ABS Sensor clearance

[rmarkr]

I recently replaced a failed front wheel bearing on the K100. (First wheel bearing failure I've ever had on a bike) Now I get some odd behavior from the brakes at very low speeds - just before coming to a complete stop they seem to let go and then grab again. Could this be the clearance adjusment between the sensor and the toothed ring? What would happen if the clearance was too small or too big?
:dunno

[deilenberger]

Mark - this doesn't sound like ABS symptoms. If the ABS was triggering it's usually rather obvious - and noisy. It sounds more as if one of the brake rotors isn't running true with the wheel.

Were the rotors removed to replace the bearing?

Could the wheel have been rested on a rotor while the work was being done?

It's also possible that a contaminated rotor could cause this sort of shuddering (grease on the rotor on one spot..)

[rmarkr]

Hi Don
Yes, I repainted the rims - so the discs, bearings and tires were removed. The tires were refitted, by me, with the discs off, so I doubt that they could have been damaged. I cleaned them well with degreaser, fitted new mounting bolts. The wheel bearings were fitted to the naked rim as well.
The symtom was more noticeable after a 100 mile ride, which should have shed any contaminates. My initial thoughts were that the new bearings may have "settled" and altered the (side mounted ring) clearance.
I'll check the clearance again, but I'm kind of curious as to what tolerance will be tolerated.
Thanks for the feedback
;)

[deilenberger]

Hi Mark,

The reason I don't think it's an ABS sensor clearance problem is - even if the sensor is too close or too far from the ABS ring - this won't cause the ABS to pulse.. only a wheel locking will do that, and that's not what you're describing.

Unfortunately - it sounds like a frictional variation - which usually means a contaminated brake rotor, or a bent one. If you want to check for run-out, a technique I found works is to place a bright light on one side of the caliper next to the rotor (back side works for me) - then sight along the rotor trying to see the light in the gap between the pad and the rotor. Slowly turn the wheel. If there is run-out - the brightness of the light coming through the gap changes quite visibly. You can spot runout of less than 0.001" using this technique... which is almost impossible to measure.

I assume when you reinstalled the rotors you torqued the mounting bolts in a cross-pattern working up to the final tightening torque?

FWIW - I had this problem on my '87 K75S - and went through 4 sets of rotors in 3 years on the bike. Plus multiple wheel-bearing replacements. Finally put EBC rotors on it - and it seemed OK when I sold it (and still does according to the new owner..) - but it was a very frustrating problem with no obvious reason for the problem. Sometimes new bearings fixed it - sometimes not. Sometimes new rotors fixed it - sometimes not. Frustrating..

[sgborgstrom]

The original description is similar to how I would describe the feedback I was getting from my front brake before I replaced the little "buttons" that hold the rotor and carrier together.

[rmarkr]

Thanks Steve and Don
I do have the floating discs with the "buttons". I assumed that the flotation would allow for some warpage (maybe when hot?) and a bit of throw-out. Is that that the reason, or is it to equalize pad pressure? I've always wondered what really happens to a (rigid) brake disk when it is hot around the circumference and cool at the hub.
I balance my own wheels and will usually give the brake disc a good eyeball as the wheel rotates; dont recall noticing anything odd. I was careful to tighten up the disc carrier progressively in a criss-cross pattern. I'll try the light behind the caliper trick tonite, if that yeilds nothing, I'll replace the buttons - thanks Steve
The brakes seem fine in normal use, just this odd behavior as I come to a stop.
The spline lube has made the bike shift like butter - what a pleasure
:blah

[rmarkr]

Well, it seems that it was the clearance at the ABS sensor - it was way off - 1.2mm, should be .35 to .55. I removed 2 shims to get it down to about .45 and took it for a test ride, and the symtom seems to have disappeared.

One concern though, the front wheel does not seem to be centered in the forks. The lateral position is controlled by the the larger (double row) wheel bearing which is retained in its recess by a circlip. The offset is quite visible, I guess about 2mm off center. The axle spacers must be correct in order to get the ABS sensor to such a close tolerance. The axle is not bent, checked it by turning it in the forks and feeling for any movement between the tire and the fork leg. Also, I dont think the forks could be bent - this bike has never been crashed, and they look good.
Could it be setup like this?

:dunno

[deilenberger]

Mark,

I can only theorize that the new bearing you installed is somehow different from the old one.. either in thickness or how deep it's installed in the rim. When I did bearings on late model 3-spoke K75 rims (which used one odd bearing that was a semi-floating center design) - I needed considerable heating of the rim, combined with freezing the bearing AND some convincing with a socket/hammer combo to get the bearings to seat to the correct position in the rim. What you're describing as the "double-row" bearing - is the one where the center of the bearing can move axially (side-to-side). (It's a very interesting bearing - I looked up the SKF specifications on it.. neat design..)

Looking at: http://www.realoem.com/bmw/showparts.do?model=0573&mospid=47906&btnr=36_0349&hg=36&fg=10 - PN#3 is the one where the center can off-set a bit. That's supposed to take up the slack so the spacer PN#2 is firmly gripped between the two bearings. The only way I can think of that the positioning changed is if the thickness (side to side) of the center of that bearing was different from the original one you replaced.

Puzzled in NJ..

[deilenberger]

Oh - Mark, not to be insulting or stupid..

You did follow the correct tightening procedure for the front wheel right?

http://www.realoem.com/bmw/showparts.do?model=0533&mospid=47924&btnr=36_0172&hg=36&fg=10

With all clamps loose - you tighten up PN#10 first. Then you tighten the slider axle clamp on that side. Next you take bike off centerstand and bounce up and down the front end to center the fork tubes/sliders. Finally you tighten the slider clamp on the right side.

??

[rmarkr]

Oh - Mark, not to be insulting or stupid..

You did follow the correct tightening procedure for the front wheel right?

[??

Hi Don
Yes, I was carefull to make sure that the fork legs were correctly spaced. Went through the sequence, then backed things off a bit and rotated the axle to see if the (right) fork leg would move laterally (with forks extended). The axle goes in very smoothly so I dont think anything is getting distorted.
Mulling over the issue of the wheel being off-center - if the wheel was brought into the center the ABS toothed ring would be right up against the flat machined on the side of the fork tube. Could this be an unique to ABS equipped bikes? It sure has me baffled!

:confused:

[rmarkr]

Don
Apologies, I missed your first reply regarding the bearings. I certainly wasnt aware that the larger bearing had some float on the inner race. Makes sense - it will take the side load off the bearings when the axle is clamped up.
I concur with the difficulty of getting the bearings in and out. I had to dismantle the smaller bearing in place - rip out the ball cage, shuttle all the balls to one side which released the inner race. Once I got there I could drift the larger bearing out, and then the remains of the smaller bearing. Lots of boiling water and freezing of bearings went on.
Thanks Don

[oilhead110]

I recently replaced a failed front wheel bearing on the K100. (First wheel bearing failure I've ever had on a bike) Now I get some odd behavior from the brakes at very low speeds - just before coming to a complete stop they seem to let go and then grab again. Could this be the clearance adjusment between the sensor and the toothed ring? What would happen if the clearance was too small or too big?
:dunno

Yes! this is possibly the ABS sensor clearance, I had the same issue with my 1991 K100rs. As I would roll to a stop and just befre stopping it felt like the ABS pump would engage (Front Brake). I has just replaced the wheel bearinga and had also has the wheels powder coated. When I measured the Front wheel sensor gap it was way out. I corrected it and the problem went away.