Strange brake problem

[alexis291]

Hello Folks,

I have a problem with my front brakes which has got me puzzled... Your thoughts would be welcome please.

The symptom is that the braking force changes with wheel rotation - ie there appears to be a loose / tight spot, once per revolution. This is very noticeable as fluctuating deceleration when riding at low speed (ie it feels just like I am repeatedly pulling and releasing the brake lever), and front wheel vibration at high speed. I can't actually feel any pulsing at the lever, just the speed change / vibration. This appeared out of the blue during an otherwise uneventful long ride.

I have jacked up the front wheel and applied a little brake pressure with a cable tie, then turned the wheel by hand. It is possible to feel that the force required to turn the wheel changes once per revolution, roughly at about the same spot.

The brake pads are relatively new and I have removed them and checked that they are free to move.

I changed my front bobbins for these

http://boxer-upgrades.webs.com/apps/webstore/products/show/3821625

about 1,000 miles ago, though I also fitted wave washers which restricts the disk float somewhat. But they do move. After fitting the new bobbins, the brakes were the best they had ever been.

I have checked that the disks (rotors) are still free to 'float' on the bobbins, and that none of the C-clips or washers have come off or broken. I may remove the wave washers from the bobbins altogether as a test.

I have not yet measured the disk thickness to see if it varies but I will be surprised if that is the problem as it appeared so suddenly.

There is no evidence on the disks of overheating that may indicate they are warped, and there was nothing in my journey that would have caused that to happen.

Has anyone had anything similar to this ? Or have any ideas what might be causing it ?

Thanks.

[Martin]

Looks like warped disc or discs. They can be checked in situ with either a dial gauge indicator or a pointer and feeler gauges. Maximum warpage 0.20 mm or .008 thou.
Regards Martin.

[Filmcamera]

I have no idea what is causing it but I would start by give the entire brake system a good clean.  The fact that it started on a long road points to road gunk being a potential culprit.


So clean all the bobbins till you have maximum movement then take the calipers off and degunk the pistons.  That should sure do it is my guess.


Are your calipers two pot or four pot? If four pot it could be that one pot is not fully retracted and that one or both discs are slightly warped to it is rubbing at that point.

[Chaos]

with the force to turn the wheel changing one per revolution, as you stated, I'd suspect the rotor's friction coefficient varies at one spot.  Could be glazed or contaminated.  I'd try cleaning the rotors with some emery paper or steel wool and acetone or brake cleaner.  I've heard of iron rotors getting a defective spot like that and the only fix is to change them, not sure it it happens with ss rotors.

[Laitch]

Have you tried to isolate the problem to the left side assembly or right side yet?

[alexis291]

Well thanks for all the suggestions.


So the update is that I took the calipers off individually to try and determine which side is causing the problem - and they both are ! Left worse than right, but definitely the problem is there with either caliper.


At this point I thought Johnny may be right and the wheel is "whack" but I've checked it and it is no more bent than it was before (as a result of the UK's pothole infestation).


I think the problem is that both brake disks are distorted. I have not been able to measure it but I can see some run-out using a pointer held on the fork leg. I'm at a loss as to why this should be the case as neither is particularly worn and nothing happened on my last journey that would cause them to overheat or get damaged.


As a temporary fix I have removed 9 of the 12 wave washers on each set of disk bobbins so that the disks are more free to float, and this does seem to have made some improvement.


Judging by all the posts on here asking where to get disks at a reasonable price, this could be an expensive fix !

[Martin]

Discs can warp from heavy braking. I have had problems with cheap discs made from inferior steel made in China. Current cheap Chinese ones started to wear really quickly. I managed to get new EBC ABS floating ones from UK for $400.00 Au including freight. I will fit them when the current ones are totalled.
Regards Martin.

[alexis291]

Maybe you're right Jo. I expect motorworks sell kryptonite ones.


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[K1300S]

99% of the accusations of "rotor warp" are not warped rotors.  rotors don't warp from heavy braking.  it is uneven brake pad material build up on the rotors.  frequently caused by incorrect pad break in procedure.  the place the rotor catches on the pad is the high spot.

[mw074]

The calipers need to " float." A much overlooked fix.

[K1300S]

K calipers do not float.  They are fixed by design.  Floating calipers slide back and forth on greased pins to stay centered on the rotors.  Most later bmw cars have that design.

[Laitch]

. . . though I also fitted wave washers

I have checked that the disks (rotors) are still free to 'float' on the bobbins, and that none of the C-clips or washers have come off or broken. I may remove the wave washers from the bobbins altogether as a test.

Do it!

[mw074]

"K calipers do not float.  They are fixed by design.  Floating calipers slide back and forth on greased pins to stay centered on the rotors.  Most later bmw cars have that design"

Good point. I stand corrected. Confusing the BMW with my American made motorcycle.

[alexis291]

Laitch, I removed nine of the 12 wave washers from each disk, leaving three per side. This gives the disks a lot more freedom to float but stops them from rattling. A quick test ride and all seemed good. However, having now ridden about 250 miles the problem is still there and as bad as ever. It looks like the solution is going to have to be to renew the disks. [emoji53]


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[Laitch]

Laitch, I removed nine of the 12 wave washers from each disk, leaving three per side

Remove the other three from the disks and wear earplugs.

