FAQ: Gear ratios and sprocket selection

[MattR]

Question:
How do I determine the gear ratio based on my front and rear sprockets?
What sprockets do I use for more torque and better acceleration?
I want more top speed... what sprockets should I change to for road use?

Answer:
The best way to explain gear ratios is to see them in tabular format. As shown below, the same gear ratio can be obtained by several combinations of front and rear sprockets. The gear ratio is calculated by dividing the "rear sprocket number of teeth" by the "front sprocket number of teeth". Lower gear ratios provide more top speed but less torque and acceleration. Whereas, higher gear ratios provide more torque and acceleration but less top speed.

An example is shown below to help interpret the table. It may be seen that a reduction/addition of 1 tooth on the front sprocket is equivalent to approximately 4 teeth on the rear sprocket. It should be noted that smaller front sprockets (such as 12T) can cause more chain wear. While swapping front sprockets may offer convenient setups between road and tight singletrack, this does result in a larger jump. By changing the rear sprocket, fine increments in gear ratio can be obtained with each tooth.

[flybars]

Thanks Matt. Very handy. My FSE450 came with 13t front and 48t rear. I'm thinking I'll drop one tooth in front for tight technical stuff, keep the 13t for some dualsport stuff, and go with a 14t for most dualsport and commuting to work etc. I just hope I don't have to change my stock chain length. It'll be close.
It's very easy and much less time consuming to change your front sprocket for different types of riding. Works great if your on a long trip and U want to go explore some interesting terrain. Takes about 5 minutes.

[widebear]

Thanks for posting but his is only part of the picture Matt. Whats more important than final drive numbers are the inter gear ratios.

[MattR]

Here is the influence of the internal and final gear ratios relative to speed. I only made plots of several gear ratios. This is good for "visual" purposes. If your looking for a particular one, I can post up others as needed.

All of these were calculated using the GasGas internal gear ratios posted in their manual. I used the stock gear ratio (13T front/48T rear) for reference in all comparison plots

Here is a screenshot of my Excel spreadsheet. The information is entered in the pink column.



Here is a comparison between (13/48) and (13/52):



Here is a comparison between (13/48) and (13/50):



Here is a comparison between (13/48) and (13/46):

[widebear]

sweeet! Thanks, now if we find out what a 250 really revs you could extropolate the numbers to reflect this and we could settle once and for all the theoretical top speed debate

[GMP]

A lot of variables there between years and diffferent components. I think the '07 revs the highest by far compared to my '00 and '03. I've seen over 70 MPH on my ICO with 13/52 several times.