Every number below is off our kart (the kart's setup record: 360 lb race weight, 54 in rear track, 41 in wheelbase; the 2026-09-19 session entry) except the ones marked (typical): the front/rear weight split, the center-of-gravity height, and the cornering g. All three get measured the day the bathroom scales come out.
The one idea
A kart has no differential. Both rear tires are bolted to one axle and must turn at the same speed, but in a corner the outside tire has farther to go. If both stay planted, one of them has to slide, and the kart pushes straight on. The fix is built into the chassis: cornering force moves weight off the inside tires onto the outside ones, and the front geometry (next lesson) adds a lift, until the inside rear is skimming the track. The kart turns on three wheels. Every setup lever you have touched (pressures, front width, caster, rear track, seat) changes how much weight moves, how fast, and which tire gets it.
Three facts worth knowing
- Moving weight always loses grip. A tire's grip goes up with load, but less than proportionally (lesson 4 will put a number on it). So taking 50 lb off one tire and putting it on the other leaves you with less total grip than you started with. Weight transfer is a cost. You pay it because rotation is worth more than the grip it costs. "More grip" and "more rotation" pull in opposite directions, which is why setup is a compromise and not a search for the right answer.
- Transfer depends on three things you can change and one you can't. Lateral transfer = (cornering g) x (weight) x (height of the center of mass) / (track width). Weight is fixed by the rules at 360 lb. Height moves with the seat. Track width moves with the hubs and spacers. Cornering g is the driver.
- Where the ballast goes is a weight-transfer decision. Moving lead forward changes the front/rear split. That changes how much of the transfer happens at the front axle versus the rear, and so how hard the inside rear gets unloaded. Ballast placement is a setup lever, not just a way to make weight.
Worksheet
Units on every line.
1. Static corner weights. 360 lb, split 43% front / 57% rear (typical for a seated LO206 kart; ours is unmeasured). What is on each of the four tires at rest, assuming left and right are equal?
2. Lateral transfer. Cornering at 1.5 g (typical for a kart on slicks), center of mass 10 in off the ground (typical), rear track 54 in, front track 42 in (from the spacer stacks; approximate). Call the cornering g "g" (1.5) and the center-of-mass height "h" (10 in). Transfer at each axle = g x (that axle's static load from Q1) x h / (that axle's track). How many pounds move from the inside tire to the outside tire at the rear? At the front?
3. Does the inside rear lift? Subtract the rear transfer from the inside rear's static load (Q1). How many pounds are left on that tire? So: does weight transfer alone lift the wheel off the ground? If not, something else has to remove the rest - what part of the kart could do that? (Lesson 8 is about the answer.)
4. Rear track: 54 vs 53.25. The Birel baseline sheet says 53.25 in for a 100cc kart; ours is at 54. Redo the rear transfer at both widths. Which width leaves more weight on the inside rear, and by how many pounds? Is that difference big or small compared to the pounds left on the inside rear in Q3? (This is the setup change we parked on 9/19.)
5. The outside rear under load. Take the outside rear's cornering load (Q1 static + Q2 transfer). Using lesson 1 Q8's rule, patch area = load / pressure, what is that tire's contact patch in square inches at 14 psi hot? What was it at rest, with only the static load? Then say in one sentence why the outside rear is always the hottest tire on the kart.
6. The cost of transfer. Say grip on a tire = k x (load)^0.9, where k is just a constant that sets the units (typical exponent). Set k = 1. (With k = 1 the answer has no real unit - pounds to the 0.9 power isn't one - so call it "grip units" and only compare totals with each other. That's all this question needs.) Add up the grip of both rear tires two ways: (a) each carrying its Q1 static load, and (b) the outside carrying static + the Q2 rear transfer, the inside carrying static - that transfer. Which total is bigger, (a) or (b)? Now push the split further, 190 lb outside / 15 lb inside: is the total bigger or smaller still?
7. Braking transfer. Braking at 1.0 g (typical), h = 10 in, wheelbase 41 in, W = 360 lb. Load moved from the rear axle to the front = braking g x W x h / wheelbase. How many pounds leave the rear axle? What fraction of the kart's weight is on the rear axle mid-braking? The brakes are on the rear axle only. Say in one sentence why the rear "steps out on entry" (what you felt on 9/19, both on and off the brakes).
8. Where the seat matters. Lower the center of mass from 10 in to 9 in (a lower seat). Redo Q2 for the rear with h = 9 in. How many fewer pounds transfer? Is that more or fewer pounds than the rear-track change in Q4 was worth?
9. The Moon again. Lesson 1 Q9: on the Moon the kart weighs 60 lb but has the same mass. Cornering at the same speed on the same corner, how many pounds transfer compared with Earth: more, fewer, or the same? (Careful: the cornering force depends on mass, the transfer formula uses that force, and the kart's weight is what it's taken out of.) Could it corner faster or slower than on Earth?
10. One measurement. Which single number on this sheet, if we measured it, would fix the most answers? How would you measure it with two bathroom scales and two planks?
Done? Q10 is the scales day. Four corner weights and a CG height turn every (typical) on this sheet into a record, and lesson 8 uses them.