# Hydraulic force question

• posted
Watching "Classic Car Restoration" the host made the statement that the
brake caliper on the restoration car (1969 Alfa Romeo) had twice the force
on the rotor (compared to single piston calipers) because it has a dual
opposing pistons, one pushing on each of the brake pads.
Is this a myth, like 2 cars, 50 mph each traveling at each other, creating
an equivalent collision of 100 mph? (In reality it is equivalent to one 50
mph car slamming into an unmovable (50mph reaction) object, e.g., concrete
wall, 5' diameter oak tree, etc.
For example, if a single piston caliper is able to apply 500lbs. of push,
there would have to be 500lbs. of resistance from the other side of the
caliper frame. Total 1000 lbs. 'squeeze'. All things being equal, if in a
dual piston caliper, with each piston pushing with 500 lbs. force, doesn't
the rotor experience the same amount of friction against turning?
Where is my thinking wrong?
Thanks for replies,
Ivan Vegvary
• posted
here is an easier way to think about it. lets say there is 100 psi pressure in the brake line. if we double the size of the piston, we double the pressure on the brake pads. if we have 2 piston calipers, and convert to 4 piston, but the total area is the same, we have the same pressure squeezing.
HOWEVER, with 4 pistons, we might have more consistent, and even pressure. Which means we could get a higher clamping force before the brakes lock up.
• posted
"Ivan Vegvary" wrote: (clip) Where is my thinking wrong? ^^^^^^^^^^^^^^^^^^ Your thinking is not wrong. How much pressure is a diver subjected to, considering that it is being exerted from all angles? Having two hydraulic cylinders pressing on opposite sides of the brake disk is a way of eliminating deflection without making the disk super heavy. Also, the way the caliper is constructed, the opposing forces balance each other, so you don't need a super heavy bracket.
• posted
Yes, a lighter bracket makes a lot of sense. Ivan
• posted
People say the darndest things. The only thing dual pistons do is to keep the whole caliper from moving. The forces on each pad are the same. Its the same as pushing against the wall or pushing against another person, the pressure is the same.
John
• posted
There's no deflection of the disc. The caliper floats, providing the pins aren't corroded.
Garrett Fulton
• posted
I think you are forgetting there are two reactions, the force of the rotor against the near brake pad and the force of the other side of the rotor against the brake pad on the other side, translated through the frame of the caliper. In this case it is the rotor that supplies the equal and opposite force. The force of a single piston would be distributed on both sides of the rotor, assuming no friction on the sliding mechanism. A second piston of the same size would double the "squeezing force", however the fallacy in the commentator's statement lies in the fact that the dual piston design would use correspondingly smaller pistons. The reason for the design is to make sure force is equal on both sides without relying on the sliding frame, not to double the braking force, as another poster pointed out.
• posted
"gfulton" wrote: There's no deflection of the disc. The caliper floats, providing the pins aren't corroded. ^^^^^^^^^^^^^^^ There is no deflection of the disk with caliper brakes. What I was trying to say is that if, instead of a caliper, you had a piston pushing on one side of the disk, there would be a deflection.
• posted
You can think about an experiment that will give you a clear anser:
You have one caliper with one piston that pushes onto the disk with say 1000N. Now you take a second caliper with the same piston diameter and slip it over the disk. You will get double the force, no matter on what side of the disk the piston of that second caliper is. You can now make a single caliper out of the two, without changing anything compared to having two calipers.
Nick
• posted
"Ivan Vegvary" wrote in news:RVcDf.7671\$Cf7.7003@trnddc06:
Force = Pressure x Area
So, If you have 1 1.5" piston, and a supply pressure of 500 PSI, the force is 500x1.7671 = 883.55, now divide that by 2, since half the pressure will go to the other side, and you have 441.775 lbs of clamping force on each pad, minus any friction losses in the caliper slider. Now, say you have 2 opposed 1.5" pistons, and the same 500 PSI supply pressure, the force is still 883.55 lbs per piston, but you do not divide by 2, since the cylinders are opposed, thus your clamping force is now 883.55 lbs per pad and you do not subtract anything for caliper slider friction. So, yes, the host is correct.
• posted
People say the darndest things. The only thing dual pistons do is to keep the whole caliper from moving. The forces on each pad are the same. Its the same as pushing against the wall or pushing against another person, the pressure is the same.
........ But the brake pad area is doubled. Twice the stopping force, which is what we're talking about. Bugs
• posted
At least absent context, this sounds incorrect. Doubling the area w/o changing the normal force would not change the (dry) friction force. Don't buy it? Remember that doubling the area (again keeping the normal force constant) results in a lower pressure between the surfaces.
Bill
• posted
No.... there is a "passive" pad on the non-cylinder side. Where have you seen a caliper brake system that had only one pad pressing on only one side of the disc?
LLoyd
• posted
When the caliper float is essentially becomes a double ended piston. So your extra 833 pounds is imposed on the closed end of the piston, forces the entire caliper against the rotor from the other side. Host was incorrect.
Anth> "Ivan Vegvary" wrote in
• posted
The great majority of OEM disk brakes are single piston and there is no deflection of the disk because the caliper body is free to move sideways on lubricated slides.
Randy
• posted
Not if in making the single caliper out of two you effectively put the cylinders in series.
The simple question would be, will the system still work if the pressure line to one cylinder is removed and capped, leaving the cylinder vented but preserving system pressure for the other one?
• posted
Ummm..No.
Remember ol' Newton said about equal and oppostite reactions? With only 1 piston to force is as you describe. The first force is from the piston, and since the disk is free to move axially on the shaft, the reaction comes from the solid leg of the caliper opposite the piston (with the disk between the piston and the leg). Now if you replace the caliper leg with another piston, the force doesn't change in magnatude. Both pistons are "putting out" 882 lb (in your example) and each is reacting the force from the other.
Think of it like this: Stand with you hands against the wall. Move your legs back away from the wall until you will surely fall over if not for leaning against the wall. Feel the force in you hands? It's coming from your body weight. Now repeat this except lean against someone elses' hands (kind of like forming an arch for the kiddies after a soccer game). Did the force you feel in your hands double? After all, the force comes from your body weight (like the wall experiment) as well as the other person.
• posted
actio = reactio
Now if you replace the wall with a leaf spring (the wall is actually a very stiff one), it will deflect with one person pressing against it. If you put the leaf spring between the two hands, it will not deflect, because equal force is coming from two opposite sides. -> Double the force.
Nick
• posted
So, Nick, try this: Put a brittle spring between your hands (I suggest an egg). Press both hands together real hard. According to your note, the egg will not break (deflect) "because equal force is coming from two opposite sides".
• posted
You are right. There is no force advantage to opposing cylinders.

PolyTech Forum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.