12 replies to this topic

[dattoman]

You'd need to figure out the pedal ratios

The master cylinder is likely to be too large

[Bigfella237]

It comes down to the surface area of the piston in the master cylinder, compared to the surface area of the piston in the slave.

 

If each has the same area then 50mm of travel at the master equals 50mm of travel at the slave and the force will also be equal.

 

If the master cylinder piston has twice the surface area of the slave, then 50mm travel at the master equals 100mm at the slave but with half the force applied (meaning the pedal will be harder than the first example).

 

You need to know:

 

1. how far the master cylinder piston travels for a full pedal sweep (will depend on how the pushrod vs. pedal pivots are set up)

2. the diameter of the master cylinder

3. the distance the throwout needs to move to fully disengage the clutch

4. the surface area of the piston in the slave cylinder

 

To work out the surface area is Area = π (3.14) x r² (radius squared)

 

So if the master has a diameter of 20mm, it would be 3.14 x (10 x 10) = 314.6mm²

 

With a concentric slave, you will have to work out the area of the outside diameter of the piston, then subtract the area of the inside diameter.

 

Now two of these values will be fixed, being the travel required for the slave to throwout the clutch, and the surface area of the slave. Once you know them you can either alter the size of the master cylinder to suit the pedal/pushrod travel, or alter the pushrod travel to suit the size of the master.

[Dr Terry]

Unless you have a problem with your current set-up, like a seized pivot or similar, I can't see how changing to a concentric slave cylinder will make the clutch any lighter than a conventional slave & fork arrangement..

 

As mentioned above it's all about forces & ratios. A force is applied by your left foot thru the pedal lever to a master cylinder of a fixed displacement & this opposes the force applied by the clutch diaphragm spring against the throwout bearing. Any frictional losses would be fairly minor by comparison.

 

Dr Terry

[Bigfella237]

With a conventional setup you also have to take into account the distance from the slave cylinder to the clutch fork pivot versus the distance from the pivot to the centre of the throwout bearing.

 

I don't have one handy to measure but I'd guess it's about 2:1, meaning that a conventional slave cylinder would have to have twice the travel of a concentric throwout.

 

Another way to look at the calculation would be if the piston in the concentric was bigger than the old slave by the same ratio of the fork fulcrum (say 2:1 or twice as big), then both cylinders would engage the throwout bearing the same distance (given that nothing else changes).

 

If we look back to the original math above, a new slave piston with twice the surface area equals half the force needed at the master, but it no longer has the leverage of the fork in it's favour so as far as pedal pressure goes we're right back where we started.

 

Of course there is always some wiggle room if you're designing a system from scratch, for example you can lengthen or shorten the distance from the pedal pivot to where the pushrod attaches to it, which will give more or less pushrod travel and also change the pedal effort required.

 

The main thing you must avoid when designing a system is either the master or slave cylinder reaching the end of it's travel before the pedal does, your pedal needs to be physically limited to stop just before the piston bottoms out (or pops out in the case of a slave).

[ozyozyozy]

The CRS m/c is way too big for the malwood concentric as Datto said.
When setting up the concentric you need to know what is the travel distance of the pressure plate of clutch from there you work out m/c size.
As a guide 3/4 is the go that works on most as has been said.
Smaller clutches with less travel can go as small as a 5/8 m/c.

[ozyozyozy]

i have seen someone fit a eg tilton m/c to the crs bracket, think it was a little screwing around, this allowed him to use the concentric slave.