Type_LT

I have some Pirelli Snowsport 240's on my 05 987 Boxster and have noticed a strong side to side shake in the steering wheel.

I just had the tires balanced at the Porsche dealership and after the balance they took it for a drive and noticed that shake was still there, so they didn't charge me.

I'm guessing at this point that the cause is tread separation, it's the only thing I can think of that causes this problem if the tires are balanced and the rims are straight. It's also a problem that is hard to spot since it usually happens from the inside out. Sure, if it's bad enough you'll see bulges and the like externally, but not always. The suspension should be perfect with only 36k miles on the clock.

The only other deviation from normal is that the rim and tire sizes are for an early boxster. The fronts are 205/50 R17's and the rears are 255's, if you look at Porsche's recommended tire sizes for N-spec (Winter) tires they are different than what I have in place. To me though this shouldn't matter a bit as far as balance and a shaking in the wheel is concerned.

Any thoughts?

Macster

Were wheel/tire assemblies load force balanced on Hunter machine? If so I would think if tire tread separation (or belt movement or breakage) presence this would have turned it up by the amount of weights needed to balance or the fact the balance didn't remain constant.

Lift each corner of the car just enough to get wheel/tire off ground and give each a spin and note runout. Could be wheels bent, tires not seated on wheel properly, or wheels not mounted to hubs properly.

I don't think different wheel size plays a role but you want to be sure wheels clear brakes and the tire is not contacting the strut or any suspension, steering, drivetrain component at any time. Look for signs of contact on tread and sidewall of tires.

Sincerely,

Macster.

Type_LT

Macster,

Thanks, you brought up one great point that I skipped, and thats simply spinning the tire when it's off the ground to note runout. I skipped that step by taking them completely off and examining them visually to look for defects. When I jack up one side of the car, I think i'll setup a rigid pointer close to the wheel and see if the distance between it and the tire deviate with rotation. Do you think it would help to under/over inflate the tires just for the purpose of highlighting defects? I'm talking about only when they aren't under load.

I'm not sure if they load force balanced the tires, it's a nice modern Porsche dealership that did the balance so I would assume they at least have a machine for that. I do remember the tech saying they "looked perfectly balanced on the machine".

I'll have a look tonight and post the results. Regardless, I have a couple new fronts on the way. If thats not the problem then I'll be putting a couple brand new 205/50 R17 Pirelli Snowsport 240's up for sale!

Macster

When you spin the wheel/tire you're looking for runout -- axial or radial -- of either the wheel or tire.

The tire sidewall may appear to run out some but that is because the lettering or something else. Watch near the sidewall near where the tread portion begins. Look at the area near the bead where the tire and wheel join.

Look at the wheel rim to see if the rim moves in or out and up and down.

Tire inflation shouldn't matter. They want to be properly inflated but there is no reason to over-inflate for this test/check.

I don't know about all tires but I know with Pirelli or Michelin tires on my Boxster if the tires are over-inflated -- the fronts especially -- the car develops a vibration at highway and highway plus speeds. Not a real bad vibration but one that had me almost ready to take car into dealer for tire/wheel rebalancing but thankfully I checked the tire pressures and found them several (3 or so) psi high.

This can happen if tires properly inflated for colder ambient temperatures then a warm spell happens which is sort of common in either the fall or spring.

You can arrange a pointer but the runout should be enough -- if present -- that you'll see it. There can be some. I can't put my hands on the allowable amount, several thousandths of an inch IIRC but don't quote me. This barely enough for an "untrained" eye to see. (When I was a machinist I could spot just a few thousandths of an inch of runout.)

Chances are you're looking at runout that approaches that of an old bicycle wheel/tire that had seen too many curbs.

Sincerely,

Macster.

Type_LT

Macster,

Looking at the area where the sidewall meets the tread I see no runout axial or radial (~< 1 mm).

If i look at the interface between the rim and tire I also see no axial or radial runout (~< 1mm).

