[Kartbuilding] gear ratio problems

Stephen Burke sburke at burkesys.com
Mon Dec 21 22:31:31 GMT 2009

Hi Bryan,

Thanks for your email. Sorry for the slow reply.
16" tires are fairly large. This fact is not helping the problem either.
Having large wheels is like reducing the gear ratio. (The bigger the 
wheel, the faster you go, as 1 RPM of rear axle will move you further).

To be honest, ratio is the main issue. The ideal ratio will depend on your 
racing track. Will you have enough of a straight even road to get the kart 
to reach its top speed, or are you more concerned with initial take off 
and speed around corners?

Racing Kart engines, have a single gear, but their RPM is around 20,000 
rpm! That is how they can achieve the speeds with a single gear. For your 
8 hp briggs (which would turn over at around 3500 or 5000 rpm), your stuck 
with low revs and high torque.

My suggestion would be to look to getting a small motorbike engine. Even a 
100cc motorbike engine with 3 gears and a clutch would be much more 
powerful than that 8hp briggs.
So its either go down the "gears" route, or tweak the ratio.

It is also possible to try different sized rear sprockets. You can cut a 
slice out of the large rear sprocket. This will allow you quickly try 
different rear sprockets, and allow you go adjust for your terrain. See 
some of the following photos of how I split the rear axle sprocket.

I hope this helps. Best of luck,

On Fri, 11 Dec 2009, Bryan Buxton wrote:

> I put an 8hp briggs on my cart with a 1"bore.  I put a comet #35 chain clutch and sprocket on it with 16" tires. I want to obtain proper gear ratio without sacrificing speed. any suggestions? would a racing clutch solve the problem? or is the ratio the main issue?
> any help would be great    thanks
> Bryan

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