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Discussion Starter #1 (Edited)
I decided to share some conception about Master cylinder and caliper as I see a lot of miss conception and there is no magic here, just a math. I am not professionally design any brake system but went through it a little bit so maybe it will be easier for everyone to make a decision about changing Master Cylinder or calipers. For different than Hydraulic leverage ratio you need to take some assumptions, that's why it most universal parameter and one where you should start your journey.
Feel free to correct me if you know more details or see mistakes.

1. MCs - parameters to look for
  • piston size (14,15,16,17,18,19 mm)
    • its size should depend on how many calipers you have and what is size of pistons. So saying that bigger one will be more rapid is only true if you compare it with the same calipers. Once you change calipers to one with different size pistons this statement doesn't have any sense.
  • pivot length (16,17,18,19,20mm etc)
    • this will affect feeling brake and lever stroke(this depends also on calipers/piston)
    • this is distance from center of piston rod to lever pivot point. In general the longer pivot length is the more force and less travel on lever is needed to fully engage brakes(but it depends on caliper configuration)
  • be careful and choose 90 degree radial pump if you decided to change it. All RCS, Brembo Racing, Galespeed are 90degree. But there are some OEM Brembo, Magura etc. which are not 90 degree pumps and they are called radial and they don't have the same effectiveness.
2. Calipers

Parameters:

  • number of calipers
  • number of pistons
  • size of pistons (in sport most people/teams use calipers with bigger pistons as they give more feeling e.g. 32mmx36mm, there is nothing wrong with Brembo M50/Stylema with 4x30mm just different approach and most probably a little cheaper to manufacture)
Mono block calipers vs 2 piece calipers
First of all mono block are not better than 2 piece calipers. It depends of quality, internal flows, seals(some race calipers have one seal per piston and they are not recommended for street use) etc. There is a lot of 2 piece calipers that are way better than popular Brembo cast mono blocks like M50, M4 etc. If you are not racer it really doesn't matter how they are made, so yes all old Brembos are not better than JJ Juan we have and there is no point to change it. I would say that M50 or Stylema don't give you much difference if you are not quite advance track day rider, it is all about pads and hydraulic leverage ratio with MC.

Types of calipers:
  • cast 2 piece calipers(classic P4, JJ Juan a lot of Nissins etc.) - still most popular solution
  • cast mono block(e.g. Brembo M50, M4, Stylema) - in theory they are more stiff than cast 2 piece, which should generate more braking power and better feeling
  • billet 2 piece calipers - e.g. Brembo P4 30/34, Hel, Beringer in theory they are better than cast mono block - stiffer, have better internal fluid flows. Some of them have room for thicker pads as they are used for endurance racing.
  • billet mono block - top quality calipers used in pro racing(motogp, sbk etc.)
3. How it works?
  • hydraulic leverage ratio between master cylinder and calipers(square area of all pistons divided by square area of MC piston) is most basic parameter which determines feeling. In general most 2 calipers system are set to have ratio 23-27. Below 20 you will end up with wooden feeling without almost no modulation and above 30 lever feeling will be soft and you will need a lot of travel to fully engage brakes.
  • Total leverage ratio - this parameter takes into account pivot length and point on lever where you usually grab(I assume 10cm from lever pivot point). In this situation total leverage ratio should be between 115:1 to 145:1, below 115 whole system will give lever which pulls back short distance and feel rock solid, above 145 lever will need a lot of travel to fully engage brakes.
4. What we have in Duke 790
  • 2 calipers with 2x30mm piston and 2x34mm piston per side
  • Axial Master cylinder with 18mm piston size
So we have hydraulic leverage ratio 25.4 which is really good. First thing you should change are pads and than start looking for more upgrades. For sure don't start with calipers. If you already changed calipers, lines, pads, MC and you compare it to stock system with stock pads it is ridiculous as all aspects of system have been changed.

