Early 911 Suspension Prescription

[RallyJon]

After a few hours reading the archives here, I'm amazed at the wide variety of suspension setups people are running. I have an interesting suspension under my '71 911E "RS Look". It was set up by a prior owner for DE use.

Front:
22mm torsion bars
22mm sway bar
Koni red-orange struts from (supposedly) a 911SC.
225/50HR15 Euro T/As on 15x7 Fuchs
Front Ride height:
Front wheel center = 298mm
Front TB center = 130mm
Difference = 168mm
Fender = 23.75"

Rear:
28mm torsion bars
22mm sway bar
white (KYB, according to some Pelicans) shocks
height adjustable spring plates.
245/50HR15 Euro T/As on 15x8 Fuchs
Rear Ride height:
Rear wheel center = 305mm
Rear TB center = 264mm
Difference = 41mm
Fender = 24.75"

Since it's a partially stripped early car with glass bumpers and decklid, it can't weigh more than 2300lbs. The car feels oversprung (or like the dampers aren't working at all). It crashes over bumps in the road and gets unsettled over ripples and sharp rises. This is a real problem in SE Pennsylvania as we have great twisty roads, but they have a lot of seams and undulations.

There is also a fair bit of sidewall bounce. I will be downsizing to 225/50 rear and 205/50 front stiff performance tires to try and remove some of the sidewall flex from the the handling equation.

The first thing I want to do is raise the ride height a half inch or so and see how it feels with a bit more up travel, but I suspect that it will need better tuned dampers to handle properly.

I'm also concerned that the sway bars may be too large--but I've always preferred to use swaybars as a tuning aid rather than such a significant part of the suspension. From what I've read a lot of people seem to run huge sway bars on 911s.

The purpose of the car will be 80% weekend driving/20% autocross, Road Rally, DE, Solo I, etc. I don't care about a smooth ride--just that the suspension allows the tires to follow the road in varying conditions while still providing buttoned down handling on the high speed turn into the infield on Pocono North.

Thanks for any advice, part or setup suggestions you can provide.

[ttweed]

Originally posted by RallyJon
There is also a fair bit of sidewall bounce. I will be downsizing to 225/50 rear and 205/50 front stiff performance tires to try and remove some of the sidewall flex from the the handling equation.

I think you're going the wrong way with this idea. You have a fairly stiff setup for street driving already, and to take some of the sting out of bumps, you should be using a tire with MORE sidewall compliance, not less. I would suggest a 205/60 series tire in front and a 225/55 in the rear. Then you could try disconnecting one droplink on each sway bar for street driving. When you go to the track, hook them back up and put on the wider, stiffer tires.

If you are going to raise the ride height, do it only in the front- it looks from your numbers like you have too much rake right now.

Good luck- that should be a fun car on the track, but hell over potholes...

TT

[911nut]

See how you like the ride after you get your ride height up a little. The suspension need to have a little travel and your car sounds like it's too low.
I've never seen torsion bars that big spec'ed for an early car. You might want to go to 20mm front/26 rear (or even 19/24) for street use if raising your car doesn't help.
PB

[Chuck Moreland]

I spent a year outside of Philidelphia. Worst roads I have ever seen, unbelievable.

With 22/28 tbars on a lightweight it is going to be a very firm ride.

The kyb shocks rear are definitely too wimpy. Your konis are probably okay. Lighter tires will help this some.

Your front ride height is way too low. You want the front about 1/2 inch HIGHER than the rear when measured at the center of the fender. At your ground-scraping height you would need raised-spindle struts or your geometry is hosed.

I'd suggest rasing the front height up to about 25 1/4, leave the rear alone. Replace the rear shocks with konis.

Try disconnecting the front and rear sway bars, just remove the drop link on one end.

Use this as your baseline. Now do one change at a time. Put the sway bar back on a soft setting. Adjust the sway bars to get the car balanced and to your liking.

I wouldn't go much lower on ride height for a street car.

[RallyJon]

Well, if the new smaller tires lower the fender height approx .25 inch front and rear, I can raise the front a full inch to 24.5" and the rear .25 inch just to get back to 24.75". I think those 25+" fender heights are for cars with rolling diameters of an inch or more larger than mine. 225/50-16 and 205/55-16 are both (nominally) 24.9" tall. 225/50-15 is 23.9" and 205/50-15 is 23.1". So, a front ride height of 24.5" is the same at 25.4" correcting for tire diameter.

From what I've been reading, the cutoff for raised spindle "RSR" front struts is right around 24.5".

I'm very willing to put up with a choppy ride over the small stuff--as long as the suspension travel is there and the dampers are responsive enough to react to medium frequency bumps, instead of letting the tires do all the work.

[jluetjen]

Disconnecting the sway bars is a cheap and easy experiment, so I agree to try it. I suspect that your car just has way too much spring on it from your description. I don't think that the swaybars will have much of an affect with the exception of single wheel bounce. It sounds like for your use you will need to back off on the spring rates to make the car easier to live with.

My personal attitude is that a car should be as softly sprung as possible while keeping the suspension in it's ideal zone. Unless you are running F1/CART levels of downforce, a suspension won't work for you if it can't work. Raising the ride height will also help to keep the suspension in it's "sweetspot" since McPherson Strut suspensions do not like to be set too low. It puts the roll center below ground level which can cause all sorts of strange handling.