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Discussion Ducati Master Cylinder

Discussion in 'Tyres Wheels and Brakes' started by Andych, Sep 3, 2021.

  1. Andych

    Andych Moderator Staff Member Premium Member Contributing Member

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    So... I needed to find a Master Cylinder for the SRX that worked with the switch block, had a rear mirror mount and was suitable for the twin disc setup and after lots of searching and comparing caliper sizes etc I found a Ducati Monster SR2 800 Dark Master... and it fits nicely, has a nicely engineered lever with lots of adjustability (as you would expect) but what I am interested in is the Union bolt at the Master.
    The hole in it is tiny compared to the other couple of masters I have (Japanese units).
    IMG_6061.jpeg

    IMG_6062.jpeg

    IMG_6064.jpeg

    Now my question to those who are versed in the mysteries of Ducati (or anyone else really) is... does the smaller hole ensure better "feel". It is a Brembo Master and for the $$ was a good buy... I am just interested in the small hole vs probably a 4mm hole in a Yamaha one I have (it wouldnt work with the switchblock).
     
  2. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    With hydraulics it's mostly about ratios. Master bore size vs caliper piston bore etc. Hydraulics fluid acts like electrical circuits in some sense, and pressure is evenly distributed in theory.

    I don't the size of the hole on the banjo bolt makes much difference, there's no huge volume of fluid moving in brake systems.
     
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  3. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Interesting

    How big is the end hole ?

    The banjo hole on the ZXR are much bigger also

    I guess a smaller hole creates more pressure? ..... or maybe the Ducati master is just very efficient compared to old school

    banjo bolts.png
     
  4. Frankster

    Frankster NFI Staff Member Premium Member Ride and Events Crew

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    I guess a smaller hole creates more pressure?

    No, it's what Linkin said. It's a small volume of liquid that is moved by the master cylinder. The size of the banjo hole is only relevant if it's blocked. Linkin's analogy of the hydraulic fluid moving like electricity is quite accurate.
     
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  5. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    looks like a sinkhole on my cheap China job :D


    IMG_3826.JPG
     
  6. Murdo

    Murdo The Good Doctor Staff Member Contributing Member Ride and Events Crew

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    For the amount of fluid that is moved when braking I don't think it would be a problem.
     
  7. Andych

    Andych Moderator Staff Member Premium Member Contributing Member

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    Well the size of the hole will have an influence as it is a restriction (I didnt think it was going to be a problem at all)... while the amount of fluid moved isnt much it is still harder to push X amount (the displacement of the piston times the travel) through a smaller hole than a larger hole... simple hydraulics.
    So based on that principle my initial thoughts were that initial grab would not be as savage (for want of a better term) and would the feel or modulation be better. Thus the brakes would also not be as "touchy" yet would potentially still give the same "ultimate" braking pressure....
    Contrary to popular opinion the pads are not really touching the discs all the time do initial pull on the lever brings the pads into contact for the "bite" point...once they bite the pressure in the system rises and then the hole size becomes less relevant.
    Not having ridden a Ducati Monster (probably never will) I dont have any idea how the brakes feel etc. I do know that the CB400SF which has similar sized (floating calipers) front brakes dont need a lot of pressure on the lever.
    I just found it interesting that there was such a large discrepancy with regard to the hole size...
     
  8. Frankster

    Frankster NFI Staff Member Premium Member Ride and Events Crew

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    Andy, you can change the 'feel' and take-up of your brakes by changing the master cylinder volume or the diameter of the hose(s). Think of it this way... if you have a twin disk front end and you block off one of those calipers, your remaining brake will activate much quicker in the lever pull as there is less volume needed for the hydraulic fluid to move the piston(s) in that caliper. By changing the master cylinder size to a larger volume (or increasing the diameter of the hose), you will change the point the brakes come on, due to the brake lever motion needing more (or less) movement to push the necessary amount of fluid through the system. Your hydraulic circuit is always 'full' and sealed. The banjo holes are just a convenient way to allow fluid to flow through a bolt. As you know, the brake piston is held in place by the o-ring seal. It's the tension of the o-ring against the piston wall that pulls the piston away from your brakes when you let the brake lever out, so you only need a little bit of fluid movement to activate a disk brake.
     
  9. Andych

    Andych Moderator Staff Member Premium Member Contributing Member

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    Frank, I am not entirely sure what most of the above has to do with the size of the hole in the banjo bolt other than 1 point where you discount it as just being a convenient way to move fluid.
    For a start... I wasnt asking how to change the "feel" of the brakes etc... simply comparing different designs and asking for informed input on the possible reasons for the difference.
    Hose size (diameter) is in "general" irrelevant to this discussion as the hose is simply a method of transferring the pressure developed at the Master Cylinder to the calipers.
    Back to the "discussion" on that hole size... as I explained further above... it is relevant, in that it is a restriction in the circuit... ie it will limit the amount of fluid that can pass through it based on the pressure applied to the lever. The harder you pull on the lever the more pressure you generate and the faster the fluid will pass through that hole... as I said.. Simple Hydraulics.
    You can test his with an old banjo bolt.. mostly block the holes and blow down the middle... you need to blow a lot harder with the holes mostly closed off than with them not closed off..
    As for the rest of the system... well it all comes down to the ratio of caliper piston area x number of pistons then divided by the area of the master cylinder piston. That then needs to multiplied by the ratio of the distance of the pivot point on the lever to where it actuates the piston and the point where you would normally pull on the lever (shorty levers have less mechanical advantage than standard levers).
    It can be calculated relatively easily but what is the magic number...hard to say but... the lower the overall ratio the "harder" the feel would be but with less actual braking pressure... the higher the ratio the more lever movement and better feel or ability to "modulate" the brake.
    You can then go down the rabbit hole of Axial MC vs Radial MC and adjustable pivot distances, lever length, rotor diameter, pad material, friction co-efficient etc etc etc... I will do all the final calculations on mine to ensure it will be a relatively "user friendly" front brake setup and not just a 'mash-up" to look good.
    Braking systems are critical and ANY modifications need to properly engineered or the consequences could well be fatal.
     
  10. Frankster

    Frankster NFI Staff Member Premium Member Ride and Events Crew

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    That's fine Andy. I was just trying to help. To move a brake piston with a 26mm diameter a distance of 1mm you'll need 0.08164ml of fluid; to move it 2mm you'll need 0.16328ml and so on. That part of your formula never changes. How you get that fluid there and how much pressure you use to get it there are the only variables. I refer to 'feel' as how the brake lever feels when I apply it; spongy, firm, slow/fast application etc. The hole in your banjo bolt is small, but if it's standard then it must be okay.
     
    Last edited: Sep 4, 2021
  11. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    Upon further thinking, I think Ducati using a small orifice in the banjo to provide an intermediate feeling of resistance until the free play is taken up and it starts activating the brakes, so it doesn't immediately go from spongey to solid. This may serve to avoid locking up the wheel from rapid brake application on bikes without ABS.

    I work with Ducatis all the time and that is the impression I get from the models using that master cylinder.
     
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  12. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    A lot of the electrical formulas can work by analogy, if you think of volts as the master cylinder pressure, current as your fluid flow, the small orifice is resistance and as @Linkin says, it gives a less abrupt feel on the lever as the volume of fluid required to flow happens over time because of the resistance to that flow

    It's a very clever way to tune brake feel and has me wondering if others are doing it also

    A whole bunch of banjo bolts with different size holes to test the different feel would be an interesting experiment, that no-one will ever do unfortunately, because bleeding brakes for air isn't anything that anyone ever wants to do even just once
     

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