Hydraulic Disc Brake Argument...

[CurtisRider]

Mount it on the front like on the new pashley forks?

[Clawz114]

So is the conclusion;

No it would not be twice as powerful?

What is the reasoning for this?

[forteh]

IMHO it would be twice as powerful, you have twice the friction area to act upon. You would need to use a longer stroke master cylinder to push the extra oil but thats of minor importance.

[craigjames]

The only people that used to run twin disc set ups on a DH bike were shimano who had a prototype twin disc that fitted in a widened calliper with a pad between the two rotors. I think Greg Minaar used to run it for a period of time when he was at Global racing. I can remember reports of it being too grabby and powerful, with GT85 being the solution....

I think if hope could sort out the consistency and reliablity issues with the mono trials brake it would be more than adequate for 99% of riders.

With the twin calliper set up you're talking about, surely you would have to have a huge master cylinder/fluid resevoir to account for the larger amount of fluid being moved? Otherwise you'd have a stupidly spongey brake!

[AdamR28]

Twice as powerful.

prototype twin disc that fitted in a widened calliper with a pad between the two rotors.

Dammit, I wanted to do that! Would be bloody beast for trials...

PS: Marzocchi Z1 Bombers from 97 had two disc mounts (one on each leg), I saw a bike with twin front discs in MBR, anyone remember what it was?

[Greetings]

Twice as powerful.

Exactly, the power of a brake depends on the ratio between the master cylinder and the slave cylinder. If you double the slave cylinder area, you're going to get twice the power. The only difference is that you're going to have to pull the lever much further to get any effect.

[Al_Fel]

Twice as powerful.

Dammit, I wanted to do that! Would be bloody beast for trials...

PS: Marzocchi Z1 Bombers from 97 had two disc mounts (one on each leg), I saw a bike with twin front discs in MBR, anyone remember what it was?

And here it is...

[AdamR28]

Haha, dammit, I was only 9 years too late

That with a tiny (120mm, 140mm) rotor on a trials bike... awesome.

[Clawz114]

Sorry, i should have specified in detail a bit more.

The lever would not be a special modified lever, it would just be a standard lever which is meant for a single caliper.

I can't understand how it would be twice as powerful...

If you had 10 bikes lined up, with all the disc brakes connected to the same lever, each piston would be moving 10 times less than it usually moves.

Are you telling me in this scenario it would have the power of 10 calipers?

[forteh]

Beringer do a quad disk setup in the same arangement, uses 4 x 230mm disks, lighter, better braking and far less inertial forces affecting the steering

Edited August 20, 2009 by forteh

[AdamR28]

Are you telling me in this scenario it would have the power of 10 calipers?

Yes: http://en.wikipedia.org/wiki/Pascal%27s_principle, http://en.wikipedia.org/wiki/Hydraulic_press

[forteh]

Sorry, i should have specified in detail a bit more.

The lever would not be a special modified lever, it would just be a standard lever which is meant for a single caliper.

I can't understand how it would be twice as powerful...

If you had 10 bikes lined up, with all the disc brakes connected to the same lever, each piston would be moving 10 times less than it usually moves.

Are you telling me in this scenario it would have the power of 10 calipers?

Of course you need to increase the amount of fluid displaced by the master cylinder, if you change the piston diameter then it will affect the power, you will need to increase the stroke of a standard master cylinder to run 2 calipers. Alternatively you could use a single fixed pad and a moving pad (as per a BB7) and just use 2 caliper cylinders of the same size as original, that way you could use the stock lever and still retain double power from the twin disks

[Clawz114]

I can't understand why you would think it would be slightly more powerful just because you have twice the pads (twice the area). I mean Monty used to sell 35mm magura pads and they were just as powerful as regular 50mm pads because smaller / bigger pads apply the same amount of force per unit of measurement. (Eg. Pushing a nail and pushing a scaffolding pole into a wall with the same force, the wall is going to feel the same force being applied just over different areas, hence why the nail would probably go through the wall and the scaffolding pole wouldn't make a dent)

So, to begin with they would be exactly the same.

Looking into it a little more there would be less heat created which technically would make the brake a insignificantly more powerful, however on the downside you have all that extra weight which in a round-a-bout way make it less powerful, also you'd have a lot more of a spongy feel due to having more hosing and all that extra friction in the twice as many pistons.

