Originally Posted by saunders1313
So if I make the rear sprocket smaller I will be able to go the same speed, but at lower rpms? So instead of say my 4th gear being good for 35mph to 50 mph it would shift to maybe 45 mph to 60 mph? Just trying to get a feel for this. How difficult was it to get used to and how hard is it to change to a smaller sprocket? Do I need to change anything else, or just the chain tension? (smaller sprocket would equal more slack in chain)
You can figure out exactly what the speed difference will be with a different size sprocket, at a given RPM.
First you count the teeth on both of your current sprockets. Lets say it is 15T front and 45T rear. That is a ratio of 45:15 or 3:1.
Now lets say you change the rear to 40T. You now have a ratio of 40:15 or 2.666:1.
If you divide the old ratio by the new ratio you get 1.125. This means that your new sprocket ratio will make the bike go 1.125 times as fast at the same RPM as the old sprocket ratio.
If it used to go 35 mph at 3000 RPM now, with the new smaller rear sprocket, it will go 35 X 1.125 (39.4 mph) mph at 3000 RPM.
The difficulty of changing the sprockets is entirely dependent on what bike you are talking about. Back in the day, it was typically a one hour job on most bikes I had ever owned. The chain will almost certainly need to be shortened if your new sprocket is more than a tooth or so smaller.
Again, depending on your bike, it may be much simpler and cheaper to change the front sprocket. Just do the math as above to learn what size will make what difference.