Disc Brake - force

[GDT]

Hey guys.

The force applied by the brake pad to the rotor equals the force generated by pressing the brake pedal and the booster....there would be some torque effect with different diameter rotors.

But for a specific size rotor the number of pistons or the contact area of the pads has no bearing on the total force applied...
ie a single piston =100% force applied, 2 pistons = 50% force applied by each, etc.

Question, if the above is correct, then why do multipot brakes work so well even in day to day driving?

[Airmon]

I would think break pads work by friction force. Greater surface area = greater friction (well not greater friction but a larger effect of it).
Here force isn't necessarily as important as contact area, in which case multiple piston callipers would spread the force over a greater area. If you had 1 piston and a large pad, when the force is applied in the middle, the ends of the pad will pull upwards reducing contact.

My understanding at least. Am more then likely wrong

[Yellow_Festiva]

Originally Posted by GDT

Hey guys.

The force applied by the brake pad to the rotor equals the force generated by pressing the brake pedal and the booster....there would be some torque effect with different diameter rotors.

But for a specific size rotor the number of pistons or the contact area of the pads has no bearing on the total force applied...
ie a single piston =100% force applied, 2 pistons = 50% force applied by each, etc.

Question, if the above is correct, then why do multipot brakes work so well even in day to day driving?

I'm no mechanic, but I think this bit is wrong.

The force you apply to the pedal is a small fraction of the force thats applied to the rotor via the pads.

The 'booster' does that I'm assuming, as well as a series of levers that translate 1.5-2 inches of pedal movement to the less than a mm of pad movement (seeing as often pads remain in light contact with the rotors all the time it could even be less than a mm).

I will have a shot at the multi pot question. They are there to handle higher pressures. This is better used by spreading the pressure over a larger contact patch to reduce wear and lower heat.

If you applied the same pressure to a single pot brake it would kill the smaller pad in no time and cause more heat build up.

I may be right... but probably not. I dropped out of yr 11 Physics after we stopped dropping tennis balls and ping pong balls from the verandah :

[mik]

i think its also about the surface area of the caliper pistons vs diameter of the the master cylander regarding mechanical advantage, not all master cylanders have the same pressure.

[Airmon]

sorry It just hit me. I think i was pretty far off the mark.

I'm fairly certain that a fluid will transmit pressure undiminished to every point in the system. Therefore if you provide a large initial pressure, each piston in the system will apply the same pressure. So 1 piston applies the entire pressure as would 2 pistons.
The more pistons the more force applied to the pad, the greater the friction.

I had it barse ackwards to begin with. For some reason I had it in my head that friction is dependent on surface area and independent of force, just came to me its the other way around!

[tpintas]

Multi piston brake calipers work better because the pressure is distributed better along the brake pad; there is less tendency for the pad to "bend" in the middle. More of the pad pressing against the rotor = better braking.

[madmelon]

Yes, a hydraulic system will apply the same PRESSURE everywhere in the system (provided there isn't really any movement of the fluid- fluid dynamics work differently). The surface area of the caliper pistons is many many times greater than the surface area of the pedal piston. Friction works of Force, not pressure.
Frictional force (Ff) = Normal force (Fn) x Friction coefficient (μ )
(notice there is no Area factor in this- it doesn't generally play a part in the frictional force generated)

So to convert pressure into force:
Pressure or Stress (P) = Force / Area
or
F=PA

So if your caliper piston(s) have 100x the surface area then you get 100x the force applied by your foot + the booster.

The reason behind increasing piston count is to more effectively apply the force to the pad. As mentioned before, one piston applied to the middle of the pad on one side of the caliper would cause the pad to bow out at the ends as pressure is applied, reducing the effective surface area of the pad. This causes uneven wear, higher thermal loading of the centre of the pad and more opportunity for brake squeal. It also has a tendency to cause the inner pad to wear faster than the outer pad, especially if caliper slides are binding. Adding pistons to both sides of the caliper and increasing their number distributes the force more evenly over the pad and improves all the previous factors. Only disadvantage really is the substantial increase in unsprung mass for the much larger calipers required.

[YOOT]

Originally Posted by tpintas

Multi piston brake calipers work better because the pressure is distributed better along the brake pad; there is less tendency for the pad to "bend" in the middle. More of the pad pressing against the rotor = better braking.

Ignoring the physics part of the question this pretty much sums up why multi piston calipers work heas better.

You'll also notice that each piston is a differant size. The leading edge of the pad grabs the hardest as it bends so its the smallest, the the last pistin is the largest as it has to overcome the bending force and then apply braking force on top of that.