What force causes a brake piston to retract, when you release the brake??

I've never understood this, and basically have just ignored it. On cars and motorcycles, the pad rides slightly on the rotor even when you release the brake. This slight amount of drag is inconsequential. However, ANY rubbing on a bike would be annoying at the very least; and, indicates something is out of adjustment. So....when you engage the hydraulic brakes, the pistons are extended, thus pinching the rotor with the pads. When you release.....what force retracts the pistons/pads slightly so they no longer contact the rotor??

 

I'm guessing the pressure produced from the lever when the brakes are applied causes a vacuum and when released, it sucks the fluid back along with the pads. But what do I know lol its probably magic

 

Wikipedia says- " Because the "motor" is small, an uncommon feature of bicycle brakes is pads that retract to eliminate residual drag when the brake is released. In contrast, most other brakes drag the pads lightly when released."

huh? "because the motor is small"? Maybe thats' the size of the stroke? But how does that cause a pad to retract? Confused.

 

Not only is it magic but it's Black Magic! Spooky stuff if you ask me.

Subsequent release of the brake pedal/ lever allows spring(s) to return the master piston(s) back into position. This relieves the hydraulic pressure on the caliper allowing the brake piston in the caliper assembly to slide back into its housing and the brake pads to release the rotor. The hydraulic braking system is designed as a closed system: unless there is a leak within the system, none of the brake fluid enters or leaves it, nor does it get consumed through use.


shitty

 

Still don't see it. Releasing the pressure shouldn't make something move the other direction. It just takes the pressure off. What makes the pistons actually move backwards so the pad is now not in contact with the rotor? On a car or motorcycle, this doesn't happen. On a bike it does (or should). How?

 

The lack of force from the hydraulic piston allows the little springs to slightly separate the pads from the rotor.

 

They move backwards due to drop in pressure in the system.

Maybe think of it if you heat up your lunch in a sealed plastic tupperware. In this case the heat excites the vapor molecules creating more pressure in the system. And you'll see your tupperware lid bubble up. But as it cools, pressure will drop and the lid will return to it's normal shape. If put the hot food in the fridge in a sealed tupperware, you eventually see the lid suck down from the drop in pressure creating a slight vacuum.

 

Your answer is here: http://www.mtbr.com/discbrakesfaqcrx.aspx

The makers of the brakes have to contend with three things that happen to all brakes - pads need "retracting" after use, pads wear and fluid gets hot and expands. Here again they use methods that are the same as in the family car. They use an ingenious method of pad retraction and self-adjustment; for expanded fluid they use a reservoir.

Retraction works like this - the caliper piston is sealed by a seal with a rectangular cross section. When the piston in the caliper is applied this seal tends to "stick" to the piston and it deforms sideways as the piston moves. When we let off the pressure in the brake, the seal wants to regain its old shape and it retracts backwards and drags the piston with it! Very ingenious! Here then we have pad retraction without the use of springs.

 

Now your'e just making me hungry!


shitty

 

Aha! I never thought of anything like that. Well that answers it, and probably also answers why my 7 year old brakes are becoming more and more a PITA in terms of finding a way to tune them so that the pads won't rub. I am betting the rubber seal is not as elastic as it needs to be, thus it isn't creating the "pull back" friction required upon releasing the brakes.

Thanks! Good info!!

 

along with what was said above, the spring in the picture below keeps the pads away from the rotor just enough to keep it from the annoying rubbing noise.

The spring is definitely something that is missing on cars/trucks, and motorcycles.

 

Yes, it is the seal that retracts the piston.
For example, if you are running DOT 4 fluid and top it off with DOT 5 then over a period of time the pistons will stick.
This is not something that I just made up but when I took over a fleet of ambulances, the commercial shop that was working on then did exactly this.
I ended up chaning a lot of calipers and master cylinders so that they could run DOT 5. Many of the ambulances did end up loosing their brakes before I finished converting the fleet.
No issues were ever came of the loose of brakes but it was not fun. I was bring in a unit to be converted and noticed that I just lost the brakes while exiting the freeway. Go thing there was no traffic in front of me.

 

Oops, the pads I've been buying the last couple times didn't come with spring, and thus I've been going without.

 

if they would have gone with DOT5.1 the issue above would not exist. DOT 4 is not compatible with DOT5 fluid nor the seals compatible with the silicone base fluid of the DOT5. DOT5 fluid would just swell and soften the seals to the point that they can no longer hold pressure.

 

There is a spring in the master cylinder that retracts the main piston in the lever. When you release the lever, the main piston is pushed back by the spring, creating a vacuum, retracting the caliper piston. The pads themselves, which are now loose, are pushed apart by the pad spring as shown above.

 

Herein lies the problem with forums: everyone has an opinion, but very few actually know the facts.

Jeff is the one who wins the prize. Max, you get runner-up for your observation about aged seals. Another key feature of this system is that as the pads wear, the piston can slide outward through the seal, making it self adjusting.

 

oh gawd.

the spring force of the caliper seals are not strong enough to push the entire system back. they neither have enough movement or force. the main piston has to retract, the seals help the caliper pistons retract and the pads are not connected to the caliper piston, nor the seals, so they wont retract due to either factor. its the pad spring that pushes them into the caliper pistons. less you believe they get sucked in by the seals.

its a system. they work together.

the self adjusting part is also a system, the caliper pistons can "float" on the seals, and the reservoir fills in fluid into the system as needed.

seals



main piston

 

Ding, ding, ding we have a winner!

Wrong and wrong.

 

dangit, beat to it....

Original reply below:

you would need a pretty heroic set of piston seals/pad spacer springs to return the caliper pistons...(seals and flimsy springs are not going to open the pistons) http://www.amazon.com/Fundamentals-...25/ref=sr_1_16?ie=UTF8&qid=1337728736&sr=8-16 your answers are there.

quick explanation:

You squeeze the lever and move the master cylinder piston. You MC piston is much smaller than your caliper piston so you get increased force and decreased displacement at the caliper. There is a spring on the backside of the MC piston that pulls it back to its neutral position of "brake off" when you release the lever. When the piston returns it pulls the fluid with it which pull the caliper pistons with it as well, the effect of the spring is enhanced in the same way application of force at the lever is. Your brakes "self-adjust" because there are small ports in the MC that get covered/exposed at specific points in the MC pistons travel so as maintain a constant distance between the rotor and pistons by allowing the system to suck in more fluid to make up the wear in the thickness of the pad material. That's why you have a reservoir.

Bing! knows what up.

to me, it sounds like Right and Wrong, he even provided an example in his post...

 

Wait a minute here.
The original question was what foce causes the brake piston to retract.
Most hydraulic braking systems work by releasing the pressure and using the seals to retract the piston so there is no force applied. This leaves the pads floating on the rotor.
For mtb's it sounds like a spring is added to actually remove the pads from contacting the rotors.