Lumpy front brake - would this have caused it?

[markyp]

Travelling home yesterday through slow town traffic, the front brake was very lumpy and wasn't smooth at all. Worried enough to get off and check everything was still attached. At home later I discoverd that one of the holes in the ABS ring was covered with a tiny piece of paper that had stuck on. I removed it and I think all is fine now (or could have imagined the whole thing.) Same I didn't try it by switching the ABS off.

Anyway my theory is that the paper fooled the ABS sensor - my question is "What happens when the ABS kicks in" - on a car you get very harsh juddering but none of that yeserday - just poor braking "feel". Does my theory seem sound.

Oh, bike is an 04 Adventure.

Thanks in advance for any advice.

Cheers, M

 

[Timolgra]

Try riding down a gravelly track, wack the brakes on and you will find out what happens yourself, much better than reading about it.






Although your results may make interesting reading for the rest of us

Tim

 

[Ian Whinnett]

No

ABS sensors work by the holes passing the end of the sensor, thereby breaking a magnetic field in he sensor. The result is a small pulse of voltage / current in the sensor. the faster the wheel rotates the more frequent the pulses. When the pulses almost stop (ie the wheel has almost stopped) and there is braking happening the ABS removes brake force to that wheel. As soon as rotation is detected the brake force is re applied. Thus keeping the tyre from skidding and yet maintaining the tyre at the point where it is about to skid. Because this is when the most braking force can be appliedwithout loosing control.

The piece of paper would have no effect as the magnetism is no affected by non ferrous objects.

I have an 1100 with ABS and when it kicks in it is blindingly obvious that it has done so, it often seems to happen when braking hard for a junction and then riding over a wet manhole cover or somesuch.

The Lumpyness is most likely to be a disk defect only obvious when hot. The BMW OE disks are not the best. If you get happening again try and feel if the lever is pulsing in your hand. Also the suspension will pulse up and down slightly under gentle braking.

I have just changed my disks as they were lumpy and gave all above symptoms.

 

[shready]

BMW OE Disks

Ian,

You say that BMW OE disks are not the best and you have replaced yours.
Which ones do you recommend?
Mine need replacing before the next MOT.

Dealer quoted me over £400 just for the parts!! Shocking

 

[Rabbitson]

Mark,

I agree with Tim, just wack the brakes on and feel the ABS kick in for yourself.

Perhaps wouldn't recommend a gravelly path as a starting point but if you brake very hard on dry tarmac you'll feel the ABS kick in anyway (actually in this scenario you'll find that you can brake better without the ABS anyway )

 

[markyp]

Thanks all- I will take it out and get the disks really hot through some mad trashing and braking and see whether that deteriorates performance. When I rotate the wheel when cold on centrestand there is some rubbing with what appears to be a single "tightspot" in the rotation. I'll try this as well when the disk is hot and see if there is more lumpiness then.

Cheers, M

 

[markyp]

disks scored

okay taken another look (and in different light). Both front disks scored with a single line halfway through its width - picture attached. Any idea what could have caused this? Nearside disk the worst but other disk scored too.

ps bike has done 2000 miles from new.

 

[Steptoe]

That is bad for 2K miles - impossible to tell from here whats caused it. But that ain't normal wear. Even with london driving.