I have been running 2 cycle motors since getting into karting and on my local track that is 4/10 of a mile we have always been forced to use the smallest front (motor) sprocket and usually adjust gearing to control max rpm.
Now I want to start running 4cycle / 206 and there is no one at our track that is running this class so I am trying to figure this out on my own. With less than half the rpm and a limiter on the 206 how do you approach gearing?
Gear for the stop watch initially. There’s normally a range of ratios on a 206 that produce the same laptimes but slightly change where you can get a run and pass.
I base my driver selection off of keeping my rear sprocket within a certain range.
If I’m on 35 chain, I stay between 55-60. If I’m on 219, I stay between 65-70.
Don’t buy into the smaller driver does this, and bigger driver does this stuff.
Do you use these rings for clearance or some other reason? On my KA we run from 79 - 84 on our track with 10t driver.
It’s always just been a personal preference and the fact that we run RPMs low.
I know guys that run bigger, and I know guys that buy the mini gear systems and run smaller.
Some chassis can run into rear cross bar clearance issues with bigger sprockets for sure.
At our track we are on the rev limiter for the 206 quite a bit. I think I was on rev limiter 5 seconds and winner had bigger rear gear than me. I am sure he was on it longer
So if you’re not gearing for minimizing how much its hitting the rev limiter / straight speed are you gearing for out of the corner? Where is the sweet spot?
Sweet spot depends on the track and your competition, honestly. Some tracks/layouts, I’m on the limiter a ton, some I barely hit it.
It’s a balance between top and bottom.
Surely someone knows a starting gear you can use?
Sweet spot is whatever the stop watch reveals. It depends on the combination of the turns, difference between lowest turn\longest straight, gradient, speed in the critical turns before the longest straights etc etc.
Start by gearing to just touch the limiter and add teeth until you find your laptimes suffer… Of course, you need to monitor track conditions too, so a test day is best as the rubber doesn’t build up as much. That will get you a base gear to start with.
It’s actually no different to (Contemporary) two strokes, both have rev limits, even if they don’t have a rev limiter per-se.
As a very rough starting point I would expect the 206 to run a ratio half that of your KA so you are running c.8-1 on the KA try 4-1 on the 206. Reliable local knowledge would get you nearer.