One of the easiest ways to give your bike snappier acceleration and feel like it has a lot more power is a simple sprocket change. It’s an easy job to do, but the hard part is figuring out what size sprockets to replace your stock ones with. We explain it all here.
It’s All About The Gearing Ratio
Your gearing ratio is, simply put, the ratio of teeth between the front and rear sprockets. This ratio determines how engine RPM is translated into wheel speed by the bike. Changing sprocket sizes, front or rear, will change this ratio, and therefore change the way your bike puts power to the ground. OEM gear ratios are not always ideal for a given bike or riding style, so if you’ve ever found yourself wishing you had better acceleration, or found that your bike lugs around at low speeds, you may simply need to alter your current gear ratio into something that’s more suitable for you.
Example #1: Street
Understanding gearing ratios is the most complex part of deciding on a sprocket combo, so we’ll start with an example to illustrate the concept. My own bike is a 2008 R1, and in stock form it is geared very “tall” in other words, geared in such a way that it could reach very high speeds, but felt sluggish on the lower end.) This caused street riding to be a bit of a hassle; I had to really ride the clutch out a good distance to get moving, could really only use first and second gear around town, and the engine felt a little boggy at lower RPM’. What I needed was more acceleration to make my street riding more enjoyable, but it would come at the expense of some of my top speed (which I’ not using on the street anyway.)
So let’s look at the factory set up on my bike, and see why it felt that way. The stock sprockets on my R1 are 17 teeth in front, and 45 teeth in the rear. Some simple math gives us the gearing ratio: 45/17=2.647. Now I have a baseline to work with. Since I want more acceleration, I’ll want a higher gear ratio than what I have, but without going too extreme to where I’ll have uncontrollable acceleration, or where my RPM’s will be screaming at highway speeds.
Example #2: Dirt
Several of our team members here ride dirt, and they change their set-ups based on the track or trails they’re going to be riding. One of our staff took his bike, a 2008 Kawasaki KX450, on a 280-mile Baja ride. Because the KX450 is a big four-stroke with gobs of torque across the powerband, it already has plenty of low-end grunt. But for a long trail ride like Baja where a lot of ground needs to be covered, he wanted a higher top speed to really haul across the desert. His solution was to swap out the 50-tooth stock rear sprocket with a 48-tooth Renthal Sprocket to increase speed and get a lower cruising RPM (or, in terms of gearing ratio, he went from 3.846 down to 3.692.)
Another one of our team members rides a 2003 Yamaha YZ125, a light, revvy two-stroke, completely different from the big KX450. His preferred riding is on short, jumpy racetracks, where maximum drive is needed in short spurts to clear jumps and power out of corners. To get the increased acceleration he wanted he geared up in the rear, from the stock 49-tooth to a 50-tooth sprocket also from Renthal, increasing his final ratio from 3.769 to 3.846 (in other words about a 2% increase in acceleration, just enough to fine tune the way the bike responds to the throttle.)
It’s All About The Ratio!
What’s important to remember is that it’s all about the gear ratio, and I have to arrive at a ratio that will help me reach my goal. There are a number of ways to do this. You’ll see a lot of talk on the web about going “-1”, or “-1/+2” and so on. By using these figures, riders are typically expressing how many teeth they changed from stock. On sport bikes, common mods are to go -1 in front, +2 or +3 in back, or a combination of the two. The problem with that nomenclature is that it only takes on meaning relative to what size the stock sprockets are. At BikeBandit.com, we use exact sprocket sizes to indicate ratios, because all bikes are different.
To revisit my example, a simple mod would be to go from a 17-tooth in the front to a 16-tooth. That would change my ratio from 2.647 to 2.813. I did this mod, and I had noticeably better acceleration, making my street riding a lot easier, but it did lower my top speed and threw off my speedometer (which can be adjusted; more on that later.) As you can see on the chart below, there are a multitude of possible combinations to arrive at the ratio you want, but your options will be limited by what’s possible on your particular bike.
For a more extreme change, I could have gone to a 15-tooth front , which would make my ratio exactly 3.0, but I thought that would be excessive for my taste. There are also some who advise against making big changes in the front, because it spreads the chain force across less teeth and around a tighter arc, increasing wear.
