Every number below is off our kart (the kart's setup record, the 2026-09-19 session entry, our speed calculator, the lesson 1 and 2 answers) except the ones marked (typical), which are round figures for a kart of this kind and not measurements of ours. Corner radii and minimum speeds at Buttonwillow are guesses until the MyChron's GPS trace replaces them.
The one idea
Kinetic energy goes as speed SQUARED: KE = 1/2 x m x v^2. Ten percent more speed is twenty-one percent more energy. The engine has to put every bit of that energy in, one hairpin exit at a time, with about 70 lb of thrust (lesson 2). So the lap is won where speed is lowest: a little more speed carried through a hairpin is energy the engine never has to make, and it pays for the whole straight that follows. The straight itself, where the kart sits on the limiter, is nearly free.
Three facts worth knowing
- Speed squared is why the slow corners matter. Going from 25 to 27.5 mph at the bottom of a hairpin is +10% speed and +21% energy. The kart then has to gain LESS energy on the way to 55 mph, and it does that with a fixed thrust, so it gets to 55 sooner, and it's going faster at every point of the straight until it hits the limiter. The gain compounds; the loss does too.
- Brakes are heaters. Every stop from 55 to 25 mph turns the difference in kinetic energy into heat in one small disc. It never comes back. The only energy that survives a corner is what you carry through it, which is why "slow in, fast out" is really "waste less, carry more."
- Your 58.33 is an average of 42.6 mph (0.69 mi / 58.33 s). The limiter is 54.9 mph on the 72. The gap between the average and the top speed is the whole lap: it lives in how low the minimum speeds are and how long the kart spends climbing back out of them.
Worksheet
Units on every line.
1. Mass again. 360 lb of kart, driver and gear. Convert to mass in slugs (lesson 1 Q9: divide by g = 32.2 ft/s^2). Keep this number; every energy problem below uses it.
2. Kinetic energy at three speeds. KE = 1/2 x m x v^2 with v in ft/s (1 mph = 1.4667 ft/s). Compute KE at 25 mph, 30 mph and 55 mph, in ft-lb. How many times more energy does the kart have at 55 than at 25? (Not 2.2 times.)
3. Ten percent. A hairpin minimum speed rises from 25 to 27.5 mph. By what percent did the speed rise? By what percent did the kinetic energy rise? Write the general rule: if speed goes up by a small fraction f, energy goes up by about what fraction?
4. What the engine has to make. Exiting at 25 mph and reaching 55 mph on the straight, how much kinetic energy does the engine add (ft-lb)? Same question exiting at 27 mph. Using the 69.4 lb of thrust on the 70 from lesson 2 (and work = force x distance), how many feet of straight does each take to reach 55? How many feet did the 2 mph of exit speed save?
5. The hairpin itself. Take a hairpin as a half circle of radius 25 ft (typical), so the arc is pi x 25 ft long. How many seconds does the kart take to go round the arc at 25 mph? At 27.5 mph? How many seconds does the faster speed save per hairpin? Buttonwillow's kart layout has five hairpin-type corners (from the track map): how many seconds per lap if all five improve by the same 2.5 mph?
6. Compare to the straight. Lesson 2 Q4 found that 1.6 mph more top speed over a 4 s run on the limiter is worth about 0.11 s. Set that next to your Q5 answer. Which is the bigger lever, and by roughly how many times? Now say in one sentence why the sprocket argument in the paddock is smaller than it sounds.
7. Brakes as heaters. From 55 to 25 mph at the end of the straight, how much energy goes into the brake disc (ft-lb)? Convert to BTU (1 BTU = 778 ft-lb). Five such stops a lap, 13 laps in the main: total BTU into the disc. For scale, a BTU raises 1 lb of water by 1 F.
8. Braking distance from energy. If the tires can hold 1.0 g of braking (typical, on a rear-brake-only kart it's less: lesson 7), the kart decelerates at 32.2 ft/s^2. Use v^2 = v0^2 - 2 x a x d to find the distance the kart covers slowing from 55 to 25 mph, in feet, and the time it takes. Then: suppose you braked 10 ft later at the same rate. That is 10 more feet at top speed. How many seconds does 10 ft at 55 mph take? Compare that to the sprocket's 0.11 s from lesson 2 Q4.
9. Average vs minimum. Your average speed on the 58.33 lap was 42.6 mph and the limiter is 54.9. Suppose a lap is one third on the limiter, one third accelerating (average of 25 and 55 mph = 40), and one third in corners at 27 mph, near the minimum (all typical). What average speed does that three-thirds model give? Is it close to 42.6? Which third would you attack first, and why does the energy idea say so?
10. The Moon again. Lesson 1 Q9: on the Moon the kart weighs 60 lb but has the same mass. Does it have more or less kinetic energy at 55 mph than on Earth? Does the engine have to add more or less energy to get the kart to 55 mph? (Then: which changes, the energy the engine adds or the grip that lets you corner? Lesson 3 and lesson 7 both live in that gap.)
Done? Next track day, pick one hairpin and note the speed you think you carry through it. When the MyChron GPS is on, read the real minimum. The difference between your guess and the trace is the first thing lesson 9 will look at, and Q5 tells you what each mph of it is worth.