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It seems that I have done more of this sort of stuff than most people on here, and a lot of people are getting confused. So here's the information you've been needing...

First to say - this post is only relevant to Nissin axial master cylinders like these, and any which have similar looking levers:

They can be found on nearly every Jap bike from about 1988 through to modern stuff. There are also types with built-in reservoirs like CBR600f4i, they are the same too. I do not know about radial brakes, so won't go into that. Except to say that radial calipers is just marketing junk, and won't make any difference to these calculations.


Front brakes are crucial on any motorcycle -
they are the bit which should stop you hitting things! So it's very important to make sure they work right. On most bikes, keeping the original parts in as-new condition should do this for you, however some bikes are poor from the factory. On modified bikes, you might want or need to mix and match parts. In principal there is no problem with this, because hydraulic fluid doesn't care which make of bike it's being used in. But if you're going to start mix and matching, there are some things you need to understand first.

Hydraulic advantage -
the first thing you need to understand is how brakes work at all. When you squeeze the lever on the handlebar, you are moving a small piston which fits into a finely machined bore, and forces fluid down the line. That's close enough to true to do for now. This fluid moves down the line, and because it is not compressible, it forces the pistons in the calipers to move out, pushing the pads against the discs, generating friction, slowing you down.

Consider the fact that the bore of the master cylinder is much smaller than the combined bores of all the pistons in your brake calipers. This means that (made up numbers coming up) for every 15mm you push the master cylinder in, you move maybe 5cc of fluid. But as that 5cc of fluid hits the pistons at the bottom, they don't move 15mm but in total like 0.5mm. This is where you get your hydraulic advantage from, and this is why you don't have to squeeze the brake lever hard enough as if you were actually grabbing the pads with your hand.

Changing hydraulic ratios -
Now that you know that, you can consider how changing the sizes of any of the pistons in the system will affect this relationship. Let's assume we've taken a standard braking system which works fine and for whatever reason, we are thinking of making the following changes.

Bigger master cylinder
If you increase the size of the master cylinder, you will actually be REDUCING the power of the brakes. Yes, with a bigger master cylinder, you will have to squeeze HARDER to get the same braking force at the calipers. This is because you are reducing the hydraulic advantage you have over the calipers. Crazy, right? Maybe but it's still a fact.

So why would anyone do this? Well, you will also be reducing the amount of lever travel needed to move the same amount of fluid, so if you've got very strong hands, or are on a race track and want/need shorter lever travel, then you might find it advantageous. Also, some bikes (t595, 600srad for example) are fitted from stock with what might be considered too small of a master cylinder, giving over-long lever travel, so some people like to upsize it to give a firmer, less vague feel at the lever. However if your bike as stock has good feel, then going up a size can make the brakes feel "wooden", that is, like an on/off switch. This happens when pulling the lever gently doesn't do much due to reduced advantage, but then with a tiny amount of lever stroke, that you have to squeeze hard potentially without finesse, you can very suddenly lock the wheel.

You'll be reducing the power of the master cylinder, so reducing the amount of flex generated through the system, as you'll now need to squeeze harder to flex the calipers than you probably can. This difference is often very noticeable while standing in the parking lot, but you won't necessarily feel this difference on the move, as if you're at that point of feeling flex from the brakes, you've probably already locked your front wheel.

Smaller master cylinder
If you decrease the master cylinder size, you actually GAIN in braking power due to the increased hydraulic advantage you gain. Brilliant, why don't we all do it?

Well, because you also increase lever travel. In some cases it's so bad that the lever hits the bar, maybe trapping your fingers before you stop hard. This is a Bad Thing! In some case, where the brakes as standard are "wooden", fitting a smaller master cylinder can improve them massively (xj600s) but if the brakes are already fine, you may end up with them feeling vague or spongy, or trapping your fingers.

You can sub bigger master for smaller calipers, or vice versa, and the principles will hold.

So wait, what fits what now? And how will I work it out?
Ideally, you will know what is standard to your calipers, and either fit the same size master or adjust by the descriptions above. However, of course, that's not always possible. So in that case, you can work it out by Doing Math. What you need to do, is work out the surface area of your slave cylinders' pistons and divide it by the area of your master cylinder piston.

