[Dropspeed]

Originally Posted by Nick S

The info pertaining to the large and small piston (which I highlighted in bolt red text above) is great info Matt. Thanks for sharing the importance of this design attribute! Many people think engineers just do stuff to be difficult but there is always a valid reason why something is done. As a mechanical engineer myself I am often frustrated by people that think the engineers did something without having a good reason to.

To expand, I was told if they are reversed this will lead to increased pedal travel due to high taper wear.


Found this on the Web that explains it pretty well.

Brake Pad Wear

Getting your car's brake pads to wear evenly is worthwhile. It both increases the longevity of the pad, and improves the "pedal feel:" the sense of firmness and response you get when you use your brakes.
Why pads wear unevenly

Disc brake systems operate by turning the kinetic energy of a car in motion into heat. Brake pads are pressed against iron rotors attached to the car's wheels. There is friction between the pads and the rotors, and this friction causes the car to slow as its momentum is dissipated as heat in the pads and rotors. Over time, this friction causes brake pads to wear. When enough of them has worn away, they need to be replaced.
Why would pads wear unevenly? The areas of a brake pad called upon to do more work, or which run hotter, will wear away faster. Although pressing a pad against a rotor may seem to distribute the load fairly evenly, it does not.
Uneven wear occurs in roughly three different forms:

• longitudinal taper,
• radial taper, and
• inner vs. outer pad depth.

The leading edge of a brake pad is the one that may be thought of as first touching the rotor as it turns. If your calipers are mounted behind the wheel axis, the bottom pad edges will lead. The friction of the pad against the turning rotor causes the pad itself to try to rotate about an imaginary axis behind it, with the effect that the leading edge of the pad presses harder against the rotor, doing more work and getting hotter. This uneven heating leads to longitudinal wear: uneven pad depth from leading to trailing edges.

Radial taper wear can be seen as a difference in pad depth along the radius of the rotor. A brake caliper is mounted to its strut on one edge. The forces created by applying the brake pedal cause yet another twisting moment, this time of the caliper assembly around an axis through its mounting points. There are other factors as well: a square-ish pad against a round rotor means that the radially inner part of the rotor will spend proportionally more time under the pad, and less in cooling air. Heat differences also exist because the inner edge of the rotor interfaces with the hub rather than air, limiting conduction.

Differences in wear between the inner and outer pads arise for two different reasons. The first is a car's tendency to "toe out" under breaking, that is, for the front half of a tire to move outward, the rear half inward. Because of this tendency, we compensate by giving cars a little toe-in in their alignment. This toeing places a little more load on the inner pad in a caliper mounted behind the axle.
The second reason for inner/outer wear differences has to do with floating caliper designs, such as used in the 944 and 944S. If the floating half of the caliper does not move absolutely freely, the fixed half's pad does a little more work. By the way, this is the reason 944's with a single pad wear sensor use it in the inside pad.

What happens when they do

Uneven wear from inner to outer pad will not itself have any effect on braking performance on the street -- unless you run out of inner pad altogether! Tapered wear, the other two kinds, will make it necessary for the pads to move greater distances when the brakes are used, meaning more brake fluid transfer, meaning longer pedal travel. This pedal travel by itself degrades the "feel" of the brakes, and adds to the time it takes to make them effective (response time). When combined with other sources of pedal fade, such as boiled fluid or glazed pads, you can find yourself running out of brakes.
Tapered pads increase pedal travel for two reasons. Pushing a longitudinally or radially tapered brake pad against its rotor, a brake piston will "cock," that is, move off-centre in its bore in the caliper. Cocking creates a greater volume for fluid in the caliper, which causes a spongy pedal.

The other source is "knock off." When the driver releases the brake pedal, the radial and longitudinal twisting moments disappear and the caliper falls back into nominal alignment. However, because the pad is worn at an angle, the rotor will push the pad and so its piston back farther into the caliper. Because of this, the pad will have to travel more to get to the rotor next time the brakes are used, meaning more fluid transfer, meaning more pedal travel. In fact, knock off can be a problem even with flat pads because of all the knocking about the unsprung parts of the suspension take on imperfect road surfaces. It is exacerbated by tapered pads.