This is a very frequently asked question among new go-kart owners, “What gear ratio should I have?” The short answer will disappoint you because there is no short answer, It depends on a lot of things!
So, what does it depends on?
- What Engine are you using?
- Is your go-kart for off road purpose or racing on flat tracks?
- What is the weight of the driver and the kart?
- How big are your tires?
- What transmission system are you using (clutch or a Torque converter)?
Even though there is no way to tell the best ratio for your exact situation without having a look at your Go-kart, I’ve made gear ratio charts for common Go-kart engines with respect to tire sizes and the Transmission system used.
If you use a gear ratio suggested in the chart, you’ll most likely face no issues.
Best Gear Ratio for Predator 212
Best Gear ratio for a 4 stroke 6.5 hp Engine like the stock Predator 212/GX200, with a basic centrifugal clutch:
Use this chart as a guideline, if you have bigger tires, use lower gearing, if you have smaller tires use higher gearing.
If you’ve got a racing engine or made big performance upgrades on a regular engine and want a lot of speed then you can use a racing clutch and gear it high.
A torque converter can work on higher gearing in comparison to a stock clutch.
(The Gear Ratio of Torque converter in the Above chart is = Teeth in Axle Sprocket / Teeth in Jackshaft Sprocket)
Please note: If you increase the weight of the kart or driver above the weight recommended by your Clutch/Torque converter manufacturer, you’ll need to lower your gearing further.
Gearing on Bigger Engines (13 Hp+)
On bigger Engines such as Predator 420 or GX390, which produces a lot more torque and power than 212cc, you can go for a little higher gearing and bigger tires, for example, you can use a 6:1 ratio in a 22″ tire with a torque converter.
But usually, people use bigger engines on big buggies (350 lbs+). In such cases, you will need to gear it low.
What is Gear Ratio?
In a go-kart or minibike, the gear ratio is the ratio of angular speed of the engine crankshaft to the angular speed of the axle.
For example, let’s say you have a stock predator 212 engine that is rotating at 3600 rpm and your axle is rotating at 600 rpm, then the gear ratio is 3600:600 or 6:1.
The principle of working is the same as any other gear, the number of teeth determines the speed of one sprocket in respect to the other:
How to calculate the Gear Ratio on your Go-kart/Minibike?
If you have got a simple setup with a chain connecting the clutch sprocket directly to the axle sprocket then,
Gear Ratio = Teeth on Axle Sprocket / Teeth on Clutch
Clutches with Jackshaft in between
If you have a jackshaft in between the clutch and the axle sprocket then,
The jackshaft will have 2 sprockets, let’s say:
Jackshaft Sprocket 1 connects to the axle sprocket with a chain
Jackshaft Sprocket 2 connects to the clutch sprocket with a chain
Gear Ratio = (Teeth on Axle Sprocket / Teeth on Jackshaft sprocket 1) x (Teeth on Jackshaft sprocket 2 / Teeth on Clutch Sprocket)
In other words, you can use a jackshaft to increase and decrease the gear ratio by the factor of Jackshaft driver / Jackshaft driven.
The beauty of torque converter is that there is no constant gear ratio. You can only find the gear ratio at one given moment.
All Torque converters have a jackshaft in between, but that jackshaft will only have 1 sprocket that connects to the axle
Gear Ratio = (Teeth on Axle Sprocket / Teeth on Jackshaft sprocket) x (Belt diameter in Driven pulley/ Belt diameter in Driver Pulley)
Since the active diameter in the Torque converter pulleys keeps on changing, you get different Gear Ratios at all times. In a 30 series Torque converter, the belt diameter ratios range from 2.7: 1 at the low end to 0.9:1 at the high end.
How Gear Ratio Affects the Speed and Torque?
As from the definition of gear ratio, it is a ratio between the angular speed of the motor to the angular speed of the axle/wheel. If your engine’s crankshaft is revolving at 3600 RPM and you have a 5:1 gear ratio, then your wheel will revolve at 3600/5 =72 RPM.
So, mathematically, you can put a smaller rear sprocket to get higher gearing and higher top speed. If we assume that your engine can create the same RPM at those higher gearing.
But, then why not just set a gear ratio to 1:10 and get 36000 RPM on the axle? You might ask.
Well, this is no magic, you can increase your gear ratio to run at an overdrive, i.e, Gear Ratio of less than 1, but the power supply from the engine is constant, regardless of your gear ratio.
Power = Torque x RPM
The Power that the engine creates is dependent on how much fuel is getting burned. The Power from the engine’s crankshaft is what gets transmitted to the wheel (at less than 100% efficiency).
So, if you’re going to change the Gear Ratio and increase the RPM on the axle, you will pay for it with your Torque. If you reduce the gear ratio too much, you will never have enough torque to start (It will be like starting your car at 4th gear).
And Even if you do manage to start moving, a high gearing will burn your clutch or wear your torque converter. Why? Higher gearing will cause slippage in the clutch, meaning more heat, and will result in failure.
And it is worth considering what do you want more? Better Takeoff acceleration? or Higher Top Speed? If you change your gear ratio to have more torque, you’ll be getting more acceleration but top speed will reduce.
Finding the right balance is the key here, you don’t want to have too much of either but you can still make little tweaks according to your liking, if you’ll like to have more acceleration, decrease the gearing by 0.5, if you want more speed, increase the gearing 0.5 more than the suggested gearing in the chart.