RSAErick

Hi all,

I have been aware of my rear tire rubbing problem for a while, but have never been too concerned about it. I have added some thin 3 mm spacers, and I'm still getting some rubbing... probably because I'm going soooo fast.

Well, during the last track day I was actually hearing the rubbing after particularly fast corners. I don't like that, and now I'm wondering if this amount of rubbing is just too much.

I'm attaching pictures of the wear that the rear tires are leaving, and would appreciate comments or suggestions. Is it safe to be driving with this amount of rubbing? Could it be affecting performance, much like shocks bottoming out?

Thanks for the advice.

carreracup21

Yikes, that looks like too much to me. I'd get a bigger spacer. What wheel / tire size are you using ? Do you still have room to come out and still stay within the fender lip ?

Arjan B.

This is not ok............ you have installed wheels who are not fitting well. A little comment her, please paint your brake rotors with zinc spray, this looks bad with all the rust.

jenskleis

Hi Arjan,

Very good point you make about the brake rotors. I have the same problem and would like to remove the ugly rust.

How do I do that?

Bill Gregory

Erick,

1) What alignment specs are you running in the rear?
2) What wheel offset and size and brand of tire?

It's not unusual to see rubbing in the 9 to 11 o'clock positions on the passenger side (1 to 3 on the drivers side), and (faint of heart should skip the remainder of the sentence) a few whacks with a hand held sledge moves the sheet metal in enough to no longer be a problem (touch up any places which break through the undercoating). Need to see your alignment specs, however.

Rubbing on the swing arm: is it more on one side than on another? If so, you may be seeing r-compound rubber that gathers at the edges of the tire as you drive around the track. Have you looked at the condition of the inner edges of the tires? Check the tires to see if that's what's happening.

Another place to look at for rubbing is the oil line. Later cars have a flattened line, which really helps as you go to wider tires.

Jerrygreene

RSA Erik - FYI - This is exactly what my 964 cup looks like. It looked this way when I got it. The factory ran the largest tire as possible on it. I'm not sure of tires size you are using, but the spacer's are needed. The deflection at higher speeds is when this occours.
I do not think it is too much if you want to go fast. I've tried narrower, shorter...just not as fast.
Hoosier P275.35-18 or P285/30-18 both rub. I know some group out there says: Rubbins racing.... Good luck!

Oddjob

Take a close look at the tires. If there are slices or groves cut out or missing from the tires, then I would worry about it. If the tires look ok, then its kind of up to you.

If you dont like the rubbing, can hear it when it happens, and/or get the sniff of burnt rubber in the cab going around certain corners, you may find that you either consciously or unconsciously go a little slower so it doesnt rub. If so, get a slightly wider spacer (but then you can start rubbing on the outer fender lip).

As an example, I was looking at a C2 at PCA club race a few weeks ago, and it was running 275/17s on the rear and the tires were within a couple mm of touching the fender lip while it was sitting in the paddock. There were obvious signs of rubbing, but nothing that was really cutting into the tire.

tonytaylor

Erik,

Those pics are pretty much an exact copy of the rub caused by my 255 MPSC with 47 offset.
I ran about 20 days@ 150-200 miles per day with no ral ill effects other than slightly odd handling at extreme manouvers which we couldn't decide if this was caused by the wheels hitting the car or compliance in the rubber.
Hitting the wheel arch was fixed by a big hammer ala Bills post. The swing arm is more difficult to understand but it hasn't reoccurred since I replaced a worn drive shaft make of that what you will.
I can't use spacers or I will hit the fender lip (unless I roll the fender lip)
My thoughts are perhaps you are running too low for the spring/damper rates you are using. ( RS have different hubs a-arms etc so their specs are not nessacerly appropiate for C2/RSAs

Christer

yeah i get alittle of this as well with 10inch wide 18's with 265 rubber. Offset 56. If I run more than 3mm spacer then I need to roll fender as well....i think...

JC in NY

I can concur with what Jerry Greene says. My Carrera Cup has the same rubbing from "the factory".

