I have a Birel kart, LO206. Clutch sprocket is 15T, chain pitch #35, axle sprocket 60T. (an even 4.0:1 gear ratio) Tire/wheel combo is 350mm tall. 40mm axle. My best laptime at my local track is just under 52 seconds, and I hit the rev limiter well before the start/finish line at 46mph.
Yesterday, a young man, I will call him, was also driving a 206. I noticed he was quite practiced and smooth, so I thought he was just a better driver. But when he passed me on the straight, I was surprised how much faster he was going. He later asked me about my gearing, as he noticed the same thing. (That I was hitting the rev limiter early) I asked him what top speed he was getting, and what his best lap was that day. HE was hitting 51mph, and doing 47sec laps!!
So obviously, he must have a taller gearing than my 4.0 ratio. Right?? But when I did the math on the ratio of the gearing he told me he was using (19T > 69T) that works out to a 3.61:1. A SHORTER ratio! So how is he going 4sec/lap and 5mph faster than me? I dont think his tires were any bigger. I’d like to be in the same speed range as the faster guys in my class, which is about what this kid was getting in his 206 yesterday. Any suggestion of what I could change my gearing to in order to achieve this?
4 seconds a lap suggests chassis/driving needs to be looked at. Compare min and exit speeds between yourself and that other driver and you’ll likely find clues.
Weight of course is a factor too, if you’re in a masters class for example. However that’s of the matter of tenths of a second.
You can try taking some rear teeth off and see if you can hold corner speed up.
OK, chassis and/or my driving certainly could be a factor. I’m certainly no pro. But what things about the chassis should I look at? It seems pretty neutral once the tires are warmed up.
But I don’t get what you mean by “taking teeth off rear and see if you can hold corner speed up”. I mean, going fewer teeth in rear will lower my gear ratio, no? Make me go slower, yes? I mean, this kid was 5mph faster! 51mph vs 46mph @rev-limiter. I don’t see where that has anything to do with exit speed. Top speed is determined by gearing, no?
Somewhat but not entirely. If you enter a straight 2,3,4 MPH slower than another driver that will carry through to the straight and consequently you may end up running a shorter ratio in order to hit the limiter.
Chassis wise, it’s not uncommon for the chassis to feel “stuck” or “bound up” from the apex to the exit of a turn. When a chassis is in this condition it will very much feel like the other drivers have more power as it basically saps up the power from your engine as you exit the turn.
To address it you can try things like narrowing the rear track width for example to encourage more load transfer to the outside tire, reducing the scrub.
Were you able to find out the other drivers gearing?
taking teeth off the rear sprocket gives you higher top speed, typically. so he’s running a 3.63, which is considerably “taller”. there’s lots of jargon when talking gearing. hell, i may not even be using taller correctly. but it’s like a bike, on the rear wheel, the little tiny cog is the one you’re on when you’re trying to go 30MPH, and the big one on the rear wheel is the one when you’re going about 2MPH up a steep climb.
long and short of it, try taking a few teeth off the back.
Assuming the other guy was correct when he told you his gearing, for those 3 things to be correct at the same time 1. he is running a shorter gearing than you 2.hitting the rev limiter after you and 3. considerably faster on top mph, then you either have an engine that taps out too early (missing rpms at the top) or your chassis is dragging (e.g. toe is way too out).
I can’t think of any other scenario for those factors to be true at the same time
Ha! You just made me realize I was thinking about the ratio BACKWARDS. I’m such a noob. Duh. Thanks for the simple answer. (Not that I don’t also appreciate the more detailed responses!)
A follow up question would then be, Is a 58T axle sprocket a common one to find? Also, can anyone give me an idea of how much, or if, my dropping a couple teeth affect my clutch? Hopefully won’t let it slip and heat too much? Is that a reasonable expectation?
No worries! Split sprockets are about $10 and easy to find. I have a pre owned kart that had a 59, so I just ordered two 58s and a 57. 206s have a 6100rpm limit if you are outside the junior or cadet classes. You should expect to spend a little time on the limiter, but how much is hard to say. Just drop teeth until the lap time suffers if you are consistent. Your track may also be able to give good starting ratio guidance.
I created a spreadsheet like that also includes speed based on gear ratio and tire circumference. It helps me visualize how big of a change 2,3,4 teeth would have. In our case we only use 3 different sprockets so the sheet isn’t quite so busy.
You are 7 teeth off (too big) on the rear sprocket or 2 teeth off (too small) on the driver. One tooth off on rear sprocket on a 206 is a big deal so being 7 teeth off absolutely justifies that speed differential.
I’d change the driver - then you are within a tooth or two of the right gearing.
For most any track you only need two drivers and three sprockets (maybe 4) to run 206.
Assuming his 19/69 was accurate for a good practice day you need:
19 - dry
18 - wet
68 - race day prefinal/final
69 - practice day or race day qual
70 - green track or race day morning warm up
Great reply Dan, Thanks! SEVEN teeth off?? Wow. Yeah, playing with the math, I can see that, when comparing his 19T>69T (3.63:1 ratio) setup. Please excuse my nooby ignorance, I’m still on the learning curve. So let me approach this another way… Maybe I should be asking, What is the standard recommended GEARING and/or Gear Ratio? The guy who owns/runs my local track (Prairie City OHV Park, All Star Karting) built my kart and selected the gearing I have. So I assumed he picked the best one for the track. I also read in the “16 Mistakes People Make with LO206 engines” that one should gear the motor for the Power Band, not Top Speed. So I’m ASSUMING that’s what he did. (I haven’t had a chance to talk with him about this choice yet.) I certainly don’t want to do any damage to my motor. Nor do I want to over tax my clutch. That said, is a gear ratio of 3.63:1 still in the acceptable range for engine and clutch health?
I AM trying to take this all in!
So Dan suggests changing my driver gear by two teeth.
so that’s 60 / 17 = 3.52:1 That’s taller than the 69/19
or, one tooth, 60 / 16 = 3.75:1 That’s a little shorter, but taller than 4.0:1
AH! But if I go UP one tooth on the driver, and down two on the axle, I get 3.62:1, which is damn close to the 3.63:1 the young man with the 19/69 gearing said he was running. Does this sound acceptable? It won’t bog the motor, will it?
Yeah, you’re probably right! Just trying to wrap me head around it.
I DO want to get a little closer to 50MPH tho. I will try the one tooth up on the clutch sprocket, and go from there. Thanks guys, really appreciate the feedback!
And BTW, I’m not entirely new. Been doing it for a couple years. Mostly practice sessions. Have done one race. Grand Masters class. I qualified mid pack, finished mid pack. So I can keep up. But I found out the guys in front were faster, not just from being better drivers, but they had higher top speed due to different gearing. So that’s why I started this thread. I wanna be able to get up with the faster guys once I am in shape to do a race day again. (I’ve been having back issues, so have missed a lot of practice.) But yesterday’s session showed I was turning laps consistent with my lowest lap times. Now I wanna go faster!
Duh. Thanks for the simple answer. (Not that I don’t also appreciate the more detailed responses!)