High Spped Vibration

[Jon Kent]

Hi all,
I'm a 968 newbie and very impressed so far with my 1994 LHD CS (though judging by the extras, it's a UK Sport equivalent - rear seats, elec. windows, aircon).
Came over to the UK on Sunday to see a 964cabrio and was a little disturbed by severe vibration through the whole car over 170kmph.
Managed to get to 214kmph (GPS speed) & was the same all the way.
Any ideas ? Geometry ? Drive/prop shaft out of balance ? Common ?

I mention GPS speed, as the speedo is about 10kmph out GPS ready 41kmph at 50 clock speed (normal?).

Otherwise, the car is storming & did the 1101km round trip without missing a beat. The car was meant to be a temporary replacement for my recently sold 964cab, until I find another. But will compliment the 964 now. Both are great !!
CS ? Club Sport ? No, Complete Schizophrenic ! As happy being thrown round corners as cruising on motorways ! Top car !

Rgds,
Jon
Luxembourg
Ex-Boxster/964C4Cabrio - Now 968CS & ???? don't know yet !

 

[968CSReading]

Hi

Glad you like your 968.

Sorry to state the obvious but have you checked the balance of the wheels?

When the belts need adjusting that can cause vibration (but mainly through steering wheel and gear stick). Have they been replaced recently and not checked (after 2000 miles I think).

I am not very technical so can not offer any more suggestions, sorry.

 

[Jon Kent]

Thanks for the quick reply.
Yes, on Wednesday last week I had 2 new fronts & the rears re-balanced.
The car has a full history and only 66000miles (110Kkms).
But full history is mainly just servicing, so geometry setup is not really part of it.
I'll get the belts (& cam chains) checked by the local specialist, I think.
Stitch in time & all that..........
Many thanks,
Jon

Air or Water ? Both !

 

[968CSReading]

When my timing needed checking the vibration was very bad at 3000-4000rpm

 

[Jon Kent]

Don't think it's this then, as the car is sweet in all areas of handling......just not over 170kmph.
Use the full rev-range daily on B roads & no other issues.
Pete Murray can confirm this, after parade laps in the 964 at Brands las year.....chasing some turbos ;-D
It's obviously not a show-stopper, but needs sorting all the same.

Many thanks,

Jon

 

[944turboS]

You could try the discs then as these are the other main component in the same assembly. Also look at the location of your locking nuts if you have them, they should be on the stud with the painted top & opposite the valve.
Do you get the vibration through the steering (front problem) or through the seat (rear wheel problem)

 

[Jon Kent]

Many thanks for the feedback Graeme.
I'll check the locking nuts, but feel it's something else.
'Normally' an incorrect wheel balance problem shows itself at 60mph-80mph (100-130kmph).
But this is only evident at 170kmph and above (105mph-130mph).
It's the whole car & can feel it through the pedals, tunnel, seat, wheel......even the GPS works it's self loose from the screen.
Wouldn't be an issue in the UK, but here I reach that speed daily (though not at the moment due to the problem).

Rgds,
Jon

 

[944turboS]

Was the car Ok at the begining of your journey?
If yes then I would take it to be balenced again, I bet you've lost a weight or two.

 

[Jon Kent]

No, it was like this from the start.
I've only had the car a week !
But it's only done 1000kms since October last year, so I'm beginning to wonder whether the rears have flat spots.
Do the propshafts go out of balance ?
I think I'll have the geometry checked, as a precaution (save on tyres in the long run).
I'll let you know the outcome.

Thanks

Jon

 

[gm997_1]

ust to add to the mix, here's another Graeme to contribute [] Call me Graeme2 if you like!

Jon, a few things:

Yes, speedos are always optimistic, and tolerance for Porsche shouldn't be any more than 10% all the way through the range. I've checked mine on by GPS and had 130mph on the speedo reading 120 on the dial. Ran out of road to go faster but presume it would be approx GPS 150 to dial 166.
The 968 is known to be pretty accurate all the way through to 100mph / 130kph as the 10% is only coming in at higher speeds.

Certainly your major-vibration issue at 170kph isn't normal. Should still be rock solid and very steady at that speed - can tour at those speeds all day, and feels relaxed with it.

Have you also considered tyre pressures? The 968 is very sensitive to those - over-inflated may cause the symptoms you mention. Should be 34-36psi [correct me if i'm wrong, think that's about 2.1 bar for Lux!] for road use, at the 34 end if you're driving fast over long distances so the heat build up will translate into approx 2-3lb extra pressure to brong you to 36.

Also re tyres: different wear on both rears? And did you get your tyres balanced in the UK or Lux? Here in the UK the machines in the regular, public access sports tyre fitters only do up to 70-80mph. If balanced in Lux though, maybe up to a higher speed? If not, could be the answer.

You say it doens't occur at the same revs in every gear, just at 170kph and above, so that rules out a lot of things.

What about suspension issues. Rear shock absorbers? Or rear wishbone mounts could be going?

Do you know if the car's been raced hard, do you know if it's on non-stock shocks, or originals?

Might be worth looking into that area.

Hope some of this helps.

 

[bernard tester]

Hi it could be the rear tyres are not round! if the car has been standing for some time or the tyres are old. I would put the car on a wheel free lift an run up to 170k, have some one watch the rear wheels! I have had this problem with a number of customers cars.You can also jack up the rear of the car and turn the wheels and note any diffence in distance between tyre and the ground, ovalaty is often not picked up when balancing Regards Berny.

 

[Jon Kent]

Thanks Berny,

It appears you're right, as after 2500kms (a busy 2 weeks ;-D) the problem has disaapeared.
What a car !
It eats up the kms on the motorway, then dives round the roundabouts & bends on the small roads.

Enjoy,

Jon