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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: cefalu on September 22, 2011, 11:14:45 AM
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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?
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Usually warped rotors.
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Is a dial gauge the best way to check? Not sure how to check if the front wheel is off the ground.
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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.
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I have seen wheel bearings do it but more of a vibration than pulsing, it's worth a look.
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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.
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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.
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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.
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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 (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 (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.jhtchoppers.co.uk/page25/page25.html)
http://www.billet.co.uk/index2.html?home.html~mainFrame (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 !
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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
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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.
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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. ??
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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"..
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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.
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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.
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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.
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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.
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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 ________________________.
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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!
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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. (http://www.gunsmoke.com/motorcycling/k75s/k75s_improvements.html) 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.
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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.
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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.
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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:
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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.
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Have you checked the rotors for runout? How uniform is the thickness
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I have not checked them no, but they have only maybe 500 miles on them so are still virtually new. I will check them later and see.
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my left front rotor is warped and my brakes pulse. anyone have a good one they want to sell? There are a few on fleabay, but I don't want to get stuck with another warped one.
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If you are heavy handed and tend to belt things with a sledge and have no patience read no further. As per Gryph remove the brake calipers and support them. Then jack up the front so the wheel spins clear of the ground. Set up a dial gauge indicator to run on the outer rim of disc on a smooth consistent track. You will have to manufacture brackets to mount the DGI off the caliper bolt.
Rotate the wheel 360 deg and mark the high or low spot with a suitable marker. Once you have found the spot you need to carefully correct it by tapping out the carrier not the disc. I use a 10" piece of broom handle and a nylon hammer to gently tap the carrier in or out to correct the warp. Use only light taps and check with the DGI after each tap, you will have to remove the DGI every time you adjust the carrier.
You don't want to be belting the carrier to and fro, you will just make things worse. You need to sneak up on it taping gently eventually you get a feel for it you may need to tap once or twice in the opposite direction if you overdo it. but this should be avoided if possible. This is a slow process but I have successfully straightened out minor warps on bikes and Karts. I learnt from a mate who was a master at staightening damaged Karts trackside.
Regards Martin.
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Having done everything I could think of to fix the brake pulsing issues I finally decided to put the old discs back on and see what happened. All pulsing gone! So I guess that means the new discs are either slightly twisted or that the bobbins are just so tight they do not let the disc float enough. Either way I am going to send the new discs back to EBC who I have been in touch with via email and see what they say.
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Did you check them for run out? :dunno
Regards Martin.
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I do not have the tools to do that but in my desperation I had taken the bike to a certified BMW mechanic and they did and said they were out, though they didn't give me exact measurements.
I actually had a new set of bobbins which I fitted to my old discs so now there is not much movement in the old discs - maybe 0.25mm in either direction but it is enough it seems.
I was thinking about taking out the bobbins on the EBC discs and using the kit but they are not the replaceable type so it would mean cutting them off. I would rather try the warranty route before going to that kind of extreme.
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A DGI can be had from $10.00 up. The clamps to attach it too the brake caliper mounting bolt can be made up out of anything that is handy. A magnetic base with adjustable arms can be had for around $20.00. The magnetic base will not work but the arms will save you some manufacturing.
Regards Martin.
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Something like this then?
DGI (https://www.amazon.com/Grizzly-G9849-Magnetic-Indicator-Combo/dp/B0000DD0VA/ref=sr_1_4?ie=UTF8&qid=1525811561&sr=8-4&keywords=dial+gauge+indicator)
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:2thumbup: However you will still have to rig something so you can attach it to the caliper mount. You need to get the indicator to run on a smooth track on the outer most edge of the disc. It is not rocket science but it can be a bit fiddly. It gets easier with practice, as do most things.
Regards Martin.
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Having done everything I could think of to fix the brake pulsing issues I finally decided to put the old discs back on and see what happened. All pulsing gone! So I guess that means the new discs are either slightly twisted or that the bobbins are just so tight they do not let the disc float enough. Either way I am going to send the new discs back to EBC who I have been in touch with via email and see what they say.
when you say disks, I assume you are referring to the rotors? are the bobbins the pads?
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when you say disks, I assume you are referring to the rotors? are the bobbins the pads?
Disks, discs = rotors bobbins = the roller assemblies allowing floating disks to float.
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I heard back from EBC today...
Great news - they are sending me two new discs/rotors under guarantee and a set of pads just because.
