Pulsing front brakes

[cefalu]

Some bikes seem to have an issue with pulsing front brakes. My '87 K75S is one of them. All the posts I have read about fixing the problem don't seem to actually work in the long run. They say change the pads, then if that's not it, change  the rotors, then if that doesn't work, rebuild the calipers. And on and on. And the pulsing comes back.

It's driving me nuts, but I don't want to launch into a useless parts changing excersise. I did change the pads to sintered, and it helped for maybe a couple of miles. Changed the fork oil, nothing. Aside from the pulsing, the brakes have excellent stopping power and modulate braking pressure perfectly.

 Has anyone actually determined what causes the problem?

[frankenduck]

Usually warped rotors.

[cefalu]

Is a dial gauge the best way to check? Not sure how to check  if the front wheel is off the ground.

[CubPilot]

It does not take much of a warp on these things to generate a pulse.   Mount a dial indicator as close as possible to the caliper, and roll the front tire around slowly. According to the tablet I have sent down from the ffaddderrrland max run out is only .2mm for both front and rear disc.   Do not install a new or different disc with old pads! The pads will develop a ware pattern in a real short time.  It is not worth the cost of the damage that old pads do to the calipers when they start to vibrate, and the rider doesn't know it is happening until the fluid starts to run out of the caliper.

[Rick G]

I have seen wheel bearings do it but more of a vibration than pulsing, it's worth a look.

[cefalu]

OK, I finally rigged a dial gauge. It's wasn't easy because the front wheel is hard to lock in place, and when you spin the wheel it wants to pull to one side. The left rotor was pretty much dead straight/flat and the right rotor was about 0.004" total out which by my calc's is 0.1016 mm. So technically even that rotor is still within factory spec. The bearing was tight.

I found a straight/true right front rotor on fleabay for $45 that was 4.25 mm thick so I am going to swap it out.

I hope this does it.

[CubPilot]

Are you sure that what you feel is a pulse.   If you lightly rest your fingers on the front brake lever there will be a "sweet spot" that point that it is felt more than any other speed.  If you wanted to get anal about it you can rig up a pressure gauge and see it, about 1/2 to 1 lbs.   I don't know anyone that rides my bikes that is anal.  My inquiring mind just wants to SEE IT!   You could also pull the front calipers, tape or wire them out of the way, be sure to put a block between the pads and see if it goes away.

[cefalu]

It's pulsing for sure, but only when the brakes are applied, even very lightly.

It causes the front end to dive up and down in an oscillation when the front brakes are applied. The harder I apply there is a reduction in surging.

Changing the fork oil did help a bit, but I think that was because the level was low. I added more oil than came out and it reduced the travel of the surging. I was displacing air which is very compressible with oil that is virtually incompressible to reduce the fork travel.

[DRxBMW]

And on and on. And the pulsing comes back.

It's driving me nuts, but I don't want to launch into a useless parts changing exercise. I did change the pads to sintered, and it helped for maybe a couple of miles. Changed the fork oil, nothing. Aside from the pulsing, the brakes have excellent stopping power and modulate braking pressure perfectly.

Has anyone actually determined what causes the problem?

Don Eilenbeger chased this issue on his K 75 S till the end of earth several years back New pads,rotors,front wheel bearing R&R only for the gremlin to rear its ugly head again.

http://www.eilenberger.net/K75S/K75S.html

Hard deposit spots on the actual rotor were confirmed by the Penn State metallurgy lab to be the culprit. OEM stainless recipe was NOT the best metal choice for a rotor period.

Similar pulse on Pete's 93 K 75. I cured it with a pair of EBC Pro Lite brand rotors. Semi pricey but there VERY smooth ________.

http://www.bikebandit.com/ebc-pro-lite-brake-rotor?mg=4392&t=1&td=1

Harrison cast iron model is another rotor that kicks ass, especially if you add a 6 pot caliper to the stopping mix.

http://www.jhtchoppers.co.uk/page25/page25.html

http://www.billet.co.uk/index2.html?home.html~mainFrame

Bearer of bad news but a OEM replacement may NOT be your best choice.

Out of curiosity, is there any squeal present ?

FWIW, folks that practice hard front braking tend to have fewer issues !

[cefalu]

Interesting. Although Don Eilenbeger described his problem initially as pulsing, then later he described it as shuddering. Maybe it's the same issue, we'll see if changing a rotor fixes it or not. Mine is definitely not a shudder, which seems more like a faster paced movement compared to surging. Who knows:

Don's comments:

BRAKE SHUDDER AGAIN! If you've been following - this is the third time I've experienced a very unnerving brake shudder on the front end of this bike. The other times, I solved this by replacing the rotors. The solution only seemed to last about 8,000 miles this time, and that was getting WAY too expensive.
I had email chats with a number of BMW wrenches. One - Tom Cutter of the Rubber Chicken Garage - told me that it took him a while to learn this lesson - but when you have repeated incidents of this sort of shudder, replace the front wheel bearings.
 
