Upgraded suspension.....Now MORE rear compression!!

[tshebib]

Hi All, Last winter I did a full suspension upgrade using all solid bushings, 29mm rear and 22 front t-bars 22mm front and rear adjustable sway bars.





Before the upgrade I had 22mm f&r sway bars only and everything else was still original. I kept the ride height the same at 24.5" front and 24" rear at the fender lips. Problem is now the right rear tire rubs the inside of the fender during heavy cornering at the track. Here's the result




I since raised the height to 25" at the rear fender lip and have 3 deg of negative camber and the tire still rubs. Oh, BTW I've been running 255/40/17 Re-11A's on Rota's for 4 year now with no tire rub before the upgrade. Do I need to go to 31mm rear bars or is there something else that can be done?
Tom

[LARRY SHARP]

What happens is that you replaced all the rubber bushings that have a built in "stiction" that acts like additional spring rate. Your suspension now is free of that stiction and moves much easier. I am sure someone on Pelican has measured the differences between stock bushings and bearing bushings. I would say yes you need to run bigger torsion bars but how much bigger I would not know

[Tom '74 911]

My first thought is to agree w/Larry - that replacing the original rubber bushings w/the Rebel Racing stuff is letting the suspension move more freely...

On my '74 w/SC flares, I run the same t-bar set up (albeit w/Elephant Racing bushings) at a similar ride height. I run a slightly smaller tire though if I remember correctly - 8x16 Fuchs w/255/50 R16 I think. I will try to remember to verify the ride height the next time I am near my car and will post back if it's wildly different...

Tom

[Driven97]

Raising the rear spring rate, raising the rear sway, raising the ride height should all help.

Just out of curiosity, are your rear bumpstops all still there?



While cornering on the bumpstops isn't ideal, they should provide a somewhat progressive stop before you get to the fender. If not, you can add some new technology multicellular progressive stops to give you a nice progressive help near the end of stroke, but keep in mind that's extra spring rate, so you'll have to tune for it - maybe with a little extra front swaybar stiffness setting.

[tshebib]

I was told to cut off one of the four bellows of the bump stops. You're right, I'm trying to stay away from using the bump stops as extra spring. My worry is having sudden increased spring rate at mid turn and unsettling the car. I would say I was definitely bottoming out on the stops before the upgrade. The car actually feels more compliant over bumps with the heavier bars. I'm also thinking about lightening up the rear end by getting rid of the heavy tail and going with a fiberglass duck. Problem is I don't know if that would be enough.

[tshebib]

Originally Posted by Driven97

Raising the rear spring rate, raising the rear sway, raising the ride height should all help.

Just out of curiosity, are your rear bumpstops all still there?



While cornering on the bumpstops isn't ideal, they should provide a somewhat progressive stop before you get to the fender. If not, you can add some new technology multicellular progressive stops to give you a nice progressive help near the end of stroke, but keep in mind that's extra spring rate, so you'll have to tune for it - maybe with a little extra front swaybar stiffness setting.

I have the rear sways full stiff and the fronts are half way there. I would actually like to have more oversteer (looseness) to help with initial rotation at turn-in.

[Dan J]

29 rears is great on the internet However in real life you need at least 33's preferably 35's
oh and 24 frt's work great too Put some spring in that thing

[Driven97]

Yeah I'm at 21/33, 29s are really soft.

[3literpwr]

Those cut jounce bumpers may be causing your issue. No factory suspensions uses the spring rate only to deal with the total amount of compression travel!

[BFT3.2]

Dan, what would you recommend for 2700 lb G50 coupe that is used street and track(I dont mind a stiff ride on the street, not a daily driver). Rebuilt the front end this Fall with Rebel solid bushings and revalved Bilsteins for 22 TBs. Planning on doing the same to the rear over the winter and was going to go 29. Would going 30-31 unbalance the front to rear? and how stiff would you go if you were already "committed" to 22 in the front? Thanks

[Flieger]

I run 22/31 on a 2,000 pound car and it's too soft. Only way forward really is coils though.

[tshebib]

Thanks guys. Keep the opinions coming. You've helped me make the decision to change out the rear t-bars. Now I have to decide on what size. I have the ER Quick change system. PP shows bars up to 33mm however Chuck's site shows sizes up to 36mm. So, how stiff should I go? I'm keeping 22mm at the front as I'm now convinced the original formula of front to rear spring ratio has too much under steer built in.

[patkeefe]

So, the unspoken meaning of all this is that the car was bottoming the rear suspension for years, but went unnoticed because there was no tire issue? Out of curiosity, what torsion bars were you using prior to the changeout?

[Dan J]

Originally Posted by BFT3.2

Dan, what would you recommend for 2700 lb G50 coupe that is used street and track(I dont mind a stiff ride on the street, not a daily driver). Rebuilt the front end this Fall with Rebel solid bushings and revalved Bilsteins for 22 TBs. Planning on doing the same to the rear over the winter and was going to go 29. Would going 30-31 unbalance the front to rear? and how stiff would you go if you were already "committed" to 22 in the front? Thanks

I like big bars and I can not lie You other fella's can't deny That all stock Porsche's are WAY undersprung
23/31 for your car with correctly valved Bilsteins with digressive pistons will be awesome

[BFT3.2]

Thanks very much Dan, really appreciate your opinion. As I've already had the front revalved digressive by Elephant for 22s I guess that kinda forces me to 30 in the rear to keep your suggested balance.

[tshebib]

Originally Posted by patkeefe

So, the unspoken meaning of all this is that the car was bottoming the rear suspension for years, but went unnoticed because there was no tire issue? Out of curiosity, what torsion bars were you using prior to the changeout?

Sort of. As mentioned earlier, I believe the original rubber bushings were contributing to the overall spring rate more than we give it credit for plus it was bottoming out on the bump stops. I wonder if anyone has ever measured the spring rate as it progresses through compression with the stock rubber bushing and torsion bars. My old bars were 24 mm in the rear.

[patkeefe]

Looks like the tire rub is because of total travel of the suspension in compression. The bump stops are just what they say...they stop any further travel. It doesn't seem to be a spring rate issue, your suspension just travels farther now, and the tire is the bump stop. I'm no expert on this, but there is a certain range of travel within which the spring rates and damper action should be controlled such that you don't get onto the bump stop under extreme corner loading. What shock absorbers are you using?

[Bill Verburg]

Originally Posted by tshebib

Thanks guys. Keep the opinions coming. You've helped me make the decision to change out the rear t-bars. Now I have to decide on what size. I have the ER Quick change system. PP shows bars up to 33mm however Chuck's site shows sizes up to 36mm. So, how stiff should I go? I'm keeping 22mm at the front as I'm now convinced the original formula of front to rear spring ratio has too much under steer built in.

You just got some of the best advice available anywhere from Dan J

[winders]

tshebib,

With 22mm front torsion bars, I would go 31mm at the rear. With a properly match damper package, of course.

[911pcars]

MHO,
It seems you have a tire/sheet metal interference issue. The solution is NOT to limit suspension travel via increasing spring rate, adding bump stops, etc. By placing a higher value on tire rub than vehicle handling, you're compromising vehicle performance (assuming the objective is the latter).

Either your wheels have too much offset/width and/or the tires are too wide for the alloted space under the rear fenders.

Sherwood