:clap: :clap: :clap: :clap: :riding: :riding: :riding:
Once i get them I will install them and see how they feel - I have a plan B to take the first pair I brought and drill out the bobbins and use the new set I have from Motobins to see if that works better.
Watch this space.
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my left front rotor is warped and my brakes pulse. anyone have a good one they want to sell? There are a few on fleabay, but I don't want to get stuck with another warped one.
well, replacing the left rotor didn't help. bucks like a bronco as you come to a stop. since I got a used left and right when I bought the replacement, I thought , what the heck, I'll replace the right one too. problem solved, it was the right that was warped. this was from a non abs bike, so now I've got the blinking light until I can source a rotor with a ring.
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I got and installed the new rotors that I was sent by EBC - pulsing gone - great news. The bad news is that they bind o muh I cannot spin the front wheel more than a quarter of a turn. Right now I am being an ostrich and ignoring the issue in the hope that as the new pads wear a bit the problem fixes itself...
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FC you need to fix it, binding discs will cause hot spots, that could cause warping. Then you will be back to square one. :musicboohoo:
Regards Martin.
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I would love to fix it but have no idea how to, I have done everything I can think of and more.. actually the only thing I have not done is have the pads made thinner by a couple of mm to give more clearance but I am loathe to do that
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Are you sure the pads are retracting properly after activation? Can you push the pads away from the discs or are they hard up against the calipers?
The seals in the calipers are designed so that after being activated they retract the pistons back into the caliper. If the pistons are retracting and every thing is free there should be a minute gap between the pads and disc and not between the pad and the piston. I know this might cause you pain but remove the calipers and see if the pads are free to slide in and out. I smear a very light coating of copper grease on my locating pins. Are the pins clean and free of pits marks corrosion? Are the pins free of bends? The pads must move smoothly in and out on the pin, check the holes in the pads to see that they are smooth. Alternatively ride over to my place and I will have a look. :dunno Remember the truth is out there.
Regards Martin.
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The pads retract but only so far, they remain just touching the discs. The inside pad in particular is pretty hard up against the disc. After pushing the pads all the way out there is room for the wheel to rotate freely so it is a retraction issue for sure. I just do not know how to fix it. So far I have replaced the master cylinder, the pistons and the seals as well as new pads and new discs. I stripped and cleaned the calipers and made sure all passages are free and clear of gunk. I have also removed all old paint or powder coating from the caliper mounting points and even replaced the front axle that was all beaten up. Lastly I have done my best to align the forks on a few occasions. The pins are straight as far as I can see and I have used copper grease when reassembling everything. It seems to be about retaction at the very end of the pistons when they are close to the caliper body because if I remount the old discs there is no problem, hence why I was considering having the pads shaved back by a couple of mm.
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Are the caliper bores pit, and mark free? Are the pistons mark free? Does the problem only appear to be on one piston not retracting fully.
Regards Martin.
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The pistons are new so yes they are pit free, the caliper bores seem good, I have also used 600 grit wet and dry to smooth them off and try and remove and pits. It appears the issue is the same or more or less the same on both sides. The culprit is, I suspect, that when I had the calipers powder coated the idiot that did the job did BOTH sides of the calipers so I got them back and had to spend hours carefully removing it from the mating surfaces. It seemed to go ok, they have never leaked etc but maybe it has caused this issue. I may have to search or a pair of used brembo 4 pt calipers.
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It is slightly possible that during the process of powder coating the calipers were slightly distorted. Have you checked the bores for ovality and are the bores parallel? Did you try the pistons in the bores lubricated with brake fluid? Did they move freely, if rotated in the bores are there any tight spots? You are on the right track, in OZ we have a local guy that sleeves master cylinders and calipers. Do you have a local brake expert who can check them for you if you don't have the equipment?
Regards Martin.
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FC it's a long shot but if you find any pistons that are not freely moving in and out or when rotated try swapping piston and bores. But make sure you can go back to where you started from. Doing this may reveal where the source of the problem may lie.
Regards Martin.
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Doing some research it seems to show that the calipers from a K1200 RS are the same as on my bike (a 1991 K100 RS 16v), can anyone confirm that?
I found a nice set of calipers from a 1999 K1200 RS for $99 the pair on eBay but want to try and make sure they are compatible before I buy them.
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Doing some research it seems to show that the calipers from a K1200 RS are the same as on my bike (a 1991 K100 RS 16v) . . .
