wheel vibration at speed

[Doug Steinel]

bought a set of fuchs with nearly new tires on our BBS. I polished the wheels, had the wheels and tires and balanced, and put them on the car. The car ran smoothly at high speed before putting these wheels/tires on. But now it shakes through the steering wheel something fierce. I am thinking two possibilities:

1) they weren't balanced properly so I have a beef with the shop which balanced them, or

2) the wheels have been bent and so never will balance properly.
That would mean my beef would be with the guy on this board who sold them to me.

If these are my only two options, how best to proceed.

Whatdaya guys think?

[Jack Olsen]

Or...

3) the tires are out of round (probably from flat-spotting) and balance on the machine fine, but still cause a frequency-type vibration at certain speeds.

I see this a lot with tires I drive (and sometimes lock up in braking) at the track. They balance okay, but still have a vibration problem once they're flat-spotted.

[Richard-Dublin]

It would appear that 911s are very sensitive to correct balancing (engine configuration?) - it took me 3 attempts to get the vibration out of mine. After 2 attempts with the local "quick fit" I brought it to an independent specialist who sorted it first time!

[Drdogface]

Jack,
Could this flat spotting be just from sitting? I thought that would work itself out quickly. Mine has a shimmy at about 60 yet the balance seems perfect. Does one perhaps need to find a shop that will balance wheels while on the car?

[Doug Steinel]

Jack,

Once they are flat spotted, are they scrap or will it improve if you drive on them?

[Jack Olsen]

Flat-spotting from sitting might drive itself out, I don't know. (I don't let mine sit.) Flat-spotting from locked brakes doesn't go away, in my experience. I think you'd need to go to a guy who shaves used tires (not easy to find) who could return the tire to round.

[asphaltgambler]

It has to be one of two things: either the rims have been damaged and are out-round at the bead area or they are bent out of plane OR the tires have a broken belt or impact damage causing them to be out-of-round.

The only way to know for sure is to spin balance the tire/ rim off of the vehicle (with the protective hood up on the balancer)and look to see where the problem is.

[NoLift911]

Wheel assemblies are just that "assemblies". Tires, rims, rotors etc. All coming together and spinning at significant RPM’s.

Vibrations are inherent in these assemblies. Wheel balancing is somewhat of an art and needs to be done by a qualified person that can take correct readings from a quality machine and apply the weight correctly.

Never static balance a rim and tire. Always use dynamic balancing. For stubborn balancing issues there is on car balancing but that has it's pitfalls as well. Once balanced you need to index the wheel in the same spot each time or your balance is off. Usually on car is masking another problem. Bearings out of spec, severely warped rotors, bad tire or bent rim.

The shorter the sidewall the better balance you will get IMO. 16's are better but not great. I am on the fanatic side of balancing. 17's with hardly any sidewall will balance even better. A Rim by itself on the balancer should require no weight on it's own without a tire. Unless you have solid valve stems - these are extremely heavy when you are talking about grams and balancing. A taller sidewall seems to give off cyclical or harmonic vibrations the faster you spin it so a balancer will balance at a particular rpm on a particular spot on the rim. Which obviously is not a solving of the issue it just makes it better.

You can always re-index the wheel in relation to the brake rotor. Porsche used to mark the lug where the valve stem should go in relation to mounting the wheel. Tires are the same way; they are usually marked with a dot where the valve stem should go.

A Hunter Road Force machine should be able to "Match Mount" a set of tires and rims for you. Tire shaving is kind of a waste as you lose so much material you are probably better off getting a new tire instead.

All tires are not created equal - cheap tires are just that, cheap. Tires that do not use equal amounts of steel or vary in band size can cause a wacky vibration that will never go away.

Anyway just me .02 I used to do my own balancing with a strobe and by feel on a few front wheel drive cars I had. Rear drivers are a little tougher due to the drive shaft and other drive train anomalies.

-Jeff

[curlesw]

I had a similar situation. The first guy tried twice to balance the tires and even after driving the car said they were balanced...gave me the "flat spots are the problem because the car has been sitting...it will smooth out in 5-10 miles" BS. Took the tires to someone else...this guy knew what he was doing. Now the car runs smooths...take the tires somewhere else.

Good luck!

v/r
Wayne C.
83 SC