Clutch Master Cylinder Swap

beemerphile

beemerphile

Joined
Mar 27, 2019
Messages
1,024
Location
Georgia
Bike
98 & 99 ST1100
There's lot's of traffic here on changing the piston diameter of the brake M/C to gain easier "two finger" braking with the general idea being that a smaller M/C diameter is better up to the point where the lever travel is too great and the lever reaches the bar. What of the same question for the clutch side? Has anyone experimented with different M/C piston diameters in order to gain easier clutch pull consistent with still having sufficient travel to completely disengage the clutch? Does anyone know what the current cylinder diameter is for the clutch M/C?
 
This article might be of interest to you: ST1100 clutch engages at very end of lever travel.

Somewhere out there on the interweb, there is an article by a ST owner who had a hand injury and modified the clutch lever / master cylinder assembly to obtain a reduced throw on the clutch. I had a quick look around but could not find it - perhaps another member might recall it and know where it is.

Michael
 
I used a VFR700 clutch lever/mc on my ST1100, ratio was the same but with the adjustable lever throw starting closer to the bar it is easier to pull. It is the same as 98-01 VFR (and maybe later) that I use on my 1300
 
CYYJ said:
.... Somewhere out there on the interweb, there is an article by a ST owner who ... modified the clutch lever / master cylinder assembly to obtain a reduced throw on the clutch....
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Here ya go, Michael:

Clutch Lever Reach Reduction Farkle ( ST1100 ) *

Clutch Lever Reach Reduction Farkle ( ST1100 ) *
www.st-riders.net
Clutch Lever Reach Reduction Farkle
By STeve Chamberlin, STOC 1835

John
 
That seems to be a different goal than I am trying to reach. I'm fine using the entire travel, but want more of it to be working travel and less free play. The thought being that a smaller diameter M/C would cause more of the travel to be used for engagement and therefore I would gain some force multiplication by the greater ratio. Still wondering what the current stock diameter is. I couldn't find it on the body of the M/C and it wasn't mentioned on the parts descriptions for either the cylinder or the repair kits.
 
Seems that a change that imparts greater travel at the slave cylinder requires greater force at the master cylinder.

Likewise, a change that requires less force applied at the master cylinder imparts less travel at the slave cylinder.

I agree with Beemer that I'd like greater travel at the slave cylinder, and I need less free-play at the clutch lever.
 
beemerphile said:
That seems to be a different goal than I am trying to reach. I'm fine using the entire travel, but want more of it to be working travel and less free play. The thought being that a smaller diameter M/C would cause more of the travel to be used for engagement and therefore I would gain some force multiplication by the greater ratio. Still wondering what the current stock diameter is. I couldn't find it on the body of the M/C and it wasn't mentioned on the parts descriptions for either the cylinder or the repair kits.
Click to expand...


I'm sorry, but I fail to see what your problem is. You pull in the clutch, only maybe a half inch or so and the clutch disengages. No need to pull it all the way. Makes for quick and slick gear changes, as the clutch system works so efficiently as is.
 
Bush said:
I'm sorry, but I fail to see what your problem is. You pull in the clutch, only maybe a half inch or so and the clutch disengages. No need to pull it all the way. Makes for quick and slick gear changes, as the clutch system works so efficiently as is.
Click to expand...

Never said it was inefficient or that everybody wants one - just that I"d like less effort to pull it. I'd rather pull it farther with less force. I have some hand pain from arthritis and on a stalled interstate where I have to inch along for quite a while it can also pump up the forearm. Hand pain is my main motivation. I have a scooter with a CVT and seriously considered a DCT equipped bike for this reason.
 
ST1100 clutch master cylinder is 14.00mm id according to the service manual (section 1-11). VFR800Fi 98-01 and ST1300 is the same, VFR800 VTEC 02-13 uses 12.7mm.

For a given slave cylinder diameter, a smaller MC will decrease effort and slave piston movement for a given lever movement.
 
TerryS said:
ST1100 clutch master cylinder is 14.00mm id according to the service manual (section 1-11). VFR800Fi 98-01 and ST1300 is the same, VFR800 VTEC 02-13 uses 12.7mm.

