Brake sizing for brake upgrade

kuksul08

kuksul08

Suh Dude
I am trying to wrap my head around some brake upgrades right now. I have a bike with twin disk brakes up front. Each caliper is a 38mm diameter single piston and the caliper slides on pins. The master cylinder is 5/8", or 15.9mm.

The brakes feel extremely "wooden". As soon as the pads make contact, the lever has no modulation at all, and there is not much braking power. I have to use 4 fingers and don't think I could lock up the front wheel in the rain if I tried.

I am thinking of putting some sport bike calipers on there, which commonly have 4 opposed pistons ranging from 30-34mm diameter, and leaving the master cylinder stock.

My question is how do I calculate the new ratio? If I count all the areas of the pistons, the ratio is MUCH larger, and I know from experience it doesn't really work that way. Do I only consider the pistons on one half of each caliper?

Any ideas?
 
ST Guy

ST Guy

Well-known member
Are the master and calipers OEM to the bike? If so, something is very wrong as the originals should not feel that way.
 
kuksul08

kuksul08

Suh Dude
Yep, all original and fully rebuilt.

It's a common complaint for this bike, compounded by tiny rotors. Also, I am spoiled by modern race brakes and demand 1 or 2-finger braking.
 
afm199

afm199

Well-known member
Have you cleaned the discs thoroughly and are using new and high quality brake pads? That makes a huge difference.
 
RickM

RickM

Well-known member
kuksul08 said:
I am trying to wrap my head around some brake upgrades right now. I have a bike with twin disk brakes up front. Each caliper is a 38mm diameter single piston and the caliper slides on pins. The master cylinder is 5/8", or 15.9mm.

The brakes feel extremely "wooden". As soon as the pads make contact, the lever has no modulation at all, and there is not much braking power. I have to use 4 fingers and don't think I could lock up the front wheel in the rain if I tried.

I am thinking of putting some sport bike calipers on there, which commonly have 4 opposed pistons ranging from 30-34mm diameter, and leaving the master cylinder stock.

My question is how do I calculate the new ratio? If I count all the areas of the pistons, the ratio is MUCH larger, and I know from experience it doesn't really work that way. Do I only consider the pistons on one half of each caliper?

Any ideas?
Click to expand...

My take:
You do count total caliper / slave piston area for ratio. Effective force applied on pads will be a hair less than calculated to account for additional frictional loss for the additional pistons.

If you have already decided on the calipers, why not mount them with current master and try first.
 
kuksul08

kuksul08

Suh Dude
RickM said:
My take:
You do count total caliper / slave piston area for ratio. Effective force applied on pads will be a hair less than calculated to account for additional frictional loss for the additional pistons.

If you have already decided on the calipers, why not mount them with current master and try first.
Click to expand...

I'm not totally set on the calipers yet. I found some with pistons varying from 28 to 34mm.
 
Tom G

Tom G

"The Deer Hunter"
You can only count the brake piston area in one direction, consider the opposite direction as being fixed (like brakes sliding on pins). Make sure you get enough oil volume to fully engage the brakes. Did you try softer pads with a higher friction coefficient? And my old cars from the snowy Germany had the brakes stop sliding on the pins b/c of rust.
 
ST Guy

ST Guy

Well-known member
I think I'd try a different pad compound first. I have a JCW Mini that I upgraded the pads on and there was no way in hell I could lock up the fronts. They, too, felt wooden and ineffective. I switched to different pads again and now they are awesome. Not only do they bite better than stock, they are extremely powerful, even when cold, and they handle heat even better too.
 
kuksul08

kuksul08

Suh Dude
TWF said:
This will give you idea.
https://www.vintagebrake.com/mastercylinder.htm
Click to expand...

Awesome! Very helpful

stangmx13 said:
What’s the lever ratio of the master?

How small are the rotors?
Click to expand...

