Gearing generally refers to the gear ratio between the two sprockets; the drive sprocket, and the driven sprocket on chain-driven mopeds.
Front and Rear Sprockets
The drive sprocket is physically connected to the engine, and is typically farther 'forward.' It is often referred to as the front sprocket.
The driven sprocket is physically connected to the wheel, and is typically behind the front sprocket. It is often referred to as the rear sprocket. The rear sprocket is connected to the front sprocket by the drive chain.
Sprockets are typically defined or described by the number of teeth they have. This number of teeth also determines the size of the sprocket. A sprocket with a higher number of teeth must also be larger, if it is to fit the same type of chain as it did when it had fewer teeth.
The number of teeth on the two sprockets also determines the gear ratio. This ratio is often expressed by listing the number of teeth on the front sprocket, then the number of teeth on the rear sprocket. For instance, the stock gearing on many single speed Puch mopeds was 16:45.
The gear ratio determines the rate at which rotation is transferred from the engine to the rear wheel. If both the front and rear sprockets had the same number of teeth, the gear ratio would be 1:1, and the rear wheel would rotate at exactly the same rate as the front sprocket. As you increase the number of teeth on the rear sprocket, it has to turn less and less for every rotation of the front sprocket. As you increase the number of teeth on the front sprocket, the rear wheel must turn farther for every rotation of the front sprocket.
A 'higher,' or 'taller' gearing is one which forces the rear wheel to rotate more for every turn of the front sprocket. A 'lower gearing' is one which allows the rear wheel to rotate less for every turn of the front sprocket. It is easier for the engine to accelerate when it does not have to turn the rear wheel as far for every rotation of the front sprocket. This improves the acceleration of the moped. Higher gear ratios make it more difficult for the engine to accelerate, especially from a dead stop, but they allow the front sprocket to rotate the rear wheel further for every rotation, improving top speed at high RPMs.
Quick Gearing Guidelines
Some simple gearing guidelines based on these principles:
- A smaller front sprocket will increase acceleration (torque) at the expense of top speed.
- A bigger front sprocket will increase top speed at the expense of acceleration (torque).
- A smaller rear sprocket will increase top speed at the expense of acceleration (torque).
- A bigger rear sprocket will increase acceleration (torque) at the expense of top speed.
Super Handy Resources
The Gearing Spreadsheet is a great way to see how fast you might be going if you changed your gearing.
Similarly, Jaap's Puch Calculator can perform this function, only it's a downloadable program rather than a spreadsheet.
The newest resource is the web-based calculator at: Marty's Garage