I go by the BMW parts fiche numbers. At Max BMW, the caliper part numbers for the '90–'92 K100RS and the K1100RS are the same; the K1200's differs. Give us a link to where your research suggests these calipers are the same.
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One of the places I was looking at to potentially replace my calipers was beemerboneyard and they have this link
Calipers (https://www.beemerboneyard.com/34112331627.html)
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One of the places I was looking at to potentially replace my calipers was beemerboneyard and they have this link
A horned dilemma for unbelievers in BMW part numbers—that's for certain. :giggles
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Also this (https://www.realoem.com/bmw/enUS/partxref?id=0533-USA-07-1990-89V1-BMW-K_100_RS_0523,0533_&mg=&q=34112331627) from real oem but it does not mention the K1200RS but does mention the R1100 etc
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Did you try Drake at http://kbikeparts.com/?
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I found a set from a 1994 R1100 RS with part numbers which do match and they were the same price so I am going to get those...
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I didn't check your link, but did Beemer Boneyard actually have a used part in stock?! :mbird
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Lol noooo of course they did not have it in stock!! It was just that it said for K100, K1100 and 1200 etc - anyway in the end I got a set from a R1100rS which is the same part number etc so is for sure the right part. Now I just have to hope it fixes the problem... I will have them in a couple of weeks due to shipping etc here but will report back when I have news. Fingers crossed!
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https://forums.bmwmoa.org/showthread.php?33917-What-calipers-and-rotors-will-fit-a-K75
See also: https://drive.google.com/open?id=1D42exu8qpPn1An3tajpCLVzIf_o_utnh
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FC when you get it sorted you might want to check out your old calipers. If it can be sorted you could either sell them and recoup some of your investment or keep as spares.
Regards Martin.
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Sigh... I cannot seem to get a break (pun intended)
I got the calipers yesterday and set about dismantling them to give them a clean etc when my trusty 6mm allen broke off inside the head of one of the bilts.
(http://www.motobrick.com/gallery/1/3823-050818104143-16172464.jpeg)
It is hardened steel (though clearly not hard enough!)
(http://www.motobrick.com/gallery/1/3823-050818104143-1617680.jpeg)
I have no clue how to get it out, I tried drilling into it and after five minutes had made a tiny dent but no more. It doesn't just pop out when banged from behind and is in too tight for a magnet to work.
Any ideas?
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Any ideas?
Neglected fasteners should always be soaked with penetrant before any attempt to remove them is undertaken.
If there are no threads engaged with that screw then heat both sides of the hole with a hair dryer or heat gun. Apply penetrant. Wait 10 minutes then put a notch with a punch at the line in the attached photo, engage the notch with a drift and pound the screw in the direction of the arrow. When it moves, you should be able to knock it from the hole on the shaft side.
(http://www.motobrick.com/gallery/1/1601-050818145853.jpeg)
If the hole is threaded, you'll just keep hammering it circularly to rotate it out.
If you ever need to drill something out like you attempted, light oil should be applied liberally to the bit's tip while drilling to keep its cutting edges sharp. Smallest bit diameters start the project and are replaced incrementally with larger bits. With right-hand fasteners I use left-hand bits—even in this case—but use what you've got if it comes to that. I use the highest drill speed possible.
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I had something like that happen to me on another project as well. I roughly did what Laitch describes. I cut a notch with a dremel and then used a cold chisel to engage the bolt. It took some doing and a couple of notch cuts, but I finally got it out with patience and finesse.
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Maybe I'm the dissenter but I'd leave well enough alone. Just clean them as much as possible as they stand, put them on the bike and ride.
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Here is my experience. First I put new brake pads in, it got worse, I mean rally bad. Next I took the calipers apart to clean them and reassembled. (I checked the rotors with a dial indicator and found no noticeable run-out, so eliminated this as a cause). When I was bleeding the brakes with my vacuum bleeder I noticed some gummy dark brown chunks coming through the bleeder line with the brake fluid. I took the bike out for a test drive and the brakes were fine, no more pulsing and very smooth at any speed!
My theory is that there was a build up of gummy brake fluid / crud? in the channel inside the caliper. One piston was working and the other was plugged, causing unequal pressure on one side of the rotor when braking and resulting in the pulsing effect.
I am going to take the calipers apart again to see if I can clean the channels to verify my theory, (although I think all the crud is now cleared).