R100GS M. A. P. Engineering Brake Rotor

If you’ve got a grip like a gorilla you’re probably satisfied with the GS front brake. But if you’d like a little more feel, and a lot less effort at the lever, then M. A. P. Engineering may have just the hop up you’re looking for. Jim Rowley, the engineer in M. A. P. Engineering, will take your stock rotor carrier and modify it to accept a 320 mm diameter replacement that floats on its own aluminum carrier. Rattle is kept to a minimum by replaceable wave washers. That’s an important point, as the BMW rotor on lots of GSs rattles like a trail of tin cans behind a newlywed’s car. To accommodate the larger rotor, Jim has machined up a mounting block for the caliper that is both sturdy and good looking. He supplies new stainless steel bolts and lock nuts (but not washers) to fasten everything together.

The upgrade is as simple as removing your stock rotor and shipping it off to Jim, who will promptly work his magic and have you back on the road in a couple of weeks. Installation is a snap, amounting to about ten minutes with a torque wrench and a pair of open end wrenches to reattach the brake line. One small niggle is the necessity of bending the steel brake line to match up with the new position of the caliper. While the original line can be made to fit, it’s difficult to make the needed bends if you don’t have a proper bending tool, and you’ll likely end up with kinks in the line. Fortunately relief is as close as your local automotive parts house, most of whom can bend a line to the needed profile. Just use a piece of coat hanger wire to make a pattern and take along the old line so you end up with the correct fittings.

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R100GS HPN Rear Brake Lever

I first ran across HPN’s replacement rear brake lever while flipping pages in a Touratech catalog, and the idea of tucking the lever up out of the way seemed like a good one. The $140 price tag is very high, however, but when some insurance claim cash came my way I decided “what the heck” and ordered one (along with an oil cooler relocation kit).

The kit comes with everything you’ll need for installation except a drill. The instructions in the package are in German, however, and like the cooler, there is a help page for the brake lever on the Touratech USA web site (though it’s not referenced from the catalog page like the cooler). I was able to piece together the path forward from looking at the illustrations on the German instruction sheet and reading the text on the Touratech web page.

The kit comes with three pieces: the replacement brake lever, a new cable stop for the rear drive end of the brake cable, and a cable alignment bracket for the pedal end of the cable. The latter is needed because the bend in the cable necessitated by the reversed lever makes the cable want to droop as it emerges from the brake pedal housing. The lever is aluminum, just like the original, and is the same length as well. It incorporates a jog so that the cable end will not hit the drive housing. The cable stops and brackets are cad plated steel, as are the mounting screws.

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R100GS Dual Pot Caliper Upgrade

I’ve always been a believer in the old “if some is good, more is better” philosophy, especially when it comes to brakes. I’d already gotten a substantial improvement in stopping power with the big rotor conversion from M.A.P. Engineering, but when I heard on the Airheads mailing list that it was possible to fit the dual piston caliper from a later oilhead BMW I couldn’t resist leaving well enough alone. I dug a little bit and found that the left caliper from an R1150GS, part number 34117670391, has the same mounting bolt spacing as the R100GS caliper, and would center over the rotor if about 0.10″ of the boss thickness was milled off (several R11XX model BMW motorcycles use this same caliper). I fished around the IBMWR classified page for a used caliper, but some folks have an exadurated sense of worth for their old parts. A fellow lister suggested Cycles Recycled as a possible source, and Fred Rowland quickly answered an email inquiry with a $35 plus shipping offer I couldn’t refuse. Two weeks later the part was in hand and I had to figure out how to get it to fit.

First thing I did was remove the brake pads and fit the new caliper to the M.A.P. bracket to make sure that the bolt spacing was indeed the same. It was. And just as my fellow listers had said, the caliper didn’t center over the rotor very well. It seemed to be about a tenth of an inch too far toward the center of the wheel. That meant that material would have to be removed from the outside of the mounting bosses on the caliper. I figured that wouldn’t be a problem for the little milling attachment I have for the ancient South Bend lathe in the corner. I used a telescoping hole guage to measure the gap between the rotor and the inside of the caliper on each side, subtracted the two distances and divided by two to get the distance that the caliper was off center. That came out to 0.109″ on my bike, and that’s the amount I’d end up taking off the mounting bosses.

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Improved ATE Braking

From the Airheads mailing list

I see a question and comments about improving front braking on a 1978 R80 from a single disk to dual. One person said go to brembo brakes.

