Topic: Master Cilinder size vs Brake Caliper size -racing sweetspot-

[CR750]

I want to double check another thing with you guys - those handy things when you arrive at the first corner - .... BRAKES !

More specific a harmonized optimal performing system...

Using Front Master Cylinder Ratio Chart found here;

https://www.vintagebrake.com/mastercylinder.htm

And beside the chart also referencing the important info mentioned on that page (text) for proper brake feel.

Input:
- I will be using two calipers with each 2 opposing cylinders of each 38 mm for my front brakes.
- 18" rim
- 2 x 310 mm front discs
- bike is CB750 ( trimmed of all unnecessary weight, standard engine weight wise (with starter etc), all tanks aluminium etc )
- I weigh about 78 KG.


My aim is proper racing brake abilities 'with feel'. I know that is somewhat subjective, but your get the point.

Output:
I reference the chart for 38(4) .... as I am using two opposing on each caliper x 2... of course to be operated by the same master cylinder.
Meaning... I should aim for a 15 mm master cilinder as 'the sweetspot' and maybe a 5/8" backup if it needs to be firmer ?

Sounds correct ? Or should I aim for the latter (5/8") when racing ?

What are the guys on here using that actually have similar bikes/systems ?

And rear brake.... same method/principle applies ? ( albeit being 1 caliper with two opposing there  )

[simon#42]

i would go for a smaller master , for those callipers i would go for 13mm , having said that i like the leaver to travel almost to the bar .

[bwaller]

Use the same plan for the rear.

The only way to gain "feel" at the lever is to have lever travel. I expect Simon is right, at that ratio his lever would indeed come to the bar. Not a bad thing, but it's a preference thing.

On mine (not a 750 but with 2 twin piston opposed calipers) the ratio is just over 28. Static, the lever can almost be pulled to the clip-on, but in use on the track it never contacts, still lots of lever travel.For my preference I'd use a 14mm.

[MRieck]

Use the same plan for the rear.

The only way to gain "feel" at the lever is to have lever travel. I expect Simon is right, at that ratio his lever would indeed come to the bar. Not a bad thing, but it's a preference thing.

On mine (not a 750 but with 2 twin piston opposed calipers) the ratio is just over 28. Static, the lever can almost be pulled to the clip-on, but in use on the track it never contacts, still lots of lever travel.For my preference I'd use a 14mm.

I agree

[CR750]

27 being advised by vintage brake for ratio - the under limit would be 23 according to him ( very stiff ).

Ok, with my 15 mm I would end up at 25,7 for ratio...

But you guys suggest 13 or 14 mm.
13 would give me a ratio of 34.2
14 would give me a ratio of 29.5

13 feels indeed too far off for me considering above, but I might be wrong..
14 moves me 2,5 above his 'feel' of where the sweet spot lies according to VB..and the 15 would put me 1,3 under it more towards the stiffer side.

I think I have either a 14mm or 15mm laying around in my stash of bikeparts, and for sure a 14 rear. I will do some testing with that, and see what feels best for me. (maybe I even have a 13 master as well ( wasn't a vfr 13 ?) ).

I will also check what my modified R1 frontend on the vfr is at in terms of ratio ( have no idea actually might be stock R1 or modified - but that setup feels good on the track )..

[MRieck]

14mm are pretty common

[CR750]

Common is usually good.
( untill you fall of a cliff and you start to wonder why the hell the guy in front of you kept walking :-) )

I hope it's indeed a 14 I have laying around, I can check tomorrow, but for matter of self education I am also diving a little deeper into these theoratical equasions, and the stroke of the master is actually also of influence ( the bore being 14 mm ) +  the distance you need to pull the lever. I am finding myself deep into some mathematics on the www I report back when I have found practical results to proof the theoritcs :-)

Quote from somewhere else suppports the VB theory.

A general design rule-of-thumb for twin rotor brake systems is as follows:

30:1 - soft feel
27:1 - sweet spot for design
23:1 - firm feel
20:1 - wooden feel

[scottly]

The Vintage brake chart is somewhat confusing regarding the different ratios of single side calipers vs opposed piston calipers. What it boils down to is that VB doubles the ratio of opposed piston calipers compared to the same size single side caliper, but the recommended MC size ends up being the same for either caliper. The only difference would be the amount of travel of the lever before the pistons are pushed out enough to build pressure; since each piston retracts the same amount, the opposed pistons would have to move twice as far as a single side caliper, requiring a greater displacement from the MC.
The stock CB MCs are all 14mm, except for the F2/3, and thousands of bikes have used the stock 14mm MCs with two 38mm calipers; I have even used one with two 42.8mm calipers with good results.

