Originally Posted by carwyn
My xt has a 39 rear sprocket fitted for some reason and a 15 front? I still think it's lacking a bit of power so what should I go for? 45 std or a 47? I thought before I bought this bike it would at least get some air under the front wheel off the throttle in first?
If you work out the power to weight ratio (and I mean real power, and real weight!) the XT, bless her, is not a powerful bike. Sure, you can lift the front, but not at will with a twist of the grip.
As for gearing, I've had a play with mine, and done the maths. (and it has improved things) If you have Excel, or open office, use the spreadsheets and formula functions to work out the effects you'll have.
In essence, (Rear tooth count / front tooth count) = Drive ratio
((New ratio / Old ratio)-1)*100 = change as a percentage
What this means is change the tooth count, as you think you want to go for, and work out what the new drive ratio is, and then work out the percentage of change this is, by dividing the new ratio by the old, and then converting to a percentage.
Once you have the percentage change (6% or whatever), RPM at a given speed will increase/decrease by the percentage change.
So, if at the moment, 60 mph is 4,000 rpm, and you lower the gearing by 10%, when you are at 60mph, you will be at 10% higher revs, which in this example = 4,400 RPM.
For me, I thought my gearing was standard, and figured losing a tooth at the front would make a big difference. As it turned out, the last owner had added a tooth at the front, so I actualy went down two teeth at the front (roughly equivalent to 6 teeth at the back). I',m glad I didn't know, because even the two front teeth drop didn't give me the increase in acceleration I was hoping for.
I wanted to be able to flick the throttle open in first, and pick the front wheel up, to get up steps, over logs etc. The problem I have now, is the chain isn't long enough to accomodate a bigger back sprocket.
My next changes will come when I need a full new set of chain and sprockets.