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[Brompton] How does a dual pivot brake caliper work?

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[Brompton] How does a dual pivot brake caliper work?

Old 04-09-20, 01:25 AM
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Winfried
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[Brompton] How does a dual pivot brake caliper work?

Hello,

I had the opportunity of riding a brand new Brompton, and noticed its brakes worked a lot smoother than those on my 2014 Brompton, which uses the same dual pivot brake calipers.

Wondering if it'd do anything, I dissassembled the front brake to add a bit of grease to the central + right-side screws… but it doesn't seem to do much. It might be the cable + housing (which I changed a couple of years ago) or the brake lever.

But anyway, it got me wondering:
1. How does a dual pivot brake caliper actually work?

When pulling the lever, it pulls the left-side arm toward the rim as shown in 1 (and thanks to the spring, it returns to its original position when releasing the brake lever), but what about the right-side arm? Does it move thanks to the pressure applied by the housing, as shown in 2?

2. As the brake pads wear out, I understand we're supposed to loosen the barrel adjuster next to the brake lever so as to push the housing towards the caliper just a fraction of a millimeter… but how does it work? If it pushes the right-side arm toward the rim thanks to the housing (as shown in 2), what about the left-side arm? How does this other guy move towards the rim?

Note: left/right as seen while sitting on the bike.

Thank you.


Last edited by Winfried; 04-09-20 at 01:29 AM.
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Old 04-09-20, 06:09 AM
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dabac
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The technical name for the cables that operate mechanical shifters and brakes is ”Bowden cable”.
The way a Bowden cable works is that the sheath/outer cable/casing will push just as hard as the inner cable pulls.
So the basic operation of the brake is that pulling on the lever pinches one end of the brake arms together, causing the other ends to pinch around the rim.

And due to this push/pull symmetry, it doesn’t matter if you shorten the inner wire, or lengthen the casing. Both actions will bring the pad ends of the brake arms closer together.
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Old 04-09-20, 07:00 AM
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Thanks.

https://en.wikipedia.org/wiki/Bowden_cable#Housing

How does the right-side brake move towards the rim?

Likewise, when loosening the barrel adjuster, how does the left-side brake move towards the rim? I can understand the casing pushes against the right-side brake as shown in 2, but what about the other part?

I can't figure it out.
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Old 04-10-20, 07:35 AM
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I think I got it: The two parts of the caliper are connected through the centering screw, the tiny bolt that requires a 2,5mm allen key.

1. Pulling the brake lever pulls down the left-side part; The right-side part moves after being pushed by the casing going up.

When releasing the brake lever, the left-side part moves away from the rim because of its spring, which also moves the right-side part thanks to the centering screw pushing down on the right-side part.

2. Loosening the barrel adjuster on the brake lever moves the non-compressing casing against the right-side part, moving that part towards the rim; As a result, thanks to the centering screw and the spring, the left-side part also moves towards the rim.

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