Where I do see a discrepancy is when I put a reference marker in the middle of the tread. As you rotate the tire it's almost as if you see a flat spot. The car was driven about 12 hours ago so there shouldn't have been time for a flat spot to develop. I would say the difference is about 2-3 mm compared to the rest of the tire. The flat spot occupies about 10-20 degrees arc. Both tires have this flat spot, so, I guess it could be from sitting for 12 hours.

Tires are exactly at the correct PSI as stated on the drivers side panel. I also looked at the tires a lot without any pointers, rotating them by hand for a few minutes and simply didn't see anything obvious.

Is ~< 1mm axial or radial runout going to cause a problem?

I wish I could take it out the track and just melt them down a bit . Sometimes I feel a good thrashing on the track could smack the tires into shape. Or, at least cause them to fail quickly to get it over with!

I sure miss my 18" Z rated summer tires, they were smooth as butter.

Thanks again for all the help. I'll report back when the new front's are put on next week.

Macster

One (1) mm is approx. 0.040 of an inch, less than a 1/16th of an inch but more than a 1/32nd of an inch.

One mm is a bit of a runout. I don't recall the amount of runout but think it is less than this but I seem to recall it was more than I thought it would be.

But don't be alarmed. One is you're just eyeballing the amount and I suspect overestimating the amount and I don't recall the max. allowed.

You do have two front and rear wheels/tires to go by. Chances are all not all wheels are running out so you can spin several and note the one that shows the most runout. This one is likely the bad one if the difference is truely significant and real.

As for the "flat spot" on the tread surface, the outer circumferance of the tire, that is a flat spot that occurs naturally as side effect of letting a hot tire sit with weight upon it while it cools down. 12 hours is plenty of time for this to occur.

The spot should go away in just a short bit of driving, until the tire develops just a bit of heat.

If you were to take the car out for a spin -- nothing frantic but long enough to get tires up to temperature -- then come back and recheck the tread face run out you'd see it gone.

Now what can happen is there can be belt breakage or separation -- radial tires with fiberglass belts were prone to this problem but with newer tires this is rare but cheap tires can still suffer from this -- inside the tire and this can show up as not only impossible to balance out vibration (load force balancing should identify a tire suffering from this though if the tech using the machine is paying attention and doing his job).

A tire suffering from belt breakage or separation may not show up as having runout anywhere. But what it has is an inconsistent rolling shape as the area with the broken or separated belt comes into contact with the road surface. Load force balancing presses the tire/wheel against a roller and notes any out of balance condition which the tire tech can compensate for with weight. Too much weight, finding a recheck of the tire locates the point where the weight needs to be in a different place, is a sign the tire's got bad belts or is in some way bad.

Sincerely,

Macster.

Type_LT

Macster,

This site had an interesting note that a vehicle is 4-8 times more sensitive to radial runout as opposed to lateral (axial) runout. I suppose that makes sense since in the radial runout case the suspension can move more freely than in the lateral case. I also found it interesting that they suggested rotating the tire two studs as a way to mitigate vibration.

http://customwheelsmarket.com/tirwheelbal.html

This page has some guideline values for runout.

http://www.aa1car.com/library/wheel_balancing.htm

Tires come in tomorrow

Type_LT

New tires are on, shake is still there

One minute you can be driving 70 mph and everything is fine. The next moment the steering wheel is vibrating something awful at 70 mph. The only thing I can think now is that as you turn, the two front tires are fighting with each other through the stabilizer bar. If the rims both have radial runout and just happen to be in sync or out of sync you'll get this nasty shaking.

Should I chill and wait for the tires to break in?

Macster

Chill if you want, but tires do not need breaking in to stop the shake symptom you describe. That shake is not due to any tire newness.

I don't recall what you've done to get to the bottom of this symptom/behavior but you need to do more. If you believe the tires are properly mounted and balanced, the wheels are not damaged from a pothole hit or some other encounter with something on or in the road surface and the car's alignment spot on, then there is a steering or suspension problem that needs to be identified. Some pivot bearing, joint or bushing is worn, damaged or for some reason loose and allowing the normal vibrations and harmonics to result in the behavior you experience.

