I must have too much free time. Here's another product of my idle mind...
I was looking at the primary chain on the 3C I'm working on, wondering how much work it had done. I figured there had to be a simple way to measure the stretch in the chain, so I dived headlong into a bit of theoretical geometry. The bloody geometry wasn't as simple as it first looked, so it took a lot of head scratching and number crunching, but I eventually got it licked and came up with the chart below.
The shaft centre spacing of 176.5mm on Laverda triples is no accident. That's the exact spacing required for the 76 pitches of 3/8" chain to have zero slack. Well, that's the theory anyway. In practice, the chain will have a little slack because of the manufacturing clearances in the chain links. Anyway, the amount of slack in the chain can be measured by pressing down on the middle of the top span of chain and measuring how much sag there is. It's easy enough to measure - just lay a straight-edge across the two sprockets, press down on the chain and measure the amount of sag with a steel rule or vernier. You don't have to be super accurate, the nearst millimeter will do.
Look up the sag on the horizontal axis of the chart, and the % stretch (in excess of the theoretical chain length of 76 x 3/8") is on the vertical axis. If you prefer using a formula, this is a near enough approximation:
% stretch = 1.69 x Sag2/1000
Incidentally, the 3C had 9.5mm of sag which is equvalent to 0.15% stretch. Using Andy's rule of thumb that 1% stretch is a serviceable limit, the chain should be good for a while yet. Mind you, 1% stretch means a sag of about 25mm. I don't know whether the tensioner shoe has enough travel to take up that amount of slack. I think I'd pension the chain off long before it got that sloppy. Maybe around 12 or 13mm (1/2") sag will give a good safety margin. That's about 0.25% stretch.
Now waiting for Andy to load his gun with experience and practical know-how and shoot holes in my purely theoretical analysis
Cheers,
Cam