RSAErick

Wow, so much to think about. Let me try to clarify a bit from my brief initial post:

- Wheels: 996 "twist": 7x17 and 9x17
- Tires: Michelin Pilot Sport CUP 205/40-17(55 offset) and 255/45-17(55 offset)
- Could I safely use larger spacers? Yes, I believe so.
- Alignment:
Left front camber: -2.4 degrees
Right front camber: -2.4 degrees
Left front caster: 3.8 degrees
Right front caster: 3.7 degrees
Left front toe: -0.01 degrees
Right front toe: 0 degrees

Front cross camber: 0 degrees
Front cross caster: 0.1 degrees
Total toe: -0.02 degrees
Set back: 0.18 degrees

Left rear camber: -2.6 degrees
Right rear camber: -2.6 degrees
Left rear toe: 0.12 degrees
Right rear toe: 0.11 degrees

Rear total toe: 0.23 degrees
Rear thrust angle: 0.01 degrees

- Ride Height: Pretty darn low... probably too low considering that I'm using wimpy H&R shock/spring coilovers.

- Smell of burning rubber? No.
- Noise? Yes, I think so. After several warm up laps, I usually hit it pretty hard. There has been a rumbling/rubbing(?) noise that I noticed starts after my first 'hardcore' corner. It concerned me enough to stay on my mind, but I couldn't detect a noticeable handling difference in the car, so I mostly I assumed it was paranoia.

- How do the inside edge of the tires look? They actually look fine, with nothing that appears to be damaging the tire... but I fear what I can't see more than what I can.

Questions/concerns:
The rubbing at the 3:00 and 9:00 position doesn't appear to be due to ride height. Rather, it seems to indicate (with my limited understanding) some sort of twisting of the tire/wheel/hub during cornering. Am I wrong?

Thanks to everyone for working through this with me.

Bill Gregory

Erick,

With 55mm offset wheels, you can easily use 1/4" or 6mm-7mm spacers, which would make the effective offset in the 48mm-49mm range. 47mm is a well used value for 964 wider wheels, although you can go down to 44mm, which may require rolling the fender lip. I currently run 10" rears with 54mm offset and a 6mm spacer (48mm effective, with spacer) with 255x17 MPSC's which fit fine with no rubbing anywhere. With a spacer, you may need to consider longer wheel studs. Stock studs are 45mm long, and you can get 52mm at 10 for $45 from Pelican. They're pretty easy to change.

If you have the stock rear swing arm bushings, remember that the Weissach axle allows for toe changes (adds toe-in) and allows some lateral arm movement at the handling extremes.

tonytaylor

FWIW I am using 47mm offset with 9inch rims 255 MPSC and had this rub as per Erik.
Since increasing spring and damper rates I have not, so far, noticed any rub, but it's early days yet.

RSAErick

Thanks all. I will first try some larger spacers (6-7mm) to see if that helps.

But now I'm curious about this Weissach axle effect. As far as I know, I have the stock rear swing arm bushings. Should I be replacing them?

Bill Gregory

Erick,

The rear stock rubber bushings were designed to allow additional toe-in to be applied to counteract the natural tendancy on the rear swing-arm suspension to toe-out on hard cornering, adding more stability. The Weissach axle was first used on 928's, as I recall. In the front, the a-arm bushings were set up to allow wheel movement to the rear to minimize impacts. (A detailed description is in the Paul Frere book 'Porsche 911 Story').

The factory didn't run the stock fittings in the Carrera RS and Cup Cars. They have harder rubber bushings with thicker embedded washers to limit the Weissach axle affect in the rear. In the front, the a-arm bushings are of harder rubber than stock. Neither RS rubber bushings are available separately, you have to buy the A-arm or rear swing arm to get them.

One reason I replaced the rear stock rubber bushings was to eliminate the Weissach axle effect, having driven a 911SC for several years, and missing the consistency of the rear end (had a few 'moments' where, at extreme cornering, the rear end wiggled, making for nervous moments. Tires were part of the problem, however, I'm not convinced the Weissach axle wasn't contributory). While I haven't gotten to the limits of the new suspension, I feel better about how it handles, as compared to before.

If you want to replace any of the rubber bushings, ERP makes replacement monoballs front and rear, as well as a swing plate with a monoball fitting (which allows lots of camber in the rear, if you need it). While not inexpensive, the monoballs are cheaper than buying the entire RS swing arm.