01-12-2005, 10:41 PM
|
#216 (permalink)
|
| Guest | Re: Rim brake heat and spoke tension
luns@aloha.EECS.Berkeley.EDU (Luns Tee) writes:
> In article <87is645v6u.fsf_-_@k-online.com>,
> Joe Riel <joer@k-online.com> wrote:
>> 1/Feff = 1/Fs + (n/2/pi)/Fr
>
> This is, I would say, the most important single equation of your
> analysis, and represents the meeting of the spokes' spring constant with
> the equivalent constant presented by the rim - everything else is just
> baby steps of definition.
Agreed.
> But, you skipped over a very interesting
> number - this constant for the rim - which should give some idea of the
> relative magnitude of what happens in the rim vs what goes on with the
> spokes.
> I'll use your cross sectional areas (2mm^2 for the spokes,
> 80mm^2 for the rim) but for my own sanity, I'll keep forces in N and
> use metric values for material constants.
80mm^2 was for a tubular rim (I had it handy); the material
cross-section for a clincher rim is larger.
> Fspoke = 2mm^2 * 200GPa = 400kN
> Frim(eff) = 80mm^2 * 70GPa * 2*pi/36 ~ 977kN
>
> This suprised me somewhat: for a uniform radial load across the
> entire rim, the rim's share of that load is more than twice that of the
> spokes! This is quite different from the intuition Jobst gives us of the
> spokes in the LAZ taking all the load with the rim serving as a
> pretensioning element for the spokes.
I'm confused as to your interpretation. The ratio of rim compression
over spoke tension is n/2/pi = 36/2/pi ~ 5.7. You are referring to the
ratio of spring constants...
> Another interesting excercise would be to look at the relative
> effective yield strengths are for each. ISTR the tensile yield strength
> of spoke steel to be around 1GPa, while 7178-T6's compressive yield
> stength is around 530MPa according to:
>
> http://asm.matweb.com/search/Specifi...ssnum=MA7178T6
>
> Yspoke = 2mm^2 * 1GPa = 2kN
> Yrim = 80mm^2 * 530MPa * 2*pi/36 ~ 7.4kN
>
> This would suggest that in tensioning a wheel, the spokes would
> yield way before the rim does! Something doesn't seem right to me,
You [and I] are ignoring the bending stress in the rim due to the
discrete spoke locations. Isn't that where the yield problem for
the rim lies?
Joe
|
|
|  |