car pulsates/hops when braking?

Strider590

Prothet_of_Doom wrote: »

Really? I've noticed if I put a dial test Indicator on my discs and rotate it they are like 7 inch singles left out in the sun. If I replace them with new discs the DTI barely moves and there's no vibration or grabbing.

That's simply uneven disc wear, the concept of disc "warping" is that they are heated and cooled unevenly, which causes them to "warp".

ie. By hard braking followed by sitting at a junction with the brakes on, or by leaving the car on the handbrake after driving the car hard enough to heat up the brakes/discs, or by driving through flood water.

Uneven wear is a different kettle of fish, but by the time that happened you'd have to be near to replacing them anyway.

Car_Complaints

Strider590 wrote: »

That's simply uneven disc wear, the concept of disc "warping" is that they are heated and cooled unevenly, which causes them to "warp".

ie. By hard braking followed by sitting at a junction with the brakes on, or by leaving the car on the handbrake after driving the car hard enough to heat up the brakes/discs, or by driving through flood water.

Uneven wear is a different kettle of fish, but by the time that happened you'd have to be near to replacing them anyway.

You post a piece about "Warped brake discs are a myth" then give examples of how warping can be caused...I thought you said it was a myth

To be honest I didnt read the story, Warped discs are just terminology used to describe discs with run out, easy to say to the customer instead of "your discs have run out" we all know what clever customer response is coming back.

Richard53

attila_ wrote: »

The sensation is like there is an oil patch on half of the disc and the other half is fine (there are no oil patches by the way). I changed new discs and pads last year and this only happened few weeks ago.

If the discs and pads check out OK, and there is no ABS fault logged, I would suspect the discs (or one of them) are contaminated with pad material. Over time, the pad material can pick up on the disc and cause roughness/harshness in the braking, and if it is unevenly distributed it might cause the on/off feeling that you describe.

Get a Scotchbrite pad and some brake cleaner, take the wheels off and give the discs a very thorough cleaning. Both sides. Total cost about a tenner, and a couple of hours of your time. Ideally you would fit new pads at the same time, but if they have life in them and are not contaminated or unevenly worn, you could put them back.

If I have the time, I do this every time I change the brake pads on one of the bikes. It can make a big difference, both to smoothness and a predictable, consistent feel at the lever.

Strider590

Car_Complaints wrote: »

You post a piece about "Warped brake discs are a myth" then give examples of how warping can be caused...I thought you said it was a myth

*facepalm*

I gave examples of things that started and still perpetuate that myth.......

attila_

Thanks for all the replies. I'm going to have a look at it this weekend, run a scan for any fault codes on abs too.

The brakes are no different in effectiveness than before at speed. Just at low speeds i get the pulsating feel.

In traffic after a long run i do hear a rhythmic squeal from the brakes if i half press the pedal. As if there is a lump, but then again this still could be abs just coming on and off...

...however (again once i've driven for a while) sometimes when i steer right and do not brake i can hear the brakes squealing rhythmically (however no binding sensation i.e car rolls freely!)


Its really doing my head it as it doesn't make sense.

Richard53

attila_ wrote: »

The brakes are no different in effectiveness than before at speed. Just at low speeds i get the pulsating feel.

If there is a fault with either discs or pads, it's often only at low speeds that you feel it. At higher speeds, there's too much going on for the friction to get a purchase, if you know what I mean. That's a lousy and unscientific explanation, but it's the end of a long shift. I know what I mean.

attila_ wrote: »

In traffic after a long run i do hear a rhythmic squeal from the brakes if i half press the pedal. As if there is a lump, but then again this still could be abs just coming on and off...

If you are half-pressing the pedal, it isn't the ABS working. Light pressure on the brakes often makes a squeal, and it's rarely anything wrong. Might need new anti-squeal shims, or possibly just a dab of copper grease on the back of the pads. A *dab*, and on the *back*, for clarity.

attila_ wrote: »

...however (again once i've driven for a while) sometimes when i steer right and do not brake i can hear the brakes squealing rhythmically (however no binding sensation i.e car rolls freely!)

That could be almost anything. I had a Discovery that used to squeal like a stuck pig every time I turned the wheel to the left. Turned out to be a tiny piece of gravel wedged between the disc and the mudshield. For some reason, the shield was distorting slightly when the steering moved, and brought the gravel into contact with the disc. Easy fix.

Strider590

Some cars (like for example Vauxhalls) really don't like having after market, non OEM pads fitted. My Vectra had these and every now and then they'd move slightly and cause an awful screechy noise with vibrations.

It could be that you or someone else has had cheap pads fitted and they're just not seating properly.

attila_

I've fitted them! Bought from ECP (Pagid i think i recall). Have been doing for years and on my other car.

mrmechanic

Check the reluctor rings on the front hubs for splits.
These split when the hub rusts and expands.
The air gap between the teeth increases at the split and at slow speed, the abs speed sensor senses the bigger gap, sees it as a brake lock and pulses the brake.
This usually only occurs just before you come to a halt, from around 10mph down.
Most likely cause.