Tuesday 9 May 2017

Dirty Weekend

My weekend was dirty, I didn't get a lot of sleep, and I achieved almost nothing useful. Stop reading now if you don't want to hear about me taking the clutch out of my motorbike.

I ride a high-powered moped called a Yamaha TMax. It has chewed through its clutch in only 25000 miles and since I've had new friction material put in, there's a nasty clunk somewhere in the drive train. In some ways the TMax is like an old British bike, it's a parallel twin with a 360 degree crankshaft, the crankcase splits into left and right halves and you can't apply the brake with your right foot.

The bike has too many gearboxes. It's got a V-belt variomatic transmission, a centrifugal clutch, a vestigial single speed gearbox in the back of the crankcase and a wildly complicated final drive chain running in an oil-batch. As the nursery rhyme goes:
The crankshaft's connected to the variator.
The variator's connected to the V-belt.
The V-belt's connected to the main sheave.
The main sheave's connected to the clutch basket.
The clutch basket's connected to friction plates.
The friction plates're connected to the plain plates.
The plain plates're connected to the clutch boss.
The clutch boss's connected to the first motion shaft.
The first motion shaft's connected to the output shaft.
The output shaft's connected to the chain wheel.
The chain wheel's connected to the drive chain.
The drive chain's connected to the intermediate sprocket.
The sprocket's connected to the other chain.
The other chain's connected to the splined hub.
And that's what turns the wheel.

If the fault was in the vestigial gearbox or the final drive it would have to be engine out, cylinder block off and split the crankcase. I think the bike would be written off. It took me all weekend to take the clutch out and put it back in again, amongst hundreds of other ridiculous things you have to take the coolant out and the water pump off. Now that I've done it once, and I've built the tool to hold the clutch basket I think I could do it again in about 8 hours.

The good news is that I've found the fault and the bike isn't written off. The bad news is that I've no idea if or how it could possibly be fixed. The gear that takes power from the clutch boss to the first motion shaft is helical and it generates an axial thrust. The clutch boss slides up the shaft until it hits the basket. The end-float is highly critical, wrong, and seemingly can't be adjusted.

Richard "And my speedo's broken" B

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