steveP911

I just returned from a PCA Potomac HPDE at Watkins Glen. My second time there. An amazing course, IMO. If you haven't been, you should go, and if you do, don't miss doing a track walk. It is incredible to see up close. Had a great time, but I did develop a car issue that I want to ask for some help on.

Background: PCA Blue Group driver, basically my second full year at DE. 2005 911S with Ohlins R&T suspension; dampers set at 5-7 turns CCW (with Track rec range). About 2-1/2 deg front and 2 deg rear camber. Continental ExtremeContactForce tires; third event since new. 13 sessions prior to WGI + 8 more at WGI. Followed the Conti recommendation of 32 psi hot pressure that does seem about right since higher pressures can be greasy in turns and even hoppy in braking.

Issue: I began to experience some slight steering wheel vibration towards the end of the previous event (VIR), so I had the wheels rebalanced/checked. Could have been clag but I wanted to check. I was concerned about numerous gator curbing contacts shaking loose any of the weights. Nothing really detected as off at the shop. At WGI, I began to feel it again after my 3rd session (first day), so I had their site speed shop redo the balance. They said the fronts had lost their weights. 4th session felt better but not where I would have liked. BTW, I do clean off notable clag inside the wheels and on the tires between sessions. Day two, things start well and the car feels good in first two sessions. Another day in the upper 70's, sunny / partly cloudy. Maybe it got to 80 deg? During third session after lunch, the wheel vibration is coming back. A few principal areas on the course are worth noting. Coming out of the esses I am doing about 105-110 and the vibration starts in earnest. Then in the back straight as I get into the low 130's it is really shaking. I can even feel it in my butt. Unnerving. As I brake hard ABS kicks in, in addition to the vibration, and I wonder if road wheel vibration is affecting grip / initiating ABS. Track grip should be good. In two other spots - both downhill braking, into turn 6 (95ish mph) and later into turn 1 (120ish mph), I feel some wandering of the rear as if it is light / not planted (insufficient toe?). So.... I go back to the speed shop and they find that weights are off on two wheels (I forget which ones...), and they rebalance all and try to do better with securing glue and cover tape. I am hopeful for my final session (4th) of the day but I can only do outlap plus two before I pit from even worse vibration. I did not exceed 115 since the vibration was so bad. A bit of a battle to control braking from that speed into the "bus stop". I realize I am feeling unsafe so I get off. Especially disappointing because my last two sessions were solo.

Other notes: I see nothing unusual with the tires and the last set of weights are all intact. I trailered the car so I didn't have a chance to sense anything on the highway before or after the event. I have never experienced anything like it on the street. I did notice that as I bled air from the rear tires during the first couple sessions of each day, the tire gauge fitting gets so hot exhausting the air that I cannot hold it with my bare hand. I had experienced this at VIR also, but never before with my previous Michelin PS4S tires. My top speed at VIR was higher (mid 130's) but I didn't feel anything significant - slight vibration but that felt normal enough for high speed.

It does seem like as the day wears on the problem clearly worsens. Last session is worse and last day is worst. To me, that means the problem item must be elastomeric? Tires? Any bushings close enough to heat sources to get soft and unstable?

If anyone in the greater DC area has a shop they recommend for troubleshooting weird stuff like this, I am all ears. I am going to talk to the tire shop tomorrow (Paul at Radial Tire) - if I can snag him...

Any thoughts/advice are very much appreciated. I hope my info is enough but not too wordy.

Thanks!
Steve

jscott82

Axles and wheel bearings are the first things that come to mind... a good shop should be able to figure this out easily.

Estoril

No specific issue jumps out at me from your description, but...

• Which tires and what are your cold and hot pressures? How does the tire face wear look?
• Have you checked wheel bearings?
• Clag pickup is a small possibility, but you can see it on the tire face and get a sense of how much you are picking up. Some tires will pick it up and hold it more than others. RT660s will exhibit major clag vibration when newish as you turn-in all the way to trackout. Then they lose that tendency with wear.
• Next time on the lift have them check the flex disc (unlikely, but...)
• Also have the shop check suspension bolts to make sure something isn't backing out changing your camber/toe. ABS coming in on braking (unless you are braking too roughly) concerns me but is likely a pointer. Also check control arm bushings.

Do you have an Instructor in the car? If so, what did they say?

