I hit a pothole last week. The wheel and tire are fine and still balanced, fortunately, but the car has developed a very annoying vibration at around 60mph. So much so that you can see it when you let go of the steering wheel.

Any ideas what might have gone wrong? My preliminary suspicion is one of the control arms but there seems to be no meaningful play in any of them, checked with a crowbar on both ends.

All help is very welcome. Thanks.

 

@hq_ ,
I would rotate the tire/wheel in question to the rear, therefore 100% ruling it out as the vibration in the steering wheel.

Next I would check the wheel bearing, and or tie rod for play, with the wheel/tire mounted on the vehicle, and the tire off the ground, grab the tire at 12 and 6 o'clock to check for play in the wheel bearing, then grab it at 9 and 3 to check for tie rod play at the steering rack.

Johnny

 

Thanks Johnny. We popped both front wheels off and started inspecting. And found this:


It's safe to say the right-hand side lower front control arm bushing is toast. The LCA has torn clean off.

So I went to Strongflex web site and ordered a pair of bushings to take care of both sides while I'm at it. Their 021773A, for Phaeton/A8/A6, matches the dimensions of FS bushings and is a part of the comprehensive polyurethane upgrade I've planned for next year. It seems I'll have to start early and proceed gradually, I just finished another all-out PU bushing project and definitely don't have time to change them all at this point.

 

The bushing has now been replaced. While we were at it we replaced both sides. When one is toast the other one will most likely fail soon.

This is what we started with. The remains of the bushing could be pushed out with a thumb.



The new Strongflex 90 shore polyurethane bushing installed, waiting for the collar. It was a breeze with a hydraulic press and lots of PU-compatible MoS2 grease.



And finally the control arm back in place. It took a bit longer than we thought but essentially it was one evening's job.



Test drive went great. No vibration whatsoever, not even under braking so the suspected brake issue weren't warped rotors. All we need to do now is to take it to a four-wheel alignment just to make sure everything is allright.

I didn't really push it during the test drive and these are just one of four each side but I have to say the car already felt sharper than it has ever been. I'll look into replacing all of them later on, this seems like a worthwhile upgrade.

 

Hmm. After a lengthier drive there's still some vibration at certain speeds. Most of it is gone but at around 60mph you can still feel something, it's not as silky smooth as it should be. The front rear LCA bushings, the big ones, felt acceptable when tested with a crowbar but they didn't feel like new bushings anymore. We checked the front toe and it turned out to be .15°, exactly in recommended specs, and rear bushings were good enough not to give a reason for concern.

Investigation continues.

 

We had a similar vibration...turned out the right side wheel bearing was past it's prime.

 

Thanks for input. We replaced the left front wheel bearing last year and it wouldn't be surprising if the right one were on its way out now. We'll have to look into this next.

 

PROBLEM SOLVED. Even though the front rear LCA bushings seemed to be OK they weren't. We popped them out today and replaced with Strongflex 90 shore PU sport bushings. Getting the old ones out wasn't easy, my 15-ton hydraulic press was struggling with the task and only after hosing them with penetrating oil and tapping them gently they yielded.

The car is rock solid, no vibration at any speed (I only got it to 75mph on a short test drive) or during braking. As a plus, the steering feels sharp and planted with no noticeable increase in NVH. Highly recommended, either as a repair or an upgrade. The downside is that the old bushings had flexed so much that the toe was way off at speed and the insides of both front tires wore out in less than 300 miles. One tire blew on the highway last week and now I'm on winter tires as a quick and dirty solution, to keep the car on the road until I get all four summer tires replaced while I'm at it.

I can't wait to drive it with new UHP tires, the difference in handling and steering feel is already substantial.

 

An update of sorts. I just drove another car for the first time in several weeks and realized the Spur is still missing out in handling department. The car in question wasn't anything special, just my youngest daughter's Volvo V50 T5 (AWD, full polyurethane bushing upgrade, brake upgrade, Bilstein B6/B8 combo) and it felt incredibly light and precise compared to the Bentley. On the twisty road leading to our estate I could easily maintain 5-10mph higher speeds than I'm comfortable with with the Bentley. And it's a fairly pedestrian VOLVO for chrissakes. One with Ford Focus RS suspension geometry and brakes, but still.

It's about time to consider a full bushing upgrade to the Spur too. Four done, about a couple of dozen to go.