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I purchased a brand new hybrid bike with mechanical disk brakes a few weeks ago

  • I have ridden the bike for a few weeks without any issues on city streets with potholes and elevations from bridges
  • I probably rode it for around 100 miles total
  • Yesterday after 45 minutes of riding, the wheel completely and suddenly locked up on a city road. There were no bumps at this point
  • I was not even touching the brakes. Luckily I was going very slowly - so I walked away with minor bruises
  • After picking myself up, I noticed that front wheel was still completely locked up
  • I saw that the front disk rotor was bent (pictures below). The bike was handling and braking perfectly fine before this point.
  • I set the bike down gently on the side. When I tried to spin the front wheel, it just popped out of the frame. I was NOT applying any force and it just came straight off
  • This is bike I was riding: https://www.rei.com/product/121596/co-op-cycles-cty-11-bike

Brakes: Tektro M300 mechanical disc

  • The leaves and debris on the photos are from sidewalk where I dragged the bike. The road where this happened was clean and did NOT have any debris or sticks

  • What could have caused this to happen without any warning? Was it an issue with the brakes or was the wheel not tightened correctly by the bike shop?

  • I am concerned why this happened and could it happen again.

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    This sounds a lot to me like your front QR skewer was loose and the wheel slightly came out, but I'm not 100% sure. How tight is your rear wheels QR lever right now?
    – whatsisname
    Commented Jul 25, 2021 at 20:11
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    Rotors bend quite easily and replacement is quick. And I tend to share @whatisname 's opinion. Was the wheel still locked in the fork?
    – Carel
    Commented Jul 25, 2021 at 20:23
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    @whatsisname : First picture! The skewer looks as if it has those anti-theft bolts that are fastened with a hex key. They need frequent checks for tightness.
    – Carel
    Commented Jul 25, 2021 at 20:28
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    Fourth and sixth pictures. Appears that paint has flaked off the fork immediately above the point where the disk brakes attach, and there is a dent/depression in the fork below the attachment point. Suggests that the fork has been damaged in the accident and should be checked for safety before riding again (with a new wheel/rotor).
    – Penguino
    Commented Jul 25, 2021 at 22:45
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    The wheel is attachment is a sub-type of quick release. It uses the same type of 5 mm bolt. And a 9 mm hollow axle. Instead of a lever it has nuts. Most likely to make wheel theft slightly more difficult. Well closed up this is a perfectly fine design. However, it seems it was not tightened well.
    – gschenk
    Commented Jul 26, 2021 at 12:03

4 Answers 4

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Re:

When I tried to spin the front wheel, it just popped out of the frame. I was NOT applying any force and it just came straight off

This suggests the quick release was not properly attached or tightened at that point in time. If it was indeed loose when you were riding, the wheel could have moved in the dropouts and the disc rotor then jammed in the brake caliper.

Do you have a good memory of checking the quick release tightness when you started your ride? If not, it may have been getting loose for some time. Another possibility is that there are questions about whether quick release-type axles (as opposed to through axles) are safe for use with front disk brakes, because the forces generated by braking can tend to unscrew the quick release in certain configurations. See James Annan's paper Disk Brakes and Quick Releases.

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    While I do not dispute Jame's work, putting it in perspective, millions of bikes made between early 2000's (when discs were first introduced) today (low cost bikes still sue QR's ad disks) use QRs, and fewer than a dozen examples have been reliably reported of wheel ejection. (.e. an individual has more chance of winning the lottery).
    – mattnz
    Commented Jul 26, 2021 at 8:42
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    @mattnz Yes; of course after the fact it's likely impossible to differentiate QR/disk-caused loosening from improper initial tightening. In any case, I suspect certain non-mainstream configurations are the most susceptible (James had his issue with a tandem bike iirc).
    – Armand
    Commented Jul 26, 2021 at 14:07
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Concur - I guess the security bolt things were not working as they should, allowing the axle to move in the dropouts subtly. This caused the rotor to bind inside the caliper and your momentum did the rest.

Had you been moving faster, you would have gone over the bars and down the road. This is not ideal.

Rotors can be trued if lightly bent. If the rotor has a crease/fold in it then it cannot be flattened and needs replacing. Check your brake pads too - they may be damaged inside the caliper.

I would suggest either replacing the security locking system with something that works better (like a common Quick Release skewer), or learn how it should be applied correctly. From now on make sure you do a monthly bike check looking for loose things, and if you continue using this bolt system, then check it before every single ride.

