Friction zone on a slipper assist clutch

Question

I have been wondering about this for a while now, and no one I asked seems to know what the answer is.

A slipper/assist clutch has a set of ramps that work to reduce the clutch engagement when the wheel is driving the engine, thus preventing hopping. They also have a set of ramps that work to increase the clutch engagement when the engine drives the wheel, so that the clutch springs can be soft (leading to a light lever force) and the camming action compensates.

I live in a city where low speed traffic crawling is inevitable - you may have to ride at 3 to 5 kph sometimes with almost no room to maneuver.

Friction zone riding is a given, and if the clutch is good, its possible to just use the clutch and rear brakes with no throttle to creep along, even when the road slopes upwards. My antiquated Enfield used to be ideal for this, because there is more than 50% torque just off a very low idle (but a terribly hard to press clutch)

Nowadays I'm riding a Duke 390 (2013) and a YZF-R3 (2018) Neither has a slipper clutch, and one can slip the clutch easily

The new generation of Dukes and almost any bike > 300cc have slipper assist clutches now. In fact the 390 ADV model that is meant to climb bad mountain roads semi-offroad style does too.

My question is this - when a bike is under load accelerating, the engine torque will assist the springs via the assist ramps and engage tighter - even if the lever is pressed, the engine will work to undo the force you apply by hand. It will just harden up the lever as if the springs were much stronger (the ramps are working with the springs).

Given this, how does one ride a bike with slipper assist at super crawly speeds - especially an engine like the 390 which has nothing below 3000 rpm and then suddenly immense torque after that. Clearly in offroad situations you need to sometimes go well below 5 kph, and need to do so on very steep sections.

The only solution I can think of is to ride it by a different technique... Disengage clutch fully, rev the bike and let the assist grab up, then as soon as it's moving, grab the clutch again to prevent going too fast - repeat this process, using the clutch as an on/off mechanism.

Ive seen many videos of powerful modern bikes on slippery offroad sections tend to either spin wheels or not move at all - because either the clutch engages fully and the engine reaches a pretty high RPM or because the clutch is fully disengaged respectively

To summarize: How can a slipper/assist clutch even have a friction zone?

Answer 1

I looked at a lot of sources and asked a few experts (including Tony Foale) and no one gave me a real answer, everyone knew how the slipper aspect worked - no one was well versed with how the assist mechanism worked.

Then I remembered that "TheWorkshop" on youtube had a video of disassembling a Kawasaki Slip/Assist clutch and he goes deep into demonstrating how the slip/assist mechanism works.

After re-watching this video, I think I understand now how you can cause clutch slip on as assist clutch.

So here is how it works: The pressure plate of an S/A clutch is not locked solid on the shafts where the springs are mounted - instead there are oval holes that let it turn back and forth a bit. Compare this with a regular one where the P.P. can only slide up and down the spring mount shafts.

At the bottom of the PP there are three parallelogram shaped ramps - which look like this:

/__/

There are corresponding slots on the clutch boss base into which these fit - the effect of this is that, if you rotate the P.P. (which connects to the engine) in the direction of the engines rotation, while holding the other side of the clutch solidly (the part that drives the transmission), the P.P. gets pulled downward assisting the springs.

If you rotate the transmission side while holding the drive side, then the PP gets pushed outward against the springs, disengaging the clutch and slipping the plates.

The key concept that I was missing is that the pressure plate is being driven by the clutch plates, by friction. There are 2 forces - the springs pushing the PP squeezing the plates, and the clutch lever pull reducing that force, unsqueezing the plates.

If the engine is rotating faster than the transmission, the clutch is slipping (friction zone), and the pressure plate gets a small amount of torsion that makes it squeeze. However, this doesn't mean that the force is heavy enough to overcome the hand force completely - so while I am right that the lever will seem to harden in the hand (needing more force to maintain the same position), unless you rev too hard, its not going to pull the lever out against your hand.

The squeezing force is proportional to the net wheel torque force - which is typically quite low when crawling at relatively low engine RPMs.

The place where the assist will pull the lever out of your hand would be well inside the midrange of the torque curve - a high level of throttle.

Hence, to conclude, while the friction zone in an S/A clutch is not as wide as a regular clutch, it is still possible to grind it out at lower RPMs (in a regular clutch you can very well ride at 10 kph at redline if you wished to and had the correct grip modulation)