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Does a tall driver bind and hop in very high grip?
A tall driver has a higher center of gravity and usually sits farther aft, so a given lateral acceleration produces more load transfer and the chassis has a larger yaw inertia to overcome. In low or mixed grip the inside rear unloads readily and the kart rotates. In very high grip the outside contact patches can generate enough force that the frame stays too square, the inside rear remains loaded, and the kart binds rather than yaws.
When that bind finally releases, the unload is abrupt and the result is hop, especially from mid-corner to exit. The usual explanation is that extra caster jacking or a narrower, more free rear lets the chassis twist in time. That part is more setup folklore than a closed-form answer. How much of this is geometry versus the driver asking for rotation after the kart is already loaded up?
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a tall fella often sits too far back and that will keep that inside rear planted when the grip is way up and the kart just wont rotate it just binds till it lets go. I reckon extra weight hung out behind the axle is enough even if the rest of the kart is pretty near right.
You can still get that snap if you keep adding lock after the chassis is already loaded. Sliding the seat forward some even if the knees feel tight takes weight off the rear and gives the frame a chance to work before you ask for the exit.
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What decides it is whether the inside rear can lighten while the outside tires are still gaining force. A high, aft mass makes that window short, so you treat height and front width before you chase extra caster. Put the frame on the lowest ride height, hang the seat 2-3mm below the rails, run two small spacers on the front, and fit MXC, MXL, or MXQ wheels.
Zero caster pill, 2mm of toe-out, seat stays, and shorter rear hubs time the jack later and help you keep the inside rear light through mid-corner to exit. Keep the chrome round front bar. Geometry does the larger share, but asking for more lock after the kart is already loaded is what turns a tight chassis into a hop: the extra steer just parks more load on the outside front and delays the unload further. That same free rear and late jack will over-rotate the moment grip falls off, so it is a high-grip package, not a general rule.
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I wouldn't burn the first session on chassis changes with a kid waiting. Jack the rears and spin them first. A sticky hub or a bearing that isn't free will hold a corner down, and you lose the morning chasing hop that might not even be a setup problem.
After that watch whether they keep adding lock once the kart is planted. High grip makes that late ask look like a geometry problem, it's really the driver still wanting rotation. Leave the bigger setup hunt for when the rear is free and the steering is quiet.
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First thing I'd check is whether that seat's mounted up high so he can get his legs in. That puts a tall guy's weight up in the air and further back, and on a tacky track the rear just plants and won't rotate.
Once it's loaded like that, if he's still cranked into the wheel and then gets in the gas it hangs on and hops when it finally lets go, and that'll chew the right rear on a heavy track. Unwind a little earlier and roll into the throttle, a lot of that hop goes away before you start chasin geometry.
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