I have a question regarding rear sprockets for a 2-cycle cadet kart. My 9-years old son has been karting for about a year. At the track he is practicing, I have tried out different sprocket sizes to see what suits his driving style and gets him the best lap times. For one-piece sprockets, we have reached the smallest sprocket size of 64 teeth. I cannot find a one-piece sprocket that is smaller than that. Also, I doubt that a 63 teeth sprocket will allow the chain to clear mounting bolts. So far, the smaller the sprocket, the faster his time gets - the rpm gets more in line with what it should be.
However, I did find 63 teeth and smaller sprockets for 4-cycle engine. My question is about the type of sprockets. These sprockets have a single slit (not completely split in half - more for “twisting” around the rear axle for 4-cycle installation). The smaller sprockets will also have to be used with smaller sprocket hubs. Can I use these for my son’s kart? He is currently running Rotax Micro Max.
You can use a split sprocket like that, but another alternative is to go up one tooth on the front sprocket. Example: 10 front/60 rear is equal to 11 front/66 rear. What is your current front sprocket now?
He is currently using 14 teeth in the front. I know for Micro Max, internationally the rules force the use of 14 teeth front sprocket. Some races, they do require rear sprockets to be within a certain range. But in some events, they do not.
14/64 seems extremely low. If the engine is not revving out, you should be going taller in the rear. What is the track layout like? Tight and Twisty or Long and Flowing?
I was told that for Rotax Micro Max, we should be around 9000 rpm. With 14/64 we were around 9000 rpm with 95 kmph / 59 mph (using “international” wheel sizes 140mm rear / 11" overall diameter). Our home track is Bira kart circuit in Thailand. Bira Kart
Looking at the power curve for the Micro Max (https://www.rotax-kart.com/upload/files/6258.pdf) it looks like at 9K RPM is where the power curve flattens out. Peak Power is around 6.5K. Instead of gearing for peak RPM, you want to try gearing for minimum. That is to say, find a gear ratio that gets the minimum RPM around 5.5K to 6K RPM at the slowest point on the track. This will put the engine just before its peak power output in the slowest parts of the track. That should allow good acceleration out of the corners and the engine to continue to pull to the next corner. The top RPM according to the chart was around 12K. I do not know if that is by design (i.e. limiter) or just that the graph falls dramatically at that point. Either way, it does not matter. As your driver gets better and is able to carry more speed out of the corners, you will find yourself dropping the rear sprocket to match the lowest RPM goal and subsequently having a higher top speed at the end of the straights.
Don’t be afraid to ask other drivers (especially quick ones) what gear they are using. Most people will tell you or give you a range. Start with a larger rear sprocket than recommended, clock the laps, then switch to a smaller sprocket and clock them again. If you have a dash that will give you data from the sessions, you should be able to compare where your driver was faster or slower with each gear change. The stopwatch is the ultimate test.
Thank you, @GregF ! That’s a great idea with gearing for minimum rpm. I will definitely try that out!! Rotax Micro Max is limited by its ECU, exhaust gasket and intake restrictor. At the last race, my son was the fastest one on the track with Micro Max class - pacing around .4 seconds faster than others. But I’m just curious to see if he can lap even faster with different sprockets - something I want to test out.
Again, I love the idea of trying to find the gearing for peak power rpm. Will definitely check that out!
Rocket sprocket makes a small hub and 219 sprockets to fit that hub. It’s a complete swap over as it’s not compatible with standard sprockets. If you do end up wanting to go smaller than 64.
Thank you everyone for your help and advices. Race organizer set up the rule which required everyone in this class to go with 12-tooth front sprocket. (We have been practicing on 14T which is the international standard for this class) We tried all sprockets ranging from 59 to 66 and found the one that gave him the best lap times. My son ended up qualifying 4th for each round. Amateur dad couldn’t find the right setup for the cold (and sometimes wet) conditions. Anyway we tried our best. Next race we will improve more. Here is one of his qualifying laps at Orlando Kart Center…
Looking really good. I like how his kart is calm and he appears to be thoughtful about not overdoing it.
Very smooth.
I have only been to OKC a couple times but it occurs to me that if he were to tighten a couple of those apexes, there might be a podium in his future quali efforts.
It might be that those are apexes that you take a bit wide, as I don’t have great track knowledge.
Thank you, @Bimodal_Rocket . I will let him read your comment. I don’t know anything about driving a race kart (I’m pretty good with a mini van tho hahaha). I’m just trying to do my best to support his passion.
Have a great weekend!
