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Rear brake hydraulic ratio questions


KevCarver

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For all you hydraulics experts:

I've changed out my rear brake setup twice now. First was removing linked brakes, adding a crossover line to the stock rear caliper and swapping out stock '99 master for a 14mm RC51 master.

Last year I got an adapter to fit a Ducati 2 piston caliper and kept the 14mm master. Turns out the 34mm rear caliper from an '08 1098 S is using an 11mm master cylinder. Stopping power isn't great, but I don't really use it much. Nice to hold it on a hill and release the fronts.

I'm using the pads that came with the caliper, I have no idea what they are. I just ordered a set of EBC HH just to match the fronts and baseline what I'm used to. 

What will going to a smaller master cylinder do for the system?

Is there a compatible Honda smaller rear master to fit the bolt spacing? I think it's 45mm, confirm or correct?

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1 hour ago, KevCarver said:

Lever travel as is seems pretty normal, on the short side. Would a smaller m/c shorten it? I think that would be too much. I'm using the 14mm now, got new pads on order and hoping that will prove to be a difference since I don't know what pads came with it. They looked brand new, so I kept them when I installed everything.

Smaller m/c = longer lever travel less force required. Larger m/c = shorter lever travel, greater force required.

 

So For the 11mm m/c bore - 1815.84 divided by 95.03 = 19.1 : 1 ratio

And for a 14mm m/c bore - 1815.84 divided by 153.94 = 11.79 : 1 ratio

 

From the calculated ratios can the following be said for your 11mm m/c (hydraulics is not my pet subject)? 

Example.

For a 1mm extension of the caliper pistons (with a ratio of 19.1:1). The m/c needs to travel 19.1mm.

Assume this is the function of the ratio?

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5 hours ago, Grum said:

Smaller m/c = longer lever travel less force required. Larger m/c = shorter lever travel, greater force required.

 

So For the 11mm m/c bore - 1815.84 divided by 95.03 = 19.1 : 1 ratio

And for a 14mm m/c bore - 1815.84 divided by 153.94 = 11.79 : 1 ratio

 

From the calculated ratios can the following be said for your 11mm m/c (hydraulics is not my pet subject)? 

Example.

For a 1mm extension of the caliper pistons (with a ratio of 19.1:1). The m/c needs to travel 19.1mm.

Assume this is the function of the ratio?

Ok, I was thinking the opposite. Never studied hydraulics, so it's all guess work on my part...

 

So, what Honda rear m/c are out there with smaller than 14mm bore and 45mm (on center) bolt spacing?

The Duc 11mm m/c I have seen on eBay are 40mm. 

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So the simple answer is work out your ratios, its pretty simple.

 

Area of ALL brake pistons in a caliper / devided by area of master cylinder = brake ratio.

So if a 4pot caliper had 4 equal sized 32mm pistons, then 32/2=16x16=256x3.14 (pye)=804mm PER piston, so x4 = 3216mm2 is the area of the pistons. 

These would normally have a 16-19mm MC piston diameter, so we will use 16/2=8x8=64x3.14=201mm2.

So devide the 3216 from the caliper pistons by the 201 from the MC. 3216/201=16/1 ratio. I hear you say thats not right & you are correct !

That 16mm MC usually works on 2x4pot calipers, so 3216x2=6432/20=32 or 32/1 ratio.

 

Now do the same for your rear brake. They normally run around 7-9/1 ratios (for a single piston, or two small piston rear caliper), because your foot has a big leg attached & can push much harder than your tiddly fingers can pull 🙂

 

For fronts the preferred range is 25-35/1 where 25/1 gives a very stiff lever but instant if wooden bite, and 35/1 gives a soft lever with travel that allows for very fine modulation. I find 32 or 33/1 is best for me but that's my preference.

 

As always YMMV.

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Aha, small cock up on my part. most rear calipers are single piston or two very small ones. Your ratio is correct due to two large pistons. As you have worked out the 14mm has nearly halved the effective effort !

Less brake piston area, means big changes in ratio for small changes in MC diameter. IIRC the stock VFR rear MC is 1/2" or 12.7mm (check that I'm sufferring brain fade ATM) which would give 14/1 ratio, which is the same as 7/1 for a single piston caliper.

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17 hours ago, KevCarver said:

Best I can come up with is the 34mm 2 piston caliper and 14mm m/c have an 11.8/1 ratio, and the Duc 1098 S with the same caliper and 11mm m/c are 19.1/1. 

Hi KevCarver, Mohawk.

You're spot on Kev. Found this handy table.

 

Using the table the nearest dimension to yours for a 2 piston Caliper is the 35mm (2). For both 11mm and 14mm Master Cylinder, your figures are close to the table, as expected.

 

So for your 34mm (2) pistons. The total Area = 1815.84 square mm. Your m/c bore at 11mm has an Area of 95.03 square mm. A 14mm m/c bore has an Area of 153.94 square mm.

 

So For the 11mm m/c bore - 1815.84 divided by 95.03 = 19.1 1 ratio

And for a 14mm m/c bore - 1815.84 divided by 153.94 = 11.79 : 1 ratio

 

image.png

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Good to know I got the math(s) correct!

On 5/14/2020 at 4:24 AM, Mohawk said:

Less brake piston area, means big changes in ratio for small changes in MC diameter. IIRC the stock VFR rear MC is 1/2" or 12.7mm (check that I'm sufferring brain fade ATM) which would give 14/1 ratio, which is the same as 7/1 for a single piston caliper.

I pulled mine off the storage shelf and it seems to say 11/16", about 8/1 on the chart.

 

On 5/14/2020 at 5:55 AM, Grum said:

Hi KevCarver, Mohawk.

You're spot on Kev. Found this handy table.

Lever travel as is seems pretty normal, on the short side. Would a smaller m/c shorten it? I think that would be too much. I'm using the 14mm now, got new pads on order and hoping that will prove to be a difference since I don't know what pads came with it. They looked brand new, so I kept them when I installed everything.

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Best I can come up with is the 34mm 2 piston caliper and 14mm m/c have an 11.8/1 ratio, and the Duc 1098 S with the same caliper and 11mm m/c are 19.1/1. 

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