Before you head to a track, check the track’s web site or, better yet, call the track. Your engine builder and the association that runs at the track are other resources. You should be able to learn at least a starting point for gearing. Bring a couple of extra rear gears to bracket the recommendation. Also ask here on the Forum.
LAD Specialties customer / tony kart / rotax / kt100
What you are looking for is the best gear to use to get you around the track the fastest. Seems logical but what you want to do is start with what Steen suggested. Highest RPM at the end of the longest straight. Then drop a couple of teeth and see if your lap time drops, if it doesn’t do the same thing but go the other way, add a couple of teeth and re-test. Many times you will find that you will need to ” stretch” the engine down the straight because you need the gearing to get you out of the slower turns. Its really about overall elapsed time of the lap, not the highest top speed. If the track is tight and twisty with one long straight, then usually over-revving is the way to go, if the track is all medium speed corners with medium to long straights then going smaller on the gear usually works out better. Best of luck.
The 2 most important things are the stopwatch and how your gear selection might affect race strategy… sometimes the gear that gives you the fastest lap time on your own will allow someone else to make an easier pass down a straight or with the draft and block you in the corners, making it harder to gain the position back from them.
Apart from that, “highest RPM at the end of the longest straight” may or may not apply – it is generally a good starting point, but where you go from there depends on the engine and the track. And it depends on how you define “highest RPM”… point being that you’ll always hit your highest RPM at the end of the longest straight, even if you have the “wrong” gear. The key here is knowing your engine’s “maximum potential RPM”.
As for my observations with the engines I’m most familiar with…
With the traditional 100cc 2-cycles, without any form of rev-limiter, more often than not, if you hit the engine’s max potential RPM just before the end of the longest straight, you were in the ballpark.
With a Briggs LO206 and it’s 6100 RPM “hard” rev-limiter, it can be all over the map. There are some tracks where you will want to hit the rev-limiter in the middle of the straight, some where you will want to hit it almost immediately upon getting onto the straight, and others where you will want to barely hit it at all.
With a Rotax Max, which is rev-limited as much with ignition timing as anything, it’s again more like the Briggs. There are some tracks where you’ll be fastest only pulling 12,500 RPM, even though the engine can hit 14,500+.
It all goes back to starting somewhere and testing to see what gets you the best results. Daryle’s advice is great. As he says, drop a couple teeth, and see what the stop watch says. If it was the wrong way, add a couple teeth.
The only other comment I’ll add is that if your carb isn’t properly tuned, you may never hit your max potential RPM, so you could end up chasing gearing when your real problem lies somewhere else.