Vibration on the track - spacer issue?
 

After a 3 month restoration, I got my 911 back on the track Sunday for the first time this season. At 100+ speeds, I have a vibration that's new and disconcerting.
I kept re-torqueing the wheels but it was not that.

The only change I made was going to 17" Rota Fuchs and needed to use 8mm spacers front and rear to clear the TT/S4 big brake calipers. I only had generic spacers on hand and they are not hub-centric, i.e. slotted holes. 8mm is too narrow to have a hub lip for the wheel center to sit on. I was in a time jam and used what I had to solve the clearance issues.

Could non-hubcentric spacers cause a severe vibration? My guess is they are not centered and throws off the balance enough to feel at speed.

Can anyone validate this theory? I have not had to use spacers before and will be buying the proper spacers before my next event.

Beyond changing back to the other wheels or spinning up the Rotas.....that would be my choice.

Not a great choice after my first taste of real racing slicks, don't want to go back to R-comps. My buddy with a 3.6 Carrera could not catch me in the infield as I barely had to brush the brakes.
Most of my buddies with big wheels are using small spacers as are people on here, to clear brakes etc.

Trying to find out if non hub-centric spacers can cause a severe vibration so I can buy the correct hub-centric spacers.

Joe Bob,

What did you mean by spinning up the Rotas,re-balance? I just balanced them, unless I lost wheel weights on track.

Old school spin balance with the wheel on the car. If you can't duplicate it, it may be suspension related.

I suppose non-hub centric spacers could lead to vibration issue, but in my experience, they don't. I've been using .375" non-hub centric spacers (custom made - not off-the-shelf) since 1997 and have never had a vibration issue. It seems that if it was statistically likely that for any given wheel installation, the position of the wheel/balance could be slightly mis-aligned due to the lack of hub centricity, I would have seen it by now. I have probably installed front tires and wheels 100 times using these spacers and have never had a problem.

Also, my spacers are aluminum and simply fit - loosely - over the wheel studs. Mass that close to the rotational center doesn't affect overall balance much. Heck, I know guys that don't even get the track tires balanced when they're installed...

Do the 'ROTA' wheels use an acorn or ball seat lug?

I use a stack of .250 and .310 spacers under my front wheels to clear my 996TT calipers as well as to bring out the top of the wheel a little due to the -2.4 front camber and i use .250 spacers in the rear where i have -2.8 for the same camber reason.
I don't feel any vibratrion due to the spacers which are just aluminium plates ...

Yes, i am using longer wheel studs !

Cheers !
Phil

I have used the road force balancing system on both my SC and 2 other vehicles and find that it does a much better job than traditional methods of spin balanciing or even the on the car spin system I had to use on my RX7 wheels. Tires and wheels today are not always perfectly round or evenly weighted.

Balance issues on tires show up around 50-60 MPH. Flat spots tend to show up around 90 to 100. You may have a flat spot (was the car parked on the slicks for 3 months?). Could be a bent wheel. I've also seen this problem with tires that were shaved out of round.

-Andy

Rota uses regular Porsche ball seat lugs. My ROH wheels use the conical seat lugs, I had to cut the closed end off to fit the longer wheel studs.

This could be it. I bought used Grand Am Conti slicks as a relatively cheap experiment, they have 50-75% left. They were just mounted last week but I don't know their history. When balanced, there were rubber chunks which I should have shaved off but didn't think about it which I'm sure affected balance when the tires heated up and the rubber came off. The wheels are brand new and I ran them on the balancer first without tires to verify they run true.

One of my buddies mentioned flat spots but I dismissed it.

The car has so much more grip even with used slicks that I became a better driver in 1 day. Braking is also improved. I just need to solve this vibration. I may have to do 1 session with former MPSC/ROH to narrow it down to wheels/tires.

+1 on possible flat spot or tire not round. Would you believe I've had to send one brand new Ventus RS3 back.

I would bet on the non hub centric as being the issue. The lugs were never intended to be the centering guide so there may be a few thousandths of misalignment just through normal tolerance. The knurling of the lugs is not intended to be really accurate.
A couple thousandths off and I bet a 30 lb tire/wheel combo would rattle your teeth at speed.

So I read the experience of IROC and Wildcat in using generic spacers for years without issues but I almost wish you're right so I can fix it easily by just buying different spacers. I'll go ahead and do that anyway, no other way to know than trying it.

I'll also do a nut and bolt torque on the suspension to ensure nothing is loose.
Note that since the spacer holes are slotted, they don't sit centered so I think the whole hub could be inherently unbalanced. It's either that or flat spotted tires.

It is pretty tough to properly balance used race tires due to all of the clag that builds up on the tire.

You might check to see if the tire has rotated on the rim, very common with Hoosier and other race tires, usually it's a bigger problem in the rear. I mark the valve stem location so I can monitor the rotation. Can you tell if it's the front or rear that's vibrating?

Inspect your hubs (front and rear), and cv joints.

It's a good idea to replace the spacers with hubcentric ones, but I would look deeper into the tire issues...

Cory, will do, can't tell for sure if front or rear, felt more like the rear. I think you had responded to another thread using the same tires in the past. I started them at 28/31 psi cold and ended up at 34/38 hot, couldn't take temps as pyrometer is broken. Does 34/38 sound in the right range? I'm using 225s/275s.
I'm just amazed at the levels of grip, couldn't make them slide even when I lifted in turns.

I agree with all the comments above regarding used race tires, etc. I enquired about the lugs due to the possibility of those being mis-matched for any given wheel, be it acorn seat or OEM Ball Seat.

Also, vibration from the front is most felt in the steering wheel, and rear vibes in the seat(s).

I hope you get this all ironed out! Best of Luck..

I had the same issue. New brake setup required spacers in the front. First track day was fine. I noticed a vibration on the way home. I thought it was rubber stuck to the tires. Turned out some of the stick on weights shifted causing the tire to be out of balance. Once the tires were re-balanced the vibration was gone.

If you liked the used Continental challenge tires don't try new Hoosiers! :D

We ran the Continentals in 245/40 fronts and 275 rears. Your pressures sound okay, I'd probably raise the front to 36. We ran them in a range of 34-36 in the front and 36-38 rear (2 pounds higher in the rear). The Hoosier folks said to run them 2 pounds higher than R6's, but as they wear you might find it beneficial to lower the pressures a bit.