How To Determine Correct Final Drive Ratio

So I have just got back into karting after many years away from the sport. I raced 250cc gearbox karts in the UK and I’ve never driven single speed karts before. I have two 2011 Rotax Max Top karts that I am just using on practice days for fun. I have little idea of how to gear the karts for specific tracks as it is totally different to gearbox racing where you have options for internal gear ratio’s as well the final drive. Last time out at Adams kart track (in CA) it felt like I was too low geared as I was at max rpm halfway down the longest straight. However if I change the gearing to a taller ratio I know it will compromise me in at least one of the slow speed turns. Is there some general guide lines to gearing single speed karts or is it just a matter of experimenting and checking lap times.

Keep taking teeth off until you go slower.


Agree with TJs statement above. Would like to add that with the Rotax you don’t have to turn it to the rev limiter. It has a decent sweet spot on gearing. Going up or down a couple teeth is just going to fine tune what parts of the track you are faster at. If you are consistent you probably won’t see any change in lap time overall.

1 Like

John have you asked the other rotaxers what ratio they are racing?

1 Like

Well that’s the thing, there is no Rotax racing here in in Cali anymore, well at least not SoCal. If there was a race series I would be straight out there :grinning: Also when I was racing no one gave up gearing info to the competition. Well that’s not exactly true, but it would be false information :rofl:

Bummer! Sad to hear that folks would give deliberately misleading info. I guess they can go “race” themselves.

Well it ain’t hard as you recall to find the gearing and it’s probably simpler in single gear. How I go about it is thusly: I initially look for a gearing that gets me to limiter somewhere towards end of straight, before I have to brake. Once I’ve established that, I’ll start adding or dropping teeth to see how the turny bits play out, time wise. If adding/dropping teeth makes me faster in one sector, how does it change the other sectors? Is the better time I make in s2 with the long straight worth the slower s1 (which is twisty without straight)?

After a bunch of laps I’ll settle on something that allows me to make the most of the total lap, where the compromises are not so bad to the overall time.

Also, it’s never “correct”. It’s a range. I’ve managed similar times within 2 teeth, just with different results in different sectors. I imagine that since the rotaxes like to be driven differently than a top endy x, gearing would be different.

Bear in mind I am a giant noob in the grand scheme of things but I do have a strange resume of messing around a lot in sim. Gearing is gearing, sim or not.

Where are you at on gearing now John?
I would guesstimated something around 12/79 as a ballpark starting point.

1 Like

Two philosophies for Rotax gearing:

“The one in the manual” - keep the engine in the powerband 6000 to 12500 RPM
“The gear everyone seems to run” - keep the engine above 7000 RPM off the corners and damn the second half of the straights

At Apex if you can keep the thing hauling around the banked corner between the two straights, the first philosophy will pay off, at Adams it’s unlikely to. Both tracks need about a 70 MPH gear.

1 Like

Thanks Mark I have 12/82 right now.

Thanks Charles, it was you that I met at Adams the other week! Just curious do you prefer Adams or Apex. I have never driven a kart at Apex only supermoto’s.

I prefer Adams for racing, but quiet practice days at Apex are fun.

1 Like

Well I have 78t and 80t sprockets so I’m going to try the 78 first as it will be easy to see how much difference it makes with a bigger jump.