Tension in rod of a bicycle wheel skewer

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Discussion Overview

The discussion revolves around the mechanics of tension in the rod of a bicycle wheel skewer, particularly focusing on the locking mechanism and its relationship to the skewer's design and operation. Participants explore theoretical aspects, mathematical modeling, and practical implications of the skewer's tension during use.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the tension in the skewer is a function of the distance between the centerline of the disk and the handle's position, which relates to the disk's arc length.
  • Another participant clarifies that the term "skewer" likely refers to the "quick release" mechanism, describing its components and operation, including the cam and handle.
  • A participant proposes that the skewer tension could follow Hooke's law, indicating that tension is proportional to the displacement caused by the cam, while noting that the out-of-round nature of the cam introduces non-linearity.
  • One participant suggests developing a cosine-based function to model the tension, proposing that integrating this function could yield insights into the tension dynamics as the handle is turned.
  • The same participant mentions that knowing the elastic properties of the rod could allow for a relationship between tension and displacement using a spring constant.

Areas of Agreement / Disagreement

Participants express varying interpretations of the skewer mechanism and its tension dynamics. While some agree on the general principles of tension and displacement, there is no consensus on the exact modeling approach or terminology used.

Contextual Notes

The discussion includes assumptions about the mechanics of the skewer and the properties of materials involved, which may not be fully defined or agreed upon by all participants.

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The locking mechanism of a bike skewer consists of a rod and slightly out of round disk on one side attached to the skewer. A handle is attached to the out of round disk as well Assuming the skewer is nearly snug with the wheel fork, I think the tension is going to be a function of the distance between the centerline of the disk where it is connected to the skewer and the position of the handle which is related to the disks arc length relative to the unlocked position.
 
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Could you provide another word for "skewer"? I don't think it's what you mean. (Spoke??)
 
sophiecentaur said:
Could you provide another word for "skewer"? I don't think it's what you mean. (Spoke??)
I believe that he is referring to the "quick release" mechanism on the axle where, for instance, the front wheel is attached to the fork.

The handle is perhaps 5 cm in length and 1 in breadth. It is attached to the "out of round disk" which is what I would have called a "cam". In my experience, the cam is hidden inside a somewhat larger than usual nut which is threaded onto one end of the axle. The handle is rotated 180 degrees between a released position and a tightened position where it holds by friction. In the released position, the wheel can be removed from the slots at the end of the fork. In the tightened position, the wheel is firmly affixed and the bicycle is ready to ride.

I would expect the skewer tension to roughly obey Hooke's law with tension directly proportional to the displacement forced by the cam from the position where the parts begin to bind. A slightly out of round cam will have a roughly sinusoidal variation in displacement as a function of handle position. So that will cause one departure from linearity. And I would not expect Hooke's law to be obeyed at all precisely.
 
Thanks for the help
How's this for a possibility
perhaps if i can come up with equation that is "close" in terms of its graph. Ie use a cosine based function where the right angular frequency will mimic the curve so that the bottom portion of functions graph is at the point where the tension begins. then integrate that function over an interval where the pressure is applied. My reasoning us that the cosine function is increasing over the interval 3pi/2 to 2pi and hence would cause an increase in the tension within the rod. if i knew the elastic properties of the rod as a fixed value then y=kx could be written with k being the spring constant and "x" being my cosine function in terms of angular change
 

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