Vibration under braking

[TeaVR]

I’ve noticed under (moderate to heavy) braking a vibration in the car.

The feeling is NOT through the steering wheel, but through the seat. There is zero vibration in the car during motorway cruising - it’s just under braking. Almost feels like the rear is loose. Quite unnerving.

Any ideas what could be causing this?

It might be a coincidence, but I’ve just had the rear tyres changed.

[Robertb]

I'd be surprised if the new rear tyres would introduce a vibration that only happened under braking.

Possibly an imperfection on one of the rear brake discs, or wear on a control arm.

[TeaVR]

Agree.

What about the diff?

[Robertb]

If it were the diff Id have thought you'd notice the vibration on overrun, not just under braking.

[EBRANDON1]

I had the same problem for a long time and done multiple thigs to try and solve the issue, including new disks and pads (twice), new tyres, and new control arm bushes, problem still persisted.

Couple of weeks ago got the two front wheel bearings replaced and seems to have finally solved the issue, fingers crossed it doesn't come back as it went away for a few weeks with each discs change. It cost just under a £1k to do [ouch], but that was a BMW specialist garage, and the car was a 430d XDrive which apparently is a more laborious job with it being AWD.

The go to with vibration when braking is usually new discs, but if you know your discs are okay or have replaced them already then it may be the bearings.

[EBRANDON1]

Also to add, when I did get my tyres changed, the vibration got slightly worse, almost as if the better grip made the vibration more prominent compared to the tyres that had worn in whilst vibrating (maybe, I'm no mechanic though)

[TeaVR]

EBRANDON1 said:

Also to add, when I did get my tyres changed, the vibration got slightly worse, almost as if the better grip made the vibration more prominent compared to the tyres that had worn in whilst vibrating (maybe, I'm no mechanic though)

That’s very interesting. Thanks

[Summit_Detailing]

Rear discs required I'd suggest, plus a set of pads obviously.

I had this on a CLS a couple of years ago, did the fronts which sorted the slight vibration through the wheel, rear's were the cause of the vibration through the seat.

[mac96]

I'd check that all the wheel nuts are tight, particularly after someone had removed the wheels.
Probably not this, but it is easy to check, and having a wheel come off is no fun! Ask me how I know....

[Rough101]

Rear discs ‘warped’?

Especially common on cars with adaptive cruise or if you lean on the stability control a lot.

[monkfish1]

Summit_Detailing said:

Rear discs required I'd suggest, plus a set of pads obviously.

I had this on a CLS a couple of years ago, did the fronts which sorted the slight vibration through the wheel, rear's were the cause of the vibration through the seat.

This as your starting point. Bet the inside faces are rusted to buggery.

[Gordon Hill]

You need discs and pads sir

[Lincsls1]

You need to look at the rear brakes IMO.
The discs are probably excessively corroded, maybe more so inboard.

[Pica-Pica]

Lincsls1 said:

You need to look at the rear brakes IMO.
The discs are probably excessively corroded, maybe more so inboard.

That was my situation, after low use during CoVID, and over-enthusiastic road-gritters/salters. Mine were heavily corroded. I had the front discs and pads replaced, as they were low anyway. Not fully cured, and had an advisory on the rears at the next MOT (brake efficiency was OK), so I had the rears done. Fully solved. This was a 335d xdrive. (I had tried all the heavy braking to try and clean the discs off first, but that did not work).
We have discussed this on here before (at least once).

[MustangGT]

Rough101 said:

Rear discs ‘warped’?

Especially common on cars with adaptive cruise or if you lean on the stability control a lot.

Not a chance. Brake discs do not warp, the temperature required to do that is far in excess of the temperature capable of being generated on road or track.

The cause of 'warping' is actually build up of pad deposit on the surface of the disc.

[monkfish1]

This 100% ^^^^^

Changed more brake discs for judder than i care to remember. None were "warped".