Every number below is off our kart (the kart's setup record, the 2026-09-19 session entry, lessons 1-3) except the ones marked (typical), which are published figures for a kart on slicks, not measurements of ours. The corner-weight session and the MyChron brake channel replace them.
The one idea
Braking throws weight forward. A kart brakes with its rear wheels only, so the harder you brake the less load sits on the tires doing the braking, and the less there is left for the rear to grip with when you turn. Corner entry is the one moment the rear has the least load, the most work, and no differential to help it. That is where a rear steps out, and it is why 9/19's "rear breaks loose on entry, on the brakes and off them" is a physics report, not a driving complaint.
Three facts worth knowing
- A kart's brakes are on the wrong axle, on purpose. Rear-only brakes keep the front free to steer and keep the kart simple, at the cost of braking with the axle that is being unloaded. Shifter karts add front brakes precisely because at their speeds the rear can't slow the kart alone. On an LO206 the rear can, barely, and the stopping distance in Q4 says how barely.
- Kinetic energy goes as speed squared. Slowing from 55 to 25 mph removes 79% of the kart's energy, all of it through two contact patches the size of playing cards, as heat. The brake is doing four times the work per mph at the top of the straight as at the end.
- The friction circle. A tire has one grip budget; braking and cornering spend from the same account, added as a vector (square root of the sum of squares), not a sum. Braking at 0.5 g while cornering at 1.2 g uses 1.3 g of grip, not 1.7. Trail braking works because of that square root, and it stops working when the total hits the rim of the circle. (Trail braking: still on the brake as you turn in, easing off as the steering goes on.)
Worksheet
Units on every line.
1. Static loads. Kart plus driver 360 lb, front/rear split 43/57 (typical for a seated LO206 senior). What is on the front axle, the rear axle, and each tire, at rest?
2. Longitudinal transfer. Center of mass 10 in off the ground, wheelbase 41 in (typical; the homologation form says 1045 mm). Weight moved forward under braking = decel in g x weight x CG height / wheel- base. Compute the transfer at 0.5 g and 1.0 g. What load is on the rear axle in each case, and what fraction of the kart's 360 lb is the rear axle carrying?
3. Rear-only brakes. The most braking force the rear tires can make is mu x (rear load), with mu = 1.2 (typical for a slick on warm asphalt). The braking force needed to slow the kart at a decel of a (in g) is a x 360 lb. But the rear load itself falls as a rises (Q2). Set the force the tires can make equal to the force the decel needs, and solve for a: the hardest a rear-brake-only kart can brake. Compare that a to the 1.2 g a kart braking on all four wheels could reach with the same tires.
4. Stopping distance. From 55 mph (the top of the straight, lesson 2), how many feet does the kart travel before it stops, at the Q3 decel? At 1.2 g? How many seconds does each stop take? (v^2 = 2 a d; convert mph to ft/s first, g = 32.2 ft/s^2.)
5. Energy. Mass = 360 lb / 32.2 = 11.2 slug. Kinetic energy = 1/2 x m x v^2. Compute it at 55 mph and 25 mph in ft-lb. What fraction of the energy is removed slowing 55 -> 25? Where does that energy go?
6. Lateral transfer at entry. Rear track 54 in, CG 10 in. In a hairpin at 1.5 g lateral, how much load moves from the inside tires to the outside tires? (Lesson 3's formula: lateral g x weight x CG height / track width, the same shape as Q2 with track in place of wheelbase.) How much at 1.0 g?
7. The friction circle. Combined grip demand = sqrt(braking g^2 + lateral g^2). Compute it for: 0.5 g braking + 1.2 g lateral; 0.3 g braking + 1.3 g lateral; 0 braking + 1.5 g lateral. If the tire's limit is 1.5 g, which of the three is over the limit, and which is using the most of the budget without exceeding it?
8. The inside rear at entry. At 0.5 g braking, take the rear axle load from Q2 and split it evenly between the two rear tires. Now add 1.0 g lateral: take the Q6 transfer at 1.0 g, assume the rear axle sees 60% of it (typical), and take that off the inside rear tire. What load is left on the inside rear? Compare it to that tire's static load from Q1. That number is the rear "breaking loose."
9. On the brakes vs off them. Q8 explains the on-brake case. Off the brakes, the transfer in Q2 is gone, but the driver said it still steps out. What else could make the rear step out with no brake on? List two chassis causes from lesson 3 (hint: lesson 3 Q3 and Q4) and one tire cause from lesson 4 (hint: lesson 4's temperature window).
10. The measurement. Next practice: three laps braking entirely in a straight line and releasing before turn-in, three laps trail-braking into the same hairpin. Stopwatch the segment, and ask the driver which one the rear stepped out on. When the MyChron brake-pressure channel is on, this becomes a trace instead of a feeling. Record both.
Done? Q10 is the only one with a stopwatch in it. Q1-Q9 all rest on the 43/57 split and the 10 in CG, which are guesses until the corner weights are on a scale. That is on the to-do list; do it first.
Going deeper why a kart brakes with the rear wheels only