Front brakes grab when I turn the handlebars

[7695]

About 6 years ago, I installed dual front discs on my 1974 R75/6, upgrading from a single disc. The new discs are perforated and are absolutely not warped. They came off a '77 R100RS along with the the second fork tube. Calipers were rebuilt as was the master cyclinder, stainless steel brake lines added and new Brembo pads were installed. New brake fluid was installed when the rebuilt master cylinder (with stainless steel bore) was added. At the same time, I changed handles bars and cables from stock to R80RT bars - slightly higher. When I am backing the bike in/out of the garage, when I turn the front wheel, the brakes activate. With the wheel turned (not very much-10 to 15 degrees), I cannot push the bike without trememndous effort. The brake fluid is on the mark, the cables and SS brake likes do not bind, and with the tank off and looking at what may be happening when turning the bars, there is NO movement of the master cylinder lever which indicates to me it is not a mechanical input. It handles fine on the road, even when making sharp slow turns. Of course, on the road I am leaning to steer rather than turning the bars as much as when when maneuvering it in the garage. No matter where I set the brake adjustment at the handlebar, the problem persists. The bike has 53,000 original miles, I am the second owner since March 1975 and I've never had a front brake problem before. I am baffled. Any comments, suggestions, please? Thanks,
Robert

 

[20774]

Robert, welcome to the forum...long overdue!! Just thought I'd post the link to your first post where you had issues with getting the wheel into the new brake setup:

http://forums.bmwmoa.org/showthread.php?50029-6-dual-disc-brake-conversion

Have to think about this for a bit. Is there any movement of the wheel on it's axles/bearings? It might move enough that doesn't affect things at speed but might be more noticeable while rolling around the driveway.

 

[lkraus]

Sounds like the cable is being stretched when the bars are turned. Probably OK turned one way, drags only in the other direction. If possible, re-route the cable to shorten the lever-to-cylinder path, or get a longer cable to work with the R80 bars.

 

[jagarra]

Has this problem persisted since you installed the dual conversion? If the pivot eccentrics are not the issue (loose). then maybe when you installed the M/C you set it too far back and the cable adjustment at the M/C end is not allowing the fluid return hole to get exposed. There is an adjustment where the groove on the piston of the M/C has to be exposed a certain amount (1.2mm) to allow the return hole to position correctly.

 

[toooldtocare]

Sounds like the cable is being stretched when the bars are turned. Probably OK turned one way, drags only in the other direction. If possible, re-route the cable to shorten the lever-to-cylinder path, or get a longer cable to work with the R80 bars.

I agree. I had the same issue on my 78 Motorsport when I swapped the stock bars for K75S bars. It took a bit of trying different routing of the brake cable to the master cylinder to almost correct it. In full left hand lock the brakes would activate. I lived with it since it never gave me a problem, but a longer cable would have fixed the problem.

 

[dennisdarrow]

I agree. I had the same issue on my 78 Motorsport when I swapped the stock bars for K75S bars. It took a bit of trying different routing of the brake cable to the master cylinder to almost correct it. In full left hand lock the brakes would activate. I lived with it since it never gave me a problem, but a longer cable would have fixed the problem.

+3..........Had this exact problem twice and both times it was with a change of a bit longer bars......Once on a R80 ST except it was doing it with the throttle....Next time was with the brake on a R75/6

 

[ccolwell]

Ditto on the brake cable but...as you said you checked this here is another thing to try. Disconnect the brake cable and see if the problem persists. If not, there's your problem. If it does persist, pull each caliper separately to see which one is causing the problem. Putting the bike on the centerstand and jacking the front up a bit will let you diagnose it without rolling it around.

 

[PGlaves]

Under the tank master cylinder? One cable bars to master cylinder?

It sounds to me like the cable is too short and going tight when the bars are turned. I've not seen this with the brake because the last under-tank MC I had was on an R90/6 in 1985. BUT, I've seen it a dozen times with throttle cables where the engine would speed up - maybe a lot - when the bars were turned sharply.

 

[jagarra]

BMW offers only one length for the brake cable, they are production year sensitive. So either the routing is creating the issue or he doesn't have enough slack in the cable. The only way I see to give any more slack is to move the M/C forward toward the steering head.

I recently ran into an issue with the current replacement brake cable BMW is providing. I couldn't get enough slack in it to hook it up at the M/C end. In comparisons to the original cable it seems the housing was just a bit longer and the inner cable was just a little shorter.

 

[7695]

Many, many thanks to all who responded. I will try each suggestion. There are many interesting thoughts here, several of which I failed to consider in my troubleshooting. Thanks again!