Spoke to soon!!!

[Johan Bornman]

I neglected to look at the breaks closely, but I suspect you are right Johan, I probably would've found signs of metal fatigue. But the bike did stand in the morning sun for about five months whilst I was playing around on my road bike (and waiting for selfish mtb'ers to me take out on trails)

 

What I did not understand though is that the bike had not seen any dirt roads up to that point. The first time I went off road (fairly rough and corrugated) three spokes broke, so I had the wheel rebuilt like you suggest. I have not had any issues since

 

Signs of metal fatigue in spokes awkwardly placed behind cassettes is not somthing you'll see with the naked eye. It'll take a trained eye and a strong magnifier to see.

 

Dirt roads don't fatigue spokes more than smooth roads. By just riding, the spokes shorten and elongate at a frequency of once per wheel revolution. Dirt roads have an insignificant change in the amplitude of this elastic movement. The fatigue is a function of the distance the bike has done ands, the stress relieving that was performed on the wheel at the time of the build. Spoke tension, or rather, the lack thereof, has a large influence on how quickly the spokes fatigue.

 

Finally, the sun itself had nothing to do with the fatigue in the run-up of the final break on that day you observed it. Wheels can stand in fluctating temperatures (that we experience here on earth) for eons before having an effect on spoke fatigue. It is your weight and the number of revolutions the wheel turns, with the two overlaying factors of stress relieve (lack thereof actually) and tension.

 

Using the same spokes, a poorly built wheel will have fatigue breaks at 2 000 kms or if built properly, at 200 000 kms. Small actions at the build stage have a dramatic effect on spoke life.

 

Metal fatigue in spokes is like the old coat hanger wire trick you did at school. A wire coat hanger is made of 3mm wire but you can easily break it by bending it a couple of times in the same place. The same happens with spokes. At the J-bend, each revolution of the wheel causes the bend to bend and unbend a little - a micron at a time. Over time, it builds up and the wire breaks even though Johnny in Grade 6 isn't all that strong.

 

Metal fatigue is cumulative. If you only use the bike once a year, it will take say ten years before breaking. If you use it every day, it will take 10 divided by 365 years to break. Point is, that it will always break after a given number of hours, no matter how they are spread.

 

In your case the spokes were at their last little bit of metal with the break already 99% through the wire. The little bit of expansion broke the camel's back and you were quite privileged to see it happen. To most people it happens whilst riding and they never witness the actual event.

 

In summary:

 

1) Dirt roads had nothing to do with it.

2) It is a function of number of wheel revolutions overlaid onto weight (which exacerbates the problem), build (stress relieving makes them resist fatigue) and tension (tighter spokes will bend less at the bends and thus fatigue less).

3) The effect is cumulative.

4) Just riding along has a far far greater negative effect on spokes than a little bit of sunshine.

[DJR]

@ Johan Bornman

 

Can you explain what is meant by "stress relieving" when hand building a wheel?

 

Can a machine built wheel be improved without taking it apart and starting over?

Edited May 20, 2011 by DJR

[Johan Bornman]

@ Johan Bornman

 

Can you explain what is meant by "stress relieving" when hand building a wheel?

 

Can a machine built wheel be improved without taking it apart and starting over?

 

Stress relieving in the process of getting rid of residual stresses. In the case of wheelbuilding, it is everything except the tension of the spoke, in other words, the stresses left behind when stamping the mushroom head, threading the nipple end and bending the J-bend elbow. The way to do it is to strain the spoke beyond yield and then release it. The exact way I do it is by gripping parallel pairs of spokes and squeezing the together as hard as possible. It hurts like hell. I've developed another technique which is easier on the hands, but I prefer to only give that away to people in my classes - sorry.

 

However, the methods are plenty and all over the net. There's even a video of a machine doing it. Unfortunately involves a press and a mandril that supports the whole wheel, not something for the homebuilder.

 

Yes, a machine built wheel can be improved. The best time to do it is when it is brand new. First thing is to improve the spoke line. This is done by making sure there is no gap between the spoke and hub flange where it exits the flange. Secondly, the nipple swivel angle should not be less than the spoke arrival angle 9at the rim). Should this be the case, the spoke will break at the first thread. To improve the spoke line here, you would have to bend the spoke with a long-nosed plier so that the nipple doesn't exceed its own maximum swivel angle in the rim. It is tricky and time-consuming and is best done with a completely slack spoke, yet with a clear picture in your mind as to where the bend should go.

 

Dont confuse stress relieving with removing residual twist in spokes. They are two different animals.

[DJR]

Thanks Johan

 

I understand your answer about stress releiving.

 

About improving a machine built wheel, however, I've translated your answer into: "Rather let a good wheel builder do it properly from scratch".