Vibrations over 120 km/h

[Xti]

Hi guys!

My Q4 starts to vibrate at speeds over 120 km/h and I feel this through the driver’s seat. No vibrations to the steering wheel at all.

At first I thought it was due to tyres possibly used unevenly. But the symptom persists also with 17" Comps on + brand new set of tyres.

Have any of you met this problem with your 155 Q4? Any sugestions? Thanks guys.

Cheers,
C.

[Steve Webb]

Hi Cristian, when did you first notice this? Was it after the trackday?

If you can't feel it through the steering and it is still there with your other set of wheels, it could be a balance problem with your prop-shaft or a worn center bearing, (but didn't you change that?) or maybe a worn rear hub bearing.

Do you get any strange noises in the car when you get the vibration?

[AlfaJack]

Any noises when cornering?
I would also suggest a bearing is on its way.
How are the prop shafts balanced on a Q4?

[Steve Webb]

How are the prop shafts balanced on a Q4?

Do you mean how do they measure if they are balanced or how do they adjust a shaft so its in balance?

Not a clue on the first part, for the second, I seem to recall seeing small plates welded onto shafts to act as balancing weights.

[Xti]

Hi Cristian, when did you first notice this? Was it after the trackday?

If you can't feel it through the steering and it is still there with your other set of wheels, it could be a balance problem with your prop-shaft or a worn center bearing, (but didn't you change that?) or maybe a worn rear hub bearing.

Do you get any strange noises in the car when you get the vibration?

These vibrations were already present way much before the trackday. Prop-shaft's intermediate bearing has been reconditioned
(Jim's solution by filling it with ''windscreen mastick'') but now that you've mentioned about it I realise I have one more possible cause to take into consideration. As "deffective tyres" possible cause was eliminated it remains 2 more: a) worn rear hub bearing; b) unbalanced prop-shaft.

Haven't noticed any strange or increasing noise when cornering but regarding it's about rear I might not have been able to hear/feel it.

Anyway, for a start in solving this vibrations issue both rear hub bearings will be changed with new ones. I wouldn't have thought worn rear hub(s) could give place to such vibrations but it's also true I've never confrunted with that before.

And if it will turn out hubs weren't the main problem then maybe I should take into consideration a new intermediate flex. support for prop-shaft. But let's hope it will not be necessary...

Tx mates, your help is very much appreciated!

Cheers,
Cristian.

[Steve Webb]

Just in keeping with the thread, this is what I imagined was being used to balance the prop-shaft. (this one is from the GTV)


http://www.berlinasportivo.com/forumpics/project_gtv/prop_weight.jpg