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    Not that quick release is too good for disc brakes either. The main problem is in the actual dropout/axle interface, and the locking mechanism just tries to somehow fix it. Perhaps in the wrong way.
    – Vladimir F Героям слава
    Commented Jul 26, 2021 at 8:36
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    @VladimirF to be honest - a disk brake wheel with axle nuts would be almost as good as a through-axle in terms of wheel retention. And a hollow QR axle might have been replaced with a solid axle at some point in the hub's history. That could be a surprisingly robust build. But I'd guess its none of that, just an old front wheel that fitted at a time a wheel was needed.
    – Criggie ♦
    Commented Jul 26, 2021 at 11:40
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Re:

I checked with REI and they said it is not a quick release but a thru axle with a nut. This is my first bike in over 30 years and I had asked REI when I purchased the bike if I needed to do any ongoing maintenance. The only thing they told me do regularly was check tyre air pressure.

This is shocking to me if accurate. While technically not a "quick release", lacking the QR easy-open lever, functionally it seems similar enough in how it secures the wheel hub in the dropouts. It merits the same caution any competent bicycle shop would give to a QR bike rider to check that the wheel is tightly secured before every single ride.

I'd be curious to know how they would explain this accident and how to prevent it in the future. Of course, if it were me, I would now certainly "trust, but super-duper-definitely verify" anything they told me.

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This danger can happen due to a flawed design of using quick releases on disc brake wheels.

The quick release was originally developed for rim brakes. Rim brakes when braking only move the wheel hub backwards in the fork due to their position on top of the wheel, so the dropout being open in one direction of four and closed in three direction of four can hold the wheel perfectly well.

However, disc brakes apply a force of moving the hub axle downwards on the dropouts. This force is not trivial as it is with rim brakes, because rims are essentially "full wheel diameter discs". In contrast, with disc brakes, the brake disc is very small. To get any torque with such a small disc, the force needs to be astronomical. Due to the position of the brake caliper behind the fork leg, the force is applied downwards. (Placing the caliper in front of the fork leg would cause the caliper retaining screws to see forces in alternating directions, causing a danger because the screws may not be strong enough and even if they are, the forces could self-unscrew them.)

The solution of the industry to this problem is thru axle. The thru axle dropouts are closed all the way around the axle so there's no way the disc brake braking force could cause the wheel to be ejected from the dropouts.

But why doesn't the quick release being tight stop this wheel ejection then? Well, that's because the quick release is not strong enough to hold the wheel still. When braking, the axle moves slightly downwards on the dropout, when going over a bump, it moves slightly upwards on the dropout, ad infinitum. This wears the dropout gradually away, causing the quick release to be gradually loosened.

You should never purchase a new bike with disc brakes and quick release attachment on the fork. If you have a bike with disc brakes and quick release, you should re-tighten the quick release before every ride.

What I suppose happened in this instance was that repeated braking have caused the quick release to become loose. Then at some point of time in the very recent past, you braked hard, causing the axle to be partially ejected from the dropout. This made the wheel a bit tilted in the dropouts, bending the brake disc too. Then you "just rode along", not braking, but some minor bump in the road caused the wheel to become again straight in the dropouts, but the brake disc was already bent to match the previous tilted position of the wheel. The re-seating of the wheel into the straight position at the same time the brake disc was bent probably caused the brake to be locked up.

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    Through axles (TA) do not hold the wheel in place by being form fitting but by their clamping force. (That's the case for practically all threaded fasteners.) Shear forces would be way too large for thin walled through axles for form fitting fastening. Practical clamping forces for quick release are higher (N/Nm) for 5 mm threads on QR than 12 mm threads on TA. You need to tighten TA with more than twice the torque than QR. TA inherently more safe than QR for front wheels are an urban legend at best, incompetent mechanical engineering at worst. Issue here is QR not closed properly.
    – gschenk
    Commented Jul 26, 2021 at 11:57
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    The difference is failure mode. With loose thru axle, the wheel rattles, but does not fail catastrophically unless you manage to lose the whole thru axle. With loose quick release the wheel can eject itself. Think of it as an extreme version of lawyer lips.
    – ojs
    Commented Jul 26, 2021 at 12:05
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    I don't believe gschenk is correct here. There are front hubs where the thru axle is the only axle in there, and 15mm bearings run directly around the 15mm thru axle. There is no secondary axle around the thru axle here. If the thru axle was not the axle that holds the wheel in place, and if the thru axle would not withstand the shear forces, such hubs with no axle around the thru axle would fail immediately. Immediate failure is not what has been observed with such hubs. Instead, the thru axle holds the bearings and hub in place perfectly well.
    – juhist
    Commented Jul 26, 2021 at 12:24
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    Thanks to everyone for their detailed comments. My hands were definitely not on the brakes. I checked with REI and they said it is not a quick release but a thru axle with a nut. This is my first bike in over 30 years and I had asked REI when I purchased the bike if I needed to do any ongoing maintenance. The only thing they told me do regularly was check tyre air pressure. Do you think this was an issue with the tyre getting unseated or the brakes malfunctioning?
    – Help
    Commented Jul 26, 2021 at 20:57
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    @Help Not a TA at all, as evidenced by the open dropouts. For REI to suggest so is highly indicative of the potential build quality for me…
    – MaplePanda
    Commented Jul 26, 2021 at 21:35

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