Every number below is off our kart (the kart's setup record: 360 lb, 54 in rear track, 42 in front track, 41 in wheelbase, 50 mm axle, hubs 4 7/8 in from the bearing both sides; the Birel baseline sheet: torsion bar "in", 0.5-degree caster pill) except the ones marked (typical): the 43/57 split, 10 in center-of-mass height, 1.5 g, and the frame stiffness, which nobody has measured. The last problem is how you would.
The one idea
Lesson 3 moved 57 lb off the inside rear with weight transfer and found 45.6 lb still sitting on it. Transfer alone does not lift the wheel. The rest of the lift comes from the frame: caster pushes the inside front tire DOWN when you steer (lesson 8), and the frame, which is a steel spring, carries that push diagonally across the kart to the inside rear and pulls it UP. A stiffer frame carries more of the push; a softer one soaks it up. Every "chassis stiffness" adjustment on the kart - torsion bar, seat struts, axle, rear track - changes how much of the inside-front drop arrives at the inside rear. The kart is a spring with an engine on it.
Three facts worth knowing
- Half the lift is geometry, not weight. With lesson 3's numbers, transfer removes 57 lb from the inside rear and caster jacking has to remove the last 45.6. They are the same size. A kart with the right caster and the wrong seat height lifts; so does one with the right seat and the wrong caster. That is why two karts with identical corner weights can handle completely differently.
- Stiffness has a lever. The frame twists; the tire moves by the twist angle times a lever arm. At the rear, that arm is half the rear track. At the axle, the load acts through the hub overhang (4 7/8 in from the bearing to the hub face on our kart). Widening the rear track makes both levers longer, which is why the simple transfer formula in lesson 3 (0.8 lb for 3/4 in of track) badly underrates what a track change does: the lever, not the transfer, is the effect.
- The torsion bar is a stiffness switch. A bar across the front of the frame, bolted in or left out, that raises front torsional stiffness (typical: 15-25%). Ours is "in" per the baseline sheet. It is the cheapest stiffness change on the kart and the one most people try first when the front won't bite or the rear won't free up.
Worksheet
Units on every line.
1. Where lesson 3 left off. Inside rear static 102.6 lb, transfer at 1.5 g 57.0 lb. What is left on the inside rear? What has to happen to that number for the wheel to leave the ground?
2. Jacking drop. Lesson 8's rule: the inside front drops about 0.05 mm per degree^2, times the steering angle times the total caster angle (typical). Total caster 10 degrees. How far does the inside front drop at 20 degrees of steering (a hairpin)? At 10 degrees (the sweeper)?
3. The frame as a spring. Say each mm of inside-front drop pulls k_frame = 5 lb (typical) off the inside rear through the frame. In the hairpin (10 mm of drop from Q2), how many pounds does jacking remove? Subtract that from the 45.6 lb left on the inside rear in Q1. Does the wheel lift (is the result zero or less)? Same for the sweeper (5 mm). Describe in one sentence how each corner would feel to the driver.
4. Torsion bar in. The bar raises k_frame by 20% (typical): 5 -> 6 lb/mm. Redo Q3 for both corners. Which corner changes category (from "stays down" to "lifts", or the reverse)? So what does "bar in" do to a kart that pushes in fast corners but is fine in hairpins?
5. Too much of a good thing. k_frame = 6 lb/mm in the hairpin. How many pounds beyond zero is the inside rear "lifted"? A wheel can't carry negative load; the frame simply carries it higher. What does a kart with the inside rear well in the air do to the outside rear's load, and to its grip? (Lesson 4: grip rises with load but less than in proportion, so piling load on one tire gives less grip than the two tires had between them.) Name the symptom the driver would report on hairpin exit.
6. Rear track as a lever. For a fixed twist angle of the frame, the inside rear rises by (angle) x (half the rear track). Compare 54 in to 53.25 in as a ratio: how much more does the inside rear rise for the same twist? Now the axle: the outside rear's cornering load (159.6 lb, lesson 3 Q5) bends the axle over the hub overhang, the distance from the bearing to the hub. Bending moment = load x overhang. Compute that moment at our 4.875 in overhang and at 4.5 in (the 53.25 setup). Which way does a longer overhang push the axle stiffness, and does that help or hurt the lift? (Two effects, opposite signs. Say which wins: you can't; that is what the stopwatch is for.)
7. Seat height vs jacking. Lesson 3 Q8: lowering the center of mass to 9 in cut rear transfer by 5.7 lb. In Q3's hairpin, what fraction of the total unloading (transfer + jacking) is that? Which is the bigger lever on lifting the wheel: 1 in of seat height, or 1 lb/mm of frame stiffness?
8. Front ballast, again. Moving 7 lb of lead to the nose changes the split from 43/57 to 45/55 (typical for that much). Redo lesson 3 Q2 for both axles. The rear transfer drops - but the front tire now carries more load into the jacking, and the frame's front end has more weight to push against. Which effect does the transfer formula see, and which one does it miss? What does that say about why a kart might run its lead on the nose?
9. The axle. Ours is 50 mm; kart axles come in the same diameter with different wall thickness and hardness, sold as "soft/medium/hard" (typical). A softer axle flexes more under the outside rear's 778 lb-in moment, which lets the inside rear settle back toward the track. Is a soft axle a stiffness increase or decrease in the sense of this lesson? When would you want it (name the symptom)?
10. Units check. k_frame is in lb per mm and the jacking drop is in mm. What are the units of their product, and what is it being added to in Q3? Now: someone says "the torsion bar adds 20 lb of grip." Fix that sentence the way lesson 1 Q10 fixed the sprocket one.
Done? The frame's stiffness is measurable in the garage in ten minutes, and it is the one number on this page nobody has. Kart on the floor, driver's weight in the seat (or ballast), floor jack under the left-front frame rail. Lift the corner slowly and watch the RIGHT rear. Measure how high the left front is when the right rear tire just spins free. That height is the diagonal drop the frame needs before the opposite rear lifts. If that height is under the 10 mm hairpin jacking drop from Q2, this frame lifts the inside rear in a hairpin at 1.5 g; if the height is over 10 mm, the frame doesn't lift it, and the caster pills are the next conversation. Do it on the other diagonal too; the two numbers should match, and if they don't, that is the chassis-crack inspection from the garage checklist, not a setup problem. Record both heights.