BMW K bikes (Bricks)

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robmack

robmack
Life time member
Life time member
Introduction
This write-up is a description of my experiences converting the 2-piston Brembo front brakes on my 1988 K100RS into 4-piston K1100 front brakes. The process required some degree of research before I undertook it to make sure that it was going to work. Others have confirmed in various forums that they were successful in performing the conversion so I had some degree of assurance that I wasn’t trailblazing. I thought I would describe the parts and process I used and some of the issues I encountered and how I resolved them.
My Franken-bike
First, I should mention that my BMW is not a stock machine. I purchased the motorcycle from a car dealership in Europe in 1997. I was naive at the time and didn’t know very much about BMW K-bikes. It was later that I learned that the entire front end on my K100 was actually that from a 1990’s vintage K1100 – forks, wheels, and brakes. The back wheel was also a K1100 to match the front. Although the front rotors had been changed to aftermarket K1100 floating rotors, the front callipers and master cylinder were stock K100. The original brake lines had been ditched and the hydraulics had been upgraded to aftermarket stainless-steel braided hoses. When it came time to replace the aftermarket rotors because they had worn to the factory minimum thickness, I came to understand how I could upgrade the front callipers as well.
My research brought to light one problem. It became apparent that I had to also abandon the stock accelerator Bowden cable setup from the K100 because the throttle mechanism changed radically between the K100 models and the K1100 models. The throttle assembly is integral with the master cylinder so adapting the K100 throttle assembly is out of the question. Further research revealed that it was possible to use the K1100 accelerator Bowden cable with the K100 throttle body take up reel assembly with slight modification to that assembly. The end was in sight.
Parts List

  • (2) K1100 4-piston callipers; 34112331627 calliper left, 34112331628 calliper right
  • K1100 front master cylinder /throttle control; 32722352165 handle / MC
  • K1100LT throttle cable; 32732311065 accelerator bowden cable
  • (4) replacement K1100 brake pads (ceramic)
  • (1) 90 – 105cm braided brake line with 10mm Brembo-compatible banjo bolt / replacement crush washers
  • (2) 20cm braided brake lines with 10mm Brembo-compatible banjo bolts / replacement crush washers
Assembly Details
Replacing the Rotors
First, I purchased replacement nearly new K1100 stock floating rotors from a used parts dealer and fastened them to the wheels. This was a direct bolt-on replacement, requiring no modifications at all. They fit the existing setup using the K100 callipers. They are only lightly used so the majority of the rotor thickness is still there and, with my riding, it should last for some years to come.
Others who have standard K100 forks have still managed to upgrade their bikes to 4-piston callipers engaging the standard K100 rotors. The following is wisdom gleaned from others’ experiences. Offset of the rotors from the callipers has been the issue others have faced. The BMW R100R Mystic uses the same calliper as the K1200RS, uses the same fork sliders as the K75 and rotors have a 4 hole mounting pattern just like the K75. Conclusion is that the R100R callipers could be drop-in replacements for K75/K100 callipers on standard wheels and forks. EBC lists replacement rotors for the K75/K100 -- MD605LS/RS. It is possible to mount the EBC Pro-Lite floating rotors on the K100 wheels with no offset issues. Dimension on the R100R rotor (MD605LS) versus the K75 rotor (MD604LS).
MD605LS (R100R):

  • Outer Diameter = 285mm
  • Bore = 62mm
  • Thickness = 4.7mm
  • Offset = 28mm
  • 4 bolts mounting
MD604LS (K75):

  • Outer Diameter = 285mm
  • Bore = 62mm
  • Thickness = 4.7mm
  • Offset = 31.5mm
  • 4 bolts mounting
The EBC specs shows that there is a 3mm difference needed to center the rotor.
Replacing the Callipers
For my calliper conversion, the process was elementary. My motorcycle is equipped with K1100 forks, wheels and brake rotors so the callipers are bolt-on, requiring no modification of the fork lugs, callipers or shimming.
Installing K1100 front Brakes on a  non-ABS K100RT 070small
I should mention that in the previous incarnation with K100 2-piston callipers, the fit was nearly perfect and all that was needed to adapt the K100 callipers to the K1100 forks was to insert a spacer washer between the fork lug and the calliper support so that the calliper was spaced properly, center over the rotor. The photos below illustrate this shimming.
Installing K1100 front Brakes on a  non-ABS K100RT IMG_5028 Installing K1100 front Brakes on a  non-ABS K100RT IMG_5029