For a given slave cylinder diameter, a smaller MC will decrease effort and slave piston movement for a given lever movement.
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Correct - but to continue the point that I believe Terry was making, it is important to bear in mind that it is not a matter what size of master cylinder you use, the volume of hydraulic fluid required to operate the clutch is determined by the diameter and required stroke of the slave cylinder.

The fluid pressure required from the master cylinder is determined by the stiffness of the clutch basket springs and the diameter of the slave cylinder. The volume of oil required from the master is determined by the stroke and diameter of the slave.

The volume of oil required is: slave stroke x (3.14159 x (slave diameter)**2) / 4

For non-math types, the "**2" means "squared". So, you multiply the (diameter x diameter) - i.e. multiply the diameter by itself, and then multiply by pi (that is the "3.14159" number) and then divide that result by 4 and THEN multiply by the stroke of the slave.

NOTE: all of the dimensions must be in millimeters (or inches) and then the volume will come out in cubic millimeters (or cubic inches).

From there, you go back to the master cylinder volume and stroke and make sure that the volume numbers match.

The trick in switching to a smaller master cylinder will be to ensure that it can still provide sufficient fluid to move the slave enough to actuate the clutch properly.

For a given hand force on the clutch lever, you will certainly build more pressure with a smaller master cylinder, but if it doesn’t push enough fluid to operate the slave with sufficient stroke to disengage the clutch, you will experience hard shifting and the bike will “creep” at idle, unless it is in neutral. Also, finding neutral will be very difficult if the clutch is not fully disengaged.

Essentially, the master and slave cylinders of any clutch system (or calipers for a brake system) are matched by the design engineers and so changing either one of them may result in a system that simply doesn’t work properly.

Pete
 
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Found and ordered a used VFR 12.7mm M/C and a rebuild kit. The experiment is underway. thx. - Lee

MaxPete said:
The trick in switching to a smaller master cylinder will be to ensure that it can still provide sufficient fluid to move the slave enough to actuate the clutch properly.
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That is the essence of the experiment. The release occurs in the first half of the travel. Once you are at mid-travel, the additional movement only moves the slave farther than it needs to go to release the disc. My hope is to use up some of that wasted travel and gain reduced grip force. Properly bled, if the disc isn't fully released before the lever hits the bar, then the experiment fails and I have wasted $40. A brighter guy would probably just calculate it based on volume ratios and travel.

MaxPete said:
Essentially, the master and slave cylinders of any clutch or brake system are matched by the designers and changing either one of them may result in a system that simply doesn’t work properly.
Click to expand...

That risk follows those of us around who cannot leave well enough alone. I"m familiar with it
 
beemerphile said:
Found and ordered a used VFR 12.7mm M/C and a rebuild kit. The experiment is underway. thx. - Lee



That is the essence of the experiment. The release occurs in the first half of the travel. Once you are at mid-travel, the additional movement only moves the slave farther than it needs to go to release the disc. My hope is to use up some of that wasted travel and gain reduced grip force. Properly bled, if the disc isn't fully released before the lever hits the bar, then the experiment fails and I have wasted $40. A brighter guy would probably just calculate it based on volume ratios and travel.



That risk follows those of us around who cannot leave well enough alone. I"m familiar with it
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Best of luck with your 'experiment'.
 
beemerphile said:
That is the essence of the experiment. The release occurs in the first half of the travel. Once you are at mid-travel, the additional movement only moves the slave farther than it needs to go to release the disc. My hope is to use up some of that wasted travel and gain reduced grip force. Properly bled, if the disc isn't fully released before the lever hits the bar, then the experiment fails and I have wasted $40. A brighter guy would probably just calculate it based on volume ratios and travel.
Click to expand...
the area difference between 12.7mm and 14.0mm is about 21.5% so as long as you can get 21.5% or more additional lever travel you should be OK. Haven't looked at the lever pivot linkage to see if that's linear or not, but I suspect it is. That would be the only other variable in recalculating the amount of travel required at the end of the lever.
 
dwalby said:
the area difference between 12.7mm and 14.0mm is about 21.5% so as long as you can get 21.5% or more additional lever travel you should be OK. Haven't looked at the lever pivot linkage to see if that's linear or not, but I suspect it is. That would be the only other variable in recalculating the amount of travel required at the end of the lever.
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It seems to use about 50% of the travel, so 21% leaves a good margin of error. I may get away with it.
 
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