Wheel Clunk

Superman · 12-19-2001, 10:11 PM

I have a vibration that is very noticeable at 75-80 mph, independent of engine speed. It's a 'seat' vibration, not a 'steering wheel' vibration. I think it's a wheel out of balance but I examined the balance by putting the rear wheels on the looser front bearings and it seems okay.

Now I notice that I can still feel, or hear, the rhythmic thunk at speeds of 40mph or so. I wonder if I have a rear tire with a 'flat spot.' The autocross guys use this term. Perhaps I'm learning its definition through experience. Last autocross I did two spectacular crowd-pleasing spins. Whaddya think? Can you feel flat spots? Can you see/verify them?

Matt Holcomb · Matt Holcomb's Garage

Superman,

I believe what you're experiencing is a classic "heart murmur", and nothing more! It's a trait that all SC engines develop, irrespective of mileage. I'm surprised you don't know about it.

You either live with it for another 400,000 miles, or crack the engine open.

Seriously, it sounds like tires to me.

Matt Holcomb
1974 911 Carrera 2.7 Euro
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Superman · 12-20-2001, 10:19 AM

I'd like one of our autocross maniacs to confirm, if they can, that tires can be 'flat-spotted' to the degree that you can feel a vibration from the flat spot. Can you, basically, ruin a tire in 20 seconds if you really try?

Don Wohlfarth · 12-20-2001, 11:59 AM

You can ruin a tire in a hell of a lot shorter time than 20 seconds.
A really nice spin in front of the spectators is one way, a less favored way is away from the spectators.

But you usually flat spot a tire by locking up the wheel. When the wheel is locked up the tire scrubs across the pavement. This friction will cause heat that actually melts away the rubber. This heat changes the rubber compound from pliable to something very hard. The tire will clunk (seat of the pants).
On a street type tire with a tred you may be able to find a tire store to shave the tire. It will have to be an upscale race shop or a very large tire distributor to have the necessary machinery, it won't be the local gas station. If you can find someone who shaves tires they will not want to shave a used treded tire as it has picked up stones and gravel. When the tool hits this stone it usually breaks it.
This is an expensive way to learn the difference between threshold braking

and lockup

.

Superman · 12-20-2001, 12:30 PM

Thanks, Don. That's exactly what I was looking for. The tire that is likely flat-spotted would be a rear tire (my spins were classic for a 911) which will be replaced shortly anyway, so shaving is not the appropriate fix in this instance. I hope my fairly new fronts are not flat-spotted.

jwetering · jwetering's Garage

I can assure you that a tire can be ruined in 3 seconds or less with over enthusiatic autocrossing. I've ruined two full sets of tires in an one autocrossing season. Also, once, on the highway some broad (apologies to our female bbs'ers) cut me off moving from the fast lane to an exit lane, which caused me to lock up and swerve which caused me to flat spot the left side tires so badly that I could actually see the flat spots. Those tires were unusable, so I had them shaved, which solved the problem enough that I could drive the car again, but the tires were toast.

Z rated tires are soft, I've determined (the hard way) that you can not seriously autocross and street the same set of tires. After two seasons of ruining perfectly good tires ..I've decided it's false economy to NOT get a set of dedicated autocross/track wheels and tires.

jasper 1978 SC

Don Wohlfarth · 12-20-2001, 02:56 PM

S'man, keep ax'ing. Great way to learn car control skills.
With a pretty much stock SC (nothing wrong with that) best guess is you are doing one of two things that you want to work on. You may not be slowing enough for your turn in and are "tossing" the car into the turn with very quick steering inputs. Try slowing you car a little more and turn the car, not toss it. Make sure you are off the brakes. Yes, you can brake into the turn (called trail braking) but is an advanced technique and takes a real good feel for what the car is doing. Get a little more experience to try this.
The second thing to think about is when you find yourself in a turn and realize you are going too fast. Your first instinct is WRONG, and that is to try and slow down. If you take your foot off the gas you will transfer weight to the front end. Since you have put weight on the front you have taken weight off the rear and upset the balance of the car in the turn. Since there is no weight pushing the rear down it will allow that large weight in the rear (motor) to try and continue in a straight line. Tie a string around a washer and swing it around your head. At whatever point in that arc you let the string go the washer will fly off in a straight line. The same thing happens when you lift.
Now that we're in this turn going too fast you want to add a little more gas that will keep the weight on the rear. But we're now going a little bit faster and may start to loose traction and the rear starts to come around. With a gradual increase of gas you will feel this. It will not be as abrupt as if you lift and the car snaps around. When you feel the rear start to loose traction you want to prepare yourself to start turning the steering wheel the opposite way (counter steering). You want to be careful here because your tendency will be to turn the wheel in the opposite direction too much (over correcting).
All of this inputs are very gentle as you are trying to keep the car balanced properly.
I know this sounds difficult and it is. But hey, you're the driver that got us going into the turn too fast. I'm just trying to tell you some of the problems and how to correct them.

jwetering · jwetering's Garage

Yes I know, it all looks good on paper but I'm from the generation that grew up learing to drive from watching the Dukes of Hazard (YeeeeeHaawww).

Seriously though, it's a fine line to tread in a 911. My times are actually quite competitive, I usually beat every other 911 that shows up on street rubber. It's the guys with the stickies that beat me. Up until now I've simply blamed my ham fisted oafish driving on bad tires. This year I am planning a set of track rubber for myself so that excuse will be gone. Wish me luck.

oldE · 12-20-2001, 03:09 PM

Please don't overlook the chance of a self destructing tire. You can get a speed related thumping if the belts are starting to delaminate. Look for bulges in the tread. (or maybe you've scuffed a few thousand miles off) Good luck!

Superman · 12-20-2001, 03:17 PM

Oh no, I'm in complete control all the time. Those two crowd-pleasing spins were probably wind-caused.

Seriously though, I am starting to 'get it' in autocross. I understand your advice, Don. At the last AX event, I had the distinct feeling I was about to 'turn a corner' so to speak in my driving skills. Watch out for me next year! Tyson, you listening?

avi8torny · 12-20-2001, 05:53 PM

Superman,
I had a vibration in the rear of the car which vibrated the rear view mirror to the point that it was unusable. I had some new rubber drop shipped to my wrench and they match mounted the set which cost a little more. It made a huge difference, real smooth ride now.

Tyson Schmidt · 12-20-2001, 06:42 PM

Superman. I might have a new theory for you. Old tires like to flat spot from just sitting in the cold. What I mean is, the flat spot on the bottom of the tire while the car sits in your driveway overnight, will kind of stay there for a while on old-hard tires.

The old set on my 72 would do this. Sometimes they would be so bad that the whole car felt like it was flying through severe turbulence. It would go away, (but not completely) after the tires got good and warm.

The same thing happens to cars that are stored for long periods.

Doug Zielke · 12-20-2001, 06:48 PM

Tyson,
You are 100% about that. My Yoko tires seem to be especially sensitive to sitting around waiting for spring.

I've heard you can stop this problem by parking the car (tires) on pieces of styrofoam. Another urban myth?

JT911 · 12-21-2001, 01:59 AM

I to have had problems with tyre clunk/vibration after track spinning, mainly at hillclimbs. Tyre brand yoko's Advan very soft road approved. I have to keep driving on these for the remainder of the season due to financial reasons. For what it is worth the Formula fords and Cosworth open wheelers at the same hill climb will shave their own tyres by jacking the frame up and putting it in gear and grinding with what looks like wood files onto the spinning wheels till the tyres are soft again. These tyres are still
an inexpensive way of making time.