Replacing the Master Cylinder
At the same this as purchasing the rotors for my motorcycle, I purchased a used master cylinder from a K1100. The K1100 master cylinder has a 20mm actuation piston as opposed to the stock K100 master cylinder which only has a 13mm diameter piston. With double the volume of fluid needed to be moved in the new callipers, the K100 master cylinder becomes too small to use. The minimum size necessary to actuate 4-piston calliper set is 16mm. The reason is the volume of fluid that must be moved to actuate the calliper pistons. Because the disk brake cylinders are now twice the volume (8 pistons now versus 4 pistons before), it takes a twice the volume of fluid from the master cylinder to move any given piston in the new configuration the same distance as before.
Installing K1100 front Brakes on a  non-ABS K100RT 029small
There is a fundamental problem with the K1100 master cylinder in this application however,. It is not mechanically compatible with the throttle assembly for the K100. There was a complete change of design going from the K100 to the K1100. The handlebar switch assembly for the K100 fits without problems but the throttle cable is a major stumbling block. There is a seldom-found 1991-95 R80R/R100R Mystic master cylinder (P/N 32722314022) which is 16mm diameter and will work in this application without any modification to any other throttle components. However, because it is so rare on the used market, the price point for a new Mystic master cylinder does not make this conversion cost effective. It is the availability of plentiful, inexpensive K1100 parts that makes this conversion worthwhile.
When buying a used K1100 master cylinder for this application, it is necessary to obtain:

  • The master brake cylinder itself
  • The throttle housing and throttle grip
  • The brake indicator switch
  • All covers
It is not necessary to obtain another handlebar switch cluster because the existing one fits, as I mentioned.
It is necessary to modify the brake indicator wire. The connector under the tank is different on the K1100 and must be removed. A K100 connector must be spliced onto the wire as shown in the photo below:
Installing K1100 front Brakes on a  non-ABS K100RT 025
Routing the Brake Lines
As part of this conversion, I moved over to stainless steel braided brake lines. The K1100 fork brace is equipped standard with provision for a “T” splitter connection to accept the brake line from the master cylinder and divide it in two, one line for each calliper.
Installing K1100 front Brakes on a  non-ABS K100RT 025
This differs from the standard K100 configuration with the brake line through the triple clamp and splitter immediately after. I required an extra long after-market brake line because, as part of the conversion from RS to RT, I had to install the high rise bars. My existing brake line was too short. Brembo brakes use a special 10mm banjo bolt which I ordered in addition to the brake line. The brake line was custom built. I routed the line down the handlebars, by the triple clamp and swung it back creating a loop that absorbs steering-induced fork movement in the system. The cable is secured to the lower triple clamp using a metal brace protected with a rubber grommet (see photo below) and continues to the “T” connector. The metal support is attached using a M6x10 stainless steel hex head machine screw through a tapped hole drilled into the triple clamp.
Installing K1100 front Brakes on a  non-ABS K100RT 026%20%282%29
Modifying the Bowden cable
The K1100 master cylinder is completely different from the K100 master cylinder and, as a result, the old K100 accelerator Bowden cable must be discarded. Key to the installation is altering the stock K1100 accelerator Bowden cable so that it functions with the existing throttle mechanism in the K100. This modification involves the cable support at the throttle bodies and the cable itself.
Remove the fairing, airbox, tank so as to get access to the throttle cable support which is bolted to the throttle bodies. This piece is Item #6 in the diagram below
Installing K1100 front Brakes on a  non-ABS K100RT 2
Remove the existing K100 throttle cable from the take-up reel and support if not already done. Once the support has been extracted, use a hack saw to saw off the top of the metal grommet that normally supports the Bowden cable termination. When this surgery is complete, it will appear as in the photos below
Installing K1100 front Brakes on a  non-ABS K100RT Cable%20support%20bracket%20modification%201 Installing K1100 front Brakes on a  non-ABS K100RT Cable%20support%20bracket%20modification%202

Insert the K1100LT Bowden cable into the bracket and make a mark with a knife about 3-4mm from the edge of the grommet on that part of the cable termination that sticks out. This mark will appear as in the photo below. This part of the Bowden cable must be removed to make sure that the take-up reel on the throttle bodies functions and is able to sweep through its arc without interference.
Installing K1100 front Brakes on a  non-ABS K100RT Cable%20termination%20scored%20line
Remove the cable from the support, take a sharp utility knife or fine saw and CAREFULLY part the termination at the knife score. Be especially careful not to nick or cut through the Teflon inner lining of the cable. This lining ensures that the cable operates smoothly and if it is damaged, the cable will bind in its movement. Once the termination is sawn through, remove the scrap piece by cutting it in half with a pair of side clippers. The plastic used in this termination is very hard so sharp tools, time and patience are a necessity.
Installing K1100 front Brakes on a  non-ABS K100RT Cable%20termination%20modification%20complete
Replace the support bracket back onto the motorcycle and finish routing and installing the Bowden cable.
Installing K1100 front Brakes on a  non-ABS K100RT 020
One wrinkle I found was that the K1100LT cable I used, although the longest available in the BMW parts repertoire, was still a bit short to allow it to be routed through the frame opening where other cables get routed. As a result, I routed it between the frame and the radiator. This routing eliminates streching the cable when the handlebars are at full right lock position. Time will tell if there are detriments to this choice.

These are pictures of the completed installation of the Bowden cable.
Installing K1100 front Brakes on a  non-ABS K100RT 068 Installing K1100 front Brakes on a  non-ABS K100RT Cable%20takeup%20reel

Bleeding and Testing
All that is required now is filling the system with DOT4 brake fluid and bleeding out any air bubbles.

June 2010

Robert MacKinnon
Toronto, Ontario. CANADA
1988 K100RT++
(photos from this tutorial and other information is available on my Picassa album )

http://k75retro.blogspot.ca/
    

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