Topic: Double disk front master - all suggestions welcome

[SKTP]

So I am finally biting the bullet and shelling out for a modern master and stainless brake lines...

I've search the forum quite extensively and now believe that a stock master is a 14mm piston and will work but may feel soft.

Many people seem to be using 5/8 masters (15.875mm) and are happy with the feel (more bite). Which I am taking as less lever travel and a hard feel...

If that is all correct. What do you recommend for a master?

I'm thinking this one:

http://www.dimecitycycles.com/vintage-caferacer-cafe-racer-bobber-brat-chopper-custom-motorcycle-brake-parts-black-aluminum-nissin-hydraulic-front-brake-master-cylinder-perch-lever-kit-17-651-b.html

[JohnG]

I converted my '76 750F from single to dual disks  (all stock parts).  I had planned to use a different master cylinder  but was so happy with the results that I left it with the stock master.  My original plan was to use the master from a '78 Gold Wing  (and actually got one and refurbished it).

So I would suggest you try the stock master and see how you like it. Go from there.

John
PS  I have the 750F calipers which have pistons a little bigger than those in the K calipers

[flybox1]

I picked up an aftermarket 14mm MC off eBay for about $50. Works great.  Firm feel.  Very similar to the unslanted one sold by slingshot.
Guess it depends on if you want that stock look.  I didn't really care, but am happy with it.

[scottly]

I'm guessing you are talking about a '78 K with dual 42.8mm calipers? A stock 14mm MC will work fine, but may "feel" a bit soft when bleeding. It's not really an issue when riding, as you won't be clamping down that hard on the brake lever in order to stop.

[Don R]

What John G said. Same pressure, more travel. I can make the front tire squeal if I want to. It does feel softer until you lean on it excellent control. I have a rock hard master cyl on my K0 (single disc)and am not as comfortable with it.

[shinyribs]

My bike has an F and an A caliper on the front. Both are the 42mm piston. I thought the front brakes felt horrible with the stock m/c,which I really wanted to retain.I wouldn't even leave with driveway with that setup it felt so bad. I ended up using a 2006 zx6 m/c (robbed from another project) and am very happy with the brakes. Rock solid lever feel with very good modulation. They fit 7/8'' bars,but admittedly look out of place. Using a matching clutch perch fixes that. They can be found cheap on ebay. I paid $55 shipped for the m/c and clutch perch with the levers pictured. M/c needed a kit,that was $18.    Hope this helps. Good luck!




EDIT: Just looked back up at your link to DCC. That appears to be a standard Nissin 5/8'' m/c.(Same as I mentioned aboove) The lever on there,I've seen those local for about $10. Honestly,I've never bought anything from DCC...since I always find it elsewhere considerably cheaper.

[shinyribs]

Here's one for $75 shipped. Lever and res. included. Not a bad buy,but with some searching you'll likely find one cheaper.

http://www.ebay.com/itm/2001-99-01-02-Kawasaki-Zx600-Zx6r-Zx6-Ninja-GENUINE-FRONT-BRAKE-MASTER-CYLINDER-/321100816015?pt=Motorcycles_Parts_Accessories&hash=item4ac3199e8f&vxp=mtr

[scottly]

My bike has an F and an A caliper on the front. Both are the 42mm piston. I thought the front brakes felt horrible with the stock m/c,which I really wanted to retain.I wouldn't even leave with driveway with that setup it felt so bad.

If you never left the driveway, then you have no idea how the 14mm MC really worked. 

[shinyribs]

lol   My driveway is right at 1/2 mile long. I made down to the bottom of the first hill and said ''no way!''.

[scottly]

lol   My driveway is right at 1/2 mile long. I made down to the bottom of the first hill and said ''no way!''.

Why?? I'm serious.

[Viktor.J]

lol   My driveway is right at 1/2 mile long. I made down to the bottom of the first hill and said ''no way!''.

Did you bled the brakes properly, new lines etc?
I have new staneless lines, EBC brake pads and stock MC. It's not perfect but much better then stock and def. not "unsafe". Many ride like this.

I agree with the others. complete the setup and try the stock out. If you dont like it then change.

