The mystical art of mixing it up
Chances are, if you're reading this, you're looking to understand how you can adjust the power delivery on your motorcycle. Perhaps you have a 125cc and you'd like to have a bit more top end speed, or your sports bike is geared too tall for city riding. Looking into gearing can start to feel a lot like a physics lesson from school, but it doesn't have to be complicated. Read on to learn about how you can adjust your gearing to suit you.
If you find your bike a bit sluggish off the line, you might be thinking about gearing it to give better acceleration. This is achieved by reducing the number of teeth on the front sprocket, or increasing the number on the rear (or a combination of both). This gives you a higher gear ratio. This is known as 'short' gearing. Better acceleration comes at the sacrifice of top end speed. You'll get there quicker, but you'll find your bike topping out at a lower speed. This is better for around town riding where you don't have the space to get up to higher speeds.
Improving top end speed is the inverse of increasing acceleration - you'll need to increase the size of the front sprocket, or decrease the size of the rear (or a combination of both). This gives you a lower gear ratio. This is knowns as 'tall' gearing. It'll take you longer to accelerate, but you'll be able to reach higher speeds. This is more suitable for long distance rides where fuel efficiency and lower revs at higher speeds is important.
Bear in mind, altering the gearing of your bike can require you to adjust the length of your chain, depending on how much of a change you make. sprocketcalculator.com will calculate the approximate length of chain required for a given sprocket setup. It probably goes without saying, but you will also have to adjust your chain tension after changing your gearing.
The gear ratio is the number of times the front sprocket has to rotate in order to turn the rear wheel once. A higher ratio tends to result in more wear. It's worth taking a look at your stock gearing setup before making any changes. For example, my MT-07 has a 16 tooth front and a 43 tooth rear sprocket. If you divide the rear by the front, you get the gear ratio: 43/16=2.6875. The JT sprocket catalogue helpfully gives you an idea of how you can change your sprocket setup in order to adjust the gearing, and also shows you the percentage change. In order to increase acceleration, I'd have to increase the gear ratio. If you look at their Gear Ratio Chart, you can see that reducing the front sprocket by two teeth would get me a ratio of 3.07. To get a similar change with the rear sprocket, I'd have to go for a 49 tooth sprocket. Changing the front sprocket has a more dramatic impact on your ratio than the rear. Equally, you can use a combination of both. A 15 tooth front sprocket and a 46 tooth rear would also give me a ratio of 3.07. That equates to a 14% alteration in the gearing, which is pretty extreme! It's worth hunting for forums on your particular bike to see what other people have tried and tested. If unsure, it's better to make small adjustments of one or two teeth, unless you are looking to become a stunt rider! Remember - altering gearing will also affect your speedo reading! It is also worth noting that standard sprockets made to fit your bike may not come in your desired sizes (the JT range for the MT-07 for example is quite limited) - in this case, you can always have custom sprockets made if you're that into it.
Ultimately, you don't have to change your gearing too much to make a noticeable difference, but it can be a bit of a faff to mess with - new sprockets and chains as well as tools to correct your speedo which makes dabbling too deeply in gearing a bit of a dark art - for those who are into track day riding and the like. Generally, most motorcycles are set up out of the factory to have a fairly balanced ratio to allow for decent acceleration and top end, but not the best of either world. If you really have a hankering to change it up, go for it! But be prepared for a fair amount of work!