Bicycle Mechanics - pitting on 1/2 of cones
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Jean Beetham Smith
12-02-02, 07:04 PM
After 4500 miles I thought it was time for some new bearings. When I got my front hub disassembled I found that the cones look fine on one side and have pitting, actually more like radial striations, on the other half of the contact circle. When I got the second cone out it was the same. I don't see any sign of pitting in the hub races. Any thoughts on this pattern of wear? I had expected it to be more uniform. BTW, the axle does not appear bent.
If the axle has been sitting in one orientation most of the time, then all of the upward forces will have been against one half of the cone circle, causing the sort of wear pattern you encountered. Naturally, as you implied, a bent axle could have made the situation even worse. (The races, continually rotating, would wear symmetrically.) Perhaps we should rotate our axles periodically.:D
12-03-02, 05:14 AM
These observations concur with my experience - Bearing cones tend to become damaged on one side only. In fact, rotating the axle periodically is probably a good idea to spread the wear. Wheel-bearings are a constant frustration to me, as they seem to wear out with alarming frequency, especially if the bearing is not sealed to keep out water and grit etc. I suspect that the failure mode is largely due to contamination. Bicycle bearings do not run at high speed, or risk overheating in use. However, I am concerned that (especially for wheel bearings) the loads experienced are very close to the design limits. Adding contamination would certainly not help matters and pitting or spalling of the cone surface results. At present I am doing a little reading up to explore these issues and also to compare the typical recommended loads for industrial bearings of similar size to bicycle bearings. The hope is to understand the problem in a scientific way. I will post again if anything interesting turns up.
keep rolling :beer:
12-03-02, 01:41 PM
But, that implies she went 4500 miles w/out ever removing the front wheel!!?? Or, replaced it with the axle oriented roughly the same.
That just strikes me as odd... my wheels comes off for one reason or another (service, transport, whatever) about once a week.
12-03-02, 02:01 PM
It is a bit late now, but when you took it apart, it would have been helpful to check the adjustment. It seems very likely the hub was also too tight in the initial set up. There should have been play in the axle outside the bike. Front axles especially will show a different when clamped in the fork versus outside the bike.
The cone does typically wear out first, as it is smaller and see more wear as opposed to the cup. For hub adjustment see http://www.parktool.com