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Old 26-04-2018, 11:57 AM   #1
Harley
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Default Rear wheel cylinder sizes

Hi all, ive done a road test of my b16 mini and found the rear brakes lock up pretty easy with the 5/8 bore rear cylinders.
Not sure if this is due to increased weight up front or what but I've had a look on the old internets and there are both 1/2 and 9/16in wheel cylinders available as a replacement size. The 9/16 ones were used on the ERA minis with 4 pot calipers the same as what i have in my car so thinking thats whats best to go with unless people here suggest i go to the next ones down again?

What are most people running on conversion cars with 4-pot brakes?
Thanks!
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Old 26-04-2018, 01:48 PM   #2
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If you are using Metro 4 pots then yes use the ERA rear cylinders. If you are using aftermarket 4 post then speak to the manufacturer (KAD, Minisport etc) and ask them the best ones to use with their calipers.
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Old 30-04-2018, 12:10 AM   #3
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The rear brakes locking don't have anything to do with the front ! Or dose it?if it does can someone explain please?
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Old 30-04-2018, 02:25 AM   #4
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Quote:
Originally Posted by ardonfast View Post
The rear brakes locking don't have anything to do with the front ! Or dose it?if it does can someone explain please?
Ardonfast: You are correct.


The master cylinder size, brake pedal ratio, and the size of the wheel cylinders dictate the nominal brake pressure.
If you are using a single circuit system with a master of 0.700 diameter, going from a 9/16 rear brake cyl to ½ inch will decrease the pressure to the rear brakes. It will not affect the fronts at all.
Same goes for a dual circuit single master with a 0.700 bore for both front and rears changing to a smaller rear cylinder decreases the pressure in the rears and again will not affect the fronts.
Even if you have a dual circuit system & a single master with a stepped bore 0.700 for front and 0.750 rear going from a 9/16 to a ½ will decrease the rear pressure and not affect the fronts at all.
If you have two master cylinders mounted on a pedal bias box, one separate master for the front and one for the rear circuit, guess what, you can manipulate both the front & rear pressure by biasing the pedal box to provide more or less pressure to either circuit. If you still have too much pressure at the rear change the rear cylinder to a smaller one.

We don’t know what "Harley" has in the way of a master cylinder set up, but obviously the front brake pressure is basically not affected by changing the rear brake cylinder size.

Harley: I threw away the BMC bias valve mounted on the rear subframe and fitted a willwood brake adjustable bias valve, am running minisport 4 pots on the front and 9/16 rear cyls
By the way, there are 5 rear cylinder sizes available. ½, 9/16,, 5/8, 11/16 & ¾.
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Last edited by 250ptm; 30-04-2018 at 03:44 AM. Reason: clarification for psuedo lawyers
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Old 30-04-2018, 08:27 AM   #5
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Quote:
Originally Posted by ardonfast View Post
The rear brakes locking don't have anything to do with the front ! Or dose it?if it does can someone explain please?
Yes and no...

No as in the two circuits are separate

but...

Yes as in if you change the front brakes to ones with a large capacity (ie 2 pots to 4 pots) then you need to shift more fluid to get the same pad movement. To do that you need to press the pedal a little further which indirectly means that you are also pushing more fluid and increasing the pressure to the rear cylinders which then "overthrow" and lock the rears. Smaller rear cylinders apply less pressure on the rear shoes to compensate for the extra pressure generated by having to press the pedal a little further hence the suggestion to fit smaller rear cylinders
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Old 30-04-2018, 11:17 AM   #6
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OZ: Sorry I do not agree:
To respond to your 2 pot and 4 pot caliper comparison.

4 pot calipers displace 0.3cc more fluid than 2 pot calipers for any given piston movement. about 2 tablespoons.
I don’t see how you could see the increase in pedal movement you have suggested!

Also when you push a brake pedal, the circuit positive pressure rise occurs when the all pads or shoes in that circuit begin to meet a resistance, ie rotor or drum contact.

Since Fluid is regarded as incompressible, any further pedal pressure applied (after the initial resistance is met) the increased force is transmitted to every extremity of that circuit. This applies to both Front and Rear circuits. The final pressure being developed at each wheel depends only on the piston size at each wheel.

When the pressure in the entire system is equalised how do does one go about as you stated: Quote “increasing the pressure to the rear cylinders which then "overthrow" and lock the rears.” unquote.
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Old 30-04-2018, 12:00 PM   #7
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Great discussion and fascinating subject!

Quote:
Originally Posted by 250ptm View Post

Since Fluid is regarded as incompressible, any further pedal pressure applied (after the initial resistance is met) the increased force is transmitted to every extremity of that circuit. This applies to both Front and Rear circuits. The final pressure being developed at each wheel depends only on the piston size at each wheel.

This is how I understand it too.

Extra factor to throw into the mix - what about the effectiveness of the friction material? And the contact area of the pad? A huge grippy brake pad would need to be pressed less hard against the disc to generate the same braking force as a smaller alternative. The fluid pressure in the line would be lower, with the rear brake shoes also being applied more lightly irrespective of wheel cylinder bore size. I am definitely going for an adjustable bias valve. My cylinders at the moment are 3/4" but as I'll be running fiesta calipers (with huge pads) hoping this will end up reasonably well balanced.