It can be done, but prepare for a snowflake front wheel, the spoked one won’t fit with Brembos. Also you will need the later model down tubes as the attachment of the dampeners is by different means. So you just can’t replace the lowers. (But you can use the originals, if one can get some parts machined).

The wheel cylinders were of different sizes, there are some 38 MM units and some 40 mm units, the 40s give more brake power, the R75s and 80 had 38s. The R100 units and later (75 – 76) and R90S had 40s. So look at the cylinders, it is stamped on there some where. You can’t go by colors, some are silver, some black, the blue anodized ones were 40s probably they came on the 77-79RS.

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Experience with DOT 5 (Silicone) Brake Fluid

Do any of you have personal experience with DOT 5 brake fluid in Beemers? If so, I’d appreciate hearing from you, as to whether it was good or bad.  In the meantime, I’ll share mine with you.

My bike is a 1974 R90/6, purchased in December 1976 with 5k on the odometer.  It has a single-caliper front disc brake.  Removal of the fuel tank to top-up the brake fluid four months later revealed a jet-black liquid in the brake system, which I flushed out with clean DOT 3. Occasional checks thereafter revealed that the fluid in the reservoir remained clean and clear, and there were no leaks in the system.  By  1981 (31k), the fluid in the reservoir was starting to look like Sierra Nevada pale ale, so I flushed the system again with DOT 3.

In 1982, having had good luck with silicone (DOT 5) brake fluid in a 1971 MGB and a 1966 Buick for about three years, I decided to try it in my bike.  Advice to the contrary notwithstanding, I disassembled the master cylinder and front caliper, found the pistons and cavities clean and bright – no rust or corrosion – wiped out all the old fluid with a clean rag, washed loose parts with soap and water, reassembled using the original seals, and filled the system with DOT 5.  There had been a very slight seep between the master cylinder and the reservoir, but decided that it wasn’t bad enough to go to the trouble of removing the reservoir.

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Changing Brake Fluid

Fellow militant Airheads, As far as general maintenance goes how often do ya’ll change your brake fluid?

The correct answer to this can vary with the amount of humidity in your local air. The manual says do it once a year. You change the brake fluid because it absorbs water from the air. As it does so it gets darker, also its boiling point goes down. This water can eventually damage parts. As I write this I have this vision of someone with a low level of anal retention insisting on using the most exotic oil but not changing the fluid because it’s not yuppie. These are the folks whose Airheads retire early as being too expensive to fix.

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Broken front & rear brake light switches

This article is not concerned with the hydraulic pressure activated brake switches. Those switches, originally 34-31-1-233-959, were replaced with 61-31-1-244-334. You can probably substitute the switch used on old VW Beetles. Napa carries that switch as number SL143. There are other such switches: 3 terminal version is NAPA SL159, VW 113945515G; 2 terminal version is NAPA SL147, VW 0344004003.

This article IS concerned with the mechanically activated brake switches.   All R series bikes FROM 1985 model year through 1988, & SOME 1989 Airheads were affected. ALL models of Airheads for those years!

BMW Service Information Bulletins were issued regarding the front & rear brake light switches, brake lever, etc. The bulletins applied to both Classic K bikes & to Airheads, but not exactly the same way. There were bulletins issued in 1987, updated in 1988 & a formal recall in 1989. Later bulletins were longer, with more details.

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Brake Fluid Maintenance

Copyright © 1999 by Oak Okleshen. Used with permission. All Rights Reserved. May not be distributed in any form without prior written permission from the author.

Most of the technical inquiries sent to my mailbox from airhead members relate to problems why their machine won’t go, or go properly, or may relate to a desire to make them go faster. Inquiries why they may not stop or questions related to brake and safety maintenance are woefully sparse. And in reviewing the technical index I put together for AIRTECH material published in the past, I find very little on the subject of brake fluid maintenance. Thus the question arises–is it really that important or is the subject embellished with a sales pitch for brake fluid and service at your local dealer?? Before you jump to any conclusions, let me cite a very real incident where I had a direct participation in brake system maintenance.

Locally, (not an airhead member…) a gentleman acquaintance called me late in the year (several years ago…) and asked if I would help him get his 1982 R100RT in running order. He stated the machine was not ridden in over 2 years and was in storage in his aircraft hangar. The battery was dead, and the windshield cracked as something had fallen on it. So he requested generic repairs and a full 10,000 mile maintenance schedule to be performed. I wasn’t fond of working on it just then but he insisted and not running at the time, trailered it out for work. I knew the machine well, as it had been in pristine condition prior to his ownership when purchased from another acquaintance locally…

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