Spring Breakage

The possible reasons for spring breakage and the possible cure for this.

Remember this picture? it was taken when the club racer was disassembled following the Victorian Road Race Titles in March because Stacey complained about the bike jumping out of third gear. As part of the tear down the clutch cush drive springs were examined and these broken springs were found. This is not the reason for jumping out of gear but is something that needs to be fixed.

There are several possible reasons for these spring breakages and I will try to explain what these reasons are and how to rectify the problem. In the past such experts as Tony Hall of Halco in England had studied this problem because there were several teams racing sidecars that had complained about these breaking cush drive springs, this can be a serious problem as a broken spring finding its way into the primary drive gears will cause a catastrophic engine failure resulting in a broken crankcase. So Tony came up with a possible solution using stronger springs and buffer blocks.

To explain this fully you must first understand how the cush drive works. To explain this I will first show you a disassembled clutch basket and cush drive set up on a XS650.

Looking at the primary drive gear, if you look carefully you will see that there are long slots and short slots cut into the gear. When the clutch is released the gear is allowed to rotate against the springs in these short slots, we'll call these primary springs, compressing them until they come into contact with the springs in the long slots, secondary springs, and these too will compress. The problem arrises when the secondary springs become weaker over time and compress too much allowing the primary springs to fully compress and cause spring bind, this is believed to be the cause of these breakages. Tony's idea was to replace these secondary springs with buffer blocks made of Nylon and replace the primary springs with much heavier primary springs. The theory being that the buffer blocks would prevent spring bind and thus prevent the breakages. It worked. Later I looked at this and decided after looking at the specs for nylon that a better solution was available using Vesconite blocks which had a higher melting point than Nylon and was impervious to oil which Nylon was not. Ivan Hoey later used all heavy duty springs and no buffer blocks in his race bike which so far is working well but he still checks these springs an a regular basis because they can still break and it is prudent to replace any broken ones as soon as possible.

Having said all this there is another possible reason for the breakages in our club racer, if you look closely at the first picture you will see some wear marks on the springs and this may be caused by the backing plate, which is very hard, rubbing the spring causing these marks. It definitely looks to be a possibility and this may cause a stress riser in the spring creating a weak spot which may fail. In case this is also a problem we will grind any sharp edges on the pockets in the backing plate before re-assembly just to be sure.

Check the picture below to see what can happen when debris finds its way into the primary gears.