Every number below is off our kart (the kart's setup record, the 2026-09-19 session entry, our speed calculator, the lesson 2, 3 and 6 answers) except the ones marked (typical), which are round figures for a kart on slicks and not measurements of ours. The 1.5 g figure in particular is a placeholder until the MyChron's accelerometer reports what this kart and these tires actually do.
The one idea
A tire has one budget of grip, and braking, cornering and accelerating all draw on the same account. Draw the budget as a circle: 1.5 g in any direction (typical). Pure braking spends it all forward; pure cornering spends it all sideways; do both at once and the two together can't exceed the circle. Fast driving is keeping the tire near the edge of that circle the whole way around a corner, not spending the budget on one thing at a time. Trail braking is the name for spending braking and cornering at the same time on the way in, and it's exactly where the 9/19 entry problem lives.
Three facts worth knowing
- The circle is a right triangle. If the tire can do 1.5 g total and you're braking at 1.0 g, the cornering left over is not 0.5 g. It's sqrt(1.5^2 - 1.0^2) = 1.12 g. Pythagoras. That's why a little braking into a corner costs so little cornering, and why a lot of braking suddenly costs all of it.
- A kart brakes with the rear wheels only. Braking throws weight forward (lesson 3), which takes load OFF the rear tires, which are the only ones doing the braking. So a kart can't use anything like 1.5 g in a straight-line stop; the rear budget shrinks as you use it. The circle is smaller under braking than under cornering, and it's the rear's circle that decides entry.
- "Rear steps out on entry, on the brakes and off them" is the 9/19 report, in the driver's words. On the brakes, the rear is spending its budget on braking while the front turns in, so the sideways share is already gone. Off the brakes, the rear still has less load than static because the kart is still pitched forward. Both are the same picture, and both are fixed by asking the rear for less at once.
Worksheet
Units on every line.
1. The circle in pounds. 1.5 g on 360 lb of kart is how many pounds of sideways force at the tires, total? (Force = g-number x weight.) Compare it to the 69 lb of thrust the engine can make (lesson 2 Q3). Which is bigger? So which end of a corner is limited by the engine, and which by the tires: the exit, where you accelerate, or the entry, where you brake and turn?
2. Cornering speed from the circle. At 1.5 g lateral, the fastest speed through a corner of radius r is v = sqrt(a x r) with a = 1.5 x 32.2 ft/s^2. Compute it for a 25 ft hairpin and a 40 ft corner, in mph. Compare the hairpin number to the 25 mph guess in lesson 6. Then turn it around: at 30 mph and 1.5 g, what is the radius of the corner the kart is on?
3. Pythagoras. With a 1.5 g circle, how much cornering g is left at 1.0 g of braking? At 0.7 g? At 0.5 g? Write the three numbers and say which one surprised you.
4. Rear-only braking. Static rear load is 57% of 360 lb (typical from lesson 3). Under 1.0 g of braking, weight transfer moves W x h / L forward, with W = 360 lb, h = 10 in (typical CG height) and L = 41 in (wheelbase). How much load leaves the rear axle? How much is left on the rear axle? With the rear now carrying that, and grip proportional to load, how hard could the rear tires actually brake, in g, if their own limit is 1.5 g of their load? (Hint: it can't be 1.0 g, because using 1.0 g removes the load needed to make it. Set a = 1.5 x (rear load at a) / 360 and solve.)
5. The entry picture. Take your Q4 answer as the kart's real straight-line braking limit. At that deceleration, how much sideways budget does the REAR have left, in g, if you also ask it to turn in? Now put the driver's report next to it: the rear steps out on the brakes. Which is more likely: the rear's braking share is too high at the turn-in point, or the tire has no cornering budget at all?
6. Two ways in. Way A: brake in a straight line at your Q4 limit from 55 to 25 mph, then turn at 1.5 g. Way B: brake at 0.7 g and turn in at the same time, keeping the tire on the edge of the circle. Lesson 6 Q8 gave the distance for that straight-line stop at 1.0 g. Which way gets from the braking point to the corner exit in less total time, and why does Way B need LESS grip from the rear at the moment of turn-in even though it's doing two things?
7. Load moves the circle. Lesson 3 said cornering moves load to the outside tires. If the outside rear carries 160 lb mid-corner and the inside rear 20 lb, and each tire's budget is 1.5 x its own load, what is each tire's budget in pounds? Add them. Compare with two tires at 90 lb each. Which pair has more total grip, and what does that say about why weight transfer costs grip (lesson 4's load sensitivity makes this worse)?
8. Spending it on the way out. At hairpin exit the rear is asked to accelerate (69 lb of thrust, lesson 2) and to corner at the same time. If both rears together carry 205 lb (static, typical) and the budget is 1.5 g of that, how much sideways force is left once the engine's 69 lb is spent? Is the exit rear-grip-limited or engine-limited on this kart? (Compare to Q1.)
9. Half a second. On 9/19 the last five laps of the main went 59.06, 59.41, 59.78, 1:00.40, 1:02.86 while something scraped. Suppose it was a dragging brake adding 0.1 g of drag all the time. Over a 4 s run on the straight, how many mph does 0.1 g of drag cost (a = 0.1 x 32.2 ft/s^2)? Does that match the size of the lap-time loss, or was something bigger going on? Lesson 6's energy idea helps: where does 0.1 g of drag hurt most?
10. The Moon, last time. Lesson 1 Q9, lesson 3 Q9, lesson 6 Q10: mass stays, weight drops to 1/6. Grip is proportional to weight, so the circle shrinks to 0.25 g. What is the cornering speed through the 25 ft hairpin (Q2 method)? What is the braking distance from 55 to 25 mph? Say in one sentence why racing on the Moon would be a slow, long-braking, wide-cornering affair even with the same engine and the same energy at 55 mph.
Done? Next session, have someone stand at the entry of the hairpin where the rear steps out and watch the brake: does the rear start to slide while the kart is still pitched nose-down, or after it settles? That's Q5 answered by eye until the MyChron's brake-pressure and accelerometer channels answer it in numbers. Record what they saw.
Going deeper the circle is an ellipse, and it moves