Tuesday, 18 August 2009
In its original state, my Husqvarna SMR ran a 48/17 sprocket combo. But that was with the dinky little 17 inchers. Now, with the huge 19-inchers, and the swollen Maxxis flat track tires, we need a different gearing altogether. There is no shortage of "gear calculators" to be found on the web and some are really OTT (here's one), but some simple maths is actually enough.
Here goes: The stock sprocket ratio is 48/17=2,83. The standard tire (150/60-17) had a circumference of about 1,92 meters.The new tire has a circumference of about 2,20 meters, i e about 15% bigger. That means we need to lower the sprocket ratio by the same amount (15%) to get the same "total gearing". 2,83/1,15 comes out to about: 2,46, which is our target ratio. I wanted to keep the front sprocket with 17 teeth and only change the rear sprocket. So I chose a 42 tooth rear sprocket, giving a sprocket ratio of 2,47 which is pretty close to what I need. I've always thought that the original gearing was a little long and I have a 16 tooth front sprocket in reserve should the gearing turn out to be on the long side.
BTW: does anyone know of a company that sells stainless Allen bolts with "countersunk" heads in 7/16''? The ugly hex bolts I'm using now have to go. I've been googling furiously, but to no avail.