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The pic below is from the fork of a 2020 Kona CoCo. Eyeballing it, it seems to me that an arc around the disc caliper contact point would pass straight through the dropout. Should this be worrisome? Does the recess around the dropout make this sufficiently (?) safer in the event the QR becomes loose?

enter image description here

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    Does the recess around the dropout make this sufficiently (?) safer in the event the QR becomes loose? No. Solution: don't let your QR come loose. – Andrew Henle Oct 23 '19 at 15:30
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    FYI, you can make an arc around a point go through any other point, so by your logic there isn't necessarily a force trying to push the axle out of the dropouts during braking. – Argenti Apparatus Oct 23 '19 at 17:57
  • I think the concept OP is looking for is fail-safe. Through axle is fail-safe, QR skewer isn't even if failure is unlikely if used correctly. I didn't save the photos at the time, but I've seen a report of disc brake tearing off the entire dropout before QR fails. – ojs Oct 23 '19 at 19:48
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    Why would the QR "become loose"? – David Richerby Oct 23 '19 at 20:35
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    @Carel In that case, it hasn’t “become loose” — it was loose all along. – David Richerby Oct 24 '19 at 12:31
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As long as the skewer is set with sufficient clamping force this QR disc brake setup should work fine and from a practical standpoint there will be very little functional difference between this setup and a thru-axle.

If the QR is not sufficiently tight (i.e., you didn't set it with enough clamping force), you may notice a small shift in the axle position under very heavy braking. This isn't dangerous but annoying as the rotor will rub against the caliper until you reset the wheel position. Thru-axles won't do this, but with some care it shouldn't happen in a QR setup either. Even though the axle can shift in some circumstances the axle will not come out as the "lawyer lips" around the outside edge of the drop out will retain the QR. Even in the worst case where the QR somehow flips open while riding lawyer lips should still retain the wheel in the drop out. This is why you have to unscrew the QR to get it out of the drop out. By this point the wheel will be rubbing on the frame, not a great situation, but you won't be without a front wheel.

Now suppose you open the QR, unscrew it a few turns, then go for a ride and heavily apply your front brake... then the wheel will come out of the drop out. By then however you have made numerous critical errors in the setup.

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    Should note if the QR is loose enough to get past the lawyer lugs, the wheel will eventually come off no matter what brakes are installed. Disc Brakes just assist the natural process of Darwinism. – mattnz Oct 23 '19 at 19:08
  • You should also note a thru axle setup doesn't have drop outs which makes the forces across the forks more even. The only way a thru axle come out the hub is if it's physically undone. – Dan K Oct 24 '19 at 4:25
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The design on your bike looks exactly like the drop out + skewer + brake arrangement on mine. In my case I ended up in a bad crash due to front wheel coming off during the ride. Trust me, if and when the crash happens it would be quick and over with under a second with no time to react or even understand why one crashed on a flat road. I was lucky to not have ended up with broken bones, just concussions and bruises which kept me out of action for full 3-4 days.

Take a look at the below picture of my setup (image courtesy Amazon listing)

Montra Helicon Disk front fork brake and disk design flaw

Does it cover all the bad engineering in one shot?

  1. Dropouts pointing downwards in the direction of the rotation of the wheel - a free fall off in case of loose QR latch.
  2. A loosened QR would yank the wheel off the drop out in case of hard emergency braking
  3. QR and the disks on the same side. In theory if the latch gets into the rotor, it would be catastrophic.
  4. No secondary fail safe mechanism to prevent a wheel from coming off.

Now that we examined why the design is lame, here is a little bit of physics and mechanics that will play its part in the rider crashing. (The below part is borrowed from a discussion in a cycling group)

In theory, it goes like this:

A bike's wheel rolls around its axle, because rolling resistance (on the axle and on the tire-road contact) is so small when compared to the sliding friction at the tire-road contact.

What happens when we apply brake?

Braking works because of three forces: pad-rotor contact point (or pad-rim in case of non-disc). Note: pad is fixed to the fork. axle-dropout fastened by a QR. tire-road contact point (a strong sliding force) All the three contact points are on the wheel one way or other.

An effective braking requires that all the three contact points can withstand the forces. In the worst case, the tire-road contact SHOULD be the first to give up and cause a skid. Second worse case is for the pad-rotor to give up and slip the braking.

What happens if the worst case scenario turns out to be a give-up in the axle-dropout contact point? Or why would this even be the case in the first place?

When the pad-rotor contact point is fixed and tire-road grip is strong, there is a massive force at the axle-dropout contact point, with the axle pushing backward. Try to imagine without the dropout on the fork. The wheel will try to rotate around the pad-rotor contact point. If the rider+bike has sufficient momentum (speed), this can cause enough force for the wheel to rotate around the pad, specially if the dropout is somewhat on the circumference of the circle drawn with the axle around the pad and in the direction of rotation of the wheel. If the dropout directly is completely opposite of the direction of rotation of this circle of axle around the pad, then the only way the axle can come out of the dropout is to break/damage the dropout.

It so happens that a Montra helicon dropout can easily cause the axle to come out. Apparently, there were so many recalls in the US on this faulty design of the dropout. I compared this bike with my KHS 650B. There is literally no room for the wheel to come out in case of KHS 650B (you can google for images of both).

You should check if your fork dropout is designed safe enough. I WILL RATE THIS A VERY HIGH RISK CATEGORY FAULT.

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