From a riding point of view it'd be exactly the same but would make the front end significantly heavier.

I never knew that...

JT! You appear to be correct on the friction thing.

So taking into account, that friction is the same, regardless of area. Isn't having 2 calipers with 4 pads, the equivalent of having 1 caliper with 2 pads that are double the size each?

[::CH!P::]

Just buy the Clim8 Six Pot disc that i used to have on my T-Pro Was a beast of a brake... wish i still had it

[LukasMcNeal]

I think someone (not me) should just try it for a laugh!

[sstein]

I never knew that...

JT! You appear to be correct on the friction thing.

So taking into account, that friction is the same, regardless of area. Isn't having 2 calipers with 4 pads, the equivalent of having 1 caliper with 2 pads that are double the size each?

No, you would need pistons and pads that are double the size, hence the fact that 4 pot brakes are a lot more powerful than the equivelant 2 pot brakes and cars use 6 pot brakes!

[Clawz114]

No, you would need pistons and pads that are double the size, hence the fact that 4 pot brakes are a lot more powerful than the equivelant 2 pot brakes and cars use 6 pot brakes!

But we are talking about the same amount of force?

because smaller / bigger pads apply the same amount of force per unit of measurement

Surely if you are pulling the brake lever the same amount on each setup, its applying the same amount of force, which means 4 pads (double the pad area) is exactly the same as 2 pads?

*wishes JT! would bring some more input in, as he seems to know a lot about friction/force and stuff *

[PaRtZ]

But we are talking about the same amount of force?

Surely if you are pulling the brake lever the same amount on each setup, its applying the same amount of force, which means 4 pads (double the pad area) is exactly the same as 2 pads?

*wishes JT! would bring some more input in, as he seems to know a lot about friction/force and stuff *

Say you have a master piston size of say (because I dont know) 15mm diameter. This pushes ONE piston of diameter 15mm similar to how a BB7 works. This gives a master/slave ratio of 1:1, obviously. Now if we say the 15mm master pushes TWO 15mm diameter pistons, this can be equivalent to a single piston of a larger size (NOT 30mm diameter btw) and thus gives a greater force applied to the pads/rotor

1.76714 * 10^-4 : 1.76714 * 10^-4 Is one master to one slave

1.76714 * 10^-4 : 3.53428 * 10^-4 is one master to two slaves. This is equivalent to a 15mm diameter master to ONE 21.21mm slave

Now I think you've grasped this bit, its for those that hadn't. What you're saying is if you had one slave or two slaves, if the pads were the same size, why is there a difference in friction/force?

Well first of all, don't confuse force with pressure:

Pressure = Force / Area

Force = Pressure * Area

Hydraulic fluid exerts pressure onto a piston surface area, which creates a force. So the piston pressure is the SAME in the same sized 4 pot/6 pot brakes, but the fact that there are more pistons, this INCREASES the area and as you can see from the formula, INCREASES the force.

So, although you're pulling on the lever with the same force, you creating the same pressure in the fluid, which creates the same pressure on the pistons, BUT because there are more pistons/larger piston area, you're creating a larger force!

Hope that helps

[JT!]

Say you have a master piston size of say (because I dont know) 15mm diameter. This pushes ONE piston of diameter 15mm similar to how a BB7 works. This gives a master/slave ratio of 1:1, obviously. Now if we say the 15mm master pushes TWO 15mm diameter pistons, this can be equivalent to a single piston of a larger size (NOT 30mm diameter btw) and thus gives a greater force applied to the pads/rotor

1.76714 * 10^-4 : 1.76714 * 10^-4 Is one master to one slave

1.76714 * 10^-4 : 3.53428 * 10^-4 is one master to two slaves. This is equivalent to a 15mm diameter master to ONE 21.21mm slave

Now I think you've grasped this bit, its for those that hadn't. What you're saying is if you had one slave or two slaves, if the pads were the same size, why is there a difference in friction/force?

Well first of all, don't confuse force with pressure:

Pressure = Force / Area

Force = Pressure * Area

Hydraulic fluid exerts pressure onto a piston surface area, which creates a force. So the piston pressure is the SAME in the same sized 4 pot/6 pot brakes, but the fact that there are more pistons, this INCREASES the area and as you can see from the formula, INCREASES the force.