I want to update Master Cylinder which one should I choose for stock calipers?
  • 17mm MC (Brembo RCS, Corsa Corta, Accossato and some others) will give you hydraulic leverage ratio 28.5 which means it will be a little spongy.
    • 17x18 settings will give total leverage ratio of 158 which means that lever travel is really long to engage brakes( personally I don't like it)
    • 17x20 settings will give total leverage ratio of 142 which works quite nice but feeling of brakes are not as nice as 17x18 settings. - my recommendation if you want to go for 17mm pump.
  • 17,5mm MC(Galespeed, Beringer) will give you hydraulic leverage ratio 26.9 which gives much, much better feeling than 17mm ones - most probably best choice for our stock calipers.
  • 19mm MC(Brembo Racing, Brembo RCS/Corsa Corta, Gale Speed, Beringer, Hel, Accossato will give you hydraulic leverage ratio 22.8 so more on stiff side. If you plan to change calipers to M50, Stylema in the future don't buy it as you will land with ratio 19.9 and system will have really wooden feeling with totally lack of modulation(it will be like on/off switch which will lead to engage ABS to often and this is not ABS fault you basically will be able to lock wheel very easy).
    • 19x18 - setting which give us good combination of modulation, lever stroke and feeling. Total leverage ratio: 126
    • 19x20 - in my opinion it gives a little bit to wooden feeling. Total leverage ratio: 113
    • 19x17(e.g. Galespeed have such a pump) - Total leverage ratio: 134.
I made some assumption in calculation to give you some overview of lever travel(to full brake power) and force needed on lever in different configuration with stock caliper and different pumps:
17x18 - 31.6mm / 31.6N
17x20 - 28.5mm / 35.1N
19x17 - 26.8mm / 37.3N
19x18 - 25.3mm / 39.5N
19x20 - 22.8mm / 43.9N
Thats why 17x20 or 19x20 are recommended as safer for street - you need more force to engage brake.

Summary for stock calipers:
As you can see there is no perfect MC for our system. It is matter of preference 19mm can be a little to wooden 17mm can be a little to soft in feeling(and only, 17x20 setting is usable), best one should be 17.5mmx18 Galespeed or Beringer. If you plan to switch for Stylema/M50 for sure don't buy 19mm MC.

M50/Stylema calipers

  • 17mm pump will give ratio of 24.9 - my recommendation
  • 17.5mm pump will give ratio of 23.5 - my recommendation
  • 19mm pump will give ratio of 19.9 - super wooden feeling
17x18 - 27.7mm / 36.1N - my recommendation
17x20 - 24.9mm / 40.1N - my recommendation
17.5x18 - 26.1mm / 38.3N - my recommendation
19x18 - 22.2mm / 45.1N
19x20 - 19.9mm / 50.1N

M4 Calipers (4x34mm pistons)

  • 17mm pump will give ratio of 32 - it will be soft and need a lot of travel
  • 17.5mm pump will give ratio of 30.2 - the same
  • 19mm pump will give ratio of 25.6 (perfect)
17x18 - 28.1mm / 35.6N
17x20 - 31.3mm / 32N
17.5x18 - 29.8mm / 33.6N
19x18 - 28.5mm / 35.1N - my recommendation
19x20 - 25.6mm / 39N

Overall summary
Changing MC, calipers to get feeling you want its not that easy. First of all you should think if you want to stay with stock calipers or not and than choose proper pump. Aim to get proper hydraulic leverage ratio (~23-27) and than choose one with proper for you preference pivot length.

As you see there is no one clear way, if you don't have experience don't risk too much and start with most simple solution: pads and rotors(you can convert to 320mm).

5. Disc size/material
I don't have any calculation for disc size, but bigger rotor is not only bigger stopping power due to bigger diameter. Bigger disc is also less prone to fading disc as it has more heat capacity. Basically with the same pads and caliper, bigger disc will cause more torque to the wheel.
  • Disc material - for the same size disc can behave totally different from one company to other company as they use different steel. Basically it is a choice between more stopping power and faster disc wear. Most probably Power Parts Galfer have a little bit softer material thats why a lot of people who change it says that stopping power and modulation is better.
  • Disc weight. In our Duke 790 case PP 300mm disc are noticeable lighter, almost 1.3kg. Removing this amount from unsprung mass is quite a lot. For sure PP 300 disc are not 20% better but they improve not only braking power but also steering.
6. Brake pads
Use whatever you like. Just remember about one thing race track pads are for race track. In normal road condition they will behave worse than standard one as they need a lot of heat to start working properly, in most cases they will also wear disc much faster than street/amateur track days pads.

