The drum spinning has no bearing on the clutch shoes. Only the motor shaft. I assume you can spin your drum on the work bench on a completed clutch?
So if the motor shaft falls below the engagement RPM, then the shoes will drop regardless of actual kart speed.
We are not talking about on a bench or when the engine speed is below clutch engagement. We’re talking full speed when the clutch is engaged. You let off the throttle and the chain is now driving the clutch which is spinning the engine. At this exact point, the drum is driving the shoes.
But its not really driving it, but i see when you think it might be.
If the drum is not driving the shoe then what is causing the engine to stay up in rpm when you let off the throttle?
Do you still disagree that the axle drives the engine when you lift at full speed?
I think the inertia of the motor spinning is the key. If you are at max rpm which should be around 6100 and lift the crank is still spinning and thus the clutch is engaged. In a racing situation, you are off the throttle so little that the motor is never spinning low enough for the clutch to be disengaged and if it does it spins back up to the engagement rpm quickly since you are moving.
Please don’t take this combatively as I am enjoying seeing things from your alternative view:
Have you ever run an engine on the bench/dyno without a chain/clutch to see how long that engine spins with inertia?
Have you ever locked up the brakes and killed an engine? Was the drum dragging the shoes to slow down then?
No offense taken and while I think I am correct I could be totally wrong too.
I have not run a motor on a bench totally disconnected but 4 strokes have a flywheel partially for this inertia purpose and even 2 strokes have some spinning weight to keep things spinning.
Now you ask an interesting question. I have not only stalled an engine with a brake lock-up but also had it bump start (it was only dead for a second). Conversely, I have stuck a piston on a yamaha at the fastest part of our track and even though I was going about 50mph, I coasted some distance meaning the clutch was totally disengaged.
I can see why you believe the chain is driving the clutch and keeping the clutch engaged but that is only happening because the motor is spinning fast enough to keep the clutch engaged. Another way to look at it… could you push start a go-kart with a centrifugal clutch?
Change my mind if you think I am wrong.
I know you’re wrong but I don’t think I can change your mind (playful shot taken) 
That’s the beauty of the internet 
You cannot push start an engine with a centrifugal clutch. I am not debating that one bit. The engine absolutely causes the shaft to spin sending the shoes up against the drum and causing the drum to spin. What I am debating is that once that happens and the shoes are engaged to the drum. Now the shoes do not care whether the rotational force is coming from the hub/shaft or the drum. The friction between the drum and the shoes can (and does) impose a forward (or rotational) force on the shoes during deceleration. That’s why when you lose a chain you roll a lot further than if you let off the gas and let the go kart coast to an idle.
Unfortunately, we might have to agree to disagree. Thats ok, though. Like I said, this has been fun.
A very Kartpulse argument. 
unless the chain pops off, it stays locked up and the clutch/motor relationship is gonna be like a kid rolling down a hill on a tricycle with his feet off the pedals. the kart itself has to slow down somewhat for the clutch to disengage.
I think you could argue that the engine brakes the axle via compression braking.
Both are true I suppose. Once the shoes are engaged the engine and axle are coupled… the forces that happen… happen.
It sounds like we agree on some points and in the statement quoted above, I would partially agree as long as the motor is spinning fast enough to keep the clutch shoes engaged. Once the RPM are below engagement speed the drum transfers no effect on the clutch shoes. The reality is that during the race the clutch is mostly if not completely engaged at all times so the effect is the clutch is always engaged and may appear as the axle forces are driving the clutch and engine but its only because the motor is spinning fast enough to keep the shoes engaged.
Yep, it appears we only disagree on the part where you believe the engine’s inertia is what drives it to keep spinning fast enough to keep clutch shoes engaged and I believe the chain is driving the engine to keep rotating. Correct?
Once the clutch is engaged, engine, clutch and wheel is a whole unit, as long as one of them is spinning, rest of will be forced to spin at the same time. If you let the throttle go on the straight, the momentum will force you go forward, the engaged clutch and force the engine to rev as it is a whole unit from wheel going forward. It is engine braking at this stage. Once you are slowing down enough, that engine/Clutch drops to disengaged RPM, clutch start to disengaged and engine will start to continue drop RPM to idle. At this stage, you are not engine braking, more like coasting.
You should be able to experience it while driving especially in a manual car or semi. When you let the throttle go. It is the wheel to force the engine to rev. If your automatic car has O/D switch, turn it off, it is really similar to a semi engine brake situation.
If you model the drive train, the engine provides torque.
The torque acts on the engine rotational components and the flywheel, the weights in the clutch and (rotationally) accelerates them. Torque = inertia * rotational acceleration + B * rotational velocity + frictional loses.
When the clutch engages (think of it as a one way valve in that) it transfers the torque from the engine through the rest of the drive train i.e. the gear on the engine, the chain, the sprocket gear, the rear axle and the wheels.
The inertia of the sprocket, rear axle, wheels and the kart are seen by the engine as 1/N^2 where N is the gear ratio.
Anyway, when you lift on the throttle, the engine keeps spinning because of inertia and until a certain engine speed it is enough to keep the clutch engaged. How long it takes the engine to slow down below the engagement speed is a function of the load the engine is seeing.
With a few tests, one can determine the time this would take…
I’m just starting to put together a kart and don’t have this data but some day…
I will even agree with you to a point. There are very brief instances where the clutch is being driven by the chain but only because the motor is spinning enough to engage the shoes. I suppose you could check this by switching the ignition on a 206 off and then back on. What would happen?
Engine keeps spinning if you kill the ignition, until
It drops below stall speed.
Look at it this way, the engine RPM causes the clutch to engage the drum, the coefficient of friction between the drum and shoes, isnt going to allow the engine to drop to idle, while the kart is at speed. At that point, ie. lifting throttle while at redline and coasting, the clutch isnt going to disengage, and the wheels are now driving the engine.
Greg, at Whiteland Raceway Park, I am off the gas for 2 seconds at a time (2x per lap, once for each hairpin) and the clutch never disengages.
Edit: In hindsight, 2 seconds might be an exaggeration. Lets say 1.5 seconds. I’ll check the Mychron tomorrow when I am up there.
Its a kind of how long is a piece of string thing. However long it takes for the kart to coast down to that engine speed given it’s gearing. As long as there’s enough RPM for the shoes to make contact, the engine and axle are coupled.
The clutch being engages is a function of crank RPM, it makes no difference how that crank speed is achieved.