But remember, it’s all about the ratio, and we can change the size of the rear sprocket to alter this ratio also. So if we went down to a 16-tooth in the front, but simultaneously went up to a 47-tooth in the rear, our new ratio would be 2.938; not quite as extreme. 16 in front and 46 in back would be 2.875, a less radical change, but still a little more than doing only the 16 in front.
(Consider this: because the ratio is what determines how your bike will behave, you could conceivably go down on both sprockets and keep the same ratio, which some riders do to shave weight and reduce rotating mass as the sprockets and chain spin.)
The important thing to keep in mind when selecting new sprockets is that it’s all about the ratio. Figure out what you have as a baseline, determine what your goal is, and adjust accordingly. It will help to search the web for the experiences of other riders with the same bike, to see what combos are the most common. It is also a good idea to make small changes at first, and run with them for a while on your favorite roads to see if you like how your bike behaves with the new setup.
There are a lot of questions we get asked about this topic, so here are a few of the most instructive ones, answered.
When choosing a sprocket, what does 520, 525, and 530 mean?
Basically, this refers to the thickness of your sprockets and chain (called the “pitch”) 520 is the thinnest and lightest of the three, 525 is in the middle, and 530 is the beefiest. Many OEM components are 525 or 530, but with the strength of a high quality chain and sprockets, there is usually no danger in switching to the lighter 520 setup. Important note: always make sure you install components of the same pitch; they are not compatible with each other! The best course of action is to buy a conversion kit so all your components mate perfectly, like the Vortex 520 Conversion Kit. (For more information on chain and sprocket sizing, check out our video below!)
Do I have to switch both sprockets at the same time?
This is a judgment call, and there are differing opinions. Generally, it is advisable to change sprocket and chain components as a set, because they wear as a set; if you do this, we recommend a high-strength aftermarket chain from a top brand like EK, RK, and DID.
However, in many cases, it won’t hurt to change one sprocket (usually the front.) If your chain is relatively new, it will not hurt it to change only one sprocket. Considering that a front sprocket is typically only $20-30, I recommend changing it as an inexpensive way to test a new gearing ratio, before you take the plunge and spend the money to change both sprockets and your chain.
How will it affect my speed and speedometer?
It again depends on your ratio, but both will generally be altered. Since most riders opt for a higher gear ratio than stock, they will experience a drop in top speed, and a speedometer readout that says they are going faster than they are. Conversely, dropping the ratio will have the opposite effect. Some riders purchase an add-on module to adjust the speedometer after modifying the drivetrain.
How will it affect my mileage?
All things being equal, going to a higher gear ratio will drop your MPGs because you will have higher cruising RPMs for a given speed. More than likely, you’ll have so much fun with your snappy acceleration that you might ride more aggressively, and further decrease mileage. But hey, it’s a bike. Have fun with it and be glad you’re not driving a car.
Is it easier to change the front or rear sprocket?
It really depends on your bike, but neither is typically very difficult to change. Changing the chain is the most complicated task involved, so if you’re changing only a sprocket and reusing your chain, you can do whichever is most comfortable for you.
An important note: going smaller in front will loosen the chain, and you’ll have to lengthen your wheelbase to make up for it; going up in the rear will likewise shorten it. Know how much room you have to adjust your chain either way before you elect to do one or the other; and if in doubt, it’s your best bet to change both sprockets and your chain all at once.
There are aluminum and steel rear sprockets, which one should I get?
Traditionally, aluminum sprockets are light but wear quickly, and steel sprockets are durable but very heavy. These days however, most aluminum sprockets are hard anodized which dramatically extends their life, and there are many new super-light steel sprockets with the strength of steel while being almost as light as their aluminum counterparts. It ultimately comes down to preference, but generally aluminum is a little lighter and steel is a little more durable.
Some interesting alternatives however are sprockets with a light aluminum center surrounded by a ring of strong steel teeth, such as the Renthal Twinring. Besides looking unique, they offer the best of both aluminum and steel in one package and are an excellent middle ground solution.
Got anymore questions about changing your sprockets? Leave them in the comments below and we’ll help you out!