Torsion bar sizes and tire sizes

[jazzbass]

After lowing my car with stock torsion bars, I'm thinking about upgrading to get a little more stiffness and avoid rubbing. I've read most of the old t-bar threads, but was wondering if anyone had any feedback as to how different tire sizes affected the equation?

The most common torsion bar recommendations for Carreras are 21/27 or 22/28. These tend to be recommeded for car with stock rubber. For me, I run 225/45 16 in the front and 245/45 16 in the rear with Bilstien HDs all around. I had decided to go 21/27, but was thinking the different tires sizes might affect this somehow. Also, everyone I talk to says 27 will be to soft in the back, but all these people are track junkies, so I have to factor that into the equation .

At this point, I'm thinking:

19/27
20/28
21/28
21/29

I am mostly afraid of unpredicable oversteer if the back is too stiff. Also, the car is 95% street car (2, maybe 3 track events a year are planned), so I don't need or want the hardcore track setup. Just a good handling fun-use only street 911. Didn't RUFs CTR use a 20/28 setup? Damn this is confusing.

[KTL]

I have 22/29 on my car, with Bilstein HD front/sport rear shocks, all rubber bushings. Car is totally acceptable on the street. Not too firm, but firm enough to feel tight.

My car doesn't have unpredictable oversteer, but i'm also running 205/55, 225/50 on 16x6/7 Fuchs with semi agressive front alignment settings. Rear alignment is stock specs.

I think you'd have a hard time telling the difference between 28 & 29 rear bars. I'd go with the 22/28. Why? Because that's what Steve Weiner @ Rennsport Systems recommends for a street/DE 84-86 Carrera. He recommends the 29 for the heavier G50 cars. Steve has been doing the Porsche thing for alot of years so i'd have to give him the nod for knowing what's a good compromise for a street/DE car.

[KobaltBlau]

I've driven cars with comparable weight to yours on the street with 22/28 and 23/29. Not 21/27, unfortunately. I expected stiffer front bars to cause a lot of harshness, but they really didn't in my exprience. I did notice harshness from the 29mm rear bars. I noticed just a little bit of harshness from 28mm bars, but at a level that is acceptable for me. On the other hand, I do a bunch of track days a year.

If you are truly going to do 2 or 3 track events a year, I would do 21/27. I think the ride will be great, and the car will be better on the track especially. If you end up doing lots of track you may want 22/28 instead. For me, 29mm rears would be too much for a 915 car that is used on the street. A G50 car like Kevin's is heavier in the rear, and 22/29 makes sense.

IMHO reducing rubbing is a silly reason to upgrade bars. fix the problem. But I realize you have other reasons also.

[Bill Verburg]

The stiffer the suspension the more tires will contribute to the suspension reponse.

The lower the profile and smaller the tire is, the more the tire stiffens the suspension response

The bigger tires will transmit a larger load to the chassis, so in general bigger tires should be accompanied by bigger t & s - bars and more rigid connections between suspension components.

[KobaltBlau]

good points, Bill.

I should mention that the 23/29 and 22/28 cars I mentioned above had 225/50-16 and 245/45-16 tires, victoracers and MPSCs respectively. But the opinions on harshness are highly subjective, of course.

[Zeke]

Originally posted by Bill Verburg
The stiffer the suspension the more tires will contribute to the suspension reponse.

The lower the profile and smaller the tire is, the more the tire stiffens the suspension response

The bigger tires will transmit a larger load to the chassis, so in general bigger tires should be accompanied by bigger t & s - bars and more rigid connections between suspension components.

All perfectly stated, IMO. In addition, if you use a wheel that produces a wider track form center of tire to center of tire, you have added suspension leverage and need to compensate with a bigger bar.

I said this in many ways before differently each time. Above may be my best interpetation yet. Whatever, if you hang the dang tires way out there, you need bigger bars! It's a moving the fulcrum thing.

[Rick Lee]

I had the exact same setup as KTL on my G50 Carrera. I did one track event with that car before I sold it. It handled SUPER on the track.

