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Old 07-04-11, 08:09 AM   #1
bluefoxicy
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Disc brake mounting, what were they thinking?

Sheldon said crappy QRs and no lawyer lips would lead to disc bikes ejecting their wheels upon braking.

I thought about this, and then (after running through a bunch of simulations in my head, before figuring out the relevant geometry) figured out that the placement of the brake itself affects this.

When braking, the brake becomes a pivot point for rotation. Let's say you're looking at the bike such that it's traveling left. The wheel turns counter-clockwise going forward, and will always do so. So if you brake, say, up and right from the axle, then the axle will go mainly down and forward--attempting to jump out.

This is where they put the brake.

It's easy to mount forcefully to the fork there, sure; the engineering problem of putting the brake elsewhere is difficult. For example, up and left is a tricky place to mount; but will pull the wheel up and back--into the drop-out--during braking. The downside, of course, is that the counter-force will pull the brake mechanism away from the fork; whereas where it's currently mounted will pull it toward the fork.

For that, I say to mount a bracket around the brake, which goes back behind the fork and mounts ... exactly where the current mount points are. This means the brake pulls forward against the bracket and ultimately into the fork, rather than away from the fork in an attempt to rip out the mounting screws. Of course, that means you have a bunch of crap in the way--a mounting bracket that can interfere with cables or brake mechanisms, depending on how it's mounted. There are many ways to get around this, not many are elegant.

Still, positioning the brake strategically would lead to forcing the wheel into the frame, rather than ejecting it from the drop-out. The rear brake... I think is mostly fine, as it pushes the wheel forward and the drop-out points back(?) (probably because its failure mode is to drive the wheel into the frame as it drives forward). Somewhere in the lower left quadrant is probably best.

None of this matters, as changing any of this would create large incompatibilities with current frame and brake design. You could technically keep the same mounts and mess with brackets up front; that's probably a good idea to avoid a rip-out style failure. Still, the exact mounting geometry for optimal braking performance may turn out slightly different, and so you get compatible-but-not-great versus great-but-let's-break-everything-again. Going the latter route with a bracket to adapt current forks to take the changes is probably the best way, as it gives a way to sidestep the incompatibility issues....

... but of course there's a large cost associated, and you create a mixed market for a while.
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Old 07-04-11, 08:18 AM   #2
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forward facing dropouts, through axle forks
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Old 07-04-11, 08:24 AM   #3
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Tout Terrain, German frame builders , use a horizontal forward opening fork tip,
on their special Disc Fork.

I've seen custom builders one at least, put the caliper
on the Front of the Right fork blade ,
which cures the issue too.
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Old 07-04-11, 08:41 AM   #4
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Well the thing with forward facing ... eh.. trick with drop-outs is if you stop the wheel, the bike frame keeps moving. Rim brakes with a drop-out where the wheel slides back to exit will eject the bike off the wheel; disc brakes will as well, while yanking the wheel into the drop-out. If the wheel slides forward to come out, then stopping the wheel forces the bike onto the axle; but braking on a disc brake as is ejects the wheel forward relative to the axle.

Front of the Right fork blade does cure the issue, which is what I was suggesting as I always see this



Which says "Throw the wheel right off the fork."
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Old 07-04-11, 08:50 AM   #5
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The Cotic Roadrat, a general purpose, doitall frame, has the disk mount on the leading edge of the right/driveside fork.
This reverses the rotational force on the axle, forcing it into the dropout. The only disadvantage of this location is that you cant use dynamo hubs since they have a preferred rotation.

Clearer photo here
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Old 07-04-11, 08:56 AM   #6
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Interesting.

Also interesting, I went looking at pics of drop-outs. There are a billion types of drop-outs. Mine go down and forward to drop out; my rear goes down and forward, too. Some go direct back on rear, and down and back on front. Some ... they've hooked these up in every possible configuration. WTF?

I had assumed there was engineering reasoning behind which way the slot in the dropout went. It seems they just cut a slot somewhere that'll let you stick a wheel on. My mistake.
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Old 07-04-11, 09:53 AM   #7
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Quote:
Originally Posted by bluefoxicy View Post
.. I went looking at pics of drop-outs. There are a billion types of drop-outs. Mine go down and forward to drop out; my rear goes down and forward, too. Some go direct back on rear, and down and back on front. Some ... they've hooked these up in every possible configuration. WTF?

I had assumed there was engineering reasoning behind which way the slot in the dropout went.
Well, kinda-sorta. For the rear: a (semi) horizontal dropout makes the length of the chain stay rather error-tolerant. Forward facing means there'll always be enough chain slack to get the wheel out. But you'll have to have decent clamping to get the wheel to stay put. Also the possibility to use the same bits for SS bikes.
Rearward facing is a bit weirder. Sure, with perfectly tweaked chain length you have the advantage of the wheel not being able to slip, as with vertical. But getting the chain off will become a PITA, comparably.
Vertical dropouts has the same ease of wheel removal as forward facing, but with the added bonus of clamping being less critical. And you need really good alignment in assembly.
For the front though I'm inclined to agree. From an engineering perspective the one with the best strength-to-weight ratio dropout is probably the one that's best aligned with the fork leg. But then came lawyer tabs, disc brake ejection and whatnot. And a bunch of minor tweaks to make it look like they're doing something.
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Old 07-04-11, 09:55 AM   #8
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The bikes with front discs have the cavity to keep the QR in it, those 'Lawyer lips' are part of it.

the fact that both brakes are on the left side, is .. ???
is requiring time travel , and mind reading, to know why..
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