In the latest issue of Inside Motorcycles, my article discusses some of the advantages of changing your motorcycle's final-drive gearing. This can make a significant difference, especially on late-model sportbikes that are equipped with tall gearing and very close-ratio gearboxes. For track-day riders and racers, there are some additional subtleties to consider when changing gearing to suit a particular track.
In general, final gearing should be selected so that your bike just reaches maximum rpm in sixth (or top) gear at the end of the longest straight on the track. This gives you full use of the gearbox without having to worry about hitting the rev limiter in top gear on the straight; each individual gear ratio will be as short as possible, for maximum acceleration and a good launch at the start. From this starting point, looking at performance in various parts of the track can guide you to make a gearing change.
If the gearing in more than one corner is not ideal, you can change the final-drive gearing to a higher or lower ratio to better suit those particular corners - although there may be a trade-off on other parts of the track. For example, when I raced my TZ250 at Shannonville, I started with final gearing to reach maximum rpm in sixth at the end of the back straight. But in the tighter turns on the front section, the engine was either revving too high in first gear or bogging in second. I switched to a taller final ratio to make first gear more useful in the tighter turns. The tradeoff was that I ended up not using sixth gear at all on the back straight, and first gear was more difficult to use at the start of the race.
One other aspect to consider is that the spacing between gears (in terms of ratio) is not the same between each gear. Almost every gearbox has a fairly large gap between first and second, and the ratios get successively closer with each higher gear. This means that if you can use the higher gears more than the lower gears, engine revs will drop less with each shift and your bike will accelerate better. Continuing with my TZ250 example above, I could have selected shorter overall gearing to not use first gear at all, and dealt with the engine over-revving a bit on the back straight. This would have also given me the benefit of using the higher, closer ratios, and also decreased the chances of missing the first-second shift. Overall, however, using first through fifth was a better combination for that particular track. Everything is a trade-off, and it's a matter of finding what works best for the greatest portion of the track.
Attached below is an Excel gearing chart that I use. You can input all the information for your bike - maximum rpm, the internal gear ratios, final-drive ratio and tire circumference - and the chart creates several tables. Top speed in each gear for a selection of sprockets is shown; this data can help you better make a decision when it comes time to change your gearing. Also included is a table showing how swingarm length changes with each sprocket selection. If the wheel has to be moved significantly forward or rearward with a gearing change, you can consider another combination that is close to what you need, or go with a shorter or longer chain.
Gearing is one setup variable that is easy to change and can make a huge difference to your bike's performance on the track. Most racers have a large selection of sprockets on hand and constantly change gearing, but even track-day riders can take advantage of a change to better suit a particular track.
Download the Gearing Excel file here.