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					Soft clutch Mark II. Detail description.
Richard Todd.


1.       Objectives

Four years ago I worked out a design for the “Soft Clutch” (see www.bmwman.biz). This worked well
and did what it said on the tin, but I accept that a couple of the elements of construction were a tad
difficult requiring a thread to be cut and a rectangular hole to made. This version overcomes those
difficulties, requiring nothing more advanced that a drill, a hacksaw and a file.

The second parameter for consideration in this version was to reduce the efficiency slightly. Mk I
produced al reduction of almost 50%. This one aims to have a leverage ration of 1.5:1 – ie a
mechanical advantage of approx 33%.

The objective therefore was to design a soft-clutch Mk II system which was:-

     a. cheap
     b. amendable to different reduction ratios
     c. required no specialist workshop equipment or skills more advanced than wielding a havksaw
        and a drill
     d. purely bolt-on and leaves the existing cable and other components unchanged
     e. capable of being easily returnable to standard configuration whilst out on the road (in case the
        whole thing goes pear shaped!)


2.       Components and Tools Lists

Components

              Item                    Number / length                    Possible Source
     6mm x 50mm aluminium bar            80 mm                Metal Supermarkets of Redditch
                                                              01527 68818
     6mm x 12mm aluminium bar             205 mm in total     Metal Supermarkets of Redditch
     (or cut the lengths out from         (45 and 160 mm      01527 68818
     the 50mm bar)                            lengths)
     12mm x 12mm square                        45 mm          Metal Supermarkets of Redditch
     aluminium bar                                            01527 68818
     8mm dia aluminium bar            1 length of 12mm        Metal Supermarkets of Redditch
                                                              01527 68818
     5mm male rod-end                           1             Moore International 01202 462214. Part
                                                              number REM5
     5mm female rod end                         1             Moore International 01202 462214. Part
                                                              number REF5B
     M6x 25mm allen head bolt (ie               2             Mr Fast’ner
     25 mms of thread)
     M4 x 25 mm bolt                            2             B&Q
     M4 lock nuts                               2             B&Q
     M5 x 50mm bolt                             1             B&Q
     M5 x 25 mm bolt                            2             B&Q
     M5 lock nut                                4             B&Q
     M5 washers                             Approx 8          B&Q
     M8 x 70mm bolt                             1             B&Q
     M8 lock nuts                               2             B&Q
     M8 washers                             Approx 14         B&Q

Tools

     Electric drill                          Hacksaw                     Assorted spanners
     4, 5, 6, 7, 8 mm drill bits             Assorted files              Metric allen keys
     Optional – Dremel type mini-drill.      Loctite


Doc Ref:       78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                                            Page: 1 of 6
Soft clutch Mark II. Detail description.
Richard Todd.


3.       Mounting Plate



                         6 mm dia holes, 60 mm apart

                                                 16
                                                 mm

                                           38
                                           mm




                                                                    5 mm dia hole


     1. Cut an 80 mm length of the 6mm x 50mm aluminium bar.
     2. Drill two 6mm holes, 60 mm apart with centres 5 mm from the edge.
     3. Drill a 5 mm hole, 38 mm down and 16 mm in from one of the 6 mm holes (measured centre
        to centre).
     4. Cut a Vee shape out of the top edge of the plate to clear the round 'bobble’ at the bottom edge
        of the gearbox end plate. The vee should be approx 28mm across the top and 17 mm deep.




Doc Ref:       78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                                    Page: 2 of 6
Soft clutch Mark II. Detail description.
Richard Todd.


4.       Operating arm

                      Side View                                       Rear View                Lengths
                                                4mm slot for cable
                                                                                                8mm


8mm diameter
 groove to fit
  cable-end                                    4mm bolts
                                                                                               50mm



                                         5mm female rod
                                        end, held by 5mm
                                           bolt and nut


                                                                                                                       160m
                                                                                                                       m
                                                                                                                       160m
                                             Hole drilled and                                                          m
                                            tapped for 5 x 0.6
                                                                                                96mm




                                       5mm male rod
                                     end, with lock nut




                                                                            Note the straight line
                                                                            between the three contact
                                                                            points



                    Lock/adjusting             5mm x 50mm
                         nut                      bolt




                                New cable                        Lock nut
                                  barrel




 Doc Ref:        78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                                               Page: 3 of 6
Soft clutch Mark II. Detail description.
Richard Todd.


