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So called cones are the the inner races in angular contact bearings in hubs. The outer races, usually simply called races, are usually integral to the hub and cannot be replaced.

Cones however are easy to replace, often one would simply get a new axle with cones and counter nuts included.

Apparently, it is quite common for cones to become pitted. Where the surface of a race is covered by a number of small depressions and in worse cases becomes rough and This may happen in a process like Brinelling where the hardened steel balls leave a small concave impression on the race or spalling where after an initial crack flakes from the surface are shed.*

Pitting is often encountered only on the cones while the outer races remain in good shape. Evidence for the frequency of this failure mode is that replacement cones are readily available, which also indicates demand.

Why is that so?

Is this perhaps coincidental due to load geometry? (Concave vs convex ball race interfaces.)

Or is this by design, for example by different hardness outer races > balls > cones?

*In case of insufficient lubrication and/or corrosion fretting may also lead to pitting.

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    How do you know it's brinelling? – ojs Jul 7 at 11:06
  • @ojs that's a pretty good question. I did not think of doubting it when I wrote it. I still think it is indeed Brinelling, at least as initial cause in a lubricated bearing. However, I cannot tell where I have this information from. Suggestion: A new question to ask that, and marking this claim as not certain here. – gschenk Jul 7 at 13:16
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    I am not a mechanical engineer, but I have a strong hunch that if this was true brinelling, the outer race, which is often made of aluminum that is softer than steel, would be dented first. I am not really interested in gaming the Stack Overflow system. If you read the linked article about fretting, there is a mention of false brinelling that occurs with lubricated components. – ojs Jul 7 at 15:55
  • @ojs, well I'm interested in what the actual process is that damages bearings. Since it is too much for comments, I thought a new question would be preferable. Do you think this one ought to be closed until the initial damage process is understood? – gschenk Jul 7 at 16:54
  • There are so many 'quality' parameters in this question and in the possible answers: of the cones, the balls, the races, the servicing, the adjustment, the road, the rider, the rider's style ... where the tiniest changes will have a massive influence, that the answer is rather more a matter of philosophy and speculation. – Carel Jul 7 at 18:05
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According to my research, which consists of a couple of web searches and looking at a few photos, the failure mechanism that causes pits in bearing cones is spalling. It is the result of metal fatigue from balls repeatedly rolling over the same part of the cone, and it can be recognized by the irregular shape of the pits. This is different from mechanical wear, which produces a smooth groove and brinelling, which produces smooth indentations at even intervals.

The reason why cone is affected first is that the load is concentrated on smaller surface. The inner diameter of the bearing is smaller than the outer bearing, so the surface is smaller to begin with, and in bicycle wheels the cone does not rotate so the load is concentrated on a small part at the lower side of the cone. Because the outer race rotates, the load is spread to the entire bearing track. The quality of materials has also some effect here: I have seen this damage only on two Alivio hubs. The cheap no-brand cup and cone hubs that I have seen had a worn groove at the bottom side of the cone, and I have not had cone failures in higher tier hubs.

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    I thought spalling is only a secondary failure mode after an initial damage to the surface. The contact interface of the ball with the concave outer race is also larger than that on the convex inner race. This might increase pressure. This would increase pressure, may this contribute to increased spalling? Lastly, according to the source static overload causes brinelling in equidistant pits. This would indeed not fit to the failure in these bearings. However, there might be a defintion where indentations after transient overload is also considered brinelling: only a misunderstanding then? – gschenk Jul 8 at 21:37
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    Anything can be called brinelling if you get to decide the definition. For contact area, you need to take into account that the bearing surfaces are not flat rings but laterally concave too. If you're interested, have a look at ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19810023007.pdf (rocket science warning: it's not really about rockets but there is a lot of math anyway, with figures at the end) – ojs Jul 9 at 4:21
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The one time I had this problem bad -- wore out several sets of cones -- it turned out that my front hub (not just the axle) was slightly bent, such that the two ends were not parallel. This caused the cone on one side to "walk" inwards while the other one would "walk" outwards, over a distance of several hundred miles. In the process the force on the cones was distorted, and the most obvious symptom would be spalling cones.

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