1150 clutch slave modification

[The Fabricator]

My 2000 R1150GS  has a dry clutch and a really short engagement range.  That, plus a tall first gear makes for some difficult take-offs [not to mention stall fall-overs].  And the clutch lever action is stiff.  If I change the ratio of master cylinder to clutch slave cylinder piston diameters, I can make the lever force easier and expand the engagement range. 

 

The clutch master is integral with the electical switches, so subbing a different master with smaller piston was not appealing.  I considered making a new clutch lever and relocating the lever pivot for increased leverage, but I didn't think the gains would be significant.  My old BMW R100/7 has a cable operated clutch and I rigged up a pulley reduction that cut the lever effort in half and doubled the engagement range.  One concern is the clutch spring may not move far enough, because, with a ratio change, the slave piston does not move as far.  Then, the clutch may drag a little, which is not good.  The clutch did dis-engage enough with the pulley set up.  Just barely.

 

I had scabbed an old Yamaha clutch slave cylinder from the motorcycle shop where I worked, that I have been saving for over 15 years.  The BMW piston is 24mm, the Yamaha piston is 38mm, a 2.5x effort reduction. Since I had the gear box out and clear access to the slave location on the box, I made an adapter to mate the larger slave in place of the OEM slave.

 

 

 

   

A dolly I made for removing the transmission.  That, together with the scissors jack makes removal and  installation easy.

 

 

  

              Stock BMW clutch slave cylinder                                   Yamaha slave cylinder. Note gap.

 

The stock BMW slave piston has a incorporated bearing, which I wanted to retain.  I made a carrier that                                                fits precisely in the slave cavity in the transmission. The stock slave piston fits inside the carrier.

 

       Push rod fits into bearing in slave piston                                   Carrier for slave piston.

 

The Yamaha slave does not fit into the slave cylinder cavity in the transmission.  And the Yamaha bolt pattern and BMW bolt pattern are different.  I made an adapter plate to mate the  Yamaha slave to the BMW transmission.

 

 

BMW slave piston, carrier, Yamaha piston, adapter plate                      Carrier in place in transmission

 

 

   

             Adapter plate installed on transmission                             Yamaha slave installed, hoses attached, bled, ready to go.    

 

     How does it work?  The lever effort is extremely soft.  But............the clutch does not disengage!    

 It does disengage if I pump the clutch lever 3 times.  So it turns out the distance moved by the clutch slave piston is too little to be effective.  

               

Since I had the Yamaha cylinder, and did not know the minimum travel necessary for clutch disengagement, I tried it.

 

So I calculated a 32mm slave piston diameter would yield a 1.77x reduction of lever effort AND slave piston movement, which I guessed would be enough.  I ordered a slave from a BMW k1200 off Ebay for $50 shipping included, made another adapter plate which mated with the first adapter plate [2 were needed], installed it.

 

How does it work?  

 

PERFECTLY!  The lever effort is much reduced and the engagement range is increased to about 2/3 of the clutch lever sweep at maximum lever travel [#4 on the lever adjuster].  

 

Formerly, I had to raise engine RPM well over 2k for a smooth take off.  Now 2k RPM is all that is necessary, even on a hill.                 

[Baba]

That was an interesting project. Since I would have trouble making adapter plates and fitting the BMW bearing into the K1200 slave, I might pick up a used master cylinder on eBay and see if I can get it sleeved down to achieve that 1.77x reduction. Any idea how small I would have to go? I think the stock master size is 13mm.