Does anyone know what amount of clearance is typically required IRO a vehicle brake caliper between the piston and the cylinder it slides inside of in order to press the pads against the disc? I don't know the proper technical terminology here, I must admit. Say you had a brand new, 4-pot brake caliper for example. How tightly do the pistons have to fit in their respective cylinders in order to operate correctly when the brake pedal is applied? And how smooth do the piston and cylinder surfaces have to be?
There's a huge amount of confusion over how this system actually works and I intend to dispel none of it with a few observations of my own here below. AFAIA, there are two types of rubber seal: an internal 'O' ring and an exterior boot or gaiter. They perform different jobs. The exterior boot is there to keep road dirt and water out of the cylinder. The inner seal is there to prevent any stray fluid from seeping out onto the disc/pads and imparing their functionality. Both seals also act in the manner of return springs, drawing the piston back into the cylinder when brake pressure is not applied so you're not continually wearing out your pads. You can sort of see this effect in the diagram you kindly pointed us to, NP.
I must respectfully disagree here. A close, zero-slop fit between clyinder and piston is essential to maintain the pressure in the system when the brakes are applied. This can be achieved without any rubber seals *at all* if the piston and cylinder are lapped together to produce a bright, mirror-like finish. It's not my field, but I'm pretty sure that's how it works.
'Fraid not if the piton and cylinder interface was exact at one temperature it would be too loose/too tight at another. There must be a small gap between piston and cylinder, any small discrepancy made up for by rubber seals, for the system to work .
C.F. Seals in hydraulic drive cylinders and piston rings in internal combustion engines .
The pistons and cylinders will heat at the pretty much the same rate so they will expand together. The brakes must still be able to work if the seal blows, though. This is a critical application unlike many where slacker tolerances could be permissible.
yes they will both expand away from the neutral axis which ,in a nutshell, means the diameter of the piston will increase whilst the bore of the cylinder decreases, leading to an interference.
They can still work just not very efficiently, they can stop the car but you wouldn't want to drive with a blown seal.
snip NO! If the piston and cylinder temperatures rise by the same amount, and if the metal of each has the same temperature coefficient of expansion, then they will Both INCREASE IN DIAMETER.
Thought experiment for you :- think of a long bar being heated what directions does it expand in ? Now form it into a "C" shape where are the dimension changes, now close the ends of the "C" into a circle and think about the dimension changes.
But, I don't care if you remain in ignorance so, you can think whatever you want.
The difference is that when the ends meet, that change in the length means that the internal diameter does not reduce as you claim. It has to increase by the same amount as the external length, and so the internal diameter does not reduce.
The piston isn't an exact fit to the bore. There is a rubber seal between it and the bore. A bit the same as an engine piston. It's the rings on that which provide the seal, not the piston itself.
They should both be in excellent condition. Any deep scoring or pitting can allow fluid to seep past the seal. Or corrosion etc can cause the piston to not move as freely as it should causing uneven braking.
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