View Full Version : Clutch hydraulics - how do they work?

07-30-2011, 10:15 AM
There are hundreds of posts here that revolve around clutch hydraulics - mostly as they relate to problems finding neutral. I've managed to get my clutch to behave (knock wood), but at this point, what I'd like to find is something that really explains exactly how the system works.

People talk about bleeding the clutch, replacing a jet and adjusting free play, but what boggles my mind is how an air bubble or whatever can make the shift pedal so intractable at just one point, when every other shift will work just fine! And I wonder, what might all of this have to do with the "patented Pneumatic Power Clutch" slipper mechanism?

I'm not an engineer, but I do appreciate an explanation of systems (managed to learn how to make 1200 psi steam push 70,000 tons through the water at 30+ knots). If I could just get my hands on the right book or a decent video.... Google and Wikipedia were pretty much a bust. Any suggestions?

07-30-2011, 11:26 AM
I'm no scientist,

But believe it all rather straight forward anyway. I don't know pneumatic clutches have any bearing on it.

The difficulty with the neutral is really that the bikes gears it prefers to be in if you like, are either side of it, and you're trying to kind of balance the selector position, firmly, but with less effort, via the lever, onto a smaller 'notch' between them, which still firmly holds 'N' in place.

If the clutch is dragging slightly when you do this, you need more, or will use more, effort. When you do this you push it too far, so end up in 1st or 2nd, and somewhat red faced if you can't get the knack.

Riding the bike you can get higher or lower gears by relieving the engine load, off the driveline momentarily, without using the clutch at all, so tiny air bubbles whilst riding and changing, are masked by this natural mechanical possibility.

At low speed or standstill however, you will rely on the entire length of the fluid pushrod being available, if you get me.

You can get neutral by not using the clutch, but you'll probably hit your mark by the time you do it , messing about with the thing.

The hydraulics are only a liquid form of a pushrod, like your steam reference, just steam compresses obviously. The clutch fluid is adversely affected by heat, and also absorbs moisture, and it goes off in time, turning to jelly if left long enough.

Free play adjustment just needs to exist enough in all temperatures, to guarantee nothing can be applying pressure to the clutch when is is released by your hand.

Using such tiny amounts of fluid in these circuits, you often need to bleed them often. Brakes aren't so affected, as not subjected to such constant heat, but all parts do the same thing, 'pushing'.

The jet sizes relate I beleive to the equivalent of a fluid gearing/pressure, so in steam I guess via valves? Some change jets to ease the lever effort, as the Ape has a heavy clutch.

I like them though, and all four of my bikes have fluid clutches, and they're brilliant imo.

No idea if that lot is what you wish to read...lol

But 'tis my 2p :cathat:

07-30-2011, 11:28 AM
A hydraulic is a basic system with a master cylinder that delivers fluid at pressure to a slave cylinder. The slave is forced to move by the fluid which releases the clutch.

The reason that any air inhibits shifting is that the air reduces the full movement of the slave cylinder so the clutch does not release fully. This is also why air increases clutch drag.

The Rotax slipper clutch is prone to drag because of certain design features. In most clutches the plates are clamped between flat and parallel surfaces. Tapered surfaces are used in this slipper design to make the slip more progressive rather than on and off. These tapered surfaces make the clutch more prone to drag. It takes more slave cylinder travel to fully disengage the plates. Eliminating drag requires all parts of the system to be working at optimum levels.

The oil jet meters oil distributed to the interior of the clutch assembly. Over the years the size of the jet has been reduced. This reduces the amount of oil and can reduce drag.

Motorcycles can be shifted without the clutch. It is a learned art to be able to do it smoothly. With air in the clutch it can make shifting difficult but sometimes you get it just right so the shift would have been completed smoothly whether you had used the clutch or not.