BXSTR986S

I bought a 2003 '53' Boxster 96S about 8 months ago, and it's really starting to frustrate me. I hope you guys can help...

Car has done 48,000 miles and overall is in great condition. Its been on the ramps at a couple of Porsche specialists who have all told me its very clean and a nice example. However, at 75MPH or so, it has developed a very annoying judder through the steering. The wheel shudders slightly and on first site, you would say wheel balancing straight away - it is not there at lower speeds and you can accelerate through it. So I headed off for a rebalance.

However, so far...

- Front wheels have been rebalanced 4 times. Every place says the last place was way off.

- I had both front wheels checked by an alloy refurber and was told they are both within 2.5% of completely true and not worth straightening - it would be 'a waste of your money, some factory wheels aren't that straight out of the box'. Car is on the 19" 5 spoke Carreras.

- New Pilot Sports fitted to the front 200 miles ago on a specialists recommendations, as the previous ones (with loads of tread) were 2011s. This has made the steering more supple but problem remains.

- Specialist replaced front control arms and front wishbones 2 weeks ago. Car feels great below 50, took it out again for first time at motorway speeds today...problem is still there.

I am stumped. Only thought I now have is try a different set of wheels on the car and see if problem remains, because when one specialist showed me my OSF wheel on the balancer it did LOOK out?

Please help, before I set fire to it.

NA6

I think your approach has been good. I assume when the front suspension work was done they checked the bearings and all bushings? If I were you I'd try the different set of wheels as you indicated and if it shows that the issue is still present I'd continue working the suspension angle. How old are the front rotors and pads?

BXSTR986S

Thanks for this,

Yeah the car was inspected all round when I brought it in to diagnose; brake pads are nearly new, the discs I'm not sure...but have plenty of life left in them from a visual inspection. All the suspension was marked as 'low' or 'med' for wear (the items I replaced) and everything has been re-torqued. It's also had a 4 wheel alignment as you would expect after the work above and all green.

So very frustrating....

Macster

Agree with NA6. Your approach has been good.

Couple of things. Hard to believe 4 different shops would botch a simple wheel/tire balance. When tires don't stay balanced this is often a sign of a bad tire. The belts are moving.

But you replaced the tires and the symptoms remain.

Did you balance the rear tires? As these tires wear they can go out of balance -- assuming they were in balance to begin with -- as the heavier spots wear away. Often when I have the rear tires replaced on my cars the fronts need to be rebalanced. The fronts have worn enough (at around 20K miles) to be out of balance a bit. The rears get out balance too but often I live with this if it is not too bad since I know the tires will soon be replaced.

Very important: The tires must be the proper tires for the rims. I trust you aren't running some unsanctioned wheel/tire combination? I'm thinking some weird width/cross section tires, or non N-rated tires. Also, if the tires are unidirectional they must be mounted in the right direction.

The tires must be identical side to side and front to back, identical in that are the same brand, style, N-rating, and of course as I touched upon above mounted properly.

Be sure the tire inflatation pressures are correct. I found in my 2002 Boxster that just a few extra PSI over the 29psi in the front tires caused the car to develop a shimmy/shake -- a mild case though -- that mimiced a tire imbalance.

That leaves the wheels.

Don't know what "2.5%" means regarding wheel straightness.

I don't have the numbers for the Boxster or those wheels but my 996 Turbo manual has this to say (regarding 18" wheels/tires): Max lateral/radial runout for "light alloy wheels" is 0.7mm. With tire: 1.25mm with < 1mm better and 0.5mm best.

So you need to find a place that will check the wheel/tire runouts to the above specs.

If you find the wheel/tire runout too much then you probably need to consider replacing the wheels. I'm not a fan of straightening alum. wheels. Once bent they lose strength and resistance to bending is reduced. They can leave the shop straight but they won't stay straight for long.

Regarding other explanations...

If the wheels/tires prove to be ok then you need to find a highly regarding Porsche suspension specialist.

Generally the suspension on these cars is long lived but of course there are exceptions.

My SOP is if the tire wear is ok, even across the tread face and even from side to side, and the car doesn't manifest any behavior to make me think alignment I leave the alignment alone.

