brakes grabbing

[cwillson]

I am cross posting this from BMW Luxury Touring forum in hopes of getting a wider audience:

 

2007 R1200RT - after 12,000 KM I continue to love this bike. However, it has developed one irritant that continues to baffle both my dealer and me. (Note, I have no dissatisfaction with my dealer - they have to work with the parameters provided by their masters at BMW Canada)

 

At 10,000 KM (6,200 miles) the brakes have developed a 'grab, release, grab, release etc.' at slow speed. A gentle application of the front brake control and it is barely noticeable. Gentle application of rear brake control and it is very noticeable. At very slow speed maintained by throttle and rear brake, there is a clear 'grab, release, grab, release'. This is making travel around town or slow speed maneuvers a bit awkward. This all developed at the end of a 5,000 KM ride, much of which was in very very heavy rain.

 

My dealer has done run-out test on rotors, check function of ABS, check calipers/pad and checked ABS error codes. All is fine and within spec. There is no pulsing in either brake control. Hard braking on dry pavement or sand, confirms that ABS is working as intended. With bike on center stand and 1st gear engaged and application of the brakes shows a clear 'grab release grab release etc".

 

Dealer and I considered that my rear tire was quite worn and front was badly cupped and decided to wait for tire replacement and this was done yesterday. Nothing has changed.

 

I again emphasize that my dealer has not abandoned me. They are committed to helping. I am just curious if anyone else has encountered this type of problem or has any suggestions. My dealer is in another province, so I would like to be as informed as possible before I visit.

 

Thanks

Craig

[motoguy128]

I wonder if there is a problem with a pressure sensor or valve that proportions the brake force front to rear.

 

Is the grab and realease constant in frequency, or does it vary at all with wheel speed?

 

Sometimes with computers, garbage in-arbage out, so if it's getting bad data from a sensor, it will give erratic outputs.

 

This is often a problem at my work. The electricians want ot blame the programming, but it's almost always a problem with an instrument.

 

[lvmyks]

My 08RT too has a rear brake funny feel. When aplying front brake and lightly pressing rear pedal. The rear pedal start (pressure) then drops a few 1/16 and then there is pressure until I stop the bike. Not all the time, but your foot feels something strange. I had my dealer check it at 600 miles and said the bike didn't do it. I have tried different senerio's ( front level "hard" rear pedal "only") Will watch for more reply's and hope my dealer can feel it.

[cwillson]

I wonder if there is a problem with a pressure sensor or valve that proportions the brake force front to rear.

 

Is the grab and realease constant in frequency, or does it vary at all with wheel speed?

 

Sometimes with computers, garbage in-arbage out, so if it's getting bad data from a sensor, it will give erratic outputs.

 

This is often a problem at my work. The electricians want ot blame the programming, but it's almost always a problem with an instrument.

 

It is only apparent at slow speeds, but is constant. Hard braking at from a higher speed is fine (ABS kicks in perfectly)- it is only when approaching a stop and rear brake applied that is is a problem. Gentle application of front brake and no issues, but more pressure on front control (presuming rear brake is now getting engaged due to integrated brakes) and it returns.

 

Note earlier comment that with bike on center stand, motor running at 1800 rpm, 1st gear engaged - apply just rear brake and it is very obvious by watching wheel and listening, that there is a 'grab. release etc'. This sure points to a rotor to me, but I am no mechanic. (...and thanks)

Craig

[FLTRI]

Sure sounds like a warped rotor to me, especially considering that it occurred after a wet ride. Hot rotor + water = bent metal...

 

Could be a glazed spot on the rotor, too. Do you see and discoloration in the surface of the rotor, or feel any???

[Draig]

Sounds eerily familiar. My 07 RT does this with the front brake. I brought this to the attention of my dealer, they found no problem. Still, it's irritating at low speeds when I'm coming to a stop as I can't tell just when the bike will come to a complete stop.

[Don_Eilenberger]

Actually - sounds awfully familiar to me - I went through 6 sets of rotors on a K75S (front rotors) for the same sort of problem. Finally had one set analyzed at a university metrology lab.. they found a change in the metal of the rotor at one spot where the brake pad had been sitting on it wet (and rusted a bit..) You couldn't see it by eyeball - it ended up needing some rather complex research to find it. The hypothesis they came up with was rust pitting was causing a difference in surface friction on the one spot on the rotors (and it was on both front rotors - the same spot relative to each other.)

 

I finally went to EBC rotors and EBC pads on the bike and that seemed to be the end of it. EBC rotors are not stainless - they are a high carbon content steel. Surprisingly they don't seem to rust much. Current owner of the bike (friend) has not experienced any problems. This is VERY common on the K75S for some reason, less common on any other K model. Dunno why.

 

Long story short - a new rotor should fix it.

[Draig]

That seems to make sense. I notice some discoloration on the front rotors a day or so after putting the bike up after a good wash. If it is due to surface pitting, then the grabbing could go away with normal use, assuming I don't repeat the process that led to the pitting in the first place. I'll try riding the bike after the next wash to dry out the pads and see if normal wear returns even braking to the front.

[cwillson]

Just an update - I had a local bike mechanic take the bike for a ride and his opinion matches Don's and others - rotor. I have now asked my dealer to refer to BMW Canada for consideration - and they have done so. I await a response.

 

Again, I want to say that I have no problem with our dealer in Atlantic Canada. They have been professional and supportive.

 

Craig

[bobbybob]

My 08RT too has a rear brake funny feel. When aplying front brake and lightly pressing rear pedal. The rear pedal start (pressure) then drops a few 1/16 and then there is pressure until I stop the bike. Not all the time, but your foot feels something strange. I had my dealer check it at 600 miles and said the bike didn't do it. I have tried different senerio's ( front level "hard" rear pedal "only") Will watch for more reply's and hope my dealer can feel it.

 

My 07 RT also does the exact same thing. Not all the time, but some times. I have decided it is a quirk of the linked system and am not worrying about it.

[Don_Eilenberger]

That seems to make sense. I notice some discoloration on the front rotors a day or so after putting the bike up after a good wash. If it is due to surface pitting, then the grabbing could go away with normal use, assuming I don't repeat the process that led to the pitting in the first place. I'll try riding the bike after the next wash to dry out the pads and see if normal wear returns even braking to the front.

Oh - that was the 2nd point of the analysis I had done - the "surface pitting" - isn't limited to the surface.

 

The rusting caused rather deep pits, that go well below any wear surface on the face of the rotor.. meaning that normal braking isn't gonna make the rotor a good one again. I tried fixing this on several rotors.. using very agressive pads (didn't work), using a sanding disk on the surface (didn't work) and having them surface ground (didn't work - but for another reason, next to impossible to maintain parallelism on something that size without some very special equiqment.) Fix was - new rotors. Got to be a bummer after the 5th set..