Radial Master Cylinder for Clutch

Herman

Herman

New member
Location
The Netherlands
Hi all,

Is there anyone who installed a Radial MC for the clutch? I have seen some discussion about it but not all give the answer I was searching for. And the people who I send a DM didn’t answer. (Maybe didn’t see it or use it)

I’m using a Redax clutch cover with a ps13 axial MC. Works fine but biting point is a little early for my comfort (I always get stuck in traffic for some reason…)

But since the brake MC will change to radial I would like to keep it symmetrical. From axial to radial I noticed that for the radial MC I needed to go to a bigger diameter piston.

Now my question is what size piston I need for a radial Clutch. Is there a way to calculate it? (I know how to calculate for the brake system) I have seen radial MC 14, 16, and 19 on Laverda’s… very confusing haha!

Who can help me out?😅

Herman
 
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you probably want a radial MC for styling as much as anything and that is your choice, but i have experimented for decades with the stock sprocket cover piston size 31mm (30mm is also used as is the very early and rare 35) and ended up with an MC from a Ducati 916, different piston size in it and different mechanical leverage advantage has given me the lightes hydraulic clutch in the Laverda world, and it fully disengages (very important)
CLEM
 
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It's worth using an AI tool for your query. On a quick look, chatgpt.com came up with a helpful answer...or recommendation. I asked it to look at Brembo RCS m/c - this is just the end of the chat.....it came up with 10mm of slave piston travel which seems generous for the Laverda arrangement....


If you like — I can simulate what happens to clutch feel and disengagement if your clutch slave requires exactly 10 mm of piston travel (typical for many clutches), and show whether 16 mm master + 31 mm slave would still deliver that 10 mm (or how much less).

You said:​

yes please

ChatGPT said:​

Perfect — let’s simulate this step by step. We’ll assume your clutch requires 10 mm of slave piston travel to fully disengage. We’ll see whether a 16 mm radial master can provide that, based on hydraulic displacement.

1️⃣ Known Values​


  • Slave piston: 31 mm → area = 754.8 mm²

  • Required slave travel: 10 mm → volume displaced =
Vslave = Aslave×travel=754.8×10≈7548 mm³
  • Master piston: 16 mm → area = 201.1 mm²

  • Master piston stroke required to generate that slave displacement:
Master Stroke = Vslave/Amaster=7548201.1≈37.5 mm

2️⃣ Interpretation​


  • To fully disengage the clutch with a 16 mm master, you need to pull the lever enough to move the master piston ~37.5 mm.

  • Compare to your current stock master (14.6 mm bore):
Master Strokestock = Vslave/Amaster,14.6 = 7548/167.4 ≈ 45.1 mm
✅ Observation:

  • Surprisingly, the smaller stock master actually requires more lever travel (~45 mm) to move the slave 10 mm.

  • The 16 mm radial master moves more fluid per mm of piston stroke, so less lever travel is needed (~37.5 mm).

3️⃣ Lever ratio considerations​

Radial masters have a mechanical lever ratio:
Lever travel at hand=Master piston stroke×lever arm lengths\text{Lever travel at hand} = \text{Master piston stroke} \times \frac{\text{lever arm lengths}}{}

  • For example, a 17–18 mm “click” ratio changes the mechanical advantage:

    • Larger offset → requires more hand force for the same piston stroke

    • Smaller offset → easier pull, less hand force
Even with 16 mm master, the RCS lever ratio can be adjusted to make the lever feel like stock or slightly firmer — without changing hydraulic displacement.

4️⃣ Practical outcome​


  • 16 mm radial master + 31 mm slave + stock lever geometry
    → Required master piston stroke = 37.5 mm
    → Slightly shorter lever travel than stock (because stock 14.6 mm master needs ~45 mm)
    → Lever pull will feel slightly firmer, but clutch can fully disengage

  • Key: You have to check actual lever travel at the handlebar — most RCS masters have ~40–45 mm piston stroke capacity, so it should work.

✅ 5️⃣ Summary Table​

ParameterStock 14.6 mm Master16 mm Radial Master
Slave piston area754.8 mm²754.8 mm²
Required slave travel10 mm10 mm
Slave volume7548 mm³7548 mm³
Master piston area167.4 mm²201.1 mm²
Required master stroke45.1 mm37.5 mm
Lever travel at handdepends on stock leverslightly shorter, can be adjusted via RCS lever ratio
Pull feelstockslightly firmer

✅ Conclusion​


  • 16 mm radial master will allow full clutch disengagement with a 31 mm slave, assuming you have sufficient master piston stroke (~37–38 mm) and properly adjusted lever ratio.

  • Travel at the handlebar will be slightly shorter, but should feel natural.

  • 17 mm or larger masters would reduce slave travel too much and likely cause clutch drag or incomplete disengagement.
 
Last edited:
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Rob said:
It's worth using an AI tool for your query. On a quick look, chatgpt.com came up with a helpful answer...or recommendation. I asked it to look at Brembo RCS m/c - this is just the end of the chat.....it came up with 10mm of slave piston travel which seems generous for the Laverda arrangement....


