[Kartbuilding] gearing

[Stephen Burke]

Hi Steven,

With those 14" (smaller than a standard motorcycle wheel) wheels, having a 
92 tooth sprocket/gear for the rear axle sounds as if the kart will be a 
little slow and might not reach 50-60MPH.

With a 14toothed sprocket on the 110cc engine and a 92toothed sprocket on 
the rear axle, this would give a ratio of 6.5:1 This is quite a high 
ratio, and this ontop of the gearbox, and ontop of the smaller than 
normal wheel diameter, the kart would only reach ~30MPH.

You can perform a simple calculation to correctly find out the speed (as 
opposed to my estimate). If you can turn over the engine by hand, and 
watch the flywheel (alternator side) as the engine turns. If you can find 
out how may turns of the engine equals 1 turn of the sprocket on the 
engine (in 4th gear), and if you can find out the rpm of the engine, you 
can calculate how many revs the sprocket will be doing in top gear. My 
estimate is that the engine would do approx 5000 rpm, and the sprocket 
(output shaft) would do 2500rpm. Based on these estimates, using a ratio 
of 6.5:1, the rear axle would turn 385 rpm (revs per minute).
14 x 25 = 350mm diameter.
350 x 3.14 = 1099mm circumference.
385 x 1099 = 252770mm per minute
Divide by 1000 = meters per minute
Divide by 1000 = kilometers per minute
Multiply by 60 = kilometers per hour
And the above roughly works out at about 26Kph.

I would suggest using a toothed sprocket of 56 on the rear axle. This 
would give a ratio of 4:1.

What I done on my karts is to get a couple of different size of sprockets 
and try them all on the kart. I cut a triangle shaped slice out of the 
sprocket allowing me to remove an old sprocket and fit a new one without 
taking out the rear axle. Once the new sprocket was fitted on the rear 
axle the cuts were welded together. See these photos for an example:
http://www.kartbuilding.net/gallery/v/Kart_Clearance_and_Memories/DSC01027.JPG.html
http://www.kartbuilding.net/gallery/v/Kart_Clearance_and_Memories/DSC01028.JPG.html
http://www.kartbuilding.net/gallery/v/Kart_Clearance_and_Memories/DSC01026.JPG.html

Best of Luck. If you have any photos of the kart etc. feel free to forward 
them on and I can put them on the www.kartbuilding.net website for other 
karting people to see and learn.

-steve



On Wed, 2 May 2007, Steven D Lane wrote:
> Hi Stephen,
> I've been following your site for awhile now. It has been very helpful in my 
> go kart project for the Grandson. I am now building a mini trike for street 
> use.
> I have a question about gearing. I am using a Lifan 110cc engine with 4 
> speed manual transmission. It has a 14 tooth drive gear. I got a good deal on 
> some 14" wheels & tires (185/60/14). Am I correct in thinking about a 92 or 
> 93 tooth gear for the rear axle? I'm thinking 50 - 60 MPH top speed.
> I would appreciate your insight.
> Thanks!
> Steven L.