So, although you're pulling on the lever with the same force, you creating the same pressure in the fluid, which creates the same pressure on the pistons, BUT because there are more pistons/larger piston area, you're creating a larger force!

Hope that helps

I'm still having a hard time understand why it would make the brake twice as powerful.

Are you saying that a 35mm magura pad is going to have less stopping power than a 50mm pad? Becuase i was always under the impression that they create the same amount of friction (one as more serface area, but the other will (i'm not sure what the right word is) apply more push to the rim. I figured the differences would equal each other out so that it wouldn't make a difference on pad size.

So i'm I wrong about that or right?

I understand what you're saying about one 15mm piston would give you a ratio of 1:1, but having two 15mm pistons would give you a much bigger piston, but that would mean you'd have to move the leaver twice as much, so if you were to run two hope mono trials one one leaver, the leavers open system would correct that so that the leaver wouldn't be pulling twice as much, so if the leaver isn't pulling twice as much, the breaking power would be the same as running one.

That's where i'm coming from, i'm not saying it's right though.

[Clawz114]

I'm still having a hard time understand why it would make the brake twice as powerful.

Are you saying that a 35mm magura pad is going to have less stopping power than a 50mm pad? Becuase i was always under the impression that they create the same amount of friction (one as more serface area, but the other will (i'm not sure what the right word is) apply more push to the rim. I figured the differences would equal each other out so that it wouldn't make a difference on pad size.

So i'm I wrong about that or right?

I understand what you're saying about one 15mm piston would give you a ratio of 1:1, but having two 15mm pistons would give you a much bigger piston, but that would mean you'd have to move the leaver twice as much, so if you were to run two hope mono trials one one leaver, the leavers open system would correct that so that the leaver wouldn't be pulling twice as much, so if the leaver isn't pulling twice as much, the breaking power would be the same as running one.

That's where i'm coming from, i'm not saying it's right though.

I have to say, JT!'s explanation sounds right imo.

But whatever the answer is, its obviously a lot more confusing than i thought it would be

[JT!]

Well if Adam says it's twice as powerful, it's twice as powerful, he's got god knows how much experience and an engineering degree, I've got 3 years working at Cost Cutters. I'm just questioning what'll happen to the leaver as it'll be pulled. I would have assumed that the open system would put the pads twice as close to the rotor that it would with one, the leaver pull would be the same as if it was connected to one, the power would be halved over each brake totaling the same stopping power as if it was connected to one.

But then again i don't really know how open systems work, anyone know? I figure this is what is up for debate now as it'll answer the original question.

God I love these threads.

Edited August 21, 2009 by JT!

[Fish-Finger-er]

shimano trialled this on a dh/all mountain bike several years ago, with tales of them putting grease on the rotors, to stop the brake locking up instantly, its in a very very old skool edition of mbuk.

[psycholist]

I can't believe this thread is still alive. The comments above for the pressure - force calculations should explain what's going on perfectly. The lever will have to pump a little more fluid before the brake bites (So more lever travel), and the lever will feel a bit spongier thanks to twice as many flexing parts (Between the brake line, pads, pistons and seals) but if the force applied to the lever at bite point is the same a double caliper setup will be twice as powerful as a singe caliper for the same force applied to the same lever.

Now to cause more trouble - While opposed piston designs are the most common setup (Done to reduce brake drag and allow the pads to adjust themselves automatically for pad wear without relying on a floating caliper), a 2 pot brake with opposed pistons will be a lot less powerful (Half as powerful assuming each piston acts on the same pad area and the fixed pads are the same as those acted on by the pistons) than a two pot brake with the same diameter pistons where both pistons are on one side and there's a fixed pad on the other. How does this work?

On opposed piston brakes while each piston creates a force based on the pressure in the fluid and its area, the force each piston applies is balanced out by the opposing piston. In the case of both pistons on one side of the caliper, each piston exerts the same force, but the forces are in the same direction and are opposed by the fixed pad on the opposite side of the caliper rather than each other, so effectively there's twice the force squeezing the pads into the disk.

Have a look at the Magura Gustav (Probably the most powerful brake I've ever used) for an existing design that proves this principle...

Edited August 21, 2009 by psycholist

[OD404]

And the laws of physics say:

Twice as powerful

[pogo]

I'm sure someone did this with maguras on a set of forks that had mounts on both sides, the pads had to be set really close to the rim but it worked stupidly well