You need to get the car to a steering/suspension specialist -- cause apparently your dealership's service department ain't one -- and have the car's steering and suspension checked.

I would think with the car safely supported with all 4 wheels off the ground a knowledgeable tech could get under the car and start checking for excessive play, slop, looseness. An experienced tech would recognize right away if there was any out of the ordinary play or looseness.

Sincerely,

Macster.

Type_LT

I guess it would be a good time to go through the list of steps I've taken to narrow down the problem.

1. Removed winter tires/rims and replaced with summer tires/rims and everything was smooth as butter with tight razor sharp steering response we know and love. This should rule out any suspension issues.

2. Verfied correct tire pressure (30 psi), verified brake/wheel well clearance.

3. Jacked up front's and checked for axial and radial runout of tire and rim. ~1 mm of axial runout was found on the rim itself. This is still an open item as to whether or not 1 mm of axial runout is acceptable. This was certainly an improvised setup but should have uncovered any gross runout's. I'll put the known-good summer tires on the car and see what kind of runout they have for comparison.

4. Porsche dealership mounted new front tires to the same rims, fixed nothing.

Given that everything is smooth with the summer tires on and that I have new tires installed on the same rims, I feel it's best to take another look at the rims. It could also be that the dealership is somehow unable to balance the tires correctly as well.

Macster, I believer you're right that I need to have a specialist check out the rims/suspension.

Pugnacious P

I had a shake on old rims when one was bent, are you sure the rims are true?

Type_LT

Pugnacious,

Thats what I'm leaning towards at this point (rims), because the tires are new.

One other thing I noticed is that my original Porsche boxster rims fit very snugly against the hub even before you insert the five lug bolts. This would certainly make sure that the rims are centered on the hub before you insert lugs. The winter rims on the other hand have a bit of play when you're first fitting them and preparing to insert the lugs. I would hope that this isn't important and as long as you follow the proper lug tightening sequence that the rims would auto-center due to the distributed force of the lugs.

Any comment on that? I'm going to make sure I beat this topic to absolute death At least when others have some issue they can come back here and get the answers, hopefully.

BTW this weekend it was 60+F so I put my summer tires back on to do a bit of spirited driving. Everything was smooth as can be, no vibrations, only grip, grip and more grip!

Macster

Not sure I like to read the "winter" rims don't have a centering diameter like the standard/summer rims do. You can't rely upon the lugs bringing the wheel to true center position.

You sure you have the *right* winter rims for your car?

Sincerely,

Macster.

Type_LT

Macster,

The rims/tires are from an early (pre-05') boxster so they should fit ok.

I know that Porsche recommends 255 rear width for the pre-05's and a 235 rear width for the 05's on. Other than that rims and tires match up.

I have a micro-displacement gauge I'm going to use to meaure the summer/rims tires for axial/radial runout and then mount up the winter set to see how they compare. I'm determined to get to the bottom of it

Type_LT

Macster,

The dial gauge tells the truth...

With the wheel lugs removed I was able to vertically move the winter wheels on the hub a total of 20 thousandths of an inch. To me this means that if you just put the rims on the hub and let gravity do its thing, the rims will be offset from the hub centerline by at least 10 thousandths.

To see if I could fix the problem I placed a small flat jack underneath the tire so that I could raise and lower the wheel relative to the hub. At the same time I had a dial gauge fixed to the hub that would measure the tire position relative to the hub. After moving it up and down a bit I determined that if I raised the wheel 9 thousandths that I would hit the centerline. After finding center, I slowly tightened the lugs and rechecked the dial gauge.

It seems to have fixed the problem. If the rims were manufactuered as well as the OEM Porsche wheels I wouldn't have had this problem. Thats what I get for choosing the cheapo winter rims from Tire Rack.