Suspension tuning with TB's

[dede911]

Hi,

Currently, my suspension setup on my 911SC (track & road)is as follows:

Front:
-21mm TB's
-carrera sway
-bilstein sport
-monoball for -1.5 camber

Rear:
-26mm TB's
-carrera sway
-bilstein sport
-max camber

Weight : 1060kg

To diminish boy roll, my idea was to install 27mm TB's...

Yesterday I read the horror story on Pelican about a snapped new TB from Sway A Way...I don't like to see this happen in the very fast left hander in Spa-Francorchamps with exit speed around 200km/h...

Does Sway A Way has really a quality issue?

Another possibility could be to stiffen up the suspension by installing an adjustable sway bar, and leaving my 2years old OEM porsche TB's($$$) as is...

What do you think?

Thx,
André

[tobluforu]

I run 29's in the rear of my 72, sway away, and they have been in there for 12 years. I would go higher then 27.

[rfuerst911sc]

I have 22/30 Sway Away in my 83SC coupe been running them now for 5 years with no issues. Yes Sway Away recently had a batch of bad bars per another post on this forum but ANY manufacturer in the metal business can have this happen. But there are other manufacturers out there if you want to stay away from SA.

[Flieger]

I would not use any Sway-a-Ways produced since about 2000. They are marketing them as aircraft quality but this is very suspect at that price. And, anything aircraft quality would be inspected to death. No flaw that big could get through, ever. So they clearly are misrepresenting their product.

I would suggest matching the torsion bar stiffnesses to the weight distribution, then tuning the oversteer/understeer and body roll with good quality anti-sway bars such as Tarett.

[dede911]

Flieger,

Could you elaborate on "matching the torsion bar stiffnesses to the weight distribution"?

Disadvantage with adj sway's is, that this modification it is legally not allowed where I live...we have a yearly mandatory technical inspection...

André

[Flieger]

By that I mean if the weight distribution is 40% front, 60% rear, then you want the rear torsion bars to be 150% the stiffness of the rears (or the fronts to be 2/3 the stiffness of the rears if you want to look at it like that). That way, the springing force is distributed 40% front, 60% rear. That way the ride should be pretty predictable/stable. Then, because the roll centers are at different heights and the track conditions change the desired balance, use the anti-sway bars to balance the oversteer/understeer how you like it. If you can't go adjustable, then just tune for your favorite track and weld the adjuster once you find a good setting. Or try a bunch of bars until you find a stock bar you like. Or else you will have to compromise somewhere else.

[dede911]

Flieger,

According to this theory, do i come close to goor "spring" distrubution with 21/26 TB's and 1060kg ?

I have seen in other threads that you know your stuff ...could you make a sort guess or calculation?

Regarding the sway bars...if I only install an adjustable bar in the rear...it would not take much work to remove and replace with stock bar once a year before going to the technical inspection...

What do you think?

Thx,

André

[Flieger]

Without knowing your weight distribution, I can't really make a completely accurate measurement, and I also need to look up the dimensions for the lever arms on the torsion bars. If you could do that for me that would help.

The measurements are the distance from the center of the spring plate cover (where the rubber bushing goes) to the center of the wheel bearing/hub. Try measuring with two or three large rulers so that you can keep one parallel to the wheel while using the others to find the centers of the torsion bar access hole and the center of the wheel hub, while keeping everything at right angles, basically like a large pair of calipers.

The other distance is ball joint to torsion bar centerline in the front which I think is 12 inches.

Then I need to look up the lengths of the torsion bars between the splines.

All of these distances effect the wheel rate (disregarding for now the tire spring rate).

But, before factoring that in, 22/26 may be a better setup. It would give you about a 33%/66% stiffness distribution, which is a little rear biased. If you can only do a rear bar, you could do 23/26 since a front stiff car understeers and the rear anti-sway bar will add oversteer.

It depends on how you use the car. For most things like street and high speed track you want stability/understeer. For autocross you want more oversteer.

You can email me at maxsluiter@earthlink.net

[dede911]

e-mail send with data

[Flieger]

I got your email and also found the other required info. You got the length right, btw.

short version:
27mm is a good choice.


long version:
Based on your car's data, I would suggest 21/27, which is a common setup for upgraded cars. Ride stiffness is a highly subjective thing, but I think you would be fine with 21/27.

I have 22/27 in my car which is lightened to 2000 pounds with no passengers and is of unknown weight distribution. A lighter car makes the suspension seem stiffer, but I think I could go much stiffer since it is just a weekend toy for carving the canyons.

21/27 should give almost a 40/60 stiffness distribution, which you could fine tune to some extent with tire pressures. If there is too much body roll and you can only use one sway bar and want to maintain the balance, the way to counter that is to upgrade both front and rear torsion bars.

Can you get a ride in a friend's upgraded car with a similar suspension? That way you know if it is too soft or too firm.

You may not need any anti-sway bars with these torsion bars, depending on how much body roll you can tolerate. Anti-sway bars make the suspension less independent so it is better to get the roll stiffness from torsion bars and use the anti-sway bar for fine-tuning the handling. The anti-sway bars will still make the ride harsh, I would say harsher than a car with only large torsion bars.

There are other things you can do to fine tune handling. Tire pressures are one, and the roll centers are another. This basically means down to the ride heights and whether you want to raise the front spindles like the RSR. But first I would try with no anti-sway bars.

Other notes: the monoballs are great. Both front and rear. They free up the suspension and remove a little spring rate so that the suspension moves much more freely. That is my experience at least. I have monoballs front strut tops and rear trailing arm. Rebel Racing A-arm bushings and polyurethane spring plate bushings (for now). No rubber anywhere. The car is much more forgiving over bumps and has more grip since the tires remain on the road when they did not with old urethane bushings everywhere.

If you get much bigger torsion bars it is best to re-valve the dampers.

If you upgrade only the anti-sway bars, then the damping can be good in ride but in the turn it will be underdamped, so there is no free lunch.

Hope that helps.

[dede911]

Thanks a lot!

This was also my first idea based on general suspension upgrades....but then I read the horror story about snapped TB's...

Maybe my 26mm OEM Porsche TB's were so expensive because Porsche runs a quality check on them...

André

[Flieger]

Yes, you get what you pay for, as they say.

I am really not sure how the hollow bars are fabricated, but if Sway-a-Way are not using quality materials in one line I would be suspect of all their parts. Perhaps Sanders are better?

The only recourse may be inspecting them for flaws and verifying spring rate by yourself which no one should have to do.