Pulsing front brakes

[john1691]

OK, I bought a salvaged bike, which, I understand is a questionable thing to do, :scratch but...........Anyway got it all back together, runs pretty good (seems to "throb" at low RPM's, but that's another thread) and under heavy braking, the front brakes pulse something terrible. It is a non ABS bike, 4000 miles on the clock. Feels like cavitation that could upset the bike if continued too long. The rotors are true, the pads look good, fluid level is correct, what am I missing? I did have the rotors off to have the wheels powder coated. Would putting them on backwards or on the other side of the rim make a differance?

I'm stumped. :banghead

[35634]

What model bike and why was it salvage-titled?

[deilenberger]

The rotors are true, the pads look good, fluid level is correct, what am I missing? I did have the rotors off to have the wheels powder coated. Would putting them on backwards or on the other side of the rim make a differance?

I'm stumped. :banghead
Are the mounting points on the rim for the rotors clear of all paint - bare metal? Paint (especially powder coating) can be uneven enough to cause the rotors to turn out of true.

[john1691]

What model bike and why was it salvage-titled?

Sorry, 2005 K1200S, was backed into while parked in a driveway. Knocked it over, brioke plastics on both sides, broke the plastic clutch fliud resevoir at the bars, broke the left switch gear, left mirror and bent the left bar.

The powder coater had put something (tape?) on the attatchment points for the rotors, and I did check them, looked to be free of anything. I did not lay a straight edge across them to verify they were flat, maybe that is my next step.

[MotorradMike]

Hi John:

How did you verify the rotors were true? It takes very little warpage to cause pulsing brakes.

Any chance you can borrow a known good front wheel?

My bet is that new pads and rotors will fix this. Hopefully you saved enough on the bike to pay for them.

[mieczkow]

I went through this on a K75S that had really bad pulsation on on the front brakes. Here's what I discovered in remedying the problem:

1. Unless the brake rotors were mechanically bent, either by the collision or something else of considerable force, spontaneous warping the rotors is very unlikely. My metalurigcal friends tell me that rotors would have to get white-hot in order to experience heat-induced warping and when rotors are warped it is almost always due to either a misalignment on the carrier or the rotor/carrier combination being mechanically bent.

2. Surface changes to the rotor can happen from a variety of things and no matter what you do, short of a major grinding of the surfaces, nothing will clean them. As far as regrinding, it was not possible for me in the Tampa Bay area to find a shop which was willing to surface grind the rotors - no one wnated to bother with all the set-up required to accomodate the rotors - one guy quoted so much that I could buy new rotors for the front and rear just for the price of the set-up charge alone. Also I decided I did not want a guy grinding the rotors who had never doe a set before.

3. What fixed everything for me was putting on new rotors, new pads, and while I was at it, a rebuild of the calipers. Brakes worked like a brand new bike and 8k miles later continue to perform flawlessly.

This was all based on a K75S, so I can't say how much this translates into your more modern bike, but that ws my experience with brakes shuddering so bad it was scary. The whole process cost me about $300 in parts - I used EBC rotors and pads and I bought the stock caliper rebuild kits from Max BMW.
:beer

[odsmissmaggie]

I had my 2007 K1200 GT in for front brake pulse at slow speed and BMW replaced the wheel , rotor and upgraded the mounting bolts where the rotors attach to the wheel. There is a bulletin out on it my tech stated.....Now perfect!

[rbryson]

I too have pulsating front brakes (on my '76 R90/6) that seem to be getting worse. What exactly is the "acid test" for determining if the rotors are warped? I have new pads and adjusted the calipers without much change in the pulsing. It is much worse at high speed...hardly any below 20MPH. :dunno

[john1691]

Well, this may not be , but it seems if you put the 2 rotors together, they should touch all the way around. Now flip one over, do they still meet? Reverse the first one back to the original position, and flip the second one. If one or both aren't true, gaps will show between them. I guess you could sight them with a lazer.......

Either way, I will be taking the K1200RS instead of the K1200S to the Rally in the Poconos this weekend, as the forecast is for rain, and the K12RS has ABS, the "S" doesn't. I'll have to work on the brake issue later, as I need to fix the heated grips on the RS now.........

Thanks for the suggestions all, I'll update when I have them working properly.

[k75sprint]

Has the bike been sitting long prior to you bringing it back to life? I'm not sure this applies to your bike, but since you have a non-abs version, check the front disc bungs. If your bike has them, they should be free to spin and float freely. If you have one or two that are sticking, that will be enough to keep the disc from spinning true while under braking. Also, it takes very little warpage to make the bike shake bad under braking. If you have access to a machine shop, have them measure them for trueness. Lastly, I've had many a warped rotor on a few different bikes over the years. I can't recall ever having one get white hot before warping.