The maximum torque
'T' that can be transmitted by a clutch depends on the following factors:
1. The number of friction surfaces in contact. 'S'
2. The spring force of the pressure plate 'P'
3. The coefficient of friction between the friction material and the plain plate surface 'C'. This must be the coefficient applying to the expected amount of oil on the surfaces.
4. The mean effective radius of the friction surfaces 'R'
The formula used for this calculation is: T=S x P x C x R
There’s no need to worry about the units for each of these parameters as we can draw general inferences without such specifics.
If we are considering any specific clutch, say Hillbilly’s in this instance, S, P and C being held constant, increasing the mean effective radius R linearly increases the torque
The mean effective radius could be increased by using a wider clutch basket with correspondingly larger radius plates, but we’re not doing that in this situation.
The mean effective radius is roughly the half-way radius of the friction pads.
This is only an illustration, to show the impact:
Say if the inner 10mm of the friction pads are removed, the mean effective radius increases by 5mm.
The usual outside radius of the friction pads is ~76mm
The usual inside radius of the friction pads is ~59mm
So the mean effective radius is ~ 67.5mm
Then removing the inner 10mm of the friction pads moves the mean effective radius to ~72.5mm.
This is an increase of torque
capability of 72.5/67.5 X 100 = 7.4%.
Not a massive gain, and you probably wouldn’t remove as much as 10mm from the inside of the pads for a road bike (you might for racing) as that would be ~half the material. So you’d likely remove somewhat less, and gain 4% at a guess.
But every little helps, as is often said, and at least some of the 7-plate suppliers do this as well as the extra plate.
I don’t know the specifics of a BSA pre-unit clutch, but a p/u Triumph had a standard 5 friction plates with 10 friction surfaces. The unit Triumph clutch had 1 extra friction surface (apart from the late ones which had yet one more due to the basket mounted pads). If you add the extra plates that the 7-plate suppliers provide, that adds 2 additional friction surfaces.
So adding an extra friction and plain plate to one of the old clutches produces 12/10 = 20% improvement in torque
capability, 13/11 = 18% in a unit clutch.
This is the main gain from the 7 plate kits, with or without the small gain from the mean effective radius change.
Of course there is a trade off when the friction pads are reduced to increase mean effective radius. The wear of the clutch is taken by reduced area, so increased wear is only to be expected. But if the clutch doesn’t slip at high torque
(which many ordinary clutches do) then it possibly doesn’t matter too much.
Finally, I would say that my experience with Triumph clutches is to minimise the amount of oil within it, by maintaining the oil level in the chaincase such that it only reaches the lower end of the chain run under the clutch. It functions at its best when as dry as possible. That alone will make a standard clutch work satisfactorily in most ordinary riding experience.