[turboguzzi]

dont forget that pad mu coefficient and "bite factor" also play a part and it doesnt show in the chart.... at the end of the day it's trial and error. would start with 14mm and then see if to go up or down.

[scottly]

True TG, the chart is only regarding the hydraulic "leverage" ratio, and does not consider the fiction coefficient of the pad material nor the diameter of the rotor.  

[turboguzzi]

forgot to add, 14mm is what i have on my Gpz750/810 racer, P08 calipers, 300mm SS discs.

[CR750]

Thanks to all for your thoughts/opinions, and TG for the specifics (pretty close to mine).
I agree with you it's not an exact science ( hence also adjustable levers etc), but after the extra study yesterday I have learned a little more/understand the underlying principles a bit better. Never hurts.

For now I conclude I agree, and I will follow the 14 mm recommendation, as the initial set-up !

[bwaller]

This is nearly identical to what you're planning.

[Ramrod_Racing]

Just to throw another wrinkle into all of this; the chart accounts for master cylinder diameter, but not the ratio of plunger diameter and its distance from the lever pivot (much less radial vs. standard configurations). So depending on WHICH 14mm master cylinder you use, you can still get a variety of different feels with the same ratio value from the VB chart.

I only know enough to bring this up because just yesterday I was diving into master cylinder theory myself. Looking to improve the feel of my own system, but I'm using at total hodgepodge of different components.

[simon#42]

yes the answer is to try one and see how it feels . it is very easy to over think building racing bikes and people who do never seem to actually finish them . its better to race a bike that is not perfect than to spend years never finishing the perfect bike !  i was in charge of eligibility for the forgotten era racing club for my sins and for years the same dozen or so people would ring up to ask long in depth questions , in the end  non of them ever actually raced .

[CR750]

@ ramrod yes, that's part of what I found as well yesterday.
@ Simon, not intending tot overthink, just well prepared, budget is limited, so a little aim before you schoot is in order. I am already racing other bikes btw

[bear]

forgot to add, 14mm is what i have on my Gpz750/810 racer, P08 calipers, 300mm SS discs.

That's identical to the front brake setup on my sidecar TG.
I hope yours is more reliable than mine was.

Cheers,
Brian

[simon#42]

sorry cr , did not not intended to have a go at you !

[CR750]

Not taken that way Simon. Your points we're legit imo, and good advise to future builders.. Just replied to clarify.

[MRieck]

Just to spike the drink are variable MC's allowed? I highly doubt it. I have a variable ratio AP unit sitting here right now.

[scottly]

How does a variable ratio MC work Mike?

[turboguzzi]

it lets you shift the fulcrum position of the plunger, i.e. the mechanical ratio. but ASFAIK, more of a superbike era thing rather than sohc

https://apracing.com/special-vehicles/motorcycle/master-cylinders/original-adjustable-ratio-master-cylinder/cp3125-family

Funny that mountain bicycles had that feature for years, even in cable operated brakes

in any case, at least iun italy, in the up to 1983 class, no adjustable levers allow (span or ratio) , only solid ones. CR750, check your rulebook...

[simon#42]

yes the name is a bit misleading . it is back to the point ramrod racing made . different masters of the same bore do not always feel the same because of the leaver pivot point .
they are allowed in post classic racing over here but real racers dont use them .... have you seen how much the leavers cost!

[CR750]

Are allowed, if I remember correctly, but would have to re-read the rulebook to be 100% sure.

No intention to go that way though, this -hobby- cost enough already, as is, and I have more bikes and also other hobbies and a girlfriend to keep happy + a kid to take care of ( both of which consume quite a bit of budget  )

So I am not going all out;
I aim for second hand parts where possible, and only buy new where needed/wise. I should be able to rebuild straightaway when crashing, not go bankrupt..(not intending to crash, but always an option) And there is future upgrades..

I also opted out of the new three spoke dymag 18" rims, even though they were in discount for 'only' 1850 euro's instead of 2500.... 

[MRieck]

This is the AP unit I have (and never used).
https://apracing.com/special-vehicles/motorcycle/master-cylinders/pull-type-adjustable-ratio-master-cylinders/variable-ratio-type-cp4125-26

[CR750]

Yowzers, that thing is motoGP expensive, but pretty cool technology.

[scottly]

I didn't find any prices, but it's a moot point as the AP MCs are too big. The "effective bore" of the one TG linked is 16-19mm, with an actual bore of 19mm. Mike's has an "effective bore" of 17-20mm, with an actual bore of 22mm.

[turboguzzi]

indeed, that diameter range is more 4 piston per caliper (x 2 calipers) applications....

[CR750]

I noticed that, and I thought TG already referred to it with his superbike comment. But thanks for pointing it out to futures readers.