is ready .

Will be a while off yet

[winders]

Originally Posted by ian c2

Scott , I thought you could go to 930 inner with stock outer .
Then an inch above that with 935 outer ?
I don’t think there’s anyway I’ll ever need that much on a street car on 15x11 ?

If you use the stock outer pickup point, you should use the stock 930 inner pickup point. That's assuming you are using 930 trailing arms.

If you use the 935 spring plates and use the upper hole raising the outer pickup point, you should raise the inner pickup by the same amount.

Originally Posted by ian c2

I sold the 930 arms , and will probobly go with stock rear arms when the time is ready .

If you use the standard 911 trailing arms, you really should use the stock inner pickup points unless you really know what you are doing and why. If you use the standard 911 trailing arms AND raise the outer pickup points, you should also raise the inner pickup points. But, you can't use the pivot boxes with the standard 911 trailing arms because those pivot boxes are located with the shorter 930 trailing arms in mind.

[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.