I took Tom at his word - and Brian Curry and I replaced the front wheel bearings. The old ones felt just fine when spinning the wheel. They felt just fine when removed from the hub/wheel. They had a very slight and hard to feel notch or hard spot when rotated on the wheel by hand (finger in the center of the bearing, wheel off the bike.) In this state - the bearings are preloaded by the spacer between the two bearings. Apparently when removed with no preload - the bearings feel just fine.
 
Again - long story short - the shudder is gone. I'm still using the same rotors that gave the shudder before. Tom was absolutely correct, and I thank him for the hint. I was attacking the symptom not the cause, although the cause was a bit obscure and hard to explain.
 
So - all is well again. The bike now has ~61,075 miles on it as of today, and is getting more and more enjoyable to ride.  Nice ride today - temps in the low 50's with bright sunshine. Doesn't get any better for Dece

[HCorn]

On mine, the rotors were darker at spots 180 degrees apart. I figured they were warped, but with the dark spots it seemed like there was material on the rotor too. Since I had also seen articles about pads "welding" material to the rotor, I opted to go with aftermarket pads and rotors. I bought mine from Motobins. It has only been a few hundred miles, but so far so good.

[ned37]

i don't have a significant pulsing on my k75, but can feel a little bit.  what i do have is a "grinding" feeling when i take a sharp turn in either direction at slow speeds.  changed the wheel bearings, even tho the old ones seemed fine.  still there.  next is a set of new pads.  the old pads still have a bit of material left.  ??

[cefalu]

A little more BMW forum searching indicates the problem seems to be caused by an interaction between the pad and rotor. Following is the final conclusion reached by one owner after a lengthy troubleshooting process. This makes sense to me. Every time the disc rotates while braking, the pads hit that rusted/welded area, the rotors coefficient of friction is different. Probably higher friction, which causes a brief grabbing and resultant front suspension dive, which turns into a surging as the rotor rotates. When I mounted the dial gauge on the right rotor and spun the wheel, the surface was definitely different. On the right rotor I heard a screeching sound that was absent on the left rotor. Although the surface on both rotors was flat and smooth visually.

Don Eilenberge says:

I finally had one of the rotors analyzed at a university metallurgy lab.. they said there was a change in composition in part of the rotor that appeared to be caused by rust.. and the area that changed was shaped a lot like a K75 brake pad. That kinda jives with the experience I had of the pads periodically rusting to the rotor (stock pads, aftermarket pads, etc..) and shortly after experiencing the shudder.

The final fix was EBC rotors with EBC HH pads (and yes - EBC recommended these pads when I discussed it with them, and they worked just fine with the ABS system..) The bike now has a new owner who has had no problem with the "shudder"..

[frankenduck]

I recently tested different four pairs of rotors on a K75S.  One set pulsed, none of the others did.  Same pads used. Conclusion: rotors warped.

[Rick G]

My 10 cents worth.
When steel rusts it swells an example is that many ships that were rivited split the seams when rusr got into the seams. Mild steel will grow by a factor of 25 with rust 1mm steel = 25mm rust.  So even a small rusty spot would be a few thousanths of an inch thicker than the parent material and hence the grabing effect when the thicker spot comes to the calipers.  Food for thought.

[cefalu]

My new rotor arrived today, I will change it out tomorrow morning. We'll see how it goes.

How tough is it to remove the front wheel and get the rotor off? The Haynes manual is pretty much worthless in explaining it.

[frodef]

How tough is it to remove the front wheel and get the rotor off? The Haynes manual is pretty much worthless in explaining it.

I never took off the rotors, but removing the wheel was pretty easy. The Haynes info made a bit more sense when I actually started working on it.

[DRxBMW]

My new rotor arrived today, I will change it out tomorrow morning. We'll see how it goes.

How tough is it to remove the front wheel and get the rotor off? The Haynes manual is pretty much worthless in explaining it.

EASY beans, remove front wheel, remove rotor bolts,install new rotors, re-bolt & torque.  There is NO blue loctite unlike the rear wheel disc !

15 minute job ________________________.

[cefalu]

OK, right front rotor replaced, and VICTORY, perfect brakes. That is aside from my rears which seem to lock up pretty easily. I just need to learn to modulate them better under hard braking.

 And yes, the Haynes manual although lacking in pictures for this process, does adequately describe it.

While I was in there, I also replaced my centerstand with a K1100 stand. perfect bolt on replacement.

The hardest part is removing and replacing the fiberglass belly pan to get the front wheel off the ground!

