Choosing correct master cylinder

C

chickenStripCharlie

Coast to Coast
Hi!
I have decided to go with Honda VFR 800 calipers for my build. They have a VERY slim inside section and i need all the clearance i can get. They will be working on 320mm rotors. How do i select a proper master cylinder? Is there a calculator of sorts where i can see the effect of choosing one with bigger/smaller piston diameter?

I understand that the VRF 800 has a "linked" braking system where the front brake applies some pressure to rear, and rear brake applies some to the front. I am hoping to just eliminate that system altogether. Have not looked into that much yet but i know it can be done.
Appreciate the advice! Thanks!

57_d5a25060-d503-4acb-acd0-c9ce4f3bfb87.jpg
 
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doc_rot said:
just run lines straight from MC to calipers. heres a chart. You will need to know the piston diameter

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Click to expand...


It's time for me to look into this in detail and it is confusing.

How do I know what ratio I want? The writer suggests 23:1 but I dont know his riding style.

For someone whose last bike had ABS I'd probably want something with a more forgiving brake feel and, dare I say, "weaker" brakes?

Likely, the best option for me is to just get the same MC that matches my caliper. However, in my case, the calipers are braking a (likely) heavier 19" wheel instead of 17" and also have two rotor adaptors added to it which is probably 3lbs extra on top of the wheel weight difference.

Any advice?
 
Just think about it as leverage.

the ratio quoted is that of the total area of the calliper pistons, all of them added up, divided by the area of the master cylinder piston

in leverage terms its just like using a longer ratchet/breaker bar - longer bar = more force but also longer range of movement

same with master cylinder - larger ratio = more pressure but longer lever travel

hth

steve
 
chickenStripCharlie said:
It's time for me to look into this in detail and it is confusing.

How do I know what ratio I want? The writer suggests 23:1 but I dont know his riding style.

For someone whose last bike had ABS I'd probably want something with a more forgiving brake feel and, dare I say, "weaker" brakes?

Likely, the best option for me is to just get the same MC that matches my caliper. However, in my case, the calipers are braking a (likely) heavier 19" wheel instead of 17" and also have two rotor adaptors added to it which is probably 3lbs extra on top of the wheel weight difference.

Any advice?
Click to expand...
determine the ratio of disk to tire size from old to new and use that to guide your mc/caliper ratio selection. Weight of the wheel is not of an issue with "feel" it just takes longer to brake. I shoot for 25-27:1 for my custom brake setups and they have all worked very well with 17", 18", or 19" wheels. What you will discover is there are only a handful of MC bore sizes and sometimes they dont get you to the correct ratio (goldilocks effect). Its possible you may need to change the calipers to get the ratio in the sweet spot. brake lever ratio is also a factor
 
There is a whole complicated calculation you can do that factors in bike weight (laden), dick diameter, piston and M/C bore sizes etc, but while those all have an effect. the most important issue is M/c bore are to caliper bore area set out in that Vintage Brake chart. As a prior poster mentioned, total bike weight will factor in to how quickly a bike stops, start with that chart.
 
doc_rot said:
determine the ratio of disk to tire size from old to new and use that to guide your mc/caliper ratio selection. Weight of the wheel is not of an issue with "feel" it just takes longer to brake. I shoot for 25-27:1 for my custom brake setups and they have all worked very well with 17", 18", or 19" wheels. What you will discover is there are only a handful of MC bore sizes and sometimes they dont get you to the correct ratio (goldilocks effect). Its possible you may need to change the calipers to get the ratio in the sweet spot. brake lever ratio is also a factor
Click to expand...

Lets see if i understand this. I have 4 pistons per caliper.
I believe they are 30 & 27mm ('99 GSXR600 tokico). So:

At = (pi(15^2) + pi(13.5^2)) x 2
At = 2558.82 mm^2

Fourth row from the top is the closest, so lets assume it is equal to that for now (four 28mm pistons)
With the stock Honda piston (5/8") I would have a ratio of 12.44

If i go for an 11mm MC, the ratio would be 25.92

So does this mean the 5/8" MC would require roughly twice the lever EFFORT to achieve the same braking as the 11mm MC? Or twice the lever TRAVEL? I don't quite understand what the author means by a "wooden" lever.

Appreciate the feedback!

mastercylinder.gif
 
Have you accounted for 2 calipers?

the difference in ratio from 12 to 24 will result in a HUGE difference in lever pressure/movement

a ‘wooden’ lever is one that ( due to an innapprorpiate hydraulic ratio) feels rock hard and lacking in ‘feel’

check your calculations, as at first glance i dont think you have accounted for the 8 pistons in the 2 calipers.

steve
 
Double the calipers and you can get a 25.8 ratio with the 5/8" MC. Increasing MC piston size would increase lever effort and decrease lever movement.
 
