Every number below is off our kart (the kart's setup record, the 2026-09-19 session entry, our speed calculator) except the ones marked (typical), which are published LO206 figures, not measurements of our engine. The MyChron will replace them.
The one idea
Gears don't make power. The engine puts out a certain torque at a certain rpm, and a sprocket pair only trades one for the other: a taller ratio gives more speed per engine rpm and less push at the tire, in exactly the same proportion. Going from a 70 to a 73 is +4.3% thrust out of every hairpin and -4.3% speed at the limiter. Same number, opposite sign, and which one wins depends on how many seconds a lap you spend accelerating versus how many you spend on the limiter. That is a measurement, not an argument.
What you're choosing between
We own four rear sprockets, all #219 pitch, on a 20-tooth driver:
| Rear | Ratio (rear / 20) | Speed at 6100 rpm | Character |
|---|---|---|---|
| 70 | 3.500 | 56.5 mph | tallest: most top speed, least push |
| 71 | 3.550 | 55.7 mph | |
| 72 | 3.600 | 54.9 mph | what's on the kart (as bought, chosen for a 100cc 2-stroke) |
| 73 | 3.650 | 54.2 mph | shortest: most push, least top speed |
Tire OD 10.9 in and limiter 6100 rpm are typical figures, not measured - see lesson 1 Q3.
Three facts worth knowing
- One tooth on the driver is worth 3.6 teeth on the rear. 72/19 = 3.79 vs 72/20 = 3.60, a 5% jump from a single tooth. One rear tooth is 1.4%. The smallest sprocket is the biggest lever, which is why people change the rear: it's the fine adjustment.
- The LO206 limiter is an ignition cut at 6100 rpm (typical). The engine is sealed and every engine in the class has the same one. So every kart on the same sprockets has the same top speed, and gearing is the only engine-side choice the rules leave you. That is why the whole class argues about it.
- A draft can't make you faster if you're already on the limiter. It only gets you there sooner, or lets you run a taller gear before you hit it. In the 9/19 main, the three leaders ran nose-to-tail at 58.4 and you ran alone at 58.8. Hold that thought for Q7.
Worksheet
Units on every line.
1. The table, by hand. Compute all four ratios and check the table. Which direction is "taller" (more speed, less push)? If we bought a 74, what would its ratio be, and would it be taller or shorter than the 73?
2. Where the engine is in a hairpin. Tire circumference 34.24 in. Exiting a hairpin at 25 mph on the 72, what rpm is the engine turning? (Reverse lesson 1 Q3: mph to in/min, divide by circumference for tire rpm, multiply by the ratio for engine rpm.) What rpm at the same 25 mph on the 73? An LO206 makes its best torque around 4000 rpm (typical). Is the engine above or below 4000 at hairpin exit? Which of the two sprockets puts it closer to 4000?
3. Thrust. Thrust is the forward push the rear tires put on the track. Peak engine torque about 9 lb-ft (typical). Torque at the axle = engine torque x ratio. Thrust = axle torque / tire radius; the radius is 5.45 in, so convert it to feet first or the units won't cancel. Compute the thrust in lb on the 70 and on the 73. Then F = ma: the kart plus driver weighs 360 lb; convert that weight to mass (lesson 1 Q9 - divide by 32.2 ft/s^2) and get the acceleration in ft/s^2, then in mph per second, for each sprocket. How much more acceleration does the 73 give, in percent? Compare that percent with the percent of top speed the 73 gives up (the table above).
4. The straight. Suppose you're on the limiter for 4 seconds on the main straight. The 70 is 1.6 mph faster there than the 72, so over those 4 seconds it pulls ahead: by how many feet? Being that far ahead means reaching the end of the straight earlier - by how many seconds, at 56 mph? That is the lap time the 70 gains on the straight. Now the honest question: to compare that with what the 73 gives you out of five hairpins, what would you need to know? Write down the two measurements. (One of them is what a rev counter on the dash is for.)
5. Chain math. #219 chain pitch is 0.2188 in per link. Swapping the 72 for a 70 removes two teeth: how much sprocket circumference is that? The engine slides on its mount to take up the slack. Roughly how far does it slide? (Hint: the chain wraps half the sprocket, so the slack is shared between the top run and the bottom run.) The spare chain has 114 links: how long is it in inches?
6. Percentages, not teeth. Lesson 1 Q10 fixed "raises top speed by 2 teeth." Now the general rule: adding one tooth to a 70 rear changes the ratio by what percent? Adding one to a 73? Why does the same tooth matter slightly less on the bigger sprocket? Then: someone at another track says "I went up two teeth" - on the DRIVER, from 16 to 18. Is that a bigger or smaller percent change than our going up two teeth on the rear, from 70 to 72?
7. The draft. Two karts, same gearing, same engine. One runs alone and hits the limiter with 100 ft of straight left. The other is tucked behind a leader, so it reaches the limiter with 200 ft left. Using the "gears don't make power" idea, explain why the second kart could gain from a taller sprocket and the first one couldn't. Which of the two karts were you in the 9/19 main, after lap 3? What does that say about picking a sprocket for qualifying (alone) versus the final (in a pack)?
8. Listen. Until the rev counter arrives, the measurement is your ears: on the 72, does the engine hit the limiter before the braking point on the main straight? If yes, for roughly how many seconds? If no, the 70 can't help you on the straight - so what would swapping to it do to the hairpin exits?
Done? Q4 and Q7 stay half-answered until there's an rpm trace. When the MyChron goes on, this sheet gets a second page with the real numbers, and you'll be able to check every figure marked (typical) above.