Horrible clonking from my gearbox ... no longer

I had noticed a loud clonking banging noise coming from my gearbox at low speed. This was especially going up hill between 10 and 30 mph. It cleared above 30mph and the rate of clanking was the same regardless of the gear (and hence rev rate) the engine was pulling. That meant whatever it was, it was on the output side of the transmission. Needless to say, I was depressed at what it might mean. Google search suggested a whole range of horrors but - wait - also a suggestion that might just save me.

Apparently, some forum posters had diagnosed a similar problem as nothing more sinister than a worn chain. Internet advice was to check for stiff or frozen links that might be causing the chain to ride up on sprocket teeth, then slap down again to create a banging noise. However, my chain looked great. I put the bike on the centre stand and checked every single joint with two pairs of pliers. All moving smoothly. Clean and well lubricated, courtesy of my Scott Oiler, and no hint of any stiff links.

Comparing standard Trophy (left) and modified Daytona (right) sprocket covers

Daytona sprocket cover with 12mm holes for inspecting gearbox sprocket

I had previously modified my Daytona sprocket cover so I could inspect the gearbox sprocket without draining the oil. At my last oil change, I had drilled three 12mm holes at an appropriate radius the gearbox output shaft. The sprockets showed some clear signs of wear but not particularly worrying, I thought. Then I noticed that the Haynes and Factory workshop manuals quoted a maximum wear limit for the chain as 319mm for a 20 link length. It isn't possible to measure this length along the bottom run of the chain because the exhaust and subframe get in the way. So I removed the chain guard and saw 20 lengths of my chain were at 320mm. Let's say, at the wear limit.

I decided to try a new set of chain and sprockets. The chain was on the bike when I bought it and so has lasted at least the 9500 miles I have covered since obtaining the bike. Who knows how many it had covered beforehand. I decided to make another trip to see Trevor. He offers DID and a cheaper (Triple S) chain options. I so rarely need to change a chain, and in my younger days suffered so often with 'bargain' chains, for me it was a no-brainer to spend a bit more for the fantastic quality of genuine DID.

Contents of a DID VX chain and sprocket kit from Sprint Manufacturing

Trevor also sells Sunsstar gearbox sprockets too - again super quality - and includes a gasket for the sprocket cover and locking tab washer in the kit. All for £140. I was happy with that.

Having split the old chain, by first grinding of the rivet heads and then pressing out the old soft link, I was able to measure the wear on the pivot pin. In the picture above, the wear line is visible but it was less than I had imagined - the step is about 0.2 mm so about 5% of the thickness of the pin. The gearbox sprocket was getting to the end of its life. 

I'm guessing the accumulation of wear on the chain 'stretching' the distance between the pins under load, plus the gearbox sprocket, was allowing the riding  up and dropping of the chain at low speed. At higher speed, the centripetal acceleration could have prevented this as the chain was more uniformly pushed out from the rear sprocket, effectively gripping the gearbox sprocket more firmly. 

Locking tab washer in place

I treated the new Sunsstar sprocket to a dousing of paint because it appeared to be in bare metal. It took quite a bit of jiggling to settle on the splines of the output shaft. It was a really satisfying fit when it went home. Beautiful close tolerance for a part that is under huge pressure in operation. 

DID VX Professional, torqued up gearbox sprocket and
tab washer flattened into place. Perfick. 

I have a chain splitter and riveter kit. It was a bit pricey but rewarding to use with a pricey C&S kit. I measured the external width of the fixed links as 20.5mm so clamped up the soft link to the same degree. The gearbox sprocket nut was then torqued down but selecting first gear, turning the rear wheel against engine compression and standing on the rear brake.

And the result?

No more clonking any more. A result indeed.