Sderbyshire said:
Have you accounted for 2 calipers?

the difference in ratio from 12 to 24 will result in a HUGE difference in lever pressure/movement

a ‘wooden’ lever is one that ( due to an innapprorpiate hydraulic ratio) feels rock hard and lacking in ‘feel’

check your calculations, as at first glance i dont think you have accounted for the 8 pistons in the 2 calipers.

steve
Click to expand...

I did not. I thought that if you account for both calipers you still have to divide by 2? (Because the stock numbers seemed off, see below)

Help me understand this better .... My Honda (stock) brake set up is 2 calipers, each with 2 30mm pistons, and a 5/8" master cylinder.

So using the chart (6th row down) that's 2827.44 / 197.93 = 14.28 ratio

That's nowhere near the "sweet spot" author talks about, which is not what I would expect out of an OEM application

But if i use the GSXR calipers with the Honda 5/8" MC i get 25.8 which seems better?


The Honda orginally had a 26" tall tire with (i believe) 280mm dual rotors.
The front wheel is kept same, but the bike will now have 320mm dual rotors.
 
the chart is a only a guide. Use the calculation to determine what you need. 8 live pistons for a total area of 5115 mm2. a 16mm MC would put you at a 25.4/1 ratio. which is ideal.
 
doc_rot said:
the chart is a only a guide. Use the calculation to determine what you need. 8 live pistons for a total area of 5115 mm2. a 16mm MC would put you at a 25.4/1 ratio. which is ideal.
Click to expand...

That's very close to what i posted above (25.8), given that the area between a 5/8" MC and 16mm MC is only 3mm^2.

I think I will just run the 5/8" i already have and save some money to start. I only need a 20 dollar rebuild kit for the one i already have so it should be a good start at least.

And thank you for the help!!!!!
 
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As a sanity check a 16-17mm mc is right for a twin caliper setup on old hondas, it’s what was fitted to the twin disk gl1000

so give your 5/8 mc a go, i think it will be fine.
 
doc_rot said:
the chart is a only a guide. Use the calculation to determine what you need. 8 live pistons for a total area of 5115 mm2. a 16mm MC would put you at a 25.4/1 ratio. which is ideal.
Click to expand...

So I found out the stock gsxr MC, running with the same 8 live pistons, is 14mm.

That would put it at a much higher ratio than 25.x:1 ... so a higher ratio (say 30:1) would be more soft, correct? (More powerful but at the expense of more lever travel so feeling soft?)

Now running that with a 19" wheel instead of the GSXR 17" would make it even MORE soft? Or would the wheel not have any effect on feel, but rather just decrease the effectiveness of the braking force??

I realized I did not account for the wheel size up above.

And I just found out that my honda MC doesnt quite fit on the bars I have. It sits way too angled and besides looking stupid it will likely leak.

So debating whether to pickup a gsxr MC meant for the calipers I'm running, or some other 5/8 or 16mm MC with remote reservoir. Wondering if the 19" wheel changes things.
 
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I dont think that the wheel size affects the working of the brakes, although it will alter their affectiveness.

i wouldnt consider wheel size, if you do then you’ll also need to consider disk size..... which is too complex imho

go with a master cylinder close to ‘correct’ ratio

you are right that a smaller diameter master = more power but more lever movement
dont go too small otherwise there may not be sufficient fluid flow to operate the calipers fully

steve
 
Sderbyshire said:
I dont think that the wheel size affects the working of the brakes, although it will alter their affectiveness.

i wouldnt consider wheel size, if you do then you’ll also need to consider disk size..... which is too complex imho

go with a master cylinder close to ‘correct’ ratio

you are right that a smaller diameter master = more power but more lever movement
dont go too small otherwise there may not be sufficient fluid flow to operate the calipers fully

steve
Click to expand...

Makes sense.

And i know doc rot mentioned the chart being only a guide, but it still does not compute with me why the OEM caliper/MC combos are so far off the so called "ideal" ratio. I mean, wouldn't the manufacturers know this already?

If you look at the stock GSXR set up. With 8 live pistons and a total area of 5117.64mm^2 and 14mm MC the ratio comes out to 33.24:1. Quite a bit off the "ideal" as per chart.

at 33:1, the GSXR brakes should feel quite soft with more lever travel, albeit stronger. So are they using a smaller MC for more power, but how are they overcoming the lever travel? because it's a sport bike. I'm sure it doesn't have some massive lever travel required to stop.
 
Fot the total picture you nee to consider :

lever length and pivot ratio
mc piston area
total calliper piston area
disk ‘size’ at the effective radius
pad coefficient of friction
wheel radius
tyre coeeficient of friction

way too complex for me ! Hence focus on mc and caliper ratio, and pad grade ( aim for hh or at least gg)

steve
 
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