gear ratio

[kenworth85]

whatis best way o figure gear ratio with a jackshaft? I've got 13hp briggs, 10t clutch 17" tall tires kart weighs about 200lbs. I've tried a
72t direct to the axle. doesn't work very well. any bigger sprocket will have a ground clearance issue. so I have a jackshaft already I thought I would try to do something with it. right now tc is not in the budget.

 

[bob58o]

Are you telling me that with a 13HP engine, 7.2:1 gear ratio, and 17" tires on a 200 lbs kart....

There is not enough torque?
Or what exact does "Doesn't work very well" mean?

I would think this is geared low enough already. Maybe there is somthing else causing issues????

Anyway...

Engine out. Clutch
JS in. Attached to clutch
JS out. Attached to Axle
Axle in. Rear Sprocket

Engine out is 10T (per your clutch)
Jack shaft in you can make maybe 15T.
Jack shaft out you can make maybe 10T.
Axle in is 72T (per your sprocket)

Gear Ratio = (Jackshaft in / Engine out) x (Axle in / JS out)

For the given example...

Gear Ratio = (15T / 10T) x (72T / 10T)

Gear Ratio = (1.5) x (7.2)

Gear Ratio = 10.8 : 1

 

[chancer]

Best way to calculate it is with this online tool.
https://www.affordablegokarts.com/GearCalc.php

It also equals the math Bob showed above.

 

[kenworth85]

That ratio (I'm sorry I left some important info out originally had 6.5hp I didn't think it would matter about HP but I guess I was mistaken)is 6.0 clutch didn't didn't last long. I thought I would go ahead and use the jackshaft. Maybe would help.

 

[itsid]

Sorry.. 200lbs kart.. NO WAY!
keep in mind that weights are WITH passengers... (we need to know the weight to move, not the dent in the gras it'll make standing still)

the engine alone weighs in what 50?.. another 30 at least for the wheels..
say you're small and only around 100lbs...
and now you explain to me how you make a frame, steering, axles and seat with 20lbs...

So no, please yes.. estimate the weight alright (I doubt anyone ever put his kart on a scale...)
but be reasonable.

Aaaanyways.. a gearing of 9:1 will certainly do with an engine that size (up to ~500lbs total weight)
Soo if you like to keep your 72 axle sprocket just add a 12T sprocket JS in and a 10T js out

and you'll be at 8.64:1 (I guess that'll do for ~450lbs)

if you don't care too much about the rear sprocket.. change it for additional clearance..

I'd suggest a 18T JS in and a 50T axle sprocket (clutch and JS out both ten as before)
that'd be exactly 9:1

'sid