But in your case it might be worth the money to have a proper alignment done. Let the suspension/alignment specialist advise you.

He should road test the car and inspect the car in the air for any signs of problems. The best alignment is no cure for a bent or damaged or worn out suspension component.

Be sure you get a detailed *before* and *after* printout.

BXSTR986S

Thanks again for the detailed help guys.

The car had a full four wheel alignment when the suspension work was done; here is the report. He said the toe and caster was off slightly but they couldn't get it any tighter than this....? Should I be complaining....? Everything was re-torqued and marked as well.

Tyres are the same Conti Sports as on the back. The rears have not been balanced as I always thought steering wobble = front, wobble through the seat = rear? But that is something I could try, as is tyre pressures (though fronts were changed 200 miles ago).

'2.5% out' referred to the % await from 'perfectly straight' they apparently are. I could change the wheels, I think I am going to ask the garage to try a spare set on the car and then take it up to speed, to see if it feels different -just to rule that out...?

BXSTR986S

Hmmm, not sure why the upload is horizontal, sorry about that

Macster

Don't know why/how caster can be out. It was within spec as shown by the *before* reading. Caster is not adjustable so I am not sure how it got out of spec in the *after* reading.

Does the car have any aftermarket suspension parts installed?

Or it could be the alignment rack is out of whack. I encountered this once. There was an alignment rack inside the Porsche service department on which my Turbo was aligned. Within 8K miles the rear tires were worn out. I had new tires installed and another alignment done at aother dealer. The tech told me the alignment was terrible and I told him I had the car alignged at another dealer. The tech told me he used to work at the other dealership and when he was there the alignment rack in the Porsche service area was junk. The best one was located in the service area of another brand this dealer sold.

Anyhow, I'd be a bit unhappy about the caster and want to know why it was within spec as shown by the *before* but out of spec in the *after*.

BXSTR986S

Good info thanks for this. I'll raise the question as I am asking them for further help on the matter anyway - they said replacing the front arms should solve it and it very much hasn't . It happened before this was done though so not sure how much impact. Suspension is al 100% standard, the car has an aftermarket exhaust but thats it.

Dealer is a well respected independent in my local area who do nothing but Porsche, so seem to know their stuff and have been helpful so far - so fingers crossed...

mikefocke

So what are rotating?

Tires, wheels, rotors, bearings.

I assume you have accelerated to the same speed in a lower gear to eliminate some sort of engine harmonic.

I like the trial swap for a known good wheel/tire set just to get 2 of the four items out of consideration.

BXSTR986S

Thanks for this,

The engine harmonic is interesting; it's got a EuroCupGT Twin High Flow System on it. This is a little boomy for long distance / motorway speeds - but I had never thought this could cause steering wobble.

Will try new wheels and accelerating to 80 in lower gears to compare...

NA6

Some other things I was thinking about:

1. Can you feel what side it is coming from?
2. Swap sides on the front wheels (I know the tires will be rotating the wrong way but for a few mile experiment it can't hurt to help isolate the issue?)
3. Under inflate the front tires by 3-4 psi
4. Over inflate the front tires by 3-4 psi (see if this changes the feel/speed at which the issue occurs, maybe you can narrow it down to one wheel or tire?)
5. As you indicated try the new rims...
I'm very curious what the root cause is.

Stephen Tinker

Try temporarily swapping the front wheels with the rear's. I know they are a different size, but for this experiment it will help diagnose an out of round wheel / tyre combo - and its free.
As you correctly pointed out in a previous post, if the vibes move from the steering wheel to the seat of your pants .......

Anker

A speed dependent shake is often caused by worn shocks/struts. Did you have them checked?

Anker

Macster

You mention "OSF" wheels. I'm not familiar with that brand/style of wheel, known nothing about them, but there have been a few cases of an impossible to eliminate balance / shimmy problem that was found to be due to wheels with the wrong size reference diameter that mates/centers to the wheel hub.

Each wheel has to be checked to ensure its hub diameter is the right size and locates the wheel accurately, and consistently.

Stephen Tinker

Macster,
OSF = offside front in Pommy English. That's the front wheel closest to the driver....