If you like — I can simulate what happens to clutch feel and disengagement if your clutch slave requires exactly 10 mm of piston travel (typical for many clutches), and show whether 16 mm master + 31 mm slave would still deliver that 10 mm (or how much less).

You said:​

yes please

ChatGPT said:​

Perfect — let’s simulate this step by step. We’ll assume your clutch requires 10 mm of slave piston travel to fully disengage. We’ll see whether a 16 mm radial master can provide that, based on hydraulic displacement.

1️⃣ Known Values​


  • Slave piston: 31 mm → area = 754.8 mm²

  • Required slave travel: 10 mm → volume displaced =
Vslave = Aslave×travel=754.8×10≈7548 mm³
  • Master piston: 16 mm → area = 201.1 mm²

  • Master piston stroke required to generate that slave displacement:
Master Stroke = Vslave/Amaster=7548201.1≈37.5 mm

2️⃣ Interpretation​


  • To fully disengage the clutch with a 16 mm master, you need to pull the lever enough to move the master piston ~37.5 mm.

  • Compare to your current stock master (14.6 mm bore):
Master Strokestock = Vslave/Amaster,14.6 = 7548/167.4 ≈ 45.1 mm
✅ Observation:

  • Surprisingly, the smaller stock master actually requires more lever travel (~45 mm) to move the slave 10 mm.

  • The 16 mm radial master moves more fluid per mm of piston stroke, so less lever travel is needed (~37.5 mm).

3️⃣ Lever ratio considerations​

Radial masters have a mechanical lever ratio:
Lever travel at hand=Master piston stroke×lever arm lengths\text{Lever travel at hand} = \text{Master piston stroke} \times \frac{\text{lever arm lengths}}{}

  • For example, a 17–18 mm “click” ratio changes the mechanical advantage:

    • Larger offset → requires more hand force for the same piston stroke

    • Smaller offset → easier pull, less hand force
Even with 16 mm master, the RCS lever ratio can be adjusted to make the lever feel like stock or slightly firmer — without changing hydraulic displacement.

4️⃣ Practical outcome​


  • 16 mm radial master + 31 mm slave + stock lever geometry
    → Required master piston stroke = 37.5 mm
    → Slightly shorter lever travel than stock (because stock 14.6 mm master needs ~45 mm)
    → Lever pull will feel slightly firmer, but clutch can fully disengage

  • Key: You have to check actual lever travel at the handlebar — most RCS masters have ~40–45 mm piston stroke capacity, so it should work.

✅ 5️⃣ Summary Table​

ParameterStock 14.6 mm Master16 mm Radial Master
Slave piston area754.8 mm²754.8 mm²
Required slave travel10 mm10 mm
Slave volume7548 mm³7548 mm³
Master piston area167.4 mm²201.1 mm²
Required master stroke45.1 mm37.5 mm
Lever travel at handdepends on stock leverslightly shorter, can be adjusted via RCS lever ratio
Pull feelstockslightly firmer

✅ Conclusion​


  • 16 mm radial master will allow full clutch disengagement with a 31 mm slave, assuming you have sufficient master piston stroke (~37–38 mm) and properly adjusted lever ratio.

  • Travel at the handlebar will be slightly shorter, but should feel natural.

  • 17 mm or larger masters would reduce slave travel too much and likely cause clutch drag or incomplete disengagement.
Click to expand...
😳 That’s what I was going to say…..
 
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Ok, that's impressive. But the few times I went AI so much was BS. Nothing beats empirical data. Be good to hear about actual events like Clems. It's a fine line between a light pull and not enough plate separation. The latter happens on my Atlas, and it's real-world painful to live with.
 
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Apparently, according to the Senite, he needed sorting out. Everybody agreed and got their turn, even his best mate. Bit academic to me, I have a cable clutch and am in no position of power, here or in the Government.
 
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I am very happy with my done for donkeys years extended lever, it's really light and gets nutral like butter. Before it was fitted, I was breaking cables 2 or 3 times a year and getting severe wrist pain after a few hours on the bike. I cannot remember when I last broke a cable since it's had this.
 
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It’s been my experience that if you don’t ride your bike then the chances of breaking a clutch cable are dramatically reduced 😂 Rough rule of thumb re radially versus axial master cylinders is two mm bigger for the radial will be close to the axial’s bore .For example 13mm axial will be close too a 15mm radial. HTH’s
Ciao Tom
 
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ozijotati said:
It’s been my experience that if you don’t ride your bike then the chances of breaking a clutch cable are dramatically reduced 😂 Rough rule of thumb re radially versus axial master cylinders is two mm bigger for the radial will be close to the axial’s bore .For example 13mm axial will be close too a 15mm radial. HTH’s
Ciao Tom
Click to expand...
That's an excellent rule-of-thumb tip, Tommy. Been trying to ascertain what front m/cyl bore is going to improve power and feel with 08 Brembos.
 
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