Dominator clutch hydraulic conversion

Chris

VIP MEMBER
Joined
Jan 21, 2008
Messages
1,626
Country flag
Option two! He asked about a hydraulic clutch! I suggested we work through & overhaul the old clutch & fit the new cable suggested levers etc first as a hydraulic clutch mechanism is a waste of time if the clutch is shagged.
I remember a bike with hydraulic clutch & a hydraulic drum brakes that used Morris minor or mini brake pistons.
Seem to remember both were cable operated with remote master cylinders. Suggestions please.
Thanks Chris
 
Option two! He asked about a hydraulic clutch! I suggested we work through & overhaul the old clutch & fit the new cable suggested levers etc first as a hydraulic clutch mechanism is a waste of time if the clutch is shagged.
I remember a bike with hydraulic clutch & a hydraulic drum brakes that used Morris minor or mini brake pistons.
Seem to remember both were cable operated with remote master cylinders. Suggestions please.
Thanks Chris
If you want to go down the hydraulic route, you might need almost a degree in engineering since evey dimension is crucial, why the insert of a Commando clutch lever instead of a normal Dommi one will affect the ease of the lever pull - it's imperative that the 7/8 inch distances between the cabe centre and the fulcrum / pivot centre is maintained. As is the leverage on the gearbox fulcrum arm must also be 7/8 inch centres.
Anything else is asking for a harder lever pull as has been discussed many time on this forum.
 
I fail to see how using hydraulic pressure to actuate the clutch release will result in less lever pull than using a cable.
Ultimately, the force required at the clutch end must be applied at the lever end.

Slick
 
Hi Bernard, Slick,
Thanks for the replys. I've already pointed out that the levers he has dont help matters. I will put together a Newby belt drive clutch & see if that helps. We might be able to improve lots of areas but the clutch might just be a heavy action. Slick it's not less lever pull it's trying to ease the action that we are after. A hydraulic clutch is seen as the way to go but as said it cant alter a poor clutch.
Thanks though.
 
Have you thought about using bicycle Bowden cables as Ludwig does
Maybe some of the friction can be reduced by using smaller cable?
I believe Ludwig said the strength of bicycle cable far exceeds the needs
Hopefully Ludwig will comment and correct me if I have miss quoted him
 
I have a650ss & a CSR 650 both fitted with that AMC clutch. Both are in very good order, set up to lift evenly, & both have levers with7/8" fulcrum centres & PTFE lined cables. They are light & neither drag nor slip, but if your friend needs a lighter clutch I believe the dreaded Norvil Co. can supply a Commando type with belt drive, which is supposedly two finger operation. Hydraulics won't make the clutch any lighter, but only more consistent as it warms up.

Martyn.
 
I fail to see how using hydraulic pressure to actuate the clutch release will result in less lever pull than using a cable.
Ultimately, the force required at the clutch end must be applied at the lever end.

Slick
Fit a servo :)
 
I fail to see how using hydraulic pressure to actuate the clutch release will result in less lever pull than using a cable.
Ultimately, the force required at the clutch end must be applied at the lever end.
That statement is wrong. Neglecting friction, the universal law of conservation of energy dictates that the product of force x displacement (= travel) is constant. For the cable operated clutch,
looking at the clutch push rod and at the lever, and accounting for torque equivalenvce at the lever, we derive at

H * a / r = F2 * d2

where
a = pivot distance between hand's force point of attack and pivot point. For simplicity, I use the maximum radial distance between pivot point and lever blade (i.e., inboard of the sphere on a ball-ended lever).
r = pivot radius
H = force applied by fingers (please note it increases with travel)
F2 = force exerted by clutch rod end
d2 = maximum displacement at clutch rod end = "clutch lift".

The AMC clutch has 3 coil spings, and assuming perfect linearity, we have

F2 = 3 * k * (s+d2)

where
k = coil spring stiffness.
s = spring pre-compression displacement

Thus, we can predict that the lever pulling force H experienced by the rider is a product of coil spring stiffness, the required lift to free the friction plates within the clutch (depende on the area of friction), and geometric parameters "a" and "r" at the lever.

Late AMC bikes used these parameters:
r = 7/8" = 22.225 mm
a = 5.315" = 135 mm

Assuming the clutch remains stock (obviously it has a large potential for improvements, and the required lift is paramount),

H = 3 * k * (s+d2) * (d2) * r / a

reducing "r" by 10% and increasing "a" by 10% will reduce the pulling force H by 18% !

Fitting a different clutch with a larger friction area and lowering the spring stiffness will be much more effective though.
The AMC clutch may require a lift of 6 mm. A modern clutch with 30% more friction area may require a lift of 5 mm only to free the clutch stack. Let's assume the same clutch has 6 springs rather than 3, each with a spring stiffness of 40% of the stiffness of the AMC clutch,
and with the same pre-compression displacement (2mm assumed here), H will be reduced by a massive 42% !

Similar considerations can be made for a hydraulic operated clutch. The lever geometry differs though.

- Knut
 
Last edited:
As mentioned by Baz, on the advice of @ludwig I fitted a linear BMX bicycle brake cable to my Morini which had a quite heavy clutch action. The difference in feel is night and day and it is now as smooth as a hydraulic clutch. Instead of the normal spiral wound housing these have strands running the length of the outer and greatly improve the feel. The one I used also has the benefit of a special friction free liner.



Hope this helps.
 
Last edited:
@Knut
How can my statement be not true? It is a generalized assertion of the Conservation Law you yourself use in your elegant analysis of the forces at each end of the the clutch cable.

Slick
 
@Knut
How can my statement be not true? It is a generalized assertion of the Conservation Law you yourself use in your elegant analysis of the forces at each end of the the clutch cable.
@ Slick
I misinterpreted your sentence "the force required at the clutch end must be applied at the lever end." You probably didn't mean those forces to be equal.
The word "apply" is ambiquous in this context.

- Knut
 
Last edited:
Back
Top