Vibration at motorway speeds - unknown cause

[benmays99]

Hi guys wonder if I get your suggestions on my latest W126 job
I have a vibration felt through the floor and the steering wheel at motorway speeds of 60-80mph. I've had the wheels welded and balanced as they were slightly bent which I thought would fix the issue but the vibration still remains but is lessened.
What could the culprit be now? I'm thinking my next step should be to get a 4 wheel alignment? If not maybe it could be to do with the prop shaft balancing or somewhere else in the driveline?
Let me know if anyone has any thoughts!
Thanks
Ben

 

[Droverunner]

OK so they've balanced the wheels but are the tyres out of round?

 

[mattkh]

The reason for al the wheels being bent might throw some more light on the issue of the vibration.

 

[brandwooddixon]

I'd jack up the car so that you can spin the front wheels freely and using a solid object with some sort of marker check that the wheels are actually round. I've used a piece of wood with a carpet tack stuck on with some bluetac before now.
Also check that you've not lost any weights since the rebalance - look for a clear patch with no weight.
Don't forget to check the back of the car as well.

 

[pgh13]

Possibly warped brake disk? Brake disk not quite flat to the hub. If you can get to see the disk and caliper, is the disk exactly the same distance from the caliper when rotated

 

[Altamar]

I have seen a few cars going down the road with the undertray flapping about because it has worked loose, which would tend to show up at motorway speeds. Is yours secure?

 

[Droverunner]

Wonder what brand the tyres are... front in particular. And their age??