Is this ratio okay?

[HONEYBADGERFTW]

4.3:1 ratio using a GX390 governed with a 40 series TC and 18” tires on a Yerf Dog adult/mid size off-road two seater kart.

I think that the general rule of thumb is 6:1 ratio but I’m not clear about if that is for flat surface riding on paved/hard surfaces or what. I ordered a jackshaft kit and it comes with a 10T or 14 sprocket to use, it hasn’t arrived yet so I’m wondering which one to install.

 

[itsid]

What? you have a lousy gear ratio and you want to make it worse with a jackshaft?
That doesn't sound like a good idea!

Okay now: 6:1 is meant for small wheels (11-13" ideally max 15" for very light applications)
and a roughly 6-9hp engine on a single seater lightweight road kart.

You doubled the weight (twices as much steel, twice as many passengers)
increased the wheels by a factor of 1.5 and are planning to go off roading..
Okay, double the hp is a good start.. that would compensate for the weight..
and the rest??

Okay let's make it short:
you want a 6.6:1 gear ratio if that's a stock 390

(either a 10T from TC to a 66T on the axle)
or if you want to stick with the 4.3 portion
you need a 15T input sprocket on the jackshaft and a 10T on the output

two jackshafts on one kart... ah well...

'sid

PS yes you can get away with a slightly higher ratio but ONLY if everything is well maintained and of good quality.. else you'll burn TC belts like crazy.

 

[Tpdingo]

Okay now: 6:1 is meant for small wheels (11-13" ideally max 15" for very light applications)
and a roughly 6-9hp engine on a single seater lightweight road kart.

Huh? My Dingo runs a 6:1 ratio on 18" tires and its fine. There is no signs its running out of powers. Granted it is a single seater but still.

I'd say probably 7:1 is fine. Its a GX390, its not like a Predator 212, its got torque, and the 40 series can push it so....I don't see a problem if its bumped to 7:1.

 

[itsid]

yes, but a Dingo also has a TC so it can cope with the large wheels.

and while HB also runs a Torque Converter it can only compensate for "so much"
..either wheels.. or weight ... or less than ideal gear ratio..

it cannot compensate all of the above at once, not by that amount.

As you can see the ratio I suggested is 6.6:1
ten percent lower than yours..
considering the power of a gx390 that should deal with the weight and wheels reliably.

with a gx200 he'd look at more like 8:1...

'sid

 

[JTSpeedDemon]

Oh crap, I just realized that that means my 5.5/1 gear ratio is too low most likely. My 66T sprocket is bent anyway, so I'll probably either get a clutch with 10-11 teeth or get a 72T sprocket. Probably both. Any thoughts?
And yeah... 4.3/1 on a heavy two seater off roader even with a TC is too low.
Obey 'sid!

 

[HONEYBADGERFTW]

Thanks again Sid! Looks like I’m buying a 66 or 70 T axle sprocket. It’s not going to be built for speed, mainly for just cruising around off-road. Yes, it’s a bone stock 390 as well, forgot to say that part.