Clutches And Wasted Horsepower.

[Pete_Muller]

Let me give you a hypothetical;
let's mount a crankshaft, just the shaft, in frictionless bearings. Now let's mount a flywheel on that shaft, with the exact same moment and size as the clutch hub with shoes, drum and springs. Just a solid piece of something. Let's spin that up to, say 6000 RPM. As you can imagine, that thing would spin for a long time. Now let's put the clutch hub on with shoes and springs and drum. Now let's spin it up to exactly the same RPM. Same shaft same exact weight distribution as before. I'm 100% sure that arrangement would start slowing down, and probably pretty rapidly, immediately after you stop applying the power that's making the thing spin.

There you go Al - Ted explained it far better than I could have.

In your first message, you said: "The stronger the spring, the heavier the shoes, the greater the loss of HP at the wheels.".

Horsepower is not compressing the springs. Horsepower is what accelerates the rotating components (hub, shoes, etc) of the clutch before friction surface contact is made. Centrifugal force compresses the springs, and that's a function of the mass of the shoe, and rotation speed and radius. I'd also point out that in most or even all clutches, this "movement" (to close up the clutch until friction surfaces are in contact) is very small.

At the point of friction surface contact, the springs have become mostly a static force as rpm rises (other than the slight influence of centrifugal force on the mass of the spring itself). What *does* change with rpm is the force of the shoe on the drum (or the force applied to the disc stack). Yes... if a heavier shoe and spring combo is in the clutch, there is more mass at a given radius to accelerate UNTIL the shoes or discs make contact, and then there will of course also be slightly more mass to accelerate in the entire system since the clutch is part of the entire drivetrain, (which includes sprockets, sprocket hubs, jackshaft, chain, rear axle, rotating components of bearings, wheels, tires, brake hub/disk).

I'm with you on one thing... I like disc clutches. Not because I think they are inherently faster, but they are just more "tunable". I personally think that nothing "pulls" like an old Burco shoe clutch (sintered shoes on a relatively soft steel drum), however they are nearly impossible to keep consistent, as they are constantly in a state of rapid wear and huge temperature swings.

PM