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Old 30-04-2018, 12:28 PM   #8
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Fluid is "uncompressible" that says it all really,
I would just fit a rear control valve and slow the rears up,
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Old 30-04-2018, 03:38 PM   #9
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Harley has suggested the heavy Honda engine is potentially the issue. I have to agree. The engine is mounted forward of the axle and wheel so means the weight of the engine is almost entirely over the front wheels and in turn will lift the rear slightly. The fronts will obviously grip more and under hard braking the weight transfers to the front, making the rear do very little.



Harley doesn't say what type of brake master he's using or if the standard bias valve is being used. This could mean he's on a single line.

I also think the adjustable brake bias is the way to get this setup correctly. That's how I have done mine and works equally now.
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Old 30-04-2018, 07:13 PM   #10
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Throw some ballast in the boot, that’ll quickly tell you if it’s the weight distribution, if it is then some corner weighting will solve the issue.


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Old 30-04-2018, 08:10 PM   #11
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Quote:
Originally Posted by Yellow View Post
Great discussion and fascinating subject!




This is how I understand it too.

Extra factor to throw into the mix - what about the effectiveness of the friction material? And the contact area of the pad? A huge grippy brake pad would need to be pressed less hard against the disc to generate the same braking force as a smaller alternative. The fluid pressure in the line would be lower, with the rear brake shoes also being applied more lightly irrespective of wheel cylinder bore size. I am definitely going for an adjustable bias valve. My cylinders at the moment are 3/4" but as I'll be running fiesta calipers (with huge pads) hoping this will end up reasonably well balanced.



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I'm running MGF 240mm vented disc's and 3/4 cylinders on the rear with a bias valve wound all the way in and I can still lock the rears when I stand on the brake pedal, going to reduce the size of the rears to 1/2" or 9/16" and hopefully that should cure it from locking up.
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Old 30-04-2018, 08:25 PM   #12
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I didn't realise the honda lump was any heavier than the A,
I think it's good that the backs are working, normally we can't make them work lol
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Old 01-05-2018, 09:24 AM   #13
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Originally Posted by 250ptm View Post
When the pressure in the entire system is equalised how do does one go about as you stated: Quote “increasing the pressure to the rear cylinders which then "overthrow" and lock the rears.” unquote.
Because with a dual braking system you do not have an "entire system" you have two completely separate systems which only share a brake fluid reservoir and are operated by one pedal. That's the whole point of a dual circuit brake system, if one fails the other keeps working by virtue of them being totally separate.

You have to think of it as two separate systems (as if it had two separate master cylinders) but being operated by one pedal, like this:-



Look how the bias works on this, you adjust the bar which alters the leverage of the pedal relative to each cylinder, each adjustment increases pressure on one and reduces it on the other which alters the pedal pressure to each separate circuit. You cannot adjust this on a standard Mini system so if you alter one circuit (eg fit calipers with different fluid capacities to the front) it will have a knock on effect on the other circuit which, because being that it is completely separate, cannot equalise the fluid as you suggest. It is simply being operated by a pedal that is having to move that slight bit more to compensate for the extra capacity at the front calipers hence the "overthrow" at the rears, they are trying to be pushed out slightly further by the extra pedal throw but because they can't as the shoe has contacted the drum they can only increase the pressure applied by the shoes to the rear drums causing them to be more likely to lock.

You either have to fit an inline bias adjuster in the rear circuit or fit smaller cylinder to reduce the pressure at the rear brakes.
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Old 01-05-2018, 01:21 PM   #14
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I think the issue is not pressure, but flow. If you want to move the 4 pot caliper pistons, this will take more fluid movement than a single pot caliper. Having a large bore rear needs quite a bit of fluid to move it. Reducing the cylinder size will increase the amount the piston comes out? Am I wrong?
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Old 01-05-2018, 02:57 PM   #15
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Quote:
Originally Posted by AGoaty View Post
I think the issue is not pressure, but flow. If you want to move the 4 pot caliper pistons, this will take more fluid movement than a single pot caliper. Having a large bore rear needs quite a bit of fluid to move it. Reducing the cylinder size will increase the amount the piston comes out? Am I wrong?
Yes although it seems totally counter intuitive because you think of putting your finger over half of a hose pipe and the water comes out with much more force so why does this not happen with the brake cylinders, right ?

My understanding is that by reducing the cylinder size you are in effect reducing the size of the piston that is pressing on the shoe so it in turn reduces the force that is applied to the shoe. It's a bit like a small engine will produce less power/torque than a larger version of that engine. In a large engine you can get more air and fuel in so the resultant pressure on the pistons produce more "push" in the piston resulting in more torque and thus power, the smaller the engine the smaller that "push" is. Similarly with the brakes, the bigger the cylinder the more "push" the fluid can exert on the brake shoe and conversely the smaller the piston the lesser that "push" is.
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