Clutch confusion
Turbo Mini Forums

Ok if friction is dimensionless then increasing the area of the clutch wont increase the torque capacity right? I'm a bit confused as to why twin plate clutches etc exist then? Prob me being stupid but any info appreciated, cheers.

The coefficient of friction is dimensionless, but in order to calculate a torque capacity you need to take into account the area the friction is acting upon, and the force normal to this area, in the case of a clutch the clamping load. These both have dimensions hence why increasing area changes the torque capacity.

...and hence why increasing the number of friction 'couples' increases the overal system torque capacity.

Ah i see, i was confused in that i didnt see how area affected the torque capacity even though its in the formula, cheers :)

Nope confused again, just read this from one of TD's posts: "It's nothing to do with area at all - friction is dimensionless (has no units). Hence you could take an organic plate and chop 90% of the material away - and it'll still hold the same torque." ??

Ok,

If you sat on a piece of wood measuring 6x6, and held a piece of 1x1 board over your head, and had a piece of rope attached to the board (contacting the ground), and a spring balance, it would (theoretically) take the exact same force to drag the board along the ground as if you were sat on the 1x1 and carrying the 6x6 above your head.

I.e. the mass of the 'system' is the same, but the area changes - and as such the units of mass per unit of area change; you are simply sharing the SAME mass over a greater area.
And as the friction is dependant on each individual area - and the mass applied to that area, you can see that the whole friction co-efficinet is constant.

Edited by turbodave16v on 27th Mar, 2007.

I didn't understand any of that at the start of the thread...Lol...all makes sense now.

A+ for the explanation Dave

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1/4mile in 13.2sec @ 111 terminal on 15psi

Ok if increasing the area of the board doesnt have any effect on the force needed to move it then i still dont see how increasing the surface area of a clutch will increase its torque capacity. What am i missing here this is driving me crazy :-

It's not the area as much as the acting radius IIRC. You don't often see people increasing the are towards the centre ....

Every day is a school day ...........

How fast and how expensive ...... the same question...

Ah i think i get it now, if you were to add extra area to the clutch plate it would make no difference at all to the torque capacity as the pressure would just decrease, however adding another plate will increase the area 100% for the same given pressure. I hope thats right?

I think you have it...

Easiest analogy is a paddle clutch - say the ones with six pads on each side (12 total). You could remove three off each side, and the torque capacity will be unchanged.

Inertia will be less of course (positive) but it'll wear faster (negative).

You have three options with regard to uprating clutches: 1) increase clamping force, 2) Increase mean radius (it has a term I don't recall right now) and 3) Increase number of friction couples (firction surfaces).

Is it radius of gyration or something similar?

Yes Tom :)

Tom .... you sure that isn't hip movement ...... maybe not

Every day is a school day ...........

How fast and how expensive ...... the same question...

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Stevie you answered your own question lol, there is less friction area but the pressure is proportionally greater so the torque capacity remains unchanged.