Cheeksyboy

Ok, so I'm not quite old enough to remember the Beach Boys...but just over a week into 996tt ownership it's developed a vibration at speed.

Now I'm considering the usual (weight has fallen off a wheel) and so I'll get the wheel balances check this weekend, but thought I'd ask you guys if there's anything else I should check?

Macster

especially the fronts can produce a vibration at "speed". Just a few PSI over recommended can do it. Check tire pressures dead cold, after sitting all night.

2) If mild weather tires get hot and then when the car parked and the temperature cools down the tires take a "set". Next day it can take a bit of driving -- my experience is sometimes a mile or more -- until tires get warm enough to lose the set.

Tire weights are taped on and are generally pretty well stuck down. A good blast from a DIY car wash wand might remove one. so at least checking wheels for a fresh spot that might be a missing weight called for.

Sincerely,

Macster.

Cheeksyboy

Hi Macster

Yes it has warmed up in the UK lately....not quite tropical, but in the low to mid 20's (which is good for the UK!!!).

I will check the cold pressures, didn't realise the 996tt was as sensitive to temperature and pressure as you suggest.

I did give the car a good presure was on the weekend, but couldn't see any dislodged weights, but will have another better check by taking each wheel off.

Wanted to be sure it's not a driveshaft problem or something, so your initial thoughts have helped take away that concern.

MechanicalEng

did you check the wheels? even if they are balanced properly, if they are bent they could cause vibrations at speed

Macster

I was relying upon my experience with my 02 Boxster which is sensitive to tire overinflation pressures.

Just something to check and eliminate as a source of the symptom before you move on to something that is more difficult or expensive or both to investigate.

Sudden onset of a vibration can be a driveshaft or drivetrain problem.

Essentially the car was smooth and now it is not. What changed?

Well, with higher ambient temperatures tires inflated to proper cold pressure before now will be overinflated.

Tire weight going missing is another possibility.

As someone else suggested a bent wheel -- hit any potholes, road debris lately? -- is another thing to consider. I will admit I've hit some pretty big potholes and while I thought the car was going to fall in and never come back again thankfully the tire, wheel or suspension has not been affected. But you may have more fragile/flexible wheels on your car.

Another possibility is a suspension component has given up. A shock.

Yet another possibility is the alignnent's out. Can be from a pothole, or brushing a curb, or ....

One thing that I see quite often is a car turning into or out of driveyway the driver cuts the corner and ends up driving a rear wheel (sometimes a front wheel in the worst cases I've observed) over the curb.

This puts tremendous and very sudden side loads on the wheel and suspension and a lesser wheel might get a bit bent of shape. Suspension alignment can be affected as well.

And one last possible cause is a tire has suffered some interal structural problem/failure of its belts. In this case while tire will continue to hold air and could even still technically be considered safe to continue to use, its roundness and ability to recover its shape as it constantly contacts the road is affected and vibration is the result. A tire that has suffered this problem or is suffering from this is hard to diagnose. One way is to load force balance and generally it will be found the tire can't be balanced cause of the shifting belt.

Sincerely,

Macster.

Cheeksyboy

Hi Macster, is that your bid to be technical support for Nascar/Indycar!?

I checked cold pressures all around and whilst the fronts were 36psi, the rears were down to 36psi (from 44)....but even after adjustment I wasn't happy so had the wheels rebalanced. This seems to have rectified the vibration issue and we're back to being able to hold 130+ without any suggestion of instability.

One thing I will do is, as you suggest, get the Geo checked, but I still need a few more miles yet to get a better basleine of the car.....will try a track day soon I think.

Thanks for all your help, much appreciated.