Simple Gear Ratio for Go Karts Explainedby John Willis
Go karts have to transfer power from the crankshaft to the wheels. The simplest setups use a centrifugal clutch and a single drive gear. But most go karts are set up more like a motorcycle, with transmission gears, a clutch sprocket and the final drive sprocket. In complex applications, like Formula One, transmission gears are changed to match every racetrack. Go karts are more likely to change only the clutch sprocket or final drive sprocket.
Engine RPM to Wheel RPM to Speed
Before you consider your go kart's gearing, it's important to understand the simplest ratios of gearing. Internal combustion engines all have a crankshaft. When the engine is running, its speed is measured in revolutions per minute. So, 3,000 RPM means the crankshaft turns 3,000 revolutions per minute. Knowing your gear ratios tells you how many times your wheels turn for every turn of the engine in any gear. If an engine has a direct drive, it has a 1:1 ratio: The wheel turns one revolution for every turn of the engine. If the engine turns 3,000 times in a minute, the wheels turn 3,000 times in a minute.
Gear Box or Transmission
Simple go karts may not have a multi-gear transmission. Go karts that do won't need to have their transmission gearing changed unless they're in an advanced racing team. The important thing to remember is that transmission gears are a ratio between the speed of the engine and the speed of the clutch sprocket. If you were to change second gear in the transmission, for example, it wouldn't affect the final drive of the go kart in any other gear. When you change the clutch sprocket or final drive sprocket, however, either one will change the characteristics in every gear.
Clutch Sprockets and Final Drive Sprockets
Drive sprockets, rather than being measured by their circumference, are typically measured by how many sprocket teeth they have for a standard-sized chain. Clutch sprockets tend to be quite a bit smaller than final drive sprockets -- sometimes one-fifth the size or smaller. You may find this counter-intuitive, but to go faster, the final drive sprocket needs to be smaller, not bigger. Conversely, to go faster, the clutch sprocket needs to be bigger. So, a simple way to change your gearing is to have one or more extra primary drive sprockets and/or final drive sprockets. Remove the chain and replace one of the sprockets with a sprocket that will either increase or decrease the wheel speed.
While it's simple enough to switch a clutch sprocket from a 10-tooth to a 12-tooth sprocket to gear the go kart faster, there's one important caveat. The gear ratios of the clutch and final drive will only make the go kart faster if the engine has the power to move it faster. If there's no modification to the engine and you gear the kart faster (taller), you'll do so at the expense of quickness. That's the trade off. Lower gearing equals quickness; taller gearing equals speed. To maximize the quickness of a kart, you may have to adjust the transmission gears, but that's not simple. To make a quicker and faster kart, you need to develop an engine with more torque. For simple adjustments, stick with having a two- or three-clutch sprocket. This will allow you to trade speed for quickness and quickness for speed without a lot of fuss.