Currently I am big fan of SBS RS pads. They are really great and cost almost 50% less than Brembo pads(which I also like but they are pricey).
If you are serious track day rider and spend a lot of time on tracks than try SBS DS1/DS2 but seriously using them on street is not a good idea.


6. Excel calculator
I saw it long time ago screen from similar calculation but as person who share it didn't want to share it with me I found all info and made my own calculation.
 

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Now that may well be one of the BEST BRAKE write ups I have read - ANYWHERE.

I would appreciate the numerical analysis of pads and rotors.

Does a 320mm disc actually brake better than a 300 - and why? - I know the rotational surface area increases - but so does the rotational speed.

There are many claims for improved braking from "wave" and similar profiled discs.
There is a 20% price premium Wave vs not (via KTM).
Is that mirrored in 20% better brakes?

Could somebody with a STOCK 890 please answer me a tiny question.
Is the disc carrier on the stock 890 disc steel or aluminium?. If so that is a big part of the weight saving of the PP wave.

Pads making a difference - why. Coeff friction?
 

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Someone’s bored.
ooooh I love numbers - I mean really love them.
There are so many interesting technical alternatives - Beringer do the twinned discs.


We've all seen 6 piston calipers. No? - look up PreTech.

Back in the early 80's Laverda experimented with front discs on a Planetary gear system. These counter rotated the wheel. It was supposedly to do with gyroscopic precession. Saw them first hand at the factory.

But on the whole - things stay unchanged. But often because it's cheaper to make and most purchasers are REALLY conservative.
Also - and this is a pet beef of mine - because some folk prefer form to function. Ductile Iron is WAY better than Stainless - but it goes rusty.
 

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Fascinating post and information. It matches exactly with my practical experience. So, now I have the numbers to explain it.Thanks!
 

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Love the post! Thanks for that. Great content to have on the forum! Keep it coming, please.

I love the harder feeling at the lever. This is why I decided to go with the RCS19, as an upgrade.
 

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The pads are definitely the best bang/$ you can get on this bike. It transforms the breaking and provides the bite that is missing from the OEM.

Changing the M/C is a simple enough procedure and there are many used on eBay that should not set you back too much $$$. Will not require any additional hose changes, etc. The RCS19 works well for the OEM calipers (which I had no intention to replace).

Although there is not one solution that will work, as we have different preferences, the guidelines you provided in your post are extremely valuable.
 

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Owning the 790 and 890 side by side, I have a pretty good comparison of the brakes. First thing I did on the 790 was swap the pads to the EBC HH. Initial bite was MUCH stronger and overall, made me not desire more. Moving to the 890, with stock pads even, it does't quite have the initial bite of the 790 with HH pads, but you can feel the power of the brakes. Initial grab is slightly softer, but with just slightly more pressure they bind so hard and linearly. After swapping back to my 790 for a bit after riding the 890, the brakes feel spongey and grabby. I know there are no hard numbers here, but doing my best to describe them. I don't know that I would want a more aggressive compound on the 890. The soft initial bite is almost needed on a bike so light with so much braking power just a finger tip away.
 

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Discussion Starter #10
I added excel file with calculations and few words about discs and pads(as I am huge fan of SBS).


@barbagris Galfer already added 320mm discs for 890 with aluminium carrier(I bet they soon will be available as PP). Galfer part number: DF787CWD and DF787CWI (right/left side). They can be used for 790, you will have to use 10mm spacer under caliper and longer bolt(e.g. from Duke 690R or Superduke 1290 (up to 2019))
 
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