924Board.org

[johnnycat]

On a recent highway trip I decided to test the upper speed limit of my '88 924S Le Mans. I was annoyed at the fact that every time I tried accelerating past 120mph (~193kph) my steering wheel would vibrate harshly. I maintained control of the car easily, however the vibration worried me. I was able to make it to 131mph (~211kph) but the vibration was so violent I stopped there.

I have two questions:

1-What could be causing the vibration, could it be a wheel alignment issue?

2-Is the top speed of the '88 924S engine-limited to ~137mph, or is this just the factory estimated top speed?

I certainly don't intend to drive this fast on highways regularly, but any other Porsche I've driven at 120mph+ has been very solid and confidence inspiring.

John C
'88 924S Le Mans
"The chicks love the teal wheels, what can I say?..."

[Peter_in_AU]

it always seems to come down to two things, wheels or alignment.

Problem is that most people think that what the kid down at the local tire shop does actually IS an alignment and balance. It's not. He does something that's suitable for chevys that spend their life driving to the supermarket. What do you expect for $20.

An alignment machine suitable for these cars is VERY expensive, you won't find one at your local Tire City.

If you're going to be up around 200Kph+ regularly you should bite down and take the car to a real Porsche service company. Their alignment machine should give a print of the before and after numbers no hand-written stuff. Ask to see their machine, they're all computer controlled, and ask to have a look at the reports the computer produces. These are very hi-tech pieces of gear and not cheap to buy so any shop should be happy to show it off. If they don't want to show you, they don't have the right gear.

A real alignment is not cheap but it is money well spent

[yOnKiNaToR]

i also have an '88 924S le mans and it vibrates the exact same way. and by the time i hit over 130, the car was swaying a little to much to handle.

i took it to the porsche dealership to have it aligned but it still pulls to the right. i also had the ride height adjusted on the rear right. i think my rear right tire is screwing it up b/c it's down to the cord on the inside edge. i would check you tire pressure, and the tread left on your tires. if that's not it, make sure your ride height and alignment is ok...

[GP510]

Are the tires rated for that kind of speed? I'm not sure how that might affect the vibration, just a thought.

[Mich924S]

I just had this problem. It turned out to be horribly out of round front tires. If you haven't had the tires rotated or changed recently, I would suggest that. My alignment was fine it wasn't pulling either way just lots of head shake. I bought two new front tires and the problem went away completely.

[wdb]

at that speed, even slight wieght imbalances will have a dramatic effect. stones wedged in the tread, chunks of rubber missing from tread, dirt on the rim, etc. the cetrifugal force of the tire spinning that fast , will cause the tire to get taller,tires not rated for that speed will deform unevenly due to variations in sidewall and tread thickness, the amount of man made materials in the tire and over heating,( rubber disapates heat faster than man made materials , all airliners have 100% rubber tires, automotive street tires are about 40% rubber 60% man made materials.)man made materials are added to make compound harder and increase life of tire. even though tire manufacturers use a nylon belt in the tread to counter the expansion (nylon shrinks when heated) it can only do so much. how did James taylor put it. "sweet dreams and fine machines in pieces on the ground " for detailed info on tire ratings
http://www.1stoptireshop.com/html/tireratings.htm
http://rma.org/tiresafety/how_tire_is_made.html
http://money.cnn.com/2002/03/06/autos/q_tire_shopping/index.htm

[ This Message was edited by: wdb on 2002-04-03 04:38 ]

[ This Message was edited by: wdb on 2002-04-03 04:55 ]

[ This Message was edited by: wdb on 2002-04-03 05:40 ]

[exc911ence]

Firstly John, WHERE were you doing that kind of speed around here?!? Secondly, why wasn't I invited?!? Seriously though, perhaps the fact that you have 2 different brands of tires on either side of your car that may contribute. Different tires act different ways. Time for a matched set of Z-rated rubber!

[johnnycat]

Thanks for the responses guys. Dave - I achieved this speed on the new Island Highway north of Nanaimo.

I think my mismatched tires do have something to do with it! I didn't remember about them being mismatched side to side.....duh.....Very observant though Dave!

[exc911ence]

I scored a set of nearly new 'real' tires from one of the guys in the Porsche club who races a 914 so I can cut you one HECK of a deal on the Snake-Skins that I have on my car now!