Vibration Under Braking- Check My Homework?

[StreetDragster]

Morning all,
I'm hoping someone can check/confirm my homework here, because a specialist has told me one thing and i just can't see how they have got there, so considering a second opinion or looking into it myself.
Keeping it all generic to keep things simple.

Car-
Front Engined Big Capacity, rear wheel drive arrangement.
Transmission is a torque-converter automatic with paddle shift over ride.
Rear diff is a self locking type.

Problem/Symptoms-
Under braking there is a vibration.
Most noticeable over 40mph, but once you have noticed it, you can feel it under 40mph. At like 70mph motorway speeds, its very noticeable.
Being an automatic, it creeps at idle. If you are creeping in traffic, like 1mph, and just holding it back with the foot brake you get a whoom-whoom-whoom type feeling as the braking isn't uniform.
All other times, there is no noises, no whining, no vibrations, nothing.

Diagnosis So Far-
Front discs and pads were checked. Found a warped passenger side disc.
Front discs and pads replaced, no change in symptoms.
When on the motorway, at 70mph in a straight line, in drive, brake with the footbrake- vibrations.
Do the same thing, but shift into neutral first, brake with the footbrake- vibrations.
Back to 70mph, no footbrake, shift into manual and aggressively come down through the gears- no vibrations.

Questions-
I'm thinking this must be either something in the rear brake discs/pads, or one of the cars 4 wheel bearings.
If it was a ball joint, bush or similar, it wouldn't be noticeable at 1mph?
If it was transmission related, the symptoms would be different between Drive/Neutral and should have definately been apparent when engine braking?
Or there would/should be clunks/whines or similar?

Anyone experienced anything similar?

[StreetDragster]

Evening all, thanks for all replies. I've had a fair few hours on the car this afternoon and I have the following to report-

Overall-
Car is a 2014, 33k miles, forgot to mention that in the original post

Front end-
Brake discs are large, fully floating, J-hook grooved discs
Calipers are 6-pots.
No noise/play in the wheel bearings, both sides.
No play in the steering linkages/ball joints both sides.
All ball joints and suspension connection points/bushes appear to be ok.
With the brake disc removed, the hub is square, with a lot of holes drilled in it, so difficult to get a DTI on it to measure runout. However, i think i managed it, and it appears to be running true on both sides.
Despite this, there does appear to be run out on the passenger side disc, which is the same one which had run out before. its not alot, maybe 0.1mm, but the drivers side is running true, and they have both done maybe 800miles since fitting.
All 12 pistons slide in and out easily, as do the pads. They come out the top of the caliper, which is radial mounted BTW.

Rear end-
Brake discs are large, solid, vented, drilled and grooved discs
Calipers are 4-pots.
Parking brake is a drum in disc configuration, so the footbrake acts on the pistons directly with hydraulics and no other mechanism in the way.
All 8 pistons slide in and out easily, as do the pads. They come out the top of the caliper also, which is again radially mounted.
No noise/play in the wheel bearings, both sides
All ball joints and suspension connection points/bushes appear to be ok.
No play in the driveshaft CV joints, or diff connections, or the prop shaft to the diff.
Turning one wheel forwards turns the other wheel forwards, so the diff is locking.
Due to the drilling/grooves, i couldn't get a DTI to read properly on the friction surface of the discs, but running it on the flat part where the wheel butts up against, both sides appear to be running true.

After all that checking, the problem remains. Definately can be felt through the pedal, and definately more prominent above 40mph. When braking down from 110mph on my local private airfield, its a big vibration, so definately scales with speed, which i would expect from something rotating.
It also definately remains if foot braked in drive, and foot braked in neutral, and does not occur if engine braking.

My friend did bring up the point, that the car has grooved brakes front and back, and appears to be most prominent at speed. Are these vibrations/noises just a by-product of having this type of brake discs?
This is the first car i've had with j-hook brakes, so I'm not sure.

In absence of any other ideas, If i was a gambling man, I'd go for front passenger side wheel bearing.
I'm also going to get the balancing on all four wheels checked, but i don't expect that to be the problem, as its not doing anything untoward when no braking.

Specialist didn't put alot of time into it TBH, and mentioned about how they tested it around the block/car park (so not at speeds) and reported it needed a £4500 rear diff. I just can't get to that part from these symptoms.

Thanks in advance

[StreetDragster]

Thanks all, I'll have the alignment checked when i have the wheel balance checked, but this is a very curious problem.

For bushes/balljoints, i can't find any play or movement in any of them, even with a big lever bar, no damaged grommets or bellows either.
Also, its just been through a full service, health check, and a MOT with a clean bill of health, so they didn't see anything either it seems.

Parts are hella expensive for this car, normally, i'd just start block changing parts until it is fixed. I'm starting to question if there is anything wrong with it at all and its just a groove brake noise/characteristic.

[StreetDragster]

richhead said:

if you feel it through the pedal and not the steering then its most likely a problem at the rear, instead of just fitting parts, a good dial gauge will tell you if the problem is the discs, they warp alot less often than people think, its often another problem like bad deposits etc, machining can sometimes fix this. If you regualy use the foot brake to hold the car at lights , this can be a problem as parts of the disc cool at a different rate to the rest where the pads are holding it. but a dial gauge on the disc will show this.

