Rim failure

[mountain_lion]

 

One for the wheel gurus...

 

What would cause a rim to crack next to the eyelets as shown in the photos below?

Obvious cracks at 3 eyelets and more that seem to be starting.

 

It is an Alex Rim R390 (road bike front).

 

I don't know much about the history of this wheel. Apparently it is about 2 years old. No noticeable signs of impact damage to the wheel.

 

 

 

 

mountain_lion2009-08-30 12:12:26

[Eugene]

Exact same thing happened to my Shimano RS10 wheels. Looks as if the spokes are about to pull right out of the rim.

 

JB believed that it was the only the anodising that cracked and it was unlikely that the wheel itself was damaged. I can't say if it is the same with yours. I'm not very technically minded.

 

Cool Heat replaced the wheel for me, no questions asked.

Eugene2009-01-16 00:55:08

[Daxiet]

No expert opinion, but I wouldn't trust those rims anymore. There looks to be a buldge around the eyelet. I would be worried about them giving in.

Could this be as the result of someone being a bit over zealous when truing the wheel last? Just a further question to those in the know.

Daxiet2009-01-16 01:04:23

[Delgado]

Anodizing!?

[Johan Bornman]

Those cracks are fatigue cracks and very common on poorly designed rims. Include just about all the fancy brand names in that class of rotten apples.

 

1) Anodising. Anodising is a hard ceramic layer that's applied on top of aluminium. It penetrates the aluminium half-way. In other words, if the anodising is 3 microns thick, 1,5 microns would be inside the aluminium and the other half on top. Since it is ceramic, its properties are like glass. It doesn't like to flex and will crack. Once it cracks, it propagates those little cracks into the aluminium much like a hard scab on your knee propagates the pain of bending into the nerves in the soft tissue underneath the scab.

 

Those little cracks work their way into the aluminium and destroys the rim as seen. Someone paraphrased me about cracked anodising. Not quite what I usually say. Anodising doesn't crack on its own like paint that sits on top of the substrate. If anodising is cracked, the substrate is cracked.

 

2) Design. A god rim will have a design where the teat that's pulled from it by the tension in the spoke, will be minimised. For instance, imaginge a broad, flat box-section rim and a spoke pulling from the centre of that. The teat will be perfectly round, circling the hole and limited by nothing other than the aluminium's resistance to flex.

 

Now imagine another rim with a deep V-section and the spoke coming out of the sharp V. You'll see that the rim's cross section prevents a proper teat being pulled and hence that rim will suffer less from that problem.

 

3) Metal fatigue. All metals fatigue when constantly manipulated. At school you used to break wire coat hangers by bending the hell out of them. Actually, you were bending them to heat them up so that you could brand little Johnny in front, but nevertheless, you discovered that too much bending of that wire eventually broke it. It's the same with rims, they bend and bend and bend and bend and break.

 

The trick is to prevent the repetitive bending in the first place through proper rim design. One strategy that works well is the so-called double eyelett or socketed rim. A bit heavier and ever so unfashionable, but durable.

 

4) Spoke tension theory. Spoke tension doesn't contribute to rim cracking. We know that by putting a stick into someone's wheel when he travels at speed. The stick breaks the spokes in pure tension but the spokes never/seldom rip right out of the rim. That's because the rim is stronger than the spoke in pure pulling force. Yet, when rims crack people blame the builder for too high tension.

 

An educated  builder understands that good tension is a friend, not an enemy. Whether the wheel is under high or low tension, the relative movement of the teat will be the same for each revolution of the wheel. It is the number of revolutions and rim design that makes the difference. DT Swiss, Rolf, Mavic and Shimano just dont get this or simply ignore it in favour of fashion and weight.

 

Summary:  The rim in question is a single eyelet rim with a heavy layer of hard anodising on top. It's a disaster engineered into the rim from the start. Keep on insisting on a guarantee replacement until the manufacturer gets tired of it and produces better rims.

 

Also, stop trying to go lighther and lighter. The less meat on a rim, the less durable it is. Period.

 

 

Edit: This week I've had two calls from people with similar rims. Bontrager and Rolf. Both people attributed the problem to an accident. This is not true. An accident will only lessen the tension on the spokes, never increase it. All that happened is that the rims were cracked and after the accident they inspected the wheels properly for the first time in a while. They discovered the cracks and blamed the accident. Blame the factory, it wasn't you.

 

Wheels that suddenly wobble are often cracked. I see this when people come to me to take out a wobble. A cracked rim obviously relaxes the spoke and hence the wobble.

 

To replace a rim on a fancy wheel is easily R1300 plus labour. To replace a rim on on a hand-built wheel done with honest-to-goodness components cost R310 plus labour. Go figure.

 

 

Johan Bornman2009-01-16 02:29:13