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Brake Caliper design

tris

Cruisin' Guzzisti
Joined
May 30, 2012
Messages
247
Location
Lincolnshire UK
I've just been servicing the front 4 pot caliper on my Roamer
I have discovered that there is 1 big piston and 1 small one in each half

I assume that is so that each side one edge of the pad is advanced before the other, the small one would move first

The question is why would you want to do that
 
Mechanical Engineering!

They actually move at exactly the same time and rate, but at different force.

Physics: Larger piston exerts more force at a given pressure than a smaller diameter piston.

Now why would we want to intentionally establish this distributed force across the surface of the pad?

Because of the forces of the coefficient of friction (μ) in this rotational system, the resultant pressure across the thick and large pad is greater in the leading edge and less in the trailing edge. This causes the pad to want to turn inward from the moment [A moment is the turning effect of a force.] on the leading edge, and pull away on the trailing edge, thereby causing uneven and tapered wear.

The use of a smaller diameter piston at the side where the disc enters the brake pads will ensure uniformity in the pressure distribution on the brake pads and hence, even wear of the pad.

In technical jargon: When the thickness of the brake pad is large compared to its length, there will be higher wear on the leading end of the pad. The frictional force acting on the disc contact surface and the reaction at the abutment of the pad will result in a turning moment on the pad, leading to higher wear on the leading end.

The following figure explains the formation of uneven wear, and illustrates how a smaller diameter piston at the leading side makes the pressure distribution more uniform.


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You are welcome Sir. My pleasure. I love stuff like this. One of my favorite books is this one which is highly technical but what fun it is to me!

Motorcycle Design and Technology Handbook, by Gaetano Coco (He was the Technical Director at Aprilia. A seriously smart engineer.)

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Todd (GTM) here is so lucky, because of his "Uncle Kevin". If I could, I would follow Kevin Cameron around all day to do nothing but talk and listen to him. He's a Tech God of the highest caliber and has my infinite respect !

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I shall look out for that

Kevin Cameron (looks like the same guy) writes in Motorcycle Sport and Leisure over here every month, and along with Maynard Hershons pieces, is one of the first parts I read
 
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