[Kartbuilding] Fwd: Gear Ratio

[Stephen Burke]

Hi Jeff,

Sounds like you have a good project on your hands.
I know exactly the difficulty you are having with gear ratio's.
With a simple direct drive for a typical engine 5-6hp and 12" diameter
wheels - I tend to recommend a ratio of 5:1 - so the rear axle will rotate
5 times slower than the engine.
If you have larger wheels - 16.5 diameter - and you prefer a higer ratio
for power rather than speed - you are looking at a ratio of 8:1. Now I am
estimating this figure too. There are a lot of variables including the
weight of the kart.
Anyways - with a ratio of 8:1 - and with 13 teeth on the engine you are
going to need 114 teeth on the rear sprocket. Now this size of sprocket is
near impossible to get in my opinion.

You are far better off - trying to get a gearbox with 3 speeds. This will
save you from having to go changing sprockets in the fture. You can leave
it in first gear for the initial build and eventually build it in so that
gears can be changed.

You should look at trying to get a gear box out of a tractor/ride-on
lawnmower as it shouldn;t be too heavy. It also might have reverse - and
you could remove the single gear reverse/forward gearbox you currently
have.

Is reverse - 1:1 ratio also?? If it was less - e.g. 2:1 then you could run
the gearbox in reverse gear all the time. But currently that gearbox you
have is of little use - especially with the size of wheels you are using.

Best of Luck,
Send on a photo if you put some of it together.
-steve

On Thu, 10 Aug 2006, JHyer at ransome.com wrote:

> Hello! My name is Jeff and I am building a gokart and have been using your
> web site for help with my design and build and it has been very helpful as
> this is the first one I am building from the ground up. I am doing pretty
> good but I have hit a road block so to speak I am not sure how to figure
> out what size sprockets to use other than trial and error which would get
> pretty costly not to mention having a ton of extra sprockets laying around
> after I'm done so I was hoping you could help. I am using a 6hp Kohler
> eng. with a centrifugal clutch w/13 teeth then I am driving a Comet
> forward/reverse gear box then a live axle. The gear box is 1:1 ratio it
> just gives reverse capabilities. I know there are a lot of variables but
> what I am looking for is more low end power than top end speed. If you
> have any info that might help I would really appreciate it. Also another
> thing that I believe is a big consideration in gear ratio is tire size,
> the tires I am using are 16.5x6.5x8 which are pretty big for a gokart but
> it is a off road kart I am building for my daughter who is currently 3
> years old but will probably be quite a bit older by the time I am actually
> done the project. It is also going to have a body either sheetmetal of
> fiberglass to make it look like a mini monster truck. Hope you can help.
> Thank You!