The master cylinder will have the size stamped on it. I am only considering axial (old-fashioned type) master cylinders, as it's all I know about from experience, so for radial stuff maybe someone who knows what they're about will chip in on this thread.

This is where you find that stamp, between the bar and the master, usually near the banjo bolt:

Most of the common Nissin type will be 1/2", 14mm, or 5/8".

Calipers won't have the sizes on them, but you can measure with a vernier gauge or to the nearest mm (they won't likely be in inches) with a tape measure. Measure the OUTside diameters! It doesn't matter much if a caliper is fixed (either radial or 4 pot like CBR900) or sliding (2 pot like vfr750) for this calculation. Measure this dimension (this is a car caliper so massive, but my google-fu is weak today) as shown here 55mm:


Then you need to work out your ratio. Here is the formula, in plain english:
(total surface area of pistons on one side of each caliper [so you will get the same result on a 2-piston sliding caliper as on a 4-piston fixed caliper]) x (number of calipers) / (surface area of master clinder piston)
In maths terminology that's:
(((Pi x R^2) + (Pi x R^2))x n) / (Pi x R^2)

But because no one really likes doing math, I have made a spreadsheet that does this for me. Download Spreadsheet here. Here's a screenshot, to show what you're getting. This shows various bikes whose brake systems I have messed about with, with those combinations I liked highlighted in green, and those I didn't highlighted in red.



Let's only take examples here with Nissin masters, as on these the lever is identical so you are comparing apples with apples, ok?

So this gives an idea that on single caliper systems, you want to aim for a ratio of about 13.5:1, and on dual caliper systems for about 17:1. On single calipers <12:1 is wooden, and >15:1 is spongey; while on duals <15:1 is wooden, and >19:1 is spongey. These comparisons are all just my personal taste though, some will like different, but it's a good place to begin from!

Sorry this is such a long post guys, but hopefully, it's been educational!

I accept payment for my time writing this in Rep ;)
Would be happy for this to go sticky.
 

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It seems that I have done more of this sort of stuff than most people on here, and a lot of people are getting confused. So here's the information you've been needing...

First to say - this post is only relevant to Nissin axial master cylinders like these, and any which have similar looking levers:

They can be found on nearly every Jap bike from about 1988 through to modern stuff. There are also types with built-in reservoirs like CBR600f4i, they are the same too. I do not know about radial brakes, so won't go into that. Except to say that radial calipers is just marketing junk, and won't make any difference to these calculations.


Front brakes are crucial on any motorcycle -
they are the bit which should stop you hitting things! So it's very important to make sure they work right. On most bikes, keeping the original parts in as-new condition should do this for you, however some bikes are poor from the factory. On modified bikes, you might want or need to mix and match parts. In principal there is no problem with this, because hydraulic fluid doesn't care which make of bike it's being used in. But if you're going to start mix and matching, there are some things you need to understand first.

Hydraulic advantage -
the first thing you need to understand is how brakes work at all. When you squeeze the lever on the handlebar, you are moving a small piston which fits into a finely machined bore, and forces fluid down the line. That's close enough to true to do for now. This fluid moves down the line, and because it is not compressible, it forces the pistons in the calipers to move out, pushing the pads against the discs, generating friction, slowing you down.

Consider the fact that the bore of the master cylinder is much smaller than the combined bores of all the pistons in your brake calipers. This means that (made up numbers coming up) for every 15mm you push the master cylinder in, you move maybe 5cc of fluid. But as that 5cc of fluid hits the pistons at the bottom, they don't move 15mm but in total like 0.5mm. This is where you get your hydraulic advantage from, and this is why you don't have to squeeze the brake lever hard enough as if you were actually grabbing the pads with your hand.

Changing hydraulic ratios -
Now that you know that, you can consider how changing the sizes of any of the pistons in the system will affect this relationship. Let's assume we've taken a standard braking system which works fine and for whatever reason, we are thinking of making the following changes.

Bigger master cylinder
If you increase the size of the master cylinder, you will actually be REDUCING the power of the brakes. Yes, with a bigger master cylinder, you will have to squeeze HARDER to get the same braking force at the calipers. This is because you are reducing the hydraulic advantage you have over the calipers. Crazy, right? Maybe but it's still a fact.

