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I know that the physics of disc brakes are different than rim brakes, but the differences are not intuitive to me so I want to double-check this. I understand that one fail point of single walled rims is the spoke hole; that they are prone to cracking. Will this tendency be exacerbated by a disc brake (compared to a rim brake)?

FWIW, this is the front wheel I am thinking of.

EDIT: if the answer to the question is no, what do I do with the various yes answers that I find? For example from Park Tool's website:

A similar torsion or twisting load is applied through the spokes by rotor disc braking.... The rim caliper brakes, however, do not apply braking force through the spokes in the same way.
http://www.parktool.com/blog/calvins-corner/wheel-tension-balance-web-app

Am I confusing the discussion of load with the forces that cause a spoke nipple to crack the rim at the hole? Or is it, as @Rider_X says, that the forces are different but the differences are negligible?

jqning
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    Possible duplicate of Will disc brakes aggravate the problem with spokes pulling through the rim? – Móż Nov 30 '15 at 03:06
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    @Mσᶎ I looked at that question. I couldn't identify a discernible answer that I can apply here. – jqning Nov 30 '15 at 03:46
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    I agree that the answers are vague, but your question is one of the unfortunate yes/no ones. So the answer there as far as you're concerned is "no". My reading of that question is that there's much discussion of possible mechanisms but no-one managed to explain how a wheel would be more likely to fail. – Móż Nov 30 '15 at 04:15
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    Given that a wheel works by being in tension, and it has to sustain your weight plus impacts when you hit something like a pot hole, I doubt that disc brake would exacerbate it much beyond what it already sees. – Rider_X Nov 30 '15 at 08:30
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    No the physics are not different. You have a given amount of inertia / momentum (mass * velocity) and that force goes thru the spokes. Rim pulling on hub or hub pulling on rim it is the same force / work. – paparazzo Nov 30 '15 at 12:57
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    @Frisbee how do you reconcile this with the warning against disc brakes on radially laced wheels? – jqning Nov 30 '15 at 16:09
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    @jqning it just needs more in the case of a disc rotor. I have disc wheels, one side radial, the other cross laced. The cross lace is on the drive side rear and brake side front. So, the rear brake force is transmitted through a radial pattern without problem. – ebrohman Nov 30 '15 at 17:17
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    I have tried to help you before and it did not go well. – paparazzo Nov 30 '15 at 17:44
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    @ebrohman - radially-laced disc side - that's rare. It's not surprising, because people can do whatever, but it's rare and ill-advised. – jqning Nov 30 '15 at 18:50
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    @ebrohman the torsion is also carried over the hub body to opposite side spokes. For example Mavic uses radial drive side and crossed non-drive side in many rear wheels in order to balance spoke tension in asymmetric wheel. – ojs Nov 30 '15 at 19:42
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    @ojs I'm sure those are hand built by a pro. I'm talking about single walled rims; wheels are likely built by a machine. – jqning Nov 30 '15 at 19:48
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    Build quality and rim don't matter in that case, it's determined by torsional rigidity of the hub. – ojs Nov 30 '15 at 20:16
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    @Frisbee I'm sorry but the rim brake and hub brake work differently. To see it, think how rim brake would work if the wheel didn't have hub and spokes at all, for example suspended by a few small bearings at the inner surface of the rim. – ojs Nov 30 '15 at 20:25
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    @ojs yeah that's really interesting. I read on zipp's site that power transfer from DS to NDS is about 20%. – jqning Nov 30 '15 at 20:37
  • @ojs I know the brakes work differently. Sorry - what are you sorry for? So you don't agree. For me it is simple physics. That spoke is between the tire and the bike - period. – paparazzo Nov 30 '15 at 20:37
  • @Frisbee I'm sorry to inform that you are wrong. You do not understand the physics. Rim brakes would work even without spokes and hubs at all. I hope this is clear enough. – ojs Nov 30 '15 at 20:42
  • @jqning It might be 20% through the hub barrel in the system with the spokes, rim and thin hub barrel that Zipp uses. With less torsionally rigid spokes and rim and more rigid hub, it is more. – ojs Nov 30 '15 at 20:48
  • @ojs Sorry but I am not wrong just because you say I am wrong. I took two year of physics with all A's and have degree in engineering. An imaginary bike with no spokes or hub adds no clarity. Move on. We don't agree. – paparazzo Nov 30 '15 at 21:01
  • Well, I happen to have an engineering degree too. No need for handwaving, you can write like an adult to me. – ojs Nov 30 '15 at 21:19
  • @ojs Handwaving, adult? You are the one stating I don't understand physics (and down voting my linked answer). That is a fair response to your assertion. Move on. – paparazzo Nov 30 '15 at 21:39
  • Your linked answer is wrong, too, and the free body diagrams do not make any sense. I understand you might be trying to dumb down the answer for general population, but you don't need to do that for me. Use the real math. – ojs Nov 30 '15 at 21:45
  • @ojs Use real math for you? I chose not not to enlighten you. I think you are rude and I don't want to interact with you. I don't care if you don't believe me. – paparazzo Dec 01 '15 at 15:19
  • Comments are not for extended discussion; this conversation has been moved to chat. – jimchristie Dec 03 '15 at 21:27

1 Answers1

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Well, let's compute the forces. For example's sake, I'm assuming 80kg of bike + rider, all braking force on front wheel and deceleration of 5 m/s^2 (extreme, but possible). This equals 400N at contact surface. The radius of 559mm rim and tire is something like 320mm, giving 128Nm of torque. The spoke hole is at 22mm radius, and cross 3 lacing places the spokes roughly tangential to hub flange. Using the axle as pivot and spoke attachment to hub as lever, we can calculate that we need force of 5818N to produce the torque.

Spread over 32 spokes, that is 181 newtons per spoke. Since spokes in a properly built wheel are already at 1000N or more, the increase is around 15-20%. This requires a rim with stronger spoke holes, but not by that large margin.

Keep in mind, that in many double wall rims the spokes attach to outer wall. In this case, the extra wall doesn't help at all.

The case with radial spokes on both sides is different: The spokes exit the hub at zero angle, there is no leverage and if the spokes wouldn't stretch and let the hub twist, the force would be infinite. Calculating the actual force is left as an exercise to the reader :)

ojs
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  • Loads (e.g., braking, bumps, etc) will not load all spokes equally at any given instantaneous moment. Peak force will be the most important when talking about fatigue. – Rider_X Dec 01 '15 at 03:24
  • True. Impacts at rim unload the spokes at contact point and only slightly increase tension elsewhere, so I didn't try anything more exact. The case with evenly spread load gets close enough for me. Let's see if anyone has time to play around with FEM. – ojs Dec 01 '15 at 17:52