[Bodi]

The problem I found when using a stock SOHC4 master on a dual disc setup was what I expected but worse. With a single disc in a "panic stop" grip I could pull the lever about halfway to the bar. And of course the brake was incapable of a stoppie or locking the wheel on dry pavement.
With the dual disk setup the lever came to the bar with more braking felt but still no stoppies.  It was scary to be pulling the lever close to the bar for normal traffic braking, realizing there wasn't much in reserve for emergency use.
With a larger bore mc the lever feel was stiffer, but maximum squeeze was again about 1/2 way to the bar and was to be avoided at speed unless I wanted to do an endo.
I used one from a late model sportbike, I think it's a Kawasaki but I picked it on looks out of a bin full of assorted M/Cs at the bike wrecker and don't know the actual model.
In theory the smaller bore will give twice the  braking for a given lever pressure: lever travel will be doubled as well. With well bled steel brake lines the lever travel could be tolerable and the stock master could work. With the stock rubber lines I don't think it will be acceptable, the lines swell under pressure and take enough fluid doing that to have the lever against the bar before maximum braking is achieved.
In general going to steel lines (steel braided hose) you get a very different lever feel, the mushy feel due to the rubber lines - that you never noticed i- s suddenly obvious once it's gone.

[Xnavylfr]

Your THEORY is flawed!!!
If your PUSHING piston(MC) is for instance 1inch and your calipers have 4 1inch piston (WORKING pistons) then you divide the PUSHING  piston size by the working piston size. In this calculation  you will need to apply 4 times the power to the MC to get the same power at the calipers. Sounds to me that your brakes aren't bled good! and you have air in them.Air compresses and fluid won't.


Xnavylfr(CHUCK)

[Viktor.J]

Your THEORY is flawed!!!
If your PUSHING piston(MC) is for instance 1inch and your calipers have 4 1inch piston (WORKING pistons) then you divide the PUSHING  piston size by the working piston size. In this calculation  you will need to apply 4 times the power to the MC to get the same power at the calipers. Sounds to me that your brakes aren't bled good! and you have air in them.Air compresses and fluid won't.


Xnavylfr(CHUCK)

I'm not sure of what you mean there's a chance that i missunderstand you. But If the area of the MC is 1/4 of the active piston size you get 4 times higher pressure at the active piston. That's the whole principle of hydraulics.
Since the pressure is constant due to incompressible media and pressure are defined as force on a area F/A the force acting on the bigger piston is Pbigger= (Fsmall*Abigger)/Asmall. Thats the basics of pascals principle and the working fork on a hydraulic brake system.

to be clear.

[MOONDOGNYC]

Your THEORY is flawed!!!
If your PUSHING piston(MC) is for instance 1inch and your calipers have 4 1inch piston (WORKING pistons) then you divide the PUSHING  piston size by the working piston size. In this calculation  you will need to apply 4 times the power to the MC to get the same power at the calipers. Sounds to me that your brakes aren't bled good! and you have air in them.Air compresses and fluid won't.


Xnavylfr(CHUCK)

I'm not sure of what you mean there's a chance that i missunderstand you. But If the area of the MC is 1/4 of the active piston size you get 4 times higher pressure at the active piston. That's the whole principle of hydraulics.
Since the pressure is constant due to incomprehensibility and pressure are defined as force on a area F/A the force acting on the bigger piston is Pbigger= (Fsmall*Abigger)/Asmall. Thats the basics of pascals principle and the working fork on a hydraulic brake system.

to be clear.

Dude....simply put, you forgot to carry the 2....   

[Viktor.J]

Your THEORY is flawed!!!
If your PUSHING piston(MC) is for instance 1inch and your calipers have 4 1inch piston (WORKING pistons) then you divide the PUSHING  piston size by the working piston size. In this calculation  you will need to apply 4 times the power to the MC to get the same power at the calipers. Sounds to me that your brakes aren't bled good! and you have air in them.Air compresses and fluid won't.


Xnavylfr(CHUCK)

I'm not sure of what you mean there's a chance that i missunderstand you. But If the area of the MC is 1/4 of the active piston size you get 4 times higher pressure at the active piston. That's the whole principle of hydraulics.
Since the pressure is constant due to incomprehensibility and pressure are defined as force on a area F/A the force acting on the bigger piston is Pbigger= (Fsmall*Abigger)/Asmall. Thats the basics of pascals principle and the working fork on a hydraulic brake system.

to be clear.

Dude....simply put, you forgot to carry the 2....   

