Bicycle Mechanics - Pitted cones

What causes hub cones to pit?

What causes hub cones to pit?

Hmm...interesting question. Perhaps this 91-page document holds a clue: http://www.vibanalysis.co.uk/technical/***%20Brg%20Damage.pdf

I'm afraid I don't care enough to read it. But if you want to summarize it when you're done that would be great.

Neglect?

Here's another publication, this time with perhaps not quite enough info: http://www.wilcoxon.com/knowdesk/bearing.pdf

I gather from the descriptions therein, pitting is due to a combination of excessive heat, vibration, breakdown of lubricant and/or presence of contaminants.

I take you question to be aimed at understanding the pitting pattern of wear - ie. why does a cone pit rather than wear evenly?

Once a pit begins, it's not hard for me to imagine it getting larger, much as our midwest potholes get larger as more cars run over them. Why the pits begin with one particular molecule and not its neighbor is beyond me - is there a metalurgist in the house?

Improper adjustment of the bearings, leaving the bearing assembly too loose, is the major cause of cone wear. In a properly adjusted bearing assembly, all the ball are in contact with the cup and cone at all times and all of the bearing loads are distributed evenly. When the adjustment is too loose, only one or two balls are in contact with cup and cone so all the load is concentrated, leading to wear in a small area.

Most people, in a misguided attempt to reduce bearing friction to a minimum, will adjust the bearings so that the bearings have th slightest hint of play or so that they are on the edge of being loose. Since bearings can only get looser with time, the bearing starts to get some play and rapidly gets damaged.

When the races are (yes it is still considered a race) made the hardening process is not equal on all parts of
the race leading to having a few soft spots, Then from the reasons listed above(too loose, lack of lube etc.)
the bearings wear on these spots and eventually take away some metal. I heard of some guys actually
parkerizing their bearing races to help the last longer. The cam in your car is parkerized so it must work and
make the part last longer. It is expensive to have done and dangerous to do yourself for most folks. Maybe a
group discount could make it worth while if a club or group of riders gathers everyones parts to have them all
done at the same time will make it cheaper?

What causes hub cones to pit?

On badly pitted hubs I've opened up, there's usually been individual bearings that are in much worse shape than the others, so I always figured that once a bearing starts to wear, it gets stuck in a specific spot on the cone. The bearings still roll in place, but tend to stay in that place. And the worst bearings just slowly tear up a specific spot on the races and cones.

That just seemed to be a sensible explanation to me. But I could be 105% wrong.

Like a crack in concrete, once you have a small failure point it will propogate from that point. All it takes is a small failure in the hardface and the failure will will work away from it. it goes down, making a pit, becuase the surrounding material is harder thatn the material below.

Improper adjustment of the bearings, leaving the bearing assembly too loose, is the major cause of cone wear. In a properly adjusted bearing assembly, all the ball are in contact with the cup and cone at all times and all of the bearing loads are distributed evenly. When the adjustment is too loose, only one or two balls are in contact with cup and cone so all the load is concentrated, leading to wear in a small area.

Most people, in a misguided attempt to reduce bearing friction to a minimum, will adjust the bearings so that the bearings have th slightest hint of play or so that they are on the edge of being loose. Since bearings can only get looser with time, the bearing starts to get some play and rapidly gets damaged.


I've seen advice somewhere that said a tiny amount of play should be allowed as the tightening of the QR will bring the exact setting required. is this wrong do you think?

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I've seen advice somewhere that said a tiny amount of play should be allowed as the tightening of the QR will bring the exact setting required. is this wrong do you think?

Yes, many people do this and if its done right, it sounds like it will work. My personal preference is to adjust bearings a little tight as the only thing they can do is loosen (assuming properly tightened cone/locknuts) so I have a margin of safety. The friction from bearings is absolutely negligible compared to all the other friction forces acting on a bicycle.

Any piece of metal has microscopic flaws even when it is brand new. My guess would be that friction is greater at these flaws and the greater friction results in more wear at these locations. More wear increases the size of the flaw which increases the friction even further eventually resulting in pitting. This is similar to Rev. Chuck's point about how concrete deteriorates.

Bearings always concentrate the load on a few points. The ball and race actually deform under the load. Usually bearing failure starts with a microscopic piece of dirt or debris but it can start from overload. The process is the same once breakdown starts. A particle is rolled over by a bearing which concentrates great force on a small area which causes a small surface fracture. Repeated flexure eventually spalls (breaks out) small particles of metal much like what happens with a pothole in a roadway. The pit (pothole) can appear either in the ball or race. The particles of metal and the edges of the pits produce additional fractures and the process multiplies rapidly.

There is no circulation and filtration to remove particles in the bicycle bearing lubrication system unlike an automobile engine. Corrosion from moisture contamination etc can also initiate the cracking and spalling process.

Keep your bearings absolutely clean! Examine bearings closely for evidence of roughness, cracks or pits, if found replace the bearing immediately. Usually if a pit or crack is found the entire bearing must be replaced which includes all the rolling elements (balls) AND both races.

Corrosion from moisture contamination etc can also initiate the cracking and spalling process.


Definitely - I have seen this several times in poorly sealed hubs on my bikes (used during UK winters, where the roads are salted etc..) After moving to similar priced, but far better sealed components, hub life was increased significantly.

Case 1. New Sora hubs (included with new bike in 2001) - major cone pitting & bearing failure in about 3 months from new. These have no rubber seal against moisture, only a dust guard.

Case 2. Wheels built for a road bike with Deore MTB hubs (about the same price and similar internal components to Sora at the time). Approx 2 years before cones replaced for minor signs of wear. Bikes used under similar conditions & mileage.

If you ride a lot in wet conditions, it is really worth investing in well sealed bearings - I have had similar experience with headsets and bottom brackets.

Ed

Sealed bearings are best. Remember that shielded bearings are not the same as sealed bearings. Shielded bearings just have a mechanical labyrinth that keeps out the big crud. There are many types of seals and materials. Even sealed bearings can be harmed by riding in deep water. A warm bearing when suddenly cooled can suck in water. When sealed or shielded bearings get rough or run out of lubrication replace them.

BTW- The inner cone or race is often the first to show extensive pitting because that surface is subjected to more repeated contact than any other surface in the bearing due to it's circumference.