How much force needed to push piston back into caliper

[ElevenFifty]

...and how do you measure it? Calipers are off the rotors. I use a couple of wedges as spacers and engage the brakes to extend the pistons. After cleaning everything, I squeeze the pistons back in with my fingers but since my hands are not calibrated, how can I be certain that the pistons are moving freely enough for normal operation?

 

DH

 

 

[dirtrider]

Afternoon ElevenFifty

 

Depends on a lot of variables.

 

Such as are the calipers hot or cold?

 

Are the calipers still hooked into the system or are they stand alone?

 

Are the I-ABS controller master cylinders completely full so fluid has to go out the vent hose?

 

As a general rule the pistons must go back SMOOTHLY with a very smooth push. If they stick or hang up in their return travel then something is not right.

 

I can't give you a force to measure as some go back in easily & others have noticeable drag.

 

 

The best test is to reassemble then ride the bike & see how much residual drag is on a free spinning wheel.

 

[ElevenFifty]

Thanks DR. The issue is that I have now warped a rotor (Grimeca) in less than 2K of riding (run out was .008). While some of the miles were spirited, there was nothing out of the ordinary. Since I had just replaced a set of warped rotors that only lasted about 30K, I've gotten paranoid that I'm the cause of the warping. So...

 

I have been checking the rotors at every stop and while the little back one will get hot as a pistol, both fronts stay from cool to warm, but never hot. If the bike is on the center stand, I can grab the brakes till the calipers lock, release them, and the wheel will spin (unless the rotor is warped) pretty freely ... 2 or 3 revolutions anyway.

 

I called MotoBins (EXCELLENT SERVICE) and they are shipping me a replacement Grimeca. In the meantime, I just had a local machine shop use their flywheel dresser and they turned the rotor flat again (still well within thickness spec).

 

I really don't believe that the caliper is dragging. I do full system flushes on a regular schedule. I just now bled off a half ounce or so of fluid at the caliper and it is a little discolored but not very bad. Come to think of it ... after my last full flush, I did have some fluid out the overflow tube because I had neglected to push the pistons all the way in ... shortly after the bleed I replaced the pads and that is when I got the overflow.

 

I am inclined to believe that the 30K I got out of the wave rotors I just retired was a reasonable life for that quality. I got nearly 80K out of the original rotors. I'm inclined to believe that this is just an anomaly - bad steel.

 

If you have any more thoughts on the subject I am an attentive listener and very grateful for your input.

 

David Hunt

[David13]

I think DR has set forth about the best response that could be set forth.

On a further note, rubber parts do not last forever. It would not hurt to rebuild the calipers.

On an unrelated note, a starter motor will not last forever.

However, a starter motor rebuild will give you, in essence, a new starter. Just one less thing that may strand you after 122,000 miles.

That's the approach I like to take to maintenance. Keep everything in good maintenance, before it malfunctions.

dc

[dirtrider]

Afternoon David

 

Rear brakes on these BMWs do tend to run hotter than the front due to reduced air flow & engine & cat heat flowing backwards.

 

If you suspect your rear is running hotter than normal make darn sure your rear master cylinder piston is returning FULLY to the stop & the pedal is adjusted properly to allow it to do so easily.

 

Also make sure there is NO chance your foot is ridding the brake pedal.

 

BUT, usually the biggest problem with the rear brake hanging is the floating caliper not moving easily on the pins, or aftermarket pads hanging up in the caliper (I have had to lightly grind the edges of a few pads).

 

[AndyS]

On the 1150's I have worked on, the Rear disc always ran warm to hot. I think the reasons covered above (Cat heat output, reduced airflow, heat generated from FD, sliders on caliper body hanging up a bit etc), but I also think the Servo is partly responsible. I found that on the ones that the iABS Servo had been removed, the rear disc ran a great deal cooler and pads lasted longer.

 

Andy