38D

Have an experienced racer drive your car and see if there a problem. The thing that caught my eye was your description of the rear wandering under braking into T1. This is completely normal, expected even, in a 911 braking inT1 at the Glen. So it’s entirely possible there is nothing wrong at all and you are just a bit too sensitive to minor things going on (which will be a good ability to have as you progress in your speed beyond Blue)

T&T Racing

There are 2 things happening
1. Tire pressure- Need to take cold tire pressure readings at the beginning of the day and also a note the air temp and sky conditions. After the 1st on track session, take the hot tire temperatures and calculate the delta for each tire.
If the next session the air temp is 10F higher and ski condition similar, drop 1 psig from each tire and record the starting pressure for each tire. Take the hot tire pressures after the session. If the hot pressures are the same as the 1 st session and you are happy with the car handling, then good adjustments made The reverse is true if the air temperature drops, then the cold starting tire pressure needs to,be increased.
1a. The rear tires feeling differnt in the late afternoon, the squimesh, with high air temp and track temp is an indication of starting with cold tire pressure too high.
2. Vibration- Check front wheel bearings for wear, rollers in have have too much play or the wheel bearing does not have enough preload.
2a. Check ball joints for play
2b. Check tie rods for play

slivel

Put the car in the air on jack stands and spin each wheel. Look for run out or wobble. In the front it could indicate the items mentioned by previous posters and it could also be a bent spindle or even a warped brake rotor. in addition to lower strut things like ball joints and tie rods look at the upper strut mount for play.

coryf

Could be the tires slipping/rotating on the wheels. The balance would change as the tire moves. I have had them go from balanced to way off and back to ok during a single run.

Frank 993 C4S

Do you mark your tires to see whether they are rotating on the wheels when cold? Do you use high temp tape to tape down your wheel weights? Have you tried scrubbing tires on your out lap? Are you consciously avoiding going off the racing line on your outlaps where you could be picking up rubber? Do you have another set of wheels/tires you could run?

dgmark

Is the vibration in the steering wheel, (front) or in your butt (rear) . If in the front biggest issue is tires or tire balance. If you feel it in your seat check your rear axles for binding and make sure the axle flange bolts are not loose.

Dwane

Frank makes great points. Had the tires rotating on the wheels many years ago after I had the wheels powdercoated. After sanding down inner-bead all was good. A simple crayon line at the valve, will establish if the tires are rotating and throwing wheels out of balance. You can use some aluminum heating duct tape over the wheel weights to see if that helps keep them in place.

steveP911

Thanks for the many helpful comments. I will respond to them ASAP but I have found something that may be the culprit.

I had my season-start alignment done in early March, with front camber at about -2-1/2 and rear at about -2 degrees. Front toe was .01' each side and rear toe was .11'-/12' each side. Pretty normal stuff I think.

I had two events - one at Summit Point (main) and VIR. I "encountered" a lot of the gator (?) curbing at VIR so I wanted to get my alignment checked (but not the same shop that did the original alignment). Had that done on 5/2. Then I went to Watkins Glen last weekend, where I had this vibration/stability issue. For some stupid reason (on my part), I must have only looked at camber values, and saw they were pretty much unchanged. I did not look at / notice the toe values until yesterday. I now see some shift up front (left side), but the biggest difference is in the rear where the toe is about 3X what it had been.

But I can't be sure what the polarity is (toe-in versus toe-out) from the little images I have circled in black and the values next them circled in red. That is, the front symbol seems to show positive toe-in but the value is negative (-.10 deg). Does that mean there is actually toe out? For the rear symbol, it seems to show toe-out and the value is positive (+.30 deg), so does that mean there actually is toe-out even though the value is positive (toe-in)? I am confused.

It seems pretty clear that my toe numbers have changed (due to some loosening?) and are not desirable. If there is any toe-out in the rear I believe that could cause the issue I was having. Perhaps the problem even gets compounded when the tires are hotter?

Thanks to anyone who can help straighten this out for me. I am going back today to the shop that did this alignment to see what they say, but I wanted to hear from others on the forum.

Steve

Frank 993 C4S

You should have some Toe OUT in the front and a bit more Toe IN in the rear. The diagram on your chart confirms that. Small Toe changes will not cause any vibration. If you end up with too much Toe in the rear on a 911, you start wearing the inside of your rear tires quite a bit because on the 911 the dynamic toe increases quite a bit with compression. I do not measure Toe in deg. but mm, so I cannot comment on your values.

steveP911

So I just went to the shop that did the alignment shown in that photo above. They clarified that the symbol/image of the wheels turned in/out means nothing as far as the result is concerned. It's strictly the polarity (+/-) of the number next to it. Maybe everyone already knew that but me. Still, I find it puzzling why they show two different images - what's the point?

Anyway, now I know that my front left toe went from +.01' (in) to -.10' (out). That's not good. No help at WGI either which is mostly right turns. And my rear toe went from .12'/.12' to .33'/.30'. Wow. Is that unusual or maybe it's not uncommon?

At any rate, I will get the car re-aligned tightly back to what I had before. Zero to +.02' (in) in front and around +.12' (in) in the rear. I hope that will tame the vibration/instability problem I was having. For the front, I prefer high speed stability over a little turn-in benefit, so that's why I chose .00'/.02' toe-in there.

Thanks!
Steve

steveP911

Thanks jscott. My tire/alignment shop put it up on the rack and found nothing wrong with those items. I had another shop examine the bearings back in late 2021 and had the front right bearing replaced due to play.

Steve

steveP911

Thanks