Metal Fatigue: can spokes be reused?

5MV-WB2 – Metal Fatigue: can spokes be reused?
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https://www.buymeacoffee.com/fiveminutevelo

In an earlier video I rebuilt a wheel (https://youtu.be/WQV2tWY0eI0?si=RRcV1S_mSl1pGOla) on my touring bike by replacing the hub; I reused the rim and spokes. Some viewers questioned my reuse of the spokes. In this video I explain why I was happy to reuse the spokes. Warning there are engineering justifications .

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About the 5MV channel
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My name is Terry and I love cycling. I have named my channel “Five Minute Velo” (5MV). I have called it 5MV because the videos I upload will (mostly) last no longer than five minutes. Each video will cover just one subject that is described in the title. Although the channel is mainly about cycling on the road, I will also cover some light (gravel) off road riding and cycle touring from time to time. No matter what, much of the advice will be pertinent to many branches of the sport/pastime.

I am new to posting videos on YouTube (May 2020) and I have much to learn, particularly in terms of presentation style and production quality. As such I welcome constructive comments that will help me improve the videos that I produce and upload.

There may be some dry humor in some of the videos, so dry it may not always be obvious :-).

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My strava profile can be found here:
https://www.strava.com/athletes/7723982

Cambridge Cycling Club website:
http://www.cambridge-cycling-club.org.uk/

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Photos of 2019 Tour of low countries:
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Random cycling photos (will be updated from time to time):
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Terry

In an earlier video which is linked above I rebuilt a wheel using some pre-used stainless steel spokes in this video I’ll carry out an engineering analysis of those spokes to determine whether it’s safe in terms of their long-term reliability to reuse these pre-used spokes unless you’re very hamfisted when

Tensioning Spokes and I’ll cover spoke tension in a future video or you have an accident the most likely cause of spoke failure will be metal fatigue metal fatigue is a non-reversible weakening of the material when it’s subject to repeated fluctuating stress there are two main phases crack initiation and

Crack propagation because there’s not much material for a crack to propagate through spoke fatigue is dominated by crack initiation bicycle spokes can accumulate metal fatigue as they are loaded and unloaded while the wheel is rotating during use Engineers use diagrams like this SN Curves in order to evaluate fatigue life

If the evaluation falls on or above the curve the component will fail if the evaluation Falls below the curve the component will remain in one piece details of the SN Curve will depend on the material’s composition and its processing in this diagram the SN curve exhibits an endurance limit not all

Materials exhibit an endurance limit but most steals do if the applied stress range is lower than the endurance limit stress fatigue cracks will never initiate and the component can receive an infinite number of Cycles without fatigue failure I selected this particular diagram because the SN curve and the

Endurance limit of this particularly old stainless steel is Low by modern standards however this diagram is useful as any assessment using it will be conservative I’ll start with the experimental data I already have those data are from the spokes that I am reusing assuming that the spokes undergo

One load cycle for every revolution of the wheel we can calculate how many load cycles each spoke has already experienced working through the numbers we can determine that each each of the spokes has undergone about 12 million load Cycles if I overlay this onto the SN

Curve you can see that the number of Cycles applied to the spokes exceeds the number of Cycles associated with the endurance limit as there were no spoke failures we can infer that the stress range Falls below the endurance limit and therefore the spokes will have infinite life however I’m a belt and

Braces sort of engineer so I’ll also look look at this from the perspective of the stress range I’ll have to make some assumptions first of all the fatigue loading is taken by just eight spokes as for that load the total mass of the bike and Rider is 120 kg and this all acts

Through one wheel I’m going to assume that stress concentration be that at the head end or the nipple end of the spoke of three when calculating the fatigue life we ignore the build or the average tension in the spokes if you do the calculations and I have the pedaling and

Braking forces are quite small also the number of load Cycles is much lower and it’s for these reasons that I have ignored both pedaling and breaking with those assumptions I can now calculate the total maximum stress range in the spokes which comes to 132 Newtons per mm

Squar if I overlay this this one to the SN curve the stress range is below the endurance limit as with the experimental data the analysis suggest that the spoke life is infinite I wanted to know if the preuse spokes would cause reliability issues I focused on metal fatigue as this is the

Most likely cause of failure I assessed this using two approaches the natural experimental data suggest the spoke dress was below the endurance limit for the material an admittedly rather crudee stress analysis also confirm this both approaches suggest that spokes will have infinite fatigue life and hence it is

Safe for me to reuse the preus spokes however this was for the spokes that I was reusing if this result is to have broader significance there are some discussion points and caveats to cover I mentioned earlier that different materials and processing would produce different different SN curves over the

Years material processing quality has improved for instance modern stainless Steels are less likely to contain micr structural defects which can encourage crack initiation these modern materials have Better Properties including higher fatigue lives and higher endurance limits unfortunately with the reputation of these older spokes made of older

Material still lingers and it bches the good quality spokes made from Modern materials and with modern processing I suspect or rather know that the quality of the hubs and to some extent the rims and nipples will also have an influence on spoke reliability so there are reasons why one

Would want to use new components particularly spokes however there is a reason you might want to actively use pre-used spokes pre-used spokes as assuming they haven’t caused any problem will have been proof tested and therefore will have proven reliability whereas new spokes are an unknown quantity even good quality

Manufacturers can make the odd bad spoke or bad batches of spokes of course even if you use good quality spokes there’s always the chance of failure as fatigue as statistical in nature there’s always the chance that a material will have a micr structural defect in the wrong place or localize damage that will

Initiate a crack having said that I haven’t had a Spoke fail in well over five years and even then those spokes failed because I broke a gear hanger and the gear changer went into the spokes and damag them causing some of them to break a few weeks later nothing not even

Good quality spokes are perfect obviously these calculations have been made for stainless steel and so they don’t apply to aluminium alloy or more modern carbon fiber spokes if your use case is much more severe than normal Road cycling like downhill mountain biking for instance then these results May not apply to your situation

Either I hope you found this video interesting and informative or preferably both if you did you’ll find a buy me a coffee Link in the description also if you can spare a second or two please give this video a thumbs up as that really helps the channel out if you

Would like me to make more engineering based videos let me know in the comments and here are some of my other bike engineering videos for you to take a look at finally if you have any comments questions or additional information feel free to moist them via the comments feed

As well from me until next time it’s goodbye