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?

[Pit Pony]

My omega had scary vibrations under braking when the front wishbone linkages were starting to wear. I put it down to warped front discs initially.

[GAjon]

Have you jacked the car up and manually rotated or checked for any back to front wobble or looseness?

If it’s a bearing you might feel it.

Are the calipers multi piston? Could need a caliper service, look at the side that had the worn disc.

[Tyrell Corp]

Maybe your "warped" disc was actually a hub with runout - but incorrectly diagnosed as warped.

Had that before, previous owner drove fast over a sped bump and broke the strut spring, but there was also unseen damage...went through several discs within a few thousand miles until a dial guage on the hub finally revealed the problem.

[Smint]

Has someone fully cleaned and lubed all calipers sliders etc, including the all important and usually forgotten pinging off the stainless sprung steel clips the pads sit in so as to scrape the inevitable grime that builds between said clips and caliper body.

Have the pistons been exercised in their bores, i see people on youtube suggesting buying special tools to force the pistons back when fitting new pads, they shouldn't need any more force than mild pressure with a decent screwdriver or old chisel as a lever.
This is especially important with multi piston calipers, there's always one piston (on Landcruisers etc its usually he inner lower of the 4) that will be the first to start seizing in its bore.

Lastly, with all wheels on the ground, park brake on, try to move the car forwards and backwards by gripping the outside of the front tyres and roll the wheel one way and then the other as if the wheel in question was powered, both sides, thereshould be almost no fore and after movement of the wheel inside its arch, i find this a good way to check for wear in wishbone bushes etc, good method when checking for rear bush wear on BMWs.

[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

[littleredrooster]

Having chased exactly this problem in the past, my money would be on front (probably lower arm) suspension bushes which are allowing the front to go into toe-out/negative camber under braking load and starting a 'wobble' which you can feel.

[E-bmw]

Have you checked all 4 wheels are straight & true?

I have had a slightly buckled one before give similar symptoms, amongst others.

Couldn't quite explain why except maybe the additional weight transfer on heavy braking was making it feel worse/bad enough to notice.

[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.

[richhead]

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.

[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.

[s p a c e m a n]

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-...

[E-bmw]

StreetDragster said:

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

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.

[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

[Tyrell Corp]

Could you bang it onto an MOT rolling road brake tester, front and then back wheels and at least try to work out which axle it seems to be coming from?

Had a wheel bearing issues, sounded 'just' like from the NSF front, so replaced both sides - but it turned out to be a OSR rear,

Op has the car been accident repaired at anytime?

[StreetDragster]

As far as known, no accidents.

[TwinKam]

StreetDragster said:

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

Would be an ideal thing to try out on that local accessible private airfield of yours...

[s p a c e m a n]

A wise man once said 'If it ain't right, do it at night'

It's the easiest way to isolate what corner is causing you problems though if it is brake related and you seem to been chasing this for a while..

The only other random thought I have is to pull the abs fuse incase that's causing pedal modulation, I wouldn't normally suggest it as a cause but stranger things have happened.

[richhead]

StreetDragster said:

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.

my point about pad deposit and cooking the disk may not show up with a dti, most other problems normaly show up under other condisions, not just braking, so i would still say its a brake issue, and my money would be on the rear, as its the pedal you feel it through.