Thanks for that, but I've already DTI'd the rear discs, the front discs and hubs as described above. The rear discs are true.
You do have to hold it on the brakes through at the lights etc, the creep is strong with this auto box.

[StreetDragster]

E-bmw said:

I didn't mean the wheel alignment, I meant the wheels themselves.

Are they actually true/round/un buckled, a small buckle can be balanced out but can cause other issues related to your problem.

Ah right, I'll get that checked whilst they are on the balancer

[StreetDragster]

s p a c e m a n said:

Quick bodge don't do this ect.

Clamp the rear brake Flexi hoses so that the rear brakes don't work, that way you'll know if it's the rear disks

https://www.machinemart.co.uk/p/blue-spot-3-piece-...

Sounds like the recipe for the next accident lol.
Would help rule out an axle though

[StreetDragster]

As far as known, no accidents.

[StreetDragster]

Afternoon all

So I'm heading towards a potential fiery death with the clamping. Brake pipe idea. But......

... the car showing this problem at sub 1mph has got me thinking.
If it was only at higher speeds I'd be fully on board with getting the wheels balanced and checked for trueness, tyres for bulges etc.
Then from there id move onto bushes, tie rod ends etc.

But if this problem shows itself at less than 1mph, creeping forward in traffic just holding the car back on the brakes. You get a definite feeling of none linear breaking, whoom-whoom-whoom if you will.
Surely, there isn't enough momentum for anything wheels related, bushes, balljoints etc.
If it was a bulge in the tyre, it would be like driving over a bumpy road if it was big enough to be noticed, and wouldn't be effected by brakes on/off.
Given that the pads in all four calipers have been checked intact, free to move, no binding etc, and all 20 pistons have been checked to be free and easy to move and also not binding. Also, brake effort at this speed has got to be majority front biased.
Then it's got to be the discs not being been at 90 degrees to the pads right, i.e, one of the wheel bearings?
Both rear discs are true when DTIed, so it's likely to be on the front end?

[StreetDragster]

Maxdecel said:

If I understand the above you measured run-out on the Top-Hat as it's sometimes referred to, the mating surface of the wheel/Disc ?
Run-out is not measured there so you cannot be certain the rear is to spec.
Pt 6 https://mintex.com/wp-content/uploads/2015/12/Disc...

Correct, front discs had run out, measured at the disc edge. 360mm odd solid steel discs i think, 1.2mm runout, i think, it was a while ago.
New discs were fitted, no run out, now have run out but not much, 0.1mm i think.
However, the originals are solid steels, replacements are fully floating 380mm ones.
Front hubs have been dtid, true.

I completely agree with what you are saying about the rear discs, and correct that it was measured at the top hat, you can't get a dti to read due to the drillings and grooves on the friction surface.
But my point is at 1mph, they are not likely to be putting any force in, and it's all from the front, right? I'm only breathing on the brake pedal to hold the car at that speed in traffic.

I imagine alot of people are wondering why i don't just replace them to rule them out, they are ~£800 a pair. Wheel bearings at the front are ~£900, the front discs and pads OEM would have been £1600. Everything's big money to be fault finding by replacing parts.
Hence, why i took it to a specialist, who half heartedly looked it over and then just quoted the entire diff needs replacing, at 15% the cars value, just for the part, plus labour.

I don't mind spending money, but it's got to be on the right bits.

[StreetDragster]

sam.rog said:

Go for a drive and see if one wheel is hotter than the others. Could be sticking calliper.
Get it on a ramp and pry on all the arms to check the bushes. A torn bush can give similar issues.
Finally check the topmounts.

No obvious wheel hotter than the others, even after extended periods of heavy braking.
All pistons in all calipers have been confirmed free and moving smoothly.
All arms and bushes have been levered/pryed and no movement.

[StreetDragster]

The discs have been changed on the front. The front I've been able check on the disc surface no problem, with the old and new brakes. And I've been able to check the hubs.

The rears, have not been replaced, and can't be checked with a dti for run out on the friction surface. Hence why i had to do it on the hubs

This problem has been building, used to do it a tiny bit, now it's more obvious.
One wheel has been refurbished in the middle of this, no change before/after. That was the front passenger wheel

Edited by StreetDragster on Friday 2nd February 20:40

[StreetDragster]

Hi All, thought i'd update this thread.

So a few months on now, and a few more thousand miles on the car, nothing is better, nothing is worse.

So what i definitely haven't done officer, is clamped off the rear brakes and took it for a drive.
And whilst i wasn't doing that, i didn't discover that the problem completely goes, then returns a bit with one side unclamped, and as per with both sides unclamped.

So, given that-
1) I wasn't able to DTI the rear discs for tru due to the grooves running all the way across the friction surface
2) The problem is present at both high speed, and also just creeping on the brakes, less that walking pace, you can feel the none linear brake effort
3) Heavy engine braking with no foot brake doesn't show any vibration, and clamping off the rear brakes would make no difference if it was in the drive train
4) The front discs and pads have been replaced

My next step is to measure the thickness of the rear discs. If thiccccc then i'll have then skimmed, if not, i'll replace and see what happens

One of its rear parking sensors has failed now, it never ends

Thanks all