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Originally Posted by cycleguy55
Nope. Overly simplistic explanation that doesn't account properly account for opposing forces.
Does anyone have any solid engineering rationale as to the use of only one side of a caliper in calculating piston area?
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Brian,
Let me take a swipe at this…
We all recognize that when we exert a force on an object, if it remains static, i.e., doesn’t move (accelerate) in the direction of the force being applied, then there is an equal and opposite force being exerted on that ‘something’ which we are pushing/pulling on. That equal and opposite force not only equals the force we are exerting but, because of its direction, cancels the force we are exerting.
Let's say the force we exert is 5 pounds, and the counteracting force is also 5 pounds, but the direction of the force is precisely opposite ours — the result is no movement. If you exert a force of 5.1 pounds on the same object that is pushing back at 5 pounds, we will begin to move the object in the direction of the force we are applying.
In the case of a brake caliper, that would cause the pistons with the lower clamping force to be pushed into their caliper side. So why is the clamping force not 2X the observed caliper clamping force of just one side?
I will use another example we are all familiar with to answer that. Earth’s gravity exerts an attractive force on all matter. That gravitational attraction/force creates the phenomena we refer to as weight. We can measure this metric by stepping on a bathroom scale. When we do, we can see what our ‘weight’ is. Think of it as a measure of the attractive force exerted by the planet’s mass on our body.
To avoid being plastered flat against the planet's surface, our body’s muscles push against that attractive force with an equal and opposite force allowing us to stand upright. The bathroom scale measures that force in pounds. To remain motionless and upright, our feet and legs need to push back on the planet’s surface with an equal but opposing (opposite direction) force.
So, the pregnant question becomes when we stand on the bathroom scale, which says 200 pounds, do we weigh 200 pounds or 100 pounds? Is that apparent second 100 pounds from gravity acting on the mass of our bodies? We all know the answer, we weigh 200 pounds, and no less an authority than our wives has confirmed it! Not just the 200 pounds — but the fat part also.
Kidding aside, the attractive force of gravity and the muscular force applied by our legs to remain upright are exactly equal and cancel each other out. Still, the scale only shows 200 pounds, not 400, and we definitely do not weigh 100 pounds. The same phenomenon occurs in the brake caliper.
As someone has already observed, the benefit of multiple pistons is to enhance the brake pad’s ability to conform to irregularities in the surface of the rotor, optimizing surface contact with the rotor for better braking performance. By using pistons on both sides of the rotor, we extend this braking performance enhancement to both the front and back side of the rotor. Significantly we do not increase the clamp load on the disc any more than gravity increases our weight when we step on the bathroom scale.