Gear ratios with manual gearbox

[nubbin89]

I'm currently in the process of building a drift trike (in the UK), mostly as a stepping stone to larger projects such as a go kart/buggy and have a few questions regarding gear ratios.

I've got a Lifan 125cc engine with a manual gearbox, and am trying to get my head around the correct gear ratios.

From what I've read countless times you need a gear ratio of about 5-6:1 in order not to destroy the clutch. With my current setup I'm managing a ratio of around 4.1 (13t front 54t back) with 11 inch wheels.

In my understanding, with a gear ratio of around 2:1 from the clutch to the front sprocket through the gearbox and in first gear, the total ratio is 2:1 x 4:1 = 8:1. By forth gear the ratio is reduces 0.96:1 x 4:1 = 3.8:1.

If my reasoning so far correct then I'm not in danger of burning up the clutch until I reach 4th gear - which is something I'm willing to live with (if anyone has a different opinion please say!).

The obvious resolution to all of this is to increase rear sprocket size or decrease front sprocket size, but due to the limitations of my setup this is not possible. I can find a minimum of 12t (428) sprockets for front and on the rear my 54t is already way too close to the ground (~8.5" sprocket on 11" wheels).

This leads me on to my next question of ground clearance of rear sprocket. My options here are to use a smaller sprocket or using a jackshaft which I'm trying to avoid for this project. Using a smaller rear sprocket will reduce my gear ratio but if I'm correct this wont cause any issues until I reach 4th gear.

How did Colin Furze get away with using two front sprockets? Just taking the hit on burning up his clutch regularly?

I hope you can help me with my many questions!

Many thanks in advance.

 

[anickode]

That 6:1 ratio is the best compromise between top speed and clutch longevity on a kart with an industrial engine and a centrifugal clutch.

Once you have a geared transmission, that changes everything. You could probably run a 40 tooth on your axle with no problem.

Plus the tires barely grip on a drift trike. They just spin, letting off all the "extra" energy instead of slipping the clutch or breaking something.

 

[itsid]

apples and oranges

you cannot compare an ungeared engine with a (mostly low priced) dry centrifugal clutch
with a geared engien that likely is equipped with a wet clutch.

Yes you need to make sure the clutch is able to fully engage rather quickly in order for it to stay alive.
wet clutches can absorb more heat (that's the point) so you have a bit more play than you'd have with a dry clutch.

Your engine is also geared internally, not only you have a gearbox it's input shaft likely is already geared down ..

And the only gear you worry about is the first, maybe the second when in doubt.
those are the ones that you need when starting to move.
Once you do move fast enough the clutch will lock much easier.
That overall ratio (primary reduction times 1st gear times final reduction times chain reduction) is what you are interested in to adjust.
everything else will follow (hopefully nicely)
That 8:1 sounds nice .. should provide enough torque for an engine that size.
buta gain.. check if your engine has a primary reduction and/or a final reduction...
and include that as well (chances are you only need 48T or such to achieve 8:1 overall reduction)

But I'D say go for it and give it a test.

Now.. you say you want to build a drifter.. so
likely you will run sleeves on the driven wheels in order to easily break traction..
and as soon as the wheels are free to spin the clutch will be fully engaged anyways.

You still need a decent bit of torque (to break traction in the first place) but depending on the sleeves that shouldn't be much of an issue.

Some people like to gear for excessive speed (the faster the wheels spin, the less likely they'll regain traction and thus gain control of the trike's direction of movement )

Others liek to stick with std gear ratios (in case they want to remove the sleeves and ride the trike with some more control and still no risk of ruining the clutch)

I'd suggest the latter tbh (in case a sleeve breaks or wears off you don't want the clutch to suffer )

So here's what I'd do..
test the 54T sprocket, forget about the sleeves for now and test with fresh racing slicks (good grip) on a dry road.
If you can accelerate nicely in second gear from a dead stop.. (don't try too hard or you'll burn the clutch remember?)
you can gear higher (remove a tooth or two from the axle sprocket)

Now shift up and see if your engine revs as nicely in fourth as it did in first..
if it doesn't it lacks a bit of torque at the top end of the spectrum
and consequently you should add a tooth or two to the axle sprocket.

That way (with usually no more than three sprockets bought) you dialed in the kart (or trike in your case) to it's ideal gear ratio.

But true, the axle sprocket might get too big (usually you don't need too much clearance really but an inch doesn't hurt)
Instead you can reduce the size of the driver sprocket (in your case the engine sprocket!)
from say a 13 to a 12..

BUT that's a far bigger step of course than adding a single tooth to the 54 axle sprocket..
in fact it's roughly equivalent to 4.5 axle sprocket teeth (at a startig point of 13-54)

'sid

 

[voided]

Man,’I had a drift trike with basic predator 212cc was the craziest thing I rode... would never think of having a faster motor on it..

Now that motor on a kart, geared right is more my style.. my shifter kart had a kx125 motor and was geared 17 motor side and 36 on the axle with 11” tires.. did 80mph in 5th never really ran 6th out to know what it did..

 

[nubbin89]

Loads of good input here guys thanks! Especially to itsid, great information there to get me started.