5

I took my bike to the bike shop to get remove the quick release wheel things, so that I can use only one U-lock, anyways they changed it for me but the bike mechanic really got mad at me and said 'you are not suppose put a u lock mount on carbon fiber'

carbon is strong one way but not the other and that i am putting too much stress on the frame etc..

is he right? the small kryptonite u-lock only weighs like 2.38 pounds!

  • 1
    yes he's right. I'm surprised he didn't also tell you that thieves often carry wrenches. – PeteH Dec 29 '15 at 20:18
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    @PeteH its a pinhead lock i got to replate the quick release thing? – nolawi Dec 29 '15 at 20:26
  • @nolawipetros pinhead or other locking skewers are a good solution, and solve the problem you have properly. A wrench won't open them. – Móż Dec 30 '15 at 0:21
  • Yeah man, don't muck up your bike by clamping lock mounts on it. – jqning Dec 31 '15 at 5:15
6

The problem is not the stress from the weight of your U-lock.

The (true) stress comes from the compressional hoop stress of the bracket to the carbon fiber tube. The bracket stays in place thanks to the friction, which requires certain amount of compression.

Unfortunately, this hoop compression coincidentally directs at the weakest point of this (tubular) design. Buckling will appear over time if enough compression is applied.

It is hard to say whether a simple bracket would cause the bike frame to fail. It depends on the user (over-torque), the design of the bracket (causing notch/localized stress on the frame), and how the frame was design (strength of carbon fiber in certain direction), etc.

There are many example of this failure type, one of which is the failure of carbon seat post. Usually, the bike frame (clamping type) was not designed to accomodate carbon seat post and cause large compressional hoop stress to the seatpost. That is why the seatpost often fail, not because of the product quality, but often the mis-use from the user.

Ref: Compressive failure in carbon composite, N.A. Fleck (Cambridge, Advances in Applied Mechanics Vol 33, 1997)

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    I see - thank you- it seems like squeezing the tube inwards is what will cause the failure. – nolawi Dec 30 '15 at 14:53
  • It is certainly discouraged. You might void the warranty and insurance for any damage that might link to U-lock mounting bracket. Although it is unlikely for a bracket to cause immediate failure, the rules are there to prevent a small percentage of humanity who are proficient in destroying delicate products. – Nhân Lê Dec 30 '15 at 16:34
4

Carbon is more resilient to this the type of stress then most people give it credit for but still it is NOT recommend for even 2.38 lb lock. When you land a jump that 2.38lbs is a decent force. If someone dropped the lock on your head from a foot a it would hurt. You will even see recommendation from the manufacturer not to even mount a carbon frame directly on a rack. I mount my lock on the handle bar stem. Even if it is a carbon stem it is replaceable.

Down on stem Down on stem.

Down and cross bars Down and cross bars

Stem and forward enter image description here

  • on the handle bar how - like this bikeblognyc.com/wp-content/uploads/2012/12/… seems unsafe – nolawi Dec 29 '15 at 19:54
  • That works but I like the stem with the U down to saves space on the bars for light, bell, narrow grip for climbing. And does not block the beam from the light. Not all mounts support that but the newer kryptonite do with pivot point(s). And a narrow lock doe not even need to pivot. – paparazzo Dec 29 '15 at 19:58
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    when you get a chance please post a pic of your solution... – nolawi Dec 29 '15 at 20:27
  • If the dog sees a camera he decides he needs to be in the picture. Really! – paparazzo Jan 1 '16 at 22:53

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