Gear ratio for my my buggy.

[Jjasion]

Hey Everyone,
I am new to this site and to working with engines. Everything I have encountered so far while building my buggy has been super fun and interesting. As I am approaching the end of my build I am facing a challenge, what gear ratio to use with my setup for optimal performance.

To start, I have a stock 420cc engine with 22"inch wheels, I plan on putting a 40 series torque converter into the setup. I intend to use this kart for mostly off-road purposes so I read somewhere that the higher the gear ratio the better for climbing steep hills and rocks. I was thinking of using a 7:1 -> 9:1 ratio??

While this does not offer high top speeds I would still like the kart to be fast generally speaking. Any help or advice with the project would be greatly appreciated!

 

[madprofessor]

Your ratio idea is actually pretty good, the 420cc has a lot of torque, usually doesn't need anything like 11:1 ratio. The drawback for you is those big 22" tires, you'll need all that 420cc torque offroad with those. Since there's no knowing for sure how much uphill torque you'll need for your varying terrain, it would be smart to put on a universal sprocket hub (5.25") sized for your axle, and use split sprockets.
Split sprockets and double-split locking collars are among the greatest gokart inventions ever. You don't even have to jack up your kart to slap them on (collars) or make ratio changes (sprocket) in minutes flat, without ever having to R&R wheels, bearings, collars, brake discs, sprockets.
There's a wide range of tooth-count for #35 split sprockets, but you need a #41 or #420 chain and sprockets for that 420cc, and the variety narrows there. I'll attach links for sizing of chains, a gear ratio calculator, double-split collars, sprocket and hubs.
Chain for Go Kart, Mini Bike | GoKarts USA®...................Go Kart Sprockets | Go Kart Sprocket Hubs (gokartsupply.com).....................1” DOUBLE SPLIT (10 PCS) STEEL NEW CLAMPING SHAFT COLLAR BLACK OXIDE Free Ship | eBay.......................Unihub - Sprocket Hub | AZ2556 | Azusa 2556 | BMI Karts And Parts...................Split Sprocket #41/420 - GoPowerSports.com

 

[Jjasion]

Thats awesome, thanks for the helpful links! I have smaller 18" wheels I could slap on there, would that make more sense instead of the 22"?

 

[madprofessor]

I personally would prefer the 18" tires for better power, but I'm a flatlander in Jacksonville, Florida. The drawbacks of smaller tires come when you want to go through holes, climb over rocks, or win a top speed race. Don't forget that for every 2" you drop from a tire's overall height, you lose 1" of ground clearance for the axle sprocket. Depends on your terrain and how much acceleration you want from your particular project, hence the reason for using split sprockets and collars so you can experiment with and change it for very little cost.
Plastic Sprocket Guard - 9" | KM1081 | KM1081 | BMI Karts And Parts

 

[panchothedog]

As Mad has explained you will lose ground clearance with the smaller tires.
With the availability of axle sprocket as well as jack shaft sprockets and the torque provided by your larger displacement engine you should be able to find a combination that works. Aside from ground clearance,the taller tires will also provide better traction.

 

[Denny]

Keep the 22” tires and start out with a 6:1 ratio and see how you like it. Should get around 40 mph with decent hill climb abilities. If the kart is heavy maybe go 7:1. Just make sure to use a Genuine Comet belt! It will solve problems before they start.

 

[madprofessor]

I plan on putting a 40 series torque converter into the setup.

If driving directly from your CVT sprocket because that's all you can do, that's fine, but limits your ground clearance. If you want high torque at lower rpm over unknown terrain, and you don't want to abandon your kart and walk half a mile for help, consider the possible 40 series' jackshafts you can make...............Jackshaft Kit for Mini-Bike or Go-Kart | 400950 | BMI Karts And Parts
While you can only get just a certain maximum number of teeth for an axle sprocket, which might be exactly what you want, your ground clearance will be reduced to getting stuck and your chain ravaged running over a lumpy diaper. A secondary chain drive (via jackshaft) solves all those problems. You could just storage the backplate if you buy a 40 series with one. A common 40 series setup has the driver/clutch mounted directly on the engine's output shaft, and the driven pulley's (driven clutch) mounted on whatever kind of jackshaft you create. I would choose a 3/4" jackshaft and matching TC driven pulley for the strength, and the availability of sprockets.
You can do 6:1 with a single primary chain (12-tooth) to the axle sprocket (72-tooth)((12:72 ratio)(6:1 ratio) and get let's guess 5" clearance. Or you can do with a 12-tooth CVT sprocket and a 24-tooth jackshaft sprocket (2:1), and a 12-tooth jackshaft secondary sprocket, and a 36-tooth axle sprocket for let's guess 8" clearance for the same 6:1 ratio.