So why would anyone do this? Well, you will also be reducing the amount of lever travel needed to move the same amount of fluid, so if you've got very strong hands, or are on a race track and want/need shorter lever travel, then you might find it advantageous. Also, some bikes (t595, 600srad for example) are fitted from stock with what might be considered too small of a master cylinder, giving over-long lever travel, so some people like to upsize it to give a firmer, less vague feel at the lever. However if your bike as stock has good feel, then going up a size can make the brakes feel "wooden", that is, like an on/off switch. This happens when pulling the lever gently doesn't do much due to reduced advantage, but then with a tiny amount of lever stroke, that you have to squeeze hard potentially without finesse, you can very suddenly lock the wheel.

You'll be reducing the power of the master cylinder, so reducing the amount of flex generated through the system, as you'll now need to squeeze harder to flex the calipers than you probably can. This difference is often very noticeable while standing in the parking lot, but you won't necessarily feel this difference on the move, as if you're at that point of feeling flex from the brakes, you've probably already locked your front wheel.

Smaller master cylinder
If you decrease the master cylinder size, you actually GAIN in braking power due to the increased hydraulic advantage you gain. Brilliant, why don't we all do it?

Well, because you also increase lever travel. In some cases it's so bad that the lever hits the bar, maybe trapping your fingers before you stop hard. This is a Bad Thing! In some case, where the brakes as standard are "wooden", fitting a smaller master cylinder can improve them massively (xj600s) but if the brakes are already fine, you may end up with them feeling vague or spongy, or trapping your fingers.

You can sub bigger master for smaller calipers, or vice versa, and the principles will hold.

So wait, what fits what now? And how will I work it out?
Ideally, you will know what is standard to your calipers, and either fit the same size master or adjust by the descriptions above. However, of course, that's not always possible. So in that case, you can work it out by Doing Math. What you need to do, is work out the surface area of your slave cylinders' pistons and divide it by the area of your master cylinder piston.

The master cylinder will have the size stamped on it. I am only considering axial (old-fashioned type) master cylinders, as it's all I know about from experience, so for radial stuff maybe someone who knows what they're about will chip in on this thread.

This is where you find that stamp, between the bar and the master, usually near the banjo bolt:

Most of the common Nissin type will be 1/2", 14mm, or 5/8".

Calipers won't have the sizes on them, but you can measure with a vernier gauge or to the nearest mm (they won't likely be in inches) with a tape measure. Measure the OUTside diameters! It doesn't matter much if a caliper is fixed (either radial or 4 pot like CBR900) or sliding (2 pot like vfr750) for this calculation. Measure this dimension (this is a car caliper so massive, but my google-fu is weak today) as shown here 55mm:


Then you need to work out your ratio. Here is the formula, in plain english:
(total surface area of pistons on one side of each caliper [so you will get the same result on a 2-piston sliding caliper as on a 4-piston fixed caliper]) x (number of calipers) / (surface area of master clinder piston)
In maths terminology that's:
(((Pi x R^2) + (Pi x R^2))x n) / (Pi x R^2)

But because no one really likes doing math, I have made a spreadsheet that does this for me. Download Spreadsheet here. Here's a screenshot, to show what you're getting. This shows various bikes whose brake systems I have messed about with, with those combinations I liked highlighted in green, and those I didn't highlighted in red.



Let's only take examples here with Nissin masters, as on these the lever is identical so you are comparing apples with apples, ok?

So this gives an idea that on single caliper systems, you want to aim for a ratio of about 13.5:1, and on dual caliper systems for about 17:1. On single calipers <12:1 is wooden, and >15:1 is spongey; while on duals <15:1 is wooden, and >19:1 is spongey. These comparisons are all just my personal taste though, some will like different, but it's a good place to begin from!

Sorry this is such a long post guys, but hopefully, it's been educational!

I accept payment for my time writing this in Rep ;)
Would be happy for this to go sticky.
The excel file for calculation the ratios is not working! Do you mind uploading another one please?
 

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Well done. Cool job and great counts. Did you develop all this yourself or did you find the idea on the Internet? I just do not know how the brain of an ordinary person without engineering education should work to work with such things.
 
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