[Martin]

Clean the rotors with wet and dry paper and clean with brake cleaner. How much run out did you have. You can also check the disc by removing it from the carrier and running a straight edge across it in multiple places. You will have to allow for the lip caused by wear if there is any.
Regards Martin.

[alexis291]

And the fix was - new disks and pads. Don't ask me why, the old ones were not warped and I spent a whole day cleaning them with wire wool and wet and dry, which made no difference. The only think I did notice was a small amount of surface pitting on one spot on one disk. I thought this was odd on a BMW disk but maybe they're not original. I have fitted Hekkane disks at £90 for a pair. Has anyone else used these? They are remarkably cheap.


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[K1300S]

yeah!

[Martin]

I while ago I came across a post where a guy cleaned his discs with a orbital sander :yow , and it worked. :dunno
Regards Martin.

[jay1622]

99% of the accusations of "rotor warp" are not warped rotors.  rotors don't warp from heavy braking.  it is uneven brake pad material build up on the rotors.  frequently caused by incorrect pad break in procedure.  the place the rotor catches on the pad is the high spot.

Do you have any idea how many "professionals" who are "in the business" still haven't figured this out? For those of you who have a few minutes to kill, here's some of the best brake-related reading around... IMHO.


http://www.hendonpub.com/resources/article_archive/results/details?id=1787

[Motorhobo]

I'm following this thread with great interest because I've got pulsing now too. About 2000 miles ago I swapped out the rotors from a parted-out bike onto my daily rider and used some pads that I inherited in a K75 purchase. Things were fine for a while but pulsing started and I need to deal with it.

I don't think it's the rotors per se but it could have been the installation -- I used a torque wrench but didn't check the Clymer for a tightening pattern. I have new pads on order and since the bike is on the lift now for the transmission swapout I'll probably gravity bleed the brake system.

I went with these recommended by Gryph:

http://www.ebay.com/itm/1989-1996-BMW-K75RT-Kevlar-Carbon-Front-Rear-Brake-Pads-/131516075180?fits=Make%3ABMW&hash=item1e9ef80cac&vxp=mtr

Beemer Boneyard is selling these at twice the price:

http://www.beemerboneyard.com/cl2258a3.html

Anyone know of a difference? If there's a difference worth noting I can always use the eBay ones for my sidecar rig where performance is less of an issue.

I'll try the clean and sand thing Martin suggested, check the install procedure and reinstall, put on the new pads and report back.

BTW I've had a tiny bit of braking irregularity and very low speeds 3mph or less coming to a full stop on my daily rider as long as I can remember but it's never been noticable braking from higher rpms so I didn't pay much attention to it.

[The Mighty Gryphon]

Do you have a tire balancing rig and a dial indicator?  I would check the rotors for runout before I did anything else. 

As far as the Volar pads, my RT now has over 20,000 miles on the set I installed back in 2014 and the brakes are working perfectly, especially since I replaced the rear master cylinder with one of those $6 Chinese units.  The pads still have a bunch of material left, probably because I do a fair amount of slabbing, but I am going to replace them next winter just to get fresh ones in there.

[Motorhobo]

Do you have a tire balancing rig and a dial indicator?  I would check the rotors for runout before I did anything else. 

No, and getting more diagnostic gear isn't going to be my first action. This started happening after I swapped disks from one wheel to another, so I think I'll start by checking that I did that properly and getting new pads -- the pads I put on there were of questionable heritage and ancient anyway. I'm pretty sure the disks are fine and if they're not I'll deal with that issue then.

[jay1622]

When installing the rotors (on any vehicle), ensure the mating surfaces are void of dirt, rust, grease... Basically, make sure the surfaces are clean. That's the hub, the rotor, the wheel. Even the most precisely torqued installations will be off if the areas are not clean and flush. 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.


Somewhere on this thread, or somewhere else, someone mentioned coming to full/abrupt stops, and then keeping the pads held in the original point of stop. It basically ruins the even coat of break-in material (burnish) of the pad on the rotor. If this occurs, it's best to re-break-in your pads. Best practice is coasting just a hair before coming to complete stops. Make sense?


There's plenty of documentation out there indicating why this is a best-practice for pad/rotor braking... Here's a simple means of burnishing. The best part is that if you've not yet done this, you can always go out and get it done. Better late than never.


http://www.wagnerbrake.com/technical/technical-tips/break-in.html

[Motorhobo]

When installing the rotors (on any vehicle), ensure the mating surfaces are void of dirt, rust, grease... Basically, make sure the surfaces are clean. That's the hub, the rotor, the wheel. Even the most precisely torqued installations will be off if the areas are not clean and flush. 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.


Somewhere on this thread, or somewhere else, someone mentioned coming to full/abrupt stops, and then keeping the pads held in the original point of stop. It basically ruins the even coat of break-in material (burnish) of the pad on the rotor. If this occurs, it's best to re-break-in your pads. Best practice is coasting just a hair before coming to complete stops. Make sense?


There's plenty of documentation out there indicating why this is a best-practice for pad/rotor braking... Here's a simple means of burnishing. The best part is that if you've not yet done this, you can always go out and get it done. Better late than never.


http://www.wagnerbrake.com/technical/technical-tips/break-in.html

Great info, jay1622 -- I will follow it from now on.