I'm just explaining the principle behind hydraulic pressure systems.
If you reefer to the second caliper you can pretend that A2 is the size of two calipers, that's not really a problem. The problem with stock MC and dual discs as I understood is not the diameter of the MC its the volume it moves (or more the volume it doesn't move). Since we now have two calipers we need to move more fluid, the volume is constant. This is done be increasing the diameter of the MC or longer stroke (moving more fluid). And since larger diameter means less difference in areas and more pressure force needed at the leverage. Therefor the problem would not need to be the diameter of the stock MC piston, Isn't this one of the reasons why modern bikes have radial Mc besides less flex in the MC ? It provides small piston size = small pressure att leverage , radial = long stroke

If you increase the diameter or ad a double piston the principle is the same, but that's one "side" of the problem. since the volume is fix you can easily calculate how many mm one caliper piston moves for every 1 mm the lever moves. And then you understand that if you increase/double the area for the caliper the distance it travels for every leverage mm will decrease, the pressure on the caliper area will however increase.

[Xnavylfr]

NOPE not wrong!!!  If you look at a BOTTLE HYDRAULIC jack, you will notice the pump PISTON is much smaller than the surface of the ram piston. That's why it take you so many pumps to move the ram up 1 inch.  If you have a brake system, for instance , you have a 1" bore MC piston and 4 1" caliper pistons, you will need 4 times the amount of pressure on the MC to equally apply all 4 calipers. The working power at the MC will be divided by the 4 calipers. If this system was reversed you would only need 1/4 the power at the MC to LOCK the calipers down HARD, because the power applied by the MC would be multiplied by 4.

I KNOW my HYDRAULICS Theory , I worked as an AIRCRAFT HYDRAULICS MECH for over 40 years!!

Xnavylfr(CHUCK)

[Viktor.J]

NOPE not wrong!!!  If you look at a BOTTLE HYDRAULIC jack, you will notice the pump PISTON is much smaller than the surface of the ram piston. That's why it take you so many pumps to move the ram up 1 inch.  If you have a brake system, for instance , you have a 1" bore MC piston and 4 1" caliper pistons, you will need 4 times the amount of pressure on the MC to equally apply all 4 calipers. The working power at the MC will be divided by the 4 calipers. If this system was reversed you would only need 1/4 the power at the MC to LOCK the calipers down HARD, because the power applied by the MC would be multiplied by 4.

I KNOW my HYDRAULICS Theory , I worked as an AIRCRAFT HYDRAULICS MECH for over 40 years!!

Xnavylfr(CHUCK)

Then we are talking about the same thing and I misunderstood your first text. But you power at the MC is multiplied by 4 (IF we are talking about this example OC) , you said it yourself with the bottle hydraulic jack, how come you could lift a 2000 kg car with one hand ?
Same principle applies on the brakes, You stop a 300 kg moving motorcycle with two fingers so yes, the power applied at the handlebar leverage is increased to the calipers
BTW caps lock/ capital letters on a forum is like yelling in real life 

[SKTP]

So, I'll begood with a 5/8ths right?

[754]

FWIW i ran stock with dual 73 calipers and dash 3 lines.. Even chromed them, and was running pods, and felt no ill effects,liked the brakes. And the pods..
 First inkling I had that either may be a problem , was 25 years later when I found this forum..

[Psychonaut]

Front Master Cylinder Ratio Chart

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

[Viktor.J]

So, I'll begood with a 5/8ths right?

Probably. assuming you have a similar stroke to the stock MC you'll move more fluid - less travel of the leverage - what most people want and some comfuses with  " better pressure, firmer brakes " etc..

 

[70CB750]

The pressure is equal throughout the hydraulic system.  Was it the first law of hydraulics? Don't remember.

Oh, it was Pascal's law:

http://www.engineersedge.com/fluid_flow/pascals_law.htm

so if the output area is double, the resulting force equals the area times the pressure, bigger area on slave cylinder, bigger force on the output.

[Viktor.J]

If anyone still have some troubles, play around with this
http://hyperphysics.phy-astr.gsu.edu/hbase/pasc.html#hpcal

scroll down to " hydraulic press calculation", play around with different MC sizes, different sizes in caliper (nbr of calipers), and see how the resulting pressure increases/decreases as well as the amount of stroke you need.

I know this is a simplification of how a MC works, but hopefully it helps someone to get a better grip around this. At least it gave be a while back.

Over and out

[70CB750]

Great link, thanks!

Did you ever fix your front wheel, Victor?