[masraum]

if you are thinking 19 27 instead of the usual 21 27 then you are going to be more likely to get oversteer. Going 19 27 instead of 21 27 puts a softer spring on the front. A softer front suspension will make your car want to oversteer more/understeer less. 225 / 245 should keep approximately the same frt/rr balance as 205 / 225, now if you were going 205 245 you would be changing the balance towards more understeer, but you aren't.

[KobaltBlau]

I see no reason to go 19/27 unless you're too cheap to buy two pairs of torsion bars, in which case you should stay stock. IMHO. Stock, 21/27, and 22/28 are all reasonable choices for a street driven 915 carrera 3.2. If you deviate much from those combinations you should know what you are doing, I think.

[jazzbass]

Thanks for all of the excellent replies. I'm sure you guys are sick of reposting this stuff over and over, but I appreciate it because all the threads I read through in the archives never seems to answer my question exaclty. This and the Suspension Tech Question thread really helped answer some questions for me (as did talking to a bunch of other local 911 owners).

I think I'm going with 21/28. Everyone I've talked to said most of the harshness from bigger tbars comes from the front, and 22 is where things start getting harsh. Given this chart from Wil Ferch:

Originally Posted by Wil Ferch
Front:
18.8= 110 lb/in
21= 173
22=210
23=250

Rear:
24=120 lb/in
25=140
26=165
27=191
28=221
29=254
30=294
31=332
33=427

a stock '86 Carrera (last of the 915 cars) has a 18.8/25 setup, which has a front:rear spring rate ratio of 1:1.27. This would mean that a 21/28 combo would match this ratio almost exactly. I know this is an oversimplification of the problem and that lots of people run bigger bars in the back, but this set up seems like it'll be about right for the stiffer-yet-not-too-stiff 95% street driven ride I'm after. Maybe some bigger sways in the future to dial out a little more roll. At least, thats the current theory.

[Carrera3.5L]

Chris,

21's or 22's in the front work quite well with Bilstein HD's, but using 28's in the rear you may want to consider Sports as an alternative to the HD's you currently have.

My .02.

Ralph

[88 Club Sport]

I'd go with 22/28's for a car that see's most of it's life on the street.

I set my 78 SCarrera up as a drive to and from the track car but the goal was to set it up for the track. I went with Bilstein Sports on all 4 corners and 23's and 29's. I installed poly bushings upfront and neatrix in the rear. By the time I added track tries (225/245 Victoracers) and a tight Corbeau seat (little padding) the ride on the street is a tad too harsh for daily driving.....great for the track though

[rdane]

I run SO3s 225/45 16 in the front and 245/45 16 in the rear with Bilstien HDs all around and plastic bushings. I also am running 22/30s hollow Sanders and have for well over a year. I am very happy with my setup as a street car and the car holds its own on the track. 95% street to 5% track.

It is not a daily driver however. Too stiff for that IMO. But not uncomfortable in any way either. Hard to get the CD player to skip. As my toy it is great. Everyone I talked to said get bigger rear bars. Glad I did. Car is amazingly neutral. Lots of comments made about that on drives. Hard to get any loss of grip no matter what you do, including serious throttle lift in the middle of a corner. Hard to mess with undeniable success. But a little softer up front would make it more comfortable, maybe not more fun.

I am mostly afraid of unpredicable oversteer if the back is too stiff. Also, the car is 95% street car (2, maybe 3 track events a year are planned), so I don't need or want the hardcore track setup.

Forget the oversteer, it won't be there. I can easily spin my tires with the new engine but no oversteer. Same set up on the 3.0 and it was fine. CRT used stock T bars and bigger sways IIRC.

21/30s is what I originally wanted (and at some point may try that set up) . Couldn't get the 21s at the time. I would do 21/30. I wouldn't do less now for your requirements if for no other reason than you have a much heavier car than mine and the others using the 22/30 set ups. I would recommend the bigger sets (21/29 and better yet 21 or 22/30s)

[KobaltBlau]

Originally posted by jazzbass
Everyone I've talked to said most of the harshness from bigger tbars comes from the front, and 22 is where things start getting harsh.

I heard that too when I was selecting bars. But my experience was exactly the opposite. 22/28 has been used with success by many many people on the heavier cars (like yours). 21/28 may not be enough of a difference to matter for your purposes, however.