Can disc brakes cause front quick-release to loosen?

Question

I just purchased a Yuba Spicy Curry Bosch cargo bike with disk brakes and quick release skewers on both wheels. I haven't had any issues with it so far, in fact, I love riding it, but I have read that using disc brakes with a quick release axle can be dangerous, as the braking force can cause the quick release to loosen and the wheel to fall out. It seems crazy to me. Nobody wants to lose their front wheel riding.

Is the risk overstated? Is it something that only happens with badly designed or improperly adjusted QR/disc setups? I check brakes and tire pressure before every time riding, if I just check the QR as well am I going to be fine?

Would it be safer to just replace the quick release on this particular bike with regular nuts?

I just can't imagine it is a common occurrence, but since I ride this bike with my kid as passenger and share it with my wife, I really don't want to leave it to chance. I am curious to hear from anyone familiar with this particular problem of disc brakes, beyond what is stated in the Sheldon Brown article. Thanks!

Answer 1

Most of the time, a bike wheel spins around the axle. When braking, the axle suddenly wants to spin around the point where the brake caliper is grabbing.

The traditional orientation for a fork dropout, with the opening a little forward of straight down, is such that on a rim brake, these forces pull the axle stubs up into the dropout, and no movement can occur.

On a disc QR fork, this orientation of the opening is a poor choice because the axle stubs want to move in that direction under braking forces. Shifting, or in the worst cases ejection, of the hub can occur. A fairly large number of QR disc brake forks have unfortunately been made that use this orientation. That's a trend that's been changing for a while now, but unfortunately slowly. It is the reason a number of QR disc forks have more forward-oriented openings, which looks funny but eliminates the problem.

Skewers that apply higher amounts of clamping force, such as good quality internal-cam ones like Shimano, counteract this tendency but will not necessarily eliminate it.

Usually lawyer lips are sufficient to make actually losing the wheel impossible, but will still tend to allow the hub to shift if the dropout opening gives it a path to do so.

Answer 2

The real problem with quick release skewers on a disc brake bike is that all skewers are not created equally, and some cheaper skewers distributed on QR disc brake bikes were not up for the task (hence recalls).

If you are using rim brakes basically any functional skewer will work, however as pointed out in the linked posts skewers on a disk brake front wheels will experience additional forces associated with braking being applied at the hub, rather than out at the rim (where you have much more favourable leverages and therefore less force required).

If you have a cheap skewer that does not produce sufficient clamping force, then you can get issues. For example, under heavy braking the quick release skewer and wheel may shift in the front drop out. This is usually pretty noticeable (i.e., sudden rotor rubbing or the wheel no longer sitting centered) and could potentially lead to loosening of the skewer over time if material begins to be rubbed away with a back and forth shifting.

This is however easy to test for. Set the front wheel QR as you typically do, take some measurements (visually is often all you need) so you can assess whether any movement occurs, then go for a short ride and try some aggressive stops with the front brake (make sure you know the body positioning for aggressive stops). If your front wheel shifts, then the skewer is supplying an insufficient clamping force for the braking application.

When in doubt internal cam skewers such as Shimano skewers tend to produce the most clamping forces. Whenever I have had issues with a front wheel shifting in the drop out of a QR disc bike, switching to a high quality Shimano Skewer has solved it.

Of course, the industry now provides thru-axle as another mounting option which eliminates this potential issue by directly bolting the axle to the frame. But Quick release can still be used safely given that you take the necessary steps to ensure you have a skewer that can produce the required clamping forces for your braking application.

Answer 3

I ended up in a bad crash due to front wheel coming off during the ride. It all happened so fast without warning, there was no time to react or even understand why I fell.

Initially though I was clueless why the wheel came off, but once I researched the link between a disk and QR on the front wheel it is now clear to me what transpired.

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.

YES there is a risk of a disk brake loosening a QR causing a wheel to come off. In my case my ride was a hybrid with disk brakes (Montra Helicon D - from TI cycles), but this can happen with any bike which has disk brakes with QR.

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

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 is how a friend explained me and he is a Trans-Am finisher, so his opinion is worth weight in gold)

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.

Answer 4

There was a huge recall of quick release bikes from Trek in 2015 (about 1000000 units in US and Canada). The reason was that the lever could, according open far enough to catch in a disk brake rotor. That could lead and in several cases did lead to serious injuries.

This particular means of failure ca be mitigated by having the lever on the drive side (opposite to the disc).

Loosening of a quick release by disc brakes is described by James Annan. I quote:

There are two main aspects to the failure.

The disk brake generates a massive force largely downwards in the direction of the open fork ends. The friction of a quick release skewer is often not sufficient to stop the axle slipping down in the dropout slot. This is explained in more detail here,

The QR is initially restrained by the retention lip on the fork (assuming it is present), however over time the slipping of the quick release leads it to unscrew, which is described here. Once it has unscrewed enough, it can be forced over the retention lip and the rider will crash.

Modern disc brake bikes tend to be sold with thru axles instead of quick releases.