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Posted
That is a fatigue crack helped along by the fact that the rim was anodised. A new rim of the same dark colour used by the same person over the same distance will again crack in one year and 3 months.

 

It is a poor design' date=' is what I used to say. However, I had a very acrimonious conversation with an importer of a similar rim that cracks prematurely and he defended his product saying it is not intended for continuous riding (or such, I'm paraphrasing here) but that it should only be used by old ladies going to church on Sundays.

 

Basicaly he admitted that these fancy lightweight wheels cannot be expected to last. They're intended for show days and race days.

 

Now I don't say it is a poor design anymore and/or a manufacturing fault, I say that you have used the rim for what it wasn't intended to do. Please be clear that you won't find the definition of what you are not intended to do with the rim in the brochure, on the website or from the importer's mounth. It's a vague, esoteric thing that's a little bit like carbon's amazing ability to be both stiff and compliant at the same time and cannot be captured in words.

 

My best suggestion is to replace the rim but make sure the next one isn't dark anodised, perhaps just silver, it makes a teeny weensy difference in longevity.

 

 

 
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Hi JB, I'm genuinely keen to know why anodizing weakens the aly, as I've used it in a few applications and do I have to be concerned about failures. Although I dis some cranks about 5 years ago & still fine.

As for wheel life, I bought a pair of light weight mavics and was told they are only for racing and will not last by the lbs I bought them from.
Posted

Anodising is a ceramic layer. Hard as glass and brittle as glass. It is applied via an electrolytic process in an acid bath.

Approximately one half of the ceramic settles on top of the aluminium and one half migrates into the top layer of the aluminium.

When subject to bending forces, the anodising forms small cracks - crazing if you like.

These cracks then propagate into the aluminium and cause stress cracks. The darker the anodising's colour, the thicker the layer and the deeper it has penetrated and the quicker/easier it cracks.

 

Think of anodising as a scab on your knee. One half of the scap sits hamlessly on top and the other half is integrated with the soft and sensitive flesh inside. Bend that knee and you'll feel the effect of the hard surface cracking and propagating its crack into your nerves.

Put that same knee in water, let the scab soften and freely flex your knee without the same pain.

 

Anodising is not suitable for structural components undergoing cyclical stresses, such as rims. I know Lefty will cite an example of of a fighter airplane's main spar that's anodised, but I have no idea what they've done to guard against cracking. Thin, cosmetic anodising like a light silver colour is much thinner than black and has less of a tendency to crack.

 

Rims undergo cyclical stresses. The tension in the spokes pull the rim into a lump (I'd like to say teat, but The Legend will get excited) around the spoke hole. With each cycle of the wheel, as the spoke is at the bottom of the wheel, the teat is relaxed. As that spoke turns away, the teat is pulled back and it travels around the wheel until it gets to the bottom again, where it is relaxed once more.

 

It is this cyclical pulling and relaxing that fatigues a rim. The fatigue is normal, all metals fatigue eventually, but anodising speeds the process up greatly.

 

Here's an example.

 

20090327_094022_Bontrager.JPG

 

This rim didn't crack because the spoke pulled through the rim, it cracked because the anodising caused a fatigue crack right around that "Teat" that I mentioned earlier on.

 

Many wheel manufacturers will quickly tell you that it was too high tension that cracked the wheel. It isn't so, no matter how often they say it. Firstly, most rims that cracked that I examine are factory assembled wheels. If the tension is too high, they should blame themselves.

 

Most cracks are anodising-induced of from rims designed to be as light as possible, but not necessarilly have the endurance that we'd like from our components.

 

It is easy to prove that spokes don't pull out of rims. You cannot pull a spoke through a rim, since the spoke breaks first.

 

Here's an example of that:

 

20090327_094405_bontrager2.JPG

 

This particular wheel was in a car accident. The aluminium nipples tore out of the rim, but the rim is intact, in spite of its anodised surface. One violent event doesn't pull a spoke out, either the spoke breaks (as in the case with brass nipples) or the nipple breaks (as in this example with aluminium nipples).

 

Anodising has a function. It gives wheels colour, it protects the aluminium from the elements and itl provides marketers with copy - "hard anodised....hurry, hurry, get yours now."

 

Hard is not better when it comes to wheels, soft is.

 

Lightly anodised silver rims are extremely durable but not very fashionable.

 

 
Posted

Thanks Johan, makes sense.I have a pair of Cosmos training wheels,black, the rims cracked after a year, I put Open pro rims, black, also cracked after a year. Now I've got Silver DRC rims, Have lasted about three years already. 

Posted

Johan.

The next question then.

What about a black anodised aluminium Mountain Bike FRAME.

 

Presumably NOT the same stresses as a wheel.
Posted

Frames dont' have the same stresses as a rim....naturally, the two do a different job.

 

However, a frame's stresses are far less than a rim's stresses and, less localised.

 

A frame flexes a bit when you're pedalling heavily. But when you coast, it doesn't flex and the wheel still flexes once per revolution. It can't escape the stresses as long as you ride and it turns.

 

A frame's stresses are also less localised. For instance a downtube and seat tube flexes sideways with pedalling, but the flex (bend) is spread out over the length of the tube. A rim on the other hand, flexes in a very localised spot around the spoke hole.

 

Anodising on a frame is thus fine.

 

 

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