[ocmsrzr]

Sorry for digging up an old thread.


I managed to address my front brake pulsing issue without replacing the rotors (too much $ for me) and I want to share in a search engine optimized place.


A little bit of history: I agonized over the pulsing front brakes for years. They were lightly pulsing when I bought the bike. Didn't have a workshop so had a independent bmw moto shop look at it. Determined that the front wheel was warped. Found a used one, put it on, new wheel bearing, problem solved for a few hundred miles then came back. This is consistent with reports i've since found.


Lived with it. Learned that settling the chassis with the rear brake helped greatly. This should have been a clue that suspension was part of problem.


Did internet research.


Course of action: 1 replace front brake pads  with EBC FA18HH sintered brake pads. The idea being that the harsher pad material will help to in effect machine either deposits or warping from the rotors. Got the idea HERE.  Make sure to thoroughly clean your calipers and ensure that your pistons are not sticking. One of my piston dust seals was not properly seated so after fiddling with it I decided to remove all 4 and clean around pistons with brake cleaner and toothbrush. Use brake lubricant to lube back side of pads and pin. Also, inspect old brake pads to make sure of even wear. 2. Shade tree rotor resurfacing: removed the rotors from the wheels. Marked L/R and bolt registration. Took 120 grit garnet (this is important) sand paper and block sanded the rotors on both sides. Spent about 10 minutes on each side using a good amount of body weight. Also used the block to clean the mounting surfaces. They were very gummed up. Try to be super consistent (flat) here because you are removing material and if the rotors don't match up to the mounting surface -> pads you are in a world of hurt. 3. Mount rotors out of phase from original position. Put the left rotor on the right and the right on the left after insuring that part numbers are the same (mine are). Rotate one 90 degrees forwards on the bolts and the other 90 degrees aft. Torque with new lock nuts. 4. With the wheel off check steering head pre-load. Squatting in front of bike grasp both forks and push fore and aft. Mine had a lot of play. I had checked this earlier with the wheel on and not touching the ground but I failed to sense it. Loosened appropriate bolts and used a cold chisel to move knurled adjustment wheel.  That's about it. Put everything back together properly.


Outcome: Immediate improvement. I've since run the bike about 1500 miles and i'm still in good shape. Occasionally, after sitting overnight at least a light pulse will come back. This leads me to believe that the metallurgy hypothesis is correct. The first stop from a speed of a t least 40 mph seems to cure the pulsing. I will also occasionally drag the brakes when safety allows. I'm happy with this outcome.


I hope this helps someone address this problem.

[lmiklosy]

When removing rotors for any reason, remount them in the same location (phase previously) with the wheel, that means align to the same bolt holes, 12 o'clock with 12 o'clock.  The rotors may be ground perfectly flat but the hubs they mount to are not. If the rotors are not clocked to the same orientation with the wheel, the breaking surface will shift laterally inducing a wobble. The pulsing you feel when applying the brake is from this wobble and shows up unexpectedly if the rotors were removed and re-fitted without paying attention to phase.  :eek:   There is a very small hole drilled through the disk in one location, the factory may use that hole for a phase reference, use that hole for realignment to the wheel or make your own.

[Motorhobo]

My pulsing problem was 99% fixed by this simple advice from in the thread below:

Also, follow the torque pattern, but not all at once; rather,  increase torque values two to three times, i.e. Pattern to initial contact and set, then 25% of rated torque, then 75%, and then final torque.

I'd recommend trying this first before moving on to other, non-installation-related issues.

[lmiklosy]

Yer darn tootin' it's torque pattern. Don't come near these bikes without a good torque wrench and pattern in hand. The older I get the more I appreciate friendly reminders from family (like Lefty-loosie and Righty-tighty), so thanks to Motorhobo and jay1622 for reminding us about torque step and pattern.  :clap:

[Filmcamera]

I had installed new EBC discs and pads a couple of months ago and was having an issue with the front wheel binding and a little bit of pulsing under braking.  I did various things to correct this including a fork alignment and having the wheel straightened and balanced.  At the same time the wheel was painted.  When I got it back from the painting the brake pulsing was much worse.  So I took off the rotor and stipped all the paint and old powdercoat etc on the disc mounting surface to bare metal and remounted the discs using a careful torque pattern.  I even changed the front axle since the one that was on the bike had at some point been beaten HARD with a hammer and was distorted leading me to think maybe the fork was not quite able to settle in the right place. All that helped for a bit but now the pulsing is back worse than ever.  Since both the pads and the discs are virtually new I do not think the phase is an issue but have run out of ideas as what to try next.  Any suggestions would be welcome.

[The Mighty Gryphon]

Have you checked the rotors for runout?  How uniform is the thickness