Brake "thump"

[Dippydog]

When I apply the brakes I can feel through the car-not the pedal-a "thump" I can also hear it.The best way I can describe it is it feels/sounds like you're running over a corrugated surface such as you find on the approach to some roundabouts etc.The brakes work perfectly well and I don't get any warning light[s]such as the ABS light on the dashboard.It sounds to me as though it's coming more from the n/s rear,but of course noise sources can be deceptive.It definitely seems to be rotational and the faster I'm going when i apply the brakes the quicker the "thump" and as the car slow down so does the feeling/noise.It's not a loud noise at any time but loud enough to be audible.Car drives fine at all times.car is a P1 V70 2.4 n/a petrol manual.Any ideas where I should begin investigations?

[stevepcar]

Originally Posted by Dippydog

When I apply the brakes I can feel through the car-not the pedal-a "thump" I can also hear it.The best way I can describe it is it feels/sounds like you're running over a corrugated surface such as you find on the approach to some roundabouts etc.The brakes work perfectly well and I don't get any warning light[s]such as the ABS light on the dashboard.It sounds to me as though it's coming more from the n/s rear,but of course noise sources can be deceptive.It definitely seems to be rotational and the faster I'm going when i apply the brakes the quicker the "thump" and as the car slow down so does the feeling/noise.It's not a loud noise at any time but loud enough to be audible.Car drives fine at all times.car is a P1 V70 2.4 n/a petrol manual.Any ideas where I should begin investigations?

A common fault with the older 850s is the handbrake linings become detached from their steel backing, it might be worth checking there. Good luck

[DaveNP]

The handbrake shoes are not used for the footbrake but definitely worth checking, while you're under the back of the car check the state of the discs both inner and outer faces, the rear brakes don't do a lot on an 850/V70 and the caliper can stick so the wear on the discs gets uneven.

[SteveSarre]

Hi Dippydog

So as you brake the thumping continues and the thumping slows down as the car slows down?

Can you give us an idea of how many thumps per second at various speeds? Or as you say, does the frequency match what you get when approaching some roundabouts?

Since the car is manual, are the symptoms different if you brake while you coast in neutral?

Steve

[Dippydog]

Yes the faster I'm going when I apply the brakes then the faster the thumping/noise and both slow as the car does.
I have no idea of frequency but it has a sound/feel of driving on a rippled surface[as you get at some roundabout approaches]but does it regardless of surface I'driving on.
It does it in gear or freewheeling in neutral.
All brake pads and handbrake shoes were changed last year as were both rear calipers.Must admit my first thought was handbrake shoes but couldn't figure out how they'd cause this as they're not footbrake dependant[I'll check them all the same]and I'll also check the discs themselves.Could a cracked ABS ring cause this without throwing the ABS light on the dash?

[SteveSarre]

I think there is not much left except a cracked brake disc.
Hopefully something would be visible if you inspect.

You could isolate the wheel by clamping off the brake hose at each corner in turn, but with appropriate precautions.


Steve

[Derek UK]

You might possibly have a damaged tyre. Have you hit any any nasty bumps/holes lately? Jack the rear up and give the wheels a spin. Might show up a problem.

[Dippydog]

Don't think I've hit anything hard,but with the state of the roads these days who knows? I'm taking all these ideas on board,but wouldn't a damaged tyre show up all the time and not just under braking? I'm grateful for all the suggetions offered and unless I drop lucky[unlikely knowing me]and find the culprit is the first thing I look at I'll certainly be looking at each suggestion.

[capt jack]

Our S70 did something similar after the front discs were replaced. The garage broke off the locating lug that holds the disc to the hub. It's the single conical pin. Everything was fine at first because the wheel bolts tighten up and hold it all tight. But once the wheel was taken off again we started to notice a thump-thump as you describe.

I carefully drilled out the remnants of the snapped pin and replaced it with a small bolt.

Possibly a bit left-field, but worth a thought as you inspect the brakes.

Jack

[SteveSarre]

Hi DippyDog

One further thought.

I had a nasty rythmic thumping when some wheel bolts were not tightened properly and came loose.
It became noticeable when I slowed down, but not sure that that was when I was using brakes, or just that I was then paying attention and the wind noise was less.

Anyway, easy to check.

HTH
Steve