Not sure about the lever ratio - that's a good question.
The rotors are tiny - 265mm.

buellistic said:
Vented brake rotors and SS lines help alot..
And how about a pic of the brakes in question..
Click to expand...

I got SS lines already.

zwcDKC6.png
 
Enchanter

Enchanter

Ghost in The Machine
Staff member
The least expensive and least labor / engineering would be to change the master cylinder to one that better matches the calipers.

The RZ350 wasn’t known for its brakes.
 
stangmx13

stangmx13

not Stan
Your hydraulic ratio compared to my R6 is only 13% more “wooden”. Same master size with 27&30mm pistons. That’s not bad. I bet the lever ratio is worse, compounding the issue.

The lack of power is probably all down to those tiny rotors.

Which Vesrah pads do you have? Some are wooden without heat, like the SRJL17. Newer race compounds like the XX do better with less heat. And I suspect the less aggressive pads like the RJL are better too.

One ez way to increase feel and power is to move the master further from the grip. This gives your fingers more leverage for power and forces them to move further for extra feel. You may need longer bars to accomplish this.
 
Eric B

Eric B

Know-it-none
Strange. My SRX 6 of similar vintage has, what I think, good braking characteristics currently. They were wooden in the past, and I think all I changed were the pads. The RZ calipers have opposed pistons, yes?
 
Eric B

Eric B

Know-it-none
TWF said:
Opposed pistons or one side pistons makes no difference for ratio. As mentioned only one side of caliper counts.
Click to expand...

Looks like the RZ has 1 piston. Would opposed pistons add better feel?
On a separate note. A google search of RZ 350 caliper upgrade, yields some feasible options.
 
TWF

TWF

training hard
Eric B said:
Looks like the RZ has 1 piston. Would opposed pistons add better feel?
On a separate note. A google search of RZ 350 caliper upgrade, yields some feasible options.
Click to expand...

Probably not since they would be bolted direct to fork and flex less.
 
kuksul08

kuksul08

Suh Dude
Enchanter said:
The least expensive and least labor / engineering would be to change the master cylinder to one that better matches the calipers.

The RZ350 wasn’t known for its brakes.
Click to expand...

You're right. However I would like to keep the cockpit look somewhat stock with the original mirrors and such. I also rebuilt it quite nicely. The calipers won't look stock, but then again if I find a set of gold calipers they might look great.

stangmx13 said:
Your hydraulic ratio compared to my R6 is only 13% more “wooden”. Same master size with 27&30mm pistons. That’s not bad. I bet the lever ratio is worse, compounding the issue.

The lack of power is probably all down to those tiny rotors.

Which Vesrah pads do you have? Some are wooden without heat, like the SRJL17. Newer race compounds like the XX do better with less heat. And I suspect the less aggressive pads like the RJL are better too.

One ez way to increase feel and power is to move the master further from the grip. This gives your fingers more leverage for power and forces them to move further for extra feel. You may need longer bars to accomplish this.
Click to expand...

My lever ratio is 3.6. You know I actually did move the master in as far as possible and it does help. Still leaves some to be desired though.

I am using Vesrah JL sintered street pads. I have used them before and been very happy. I have a set of SRJL8's that will fit if I go with 929/954/600 calipers and those are insanely powerful yet progressive.

Eric B said:
Looks like the RZ has 1 piston. Would opposed pistons add better feel?
On a separate note. A google search of RZ 350 caliper upgrade, yields some feasible options.
Click to expand...

Yeah now that I think about it, TWF is right that two opposed pistons won't be any better than a single one. The hydraulic force at each axis is the same whether the reaction force is fixed or another piston.

The caliper upgrades are pretty well documented, but I like understanding why it works that way and if I can improve on it.


Now don't get me wrong. I can ride this bike normally and it stops fine, but just gets tiring on the hand after an hour or so. My goal is to just increase the leverage a known amount and then test the result. I can't go too crazy or the little 35mm forks might snap in two :rofl
 
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