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Old 09-25-2020, 02:16 PM   #1
kuksul08
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Brake sizing for brake upgrade

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?
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Old 09-25-2020, 04:03 PM   #2
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Are the master and calipers OEM to the bike? If so, something is very wrong as the originals should not feel that way.
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Old 09-25-2020, 05:30 PM   #3
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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.
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Old 09-25-2020, 05:34 PM   #4
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Have you cleaned the discs thoroughly and are using new and high quality brake pads? That makes a huge difference.
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Old 09-25-2020, 05:53 PM   #5
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Have you cleaned the discs thoroughly and are using new and high quality brake pads? That makes a huge difference.
Yep they were honed and using Vesrah sintered pads. Same pads in another bike work excellent.
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Old 09-25-2020, 07:45 PM   #6
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Quote:
Originally Posted by kuksul08 View Post
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?
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.
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Old 09-25-2020, 10:53 PM   #7
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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.
I'm not totally set on the calipers yet. I found some with pistons varying from 28 to 34mm.
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Old 09-26-2020, 06:36 AM   #8
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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.
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Old 09-26-2020, 11:29 AM   #9
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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.
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Old 09-26-2020, 11:35 AM   #10
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This will give you idea.
https://www.vintagebrake.com/mastercylinder.htm
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Old 09-26-2020, 05:03 PM   #11
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What’s the lever ratio of the master?

How small are the rotors?
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Old 09-26-2020, 05:13 PM   #12
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Vented brake rotors and SS lines help alot..
And how about a pic of the brakes in question..
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Old 09-26-2020, 05:34 PM   #13
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Awesome! Very helpful

Quote:
Originally Posted by stangmx13 View Post
What’s the lever ratio of the master?

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

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Originally Posted by buellistic View Post
Vented brake rotors and SS lines help alot..
And how about a pic of the brakes in question..
I got SS lines already.

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Old 09-26-2020, 06:06 PM   #14
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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.
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Old 09-26-2020, 06:18 PM   #15
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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.
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