Where am I going wrong here: Clutch slave/master sizing ?

[AC Ventura]

Okay, so I have a 12" McLeod Super Street Pro and a 1" bore slave cylinder.

McLeod say 0.5" travel + say 0.12" free play at the operating end. So that's 0.62" movement required inside the bellhousing. My clutch fork ratio is 2:1, so I need 1.24" movement at the slave cylinder end of the fork.

My pedal box will only give 1.25" of master piston rod movement, so it seems I need a 1" bore master cylinder at the master to get the same at the slave. But nobody seemingly recommends that.

So which parts or theory have I got wrong here?

[spdbrake]

Your theory and math look correct. With a 1.25" throw on the 3/4" master cyl your only going to see 3/4 of the resulting travel on the 1.0" slave. You'd need to get a 1.0" master to achieve the same travel at the fork. Which it looks like it would just make your numbers needed.
Using Wilwod masters as a base a 1.125" master would overdrive your slave a bit if you wanted to go that far and be safe.
Wilwood Disc Brakes - MasterCylinder No: 260-10376

Or drop the dia of the slave to 3/4" to maintain a 1:1 movement-reaction ratio with your current master.

[AC Ventura]

Thanks just concerned it’s all very close and 1” is a big enough master to depress anyhow, plus it’s a 3 pedal self contained box and 1” is the max that will fit. It’s an all new installation and only went this route to avoid the reported HTOB issues. However I always wonder if the issues on those are clutch stop related. All modern European cars use the system. I think I will run with the 1” and hopefully I won’t need the full 1/2” to disengage. In that case I can set the pedal stop to ensure it doesn’t bottom out so much,

[jon@harrison.ne]

spdbrake, maybe I'm missing something but this is what I see: first, "with a 1.25" throw on the 3/4" master" is not relevant- the ratio of slave to master throw is the same for any input throw. Second, and more import, the slave to master throw ratio is a function of the area of the cylinders, not the diameters since the volume of fluid pumped equals the area times the throw and area is proportional to the diameter squared (area of a circle = pi x diameter squared/4). So a one inch throw on a 3/4" master driving a 1" slave will result in a 9/16 "(3/4 squared) throw at the slave, and a 1.25 " throw on a 3/4" master will produce a 0.703" throw on a 1" slave.

[spdbrake]

Originally Posted by jon@harrison.ne

spdbrake, maybe I'm missing something but this is what I see: first, "with a 1.25" throw on the 3/4" master" is not relevant- the ratio of slave to master throw is the same for any input throw. Second, and more import, the slave to master throw ratio is a function of the area of the cylinders, not the diameters since the volume of fluid pumped equals the area times the throw and area is proportional to the diameter squared (area of a circle = pi x diameter squared/4). So a one inch throw on a 3/4" master driving a 1" slave will result in a 9/16 "(3/4 squared) throw at the slave, and a 1.25 " throw on a 3/4" master will produce a 0.703" throw on a 1" slave.

Correct. Sorry if I'm confusing. My opening statement is only a "check my understanding" feedback. (Habit from thousands of flight crew debriefs.)
Volume is volume, thats Why I recommended a a 1.125" dia coupled with his current 1.0" slave for overdrive. Or possibly reducing the slave to 3/4" giving him a 1:1 movement on input and output.

[Gaz64]

If you currently have 3/4 master, 1 inch slave, I would either increase the master in increments of 1/16 (From 3/4 to 13/16),
or decrease the slave from 1 inch to 15/16, until the correct disengagement versus pedal feel happens.

A jump from 3/4 to 1 inch is way too much in one go, 78% increase in area.

Don't want a pedal you can't operate.

Gary

[AC Ventura]

Does anybody actually use a 1” Master and a 1” slave? I’m still thinking with 1.25” needed on both there’s no room to play with except the clearance between the bearings and fingers. But would like to know if pedal pressure is tenable.

[joyridin']

My car originally had a 1-1/16” master and slave cylinder. It worked really nice. The slave was a push thpe that pushed on the throwout arm. The master cam from an old Mustang from what I could find and I have no idea what the slave came from.

[RockBit]

AC Ventura - I sent you a message regarding my set up.