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I'm thinking that it provides extra tension to the brake line. Would this hinge be set in it's "down" position when installed (as seen in the picture)? If so, wouldn't that be considered a risk in the case that if the hinge flips up during rides, the cable gains slack, thus losing brake-pulling capability?

enter image description here

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    The purpose is to release cable tension so that the tire can be removed without having to use a wrench to loosen the cable, or otherwise muck with the adjustments. Jul 3 '16 at 18:35
  • These QRs can be pretty useless - Mine gives about 0.9 mm of additional space, so a fully inflated tyre still doesn't fit through without a squeeze and a twist. A flat tyre goes through perfectly, whether the QR is open or closed.
    – Criggie
    Jul 6 '16 at 1:02
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The lever pointed down is in the closed position and is how the manufacturer intended for the brakes to be used. The brake pad clearance should be set in this position. If the brake cable is routed correctly and the lever fully closed (and not damaged), tension on the brake cable will further lock the mechanism in place as it is on a slight angle causing the lever to be further pulled closed under braking tension.

The lever in the up position opens the caliper arms up wider to facilitate wheel changes.

Some people flip up the lever as a lazy way to get extra brake pad clearance when the rim gets out of true, but it isn't the best idea. That said, the brakes will still work, but are generally less effective with the lever positioned up.

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  • Braking with untrue rims makes for ineffective braking. I don't think the lever position being raised in itself leads to ineffective braking. Besides ease of wheel removal, another good reason to adjust the brakes with the lever down is so that there is margin to open the lever if the wheel does go out of true during a ride when you don't have time or tools to adjust the wheel. Jul 5 '16 at 20:45
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It's a quick release to allow the brakes to open wide enough for the tire to pass through when you are removing the wheel. I've never heard of it being accidentally activated while riding; the only danger I know of is forgetting to reset after you've put the wheel back on (in which case, as you supposed, the brake in question will either not work at all or be very feeble).

The rear brake will have the same lever, unless you have mismatched brakes, which is vanishingly unlikely.

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    I HAVE! I was riding in a downpour and my brake performance dropped dramatically. I thought it was just wet rims, but on the next trip after it dried out, braking was still dreadful. Turns out the brake QR had popped open somehow, and I had almost nothing. It was an eye opener alright!
    – Criggie
    Jul 3 '16 at 20:16
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    @criggie - You should consider whether it might have been done by a hostile or mischievous third party such as a Leprechaun, or a bicycle thief preparing to take your wheel who was scared off partway through. (I once scared off a bicycle thief, and was shocked to find my front QR untightened, even though the front wheel was locked to a rack with a U-lock). Jul 5 '16 at 20:55

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