   1. Cut a 160 mm length of 6mm x 12mm aluminium bar.
   2. Drill a 5mm hole 6mm up from the bottom edge and another 5mm hole 102 mm up from the
      bottom edge (ie the two holes are 96mm apart)
   3. Cut a 45 mm length of 6mm x 12mm aluminium bar.
   4. Drill and clamp the two bars together using 4mm bolts as shown in the diagram. Cut off any
      excess bolt length.
   5. At the top end of the bar, cut a 4 mm wide slot approx 15mm deep. This is for the existing
      clutch cable. The way I achieved this was using a junior hacksaw to cut two sides progressing
      just one or two mm per side alternately.
   6. On the rear-facing side file out an 8mm diameter groove to accept the existing cable barrel.
      (Or use the Dremel.)
   7. Mount the central female rod end by means of the 5mm dia hole and a 5mm bolt and nut. Cut
      off any excess of bolt length.
   8. Mount the bottom “hinge” male 5mm rod end as shown above. Ensure that the three contact
      points are in a straight line as shown by the dotted line on the diagram..
   9. Lastly, make up a new barrel end from a piece of 8mm dia rod, 12 mm long and drilled with a
      5mm hole. Assemble the 5mm x 50mm bolt with two lock nuts and the barrel onto the female
      rod end as per the diagram above.




Doc Ref:     78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                                  Page: 4 of 6
Soft clutch Mark II. Detail description.
Richard Todd.


5.        Cable spacer


                                  Side View                            Rear View          Lengths
        Lock nut
                                                    7mm dia hole for
                                                       cable end
                                                                                                 30 mm
                                                                                                 centres




                                                       8mm dia hole
     Spacing                                                                       3mm slot for cable
     washers              18 mm



                                               8 mm x 70 mm
                                                   bolt


      1. Cut a 45 mm length of the 12mm square bar.
      2. Drill an 8mm dia hole at one end, and a 7 mm dia hole at the other. These holes should be 30
         mm apart (centre to centre).
      3. Using the same technique as previously, create a 3 or 4mm cable slot at the end with the 7mm
         dia hole. This end should now be able to accept the existing clutch outer cable end.
      4. Assemble the 8mm bolt plus spacing washers and lock nut. As a guide, start with washers and
         lock nut having a combined length of 18 mm.




Doc Ref:           78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                               Page: 5 of 6
Soft clutch Mark II. Detail description.
Richard Todd.


6.      Assembly




               Approx 8mm of spacer
               washers between plate
                   and rod-end


Before installing this ‘bolt-on’ ensure that your current clutch cable is in good condition and correctly
adjusted as per the manual with a couple of mm of ‘free-play’ at the handlebar lever end. This is your
fallback position should this whole box of tricks fail whilst you are out on the road !

With the operating arm fully assembled as per section 4, mount the arm onto the plate using a 5mm
bolt and lock nut. Use washers as spacers to provide approx 8 mm of packing between the rod-end and
the mounting plate.

Using a 5mm allen key remove the two bolts from the bottom edge of the gearbox end-cover and,
using the two replacement 6mm x 25 mm bolts, mount the whole assembly into place. Note, the
replacement bolts should have 25 mm of thread, no more.

Disconnect the clutch cable from the original mounting arm and connect the 5mm bolt and barrel in its
place (the 5mm bolt should just fit between the prongs of the original clutch arm, if not then a small
amount of “easing” of the arm prongs with a Dremel will do the trick).

Now withdraw the clutch cable outer from its location. Install the new cable spacer in its place,
position the cable spacer so that it is twisted as high as possible and then secure it in place with a lock-
not.

Install the cable outer into its new location and insert the cable inner with its barrel onto the new
operating arm.

Final adjustment is a little bit of trial-and-error by adjusting the number of spacer washers in the new
cable spacer and the adjustment of the lock-nut / barrel on the 5mm bolt. Each bike will vary, but the
ideal is that the arm should probably touch the bend in the frame tube just before it comes to the end of
its travel.

Ride it – and enjoy it!

I’d welcome any feedback to richard.todd@bmwman.biz




Doc Ref:      78f25b76-fc51-42ef-8f7a-132972b0fc3f.doc                                         Page: 6 of 6

				
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