Belt drive systems from Gates Carbon Drive offer a multitude of advantages over traditional, chain-driven drivetrains: Belts do not rust or require lubrication, offering year-round reliability with minimal mess and virtually no upkeep. You can simply wipe ‘em down after a messy ride and immediately be ready to roll again—no chain cleaner or removal necessary. Belts also last 5-10x longer than most chains and don’t stretch. As a result, belts and sprockets tend to wear at the same (significantly slower) rate. The lack of belt stretch also means that belt tension typically does not require adjustment over the entire life of the belt. Set and forget!
So, what do you need to know to run one yourself?
Gates Carbon Drive systems contain three essential components: a front sprocket, rear sprocket, and the belt itself. There’s a huge range of sprocket interfaces and belt lengths available, and here at Cycle Monkey, we often field questions from riders wondering about compatibility with their current bike or the best choices for a custom build project. Many of these questions relate to sprocket sizes and belt length.
Luckily, Gates has created a handy app for mobile and web that’s incredibly helpful in answering these types of questions. To get started, all you need to know is your desired gear ratio and the minimum and maximum effective chainstay lengths for the frame you’ll be working with.
Gear ratio is the final drive ratio you’d like to achieve, and is calculated as front sprocket size divided by rear sprocket size, so that a drivetrain with equally sized front and rear sprockets has a ratio of 1 (ex. 32 Tooth Front / 32 Tooth Rear = 1 / 1 = Gear Ratio of 1. Smaller gear ratios make it easier to climb hills, while larger gear ratios offer a higher top speed and prevent you from spinning out at too high a cadence once you get up to speed. If you live in a hilly area or are a mountain biker, you’ll want to consider a smaller ratio. Stronger riders or those primarily riding on flat ground will benefit from a larger one.
Riders accustomed to calculating gear ratios with their chain-driven setups will be pleased to hear that this ratio calculation is the same. The number of teeth on belt sprockets will typically be greater, but gearing will be the same as long as the ratio is maintained.
Effective chainstay length refers to the distance from the center of the bottom bracket to the center of the rear axle. Belt-driven bicycles require this length to be adjustable in to apply proper belt tension and account for different gear ratios. As a result, belt-compatible frames have a range of chainstay length, rather than a single value like that you’d typically see on a manufacturer’s geometry chart. This adjustability is achieved with adjustable rear dropouts, an eccentric bottom bracket, or horizontal dropout slot lengths.
|Sliding rear dropouts are one way to achieve the flexibility in chainstay length that's required by belt systems|
Once you’ve determined these two values, you can use the Gates calculator tool to determine which sprocket sizes will provide your desired gear ratio, along with the correct belt length. If you get stuck, feel free to drop us a line at firstname.lastname@example.org, and we’ll be happy to give you a hand.
The interface differs slightly between the mobile and web apps. Generally speaking, the web app is easier to use if you’re trying to determine which components are compatible with your bike, while the mobile app allows you to explore the gear ratio and compatible chainstay lengths of the various sprocket/belt combinations more quickly.
On the web: Enter your desired gear ratio (or front and rear sprocket sizes) along with your chainstay length (in mm), and select the “Find Solutions” box. From there, you’ll receive a complete list of sprocket and belt combinations that will fit your needs. They’re organized from top-to-bottom in terms of compatibility.
On mobile: You’ll see three columns on the mobile app which allow you to scroll between various sizes for the front sprocket, rear sprocket, and belt. The “center distance” (chainstay length) and gear ratio will be displayed below for any given combination. From there, you can select the “catalog” tab in the bottom navigation to find sprockets and belts in the sizes you’re looking for. The mobile app also features a nifty belt tension meter that measures sound waves with your phone’s microphone to evaluate belt tension!
If you’d like to further refine your search based on desired wheel/tire size and crank length, the “advanced options” selection on both interfaces will allow you to do so.
There are a few additional considerations when it comes to building your ideal Gates Carbon Drive system, As mentioned above, belt systems require some degree of flexibility when it comes to chainstay length or “center distance” in order to properly install and tension the belt. If you plan on only running one gear ratio throughout the life of your bike, you’ll need a 12mm range of movement at a minimum. If you want to experiment with different sized sprockets and gear ratios, Gates recommends a 25mm range of movement.
You’ll also need to make sure that the front sprocket you select is small enough to clear the frame/chainstay. Note that belt sprockets are wider than chain sprockets, and that you’ll need at least 2mm of clearance between the sprocket and chainstay. Some crafty guesswork with a ruler can go a long way in determining whether or not the sprocket size you’ve selected will clear your frame.
Of course, you’ll need to select sprockets with the correct interface for your bottom bracket and rear hub. Unless you plan to go singlespeed, you’ll need to run an internal gearing system, such as Rohloff’s SPEEDHUB 500/14 internal gear hub or Pinion’s gearbox. These systems share many benefits with Gates Carbon Drive, including a low maintenance burden, ease-of-use, and incredible durability.
Of course, adding multiple gears into the equation with an internal gearing system means that you’ll have more than one gear ratio to consider when selecting sprockets. If you’re interested in a more detailed comparison of gearing between derailleurs and these internal systems, check out our posts on Calculating Gear Ratios with Rohloff Hubs and Calculating Gear Ratios with Pinion Gearboxes. If you’d like to compare systems, check out this helpful online bicycle gear calculator.
At Cycle Monkey, we love to help riders navigate the process of retrofitting an existing bike or bring their vision to life with a custom build. If you’re curious about running a Gates Carbon Drive system yourself, don’t hesitate to give us a shout!
Find the web app here
Download the mobile app from the Apple and Android App Store.