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After reading this (anxiety inducing) question on a disc brake dislodging a front wheel on a brand new bike with a QR axle I wondered why the brake calipers are placed behind the axle in the first place.

My understanding (which might be incorrect) of how forces and torques more or less work on a front disc brake:

  • Given a wheel rotating CCW (blue arrow)
  • The brake caliper application results in a CCW torque on the disc (green arrows), with a pivot point somewhere on the caliper.
  • This results in a downward facing force (red arrow), away from the fork (not drawn), possibly ejecting the wheel from the fork.

enter image description here

If the brake caliper would be in front of the axle, the resulting force would push the wheel into the fork, preventing ejection.
So (according to my possibly incorrect understanding):

  • The wheel rotates CCW (blue arrow)
  • The brake caliper application results in a CCW torque on the disc (green arrows), with a pivot point somewhere on the caliper.
  • This results in an upward facing force (red arrow), into the fork (not drawn), preventing the wheel from ejecting out of the fork.

enter image description here

So, what am I missing? Why are brake calipers not in front of the axle to prevent wheel ejection?

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  • Aside - note that your linked question specifically says "I was not braking at the time" so while that OP's brake was related, it wasn't the root cause. However this is a good question on its own.
    – Criggie
    Jul 26, 2021 at 19:39
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    As I commented in that post, the reality of the world we live in is tens of millions of QR bikes with disk brakes have been made sine early 2000's, and less than a dozen reliably documented examples of front wheel ejection with correctly tightened QR's. The problem, while theoretically real, is overstated. If you have concerns, make sure you do the QR up correctly and your bike is equipped with Lawyer lugs (Which 100% mitigate the issue).
    – mattnz
    Jul 26, 2021 at 21:10
  • A correctly tightened QR that is used as intended will not open.
    – Carel
    Jul 27, 2021 at 7:36
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    And exactly the words "correctly" and "used as intended" are what made me write this question. What if you don't use the QR as intended, without knowing? I don't inspect every bolt and nut before I ride, because I forget, I'm in a hurry to catch the train or other aspects of life interfere. I never knew that intended use of brake might lead to wheel ejection. Now I learned that the combination of QR's and disc brakes has a very tiny fraction of a chance to eject the wheel and I try to understand the design decisions behind this. Jul 27, 2021 at 8:16
  • One thing I have noticed is that many rigid and suspension forks designed for disk brakes do not have vertical dropouts, but slanted towards the front. If you take into account that the caliper is behind the axle, the reaction force in said axle is not directly downward, it has a rearward component, that is, the reaction force is nearly perpendicular to the dropout. That, and the feature known as "lawyer lips" may prevent many wheel ejection events.
    – Jahaziel
    Feb 16, 2023 at 14:57

1 Answer 1

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I assume this is due to the bolts that mount the caliper to the fork. Do note that if the brake caliper is behind the fork, the force pushes the brake caliper slightly upwards and towards the fork.

If the brake caliper is in front of the fork, the force pushes the brake caliper slightly downwards and away from the fork.

The force component pushing the brake caliper away from the fork is the problematic one. For example, a 110kg rider + 20kg e-bike + 15kg cargo means 145kg has to be stopped. If braking at 0.6 g, that's 853 Newtons of braking force.

The wheel rolling diameter is 677 mm, whereas the rotor diameter is 160 mm. Thus the force at the caliper is 677/160 = 4.23 times the force at the tire. That's 3610 Newtons of force either pushing the caliper towards the fork or pulling the caliper away from the fork. Your choice -- I will choose that force to push the caliper towards the fork and not pull the caliper away from the fork. My choice means the caliper has to be behind the fork, not ahead of the fork.

If the caliper mounting bolts are tightened to the full recommended torque, they probably can withstand this 3610 Newtons of force. Tightening to the recommended torque should ensure about 5000 Newton clamping force. But the problem is, brakes are a safety critical item, and bolts that are working so close to their limit even if correctly torqued, and likely to be torqued by amateur mechanics, would be very likely to fail in case someone puts only 3000 Newtons of clamping force to the bolt.

Also, torque is actually a poor indication of bolt preload. If you torque to let's say 5 Newton meters, the preload could be anywhere between 4000 Newtons and 6000 Newtons depending on the quality of the threads, the type of grease, the amount of grease, etc. It isn't as simple as saying that 5 Newton meters for a M5 bolt equals 5000 Newtons of preload.

Remember how a bolted joint works: if the piece attached with a bolt to a plate is pulled, the bolt load does not change. Instead, what changes is the load between the piece and the plate. However, that only works to the point where the load between the piece and the plate is zero. At that point, the force begins to increase the load at the bolt. Quite soon the bolt fails, and if it doesn't fail by separating in two pieces, it's possible the movement at the joint could cause the bolt to self-unscrew.

However, if you push a piece towards the plate in which it has been attached, in every case the load fully goes via the interface of the piece and the plate. The bolt preload is never affected, no matter if the bolt is loose or tight.

Now, if the fork extended below the dropout, then you could attach the caliper at the extending portion in front of the fork, so the force would then not pull the caliper away from the fork but rather push it towards the extended portion of the fork. However, such an extended fork is (1) heavy, and (2) can't have dropout slots that are open towards the bottom but rather must use thru-axle, and thru-axle anyway solves the QR unscrewing issue with disc brakes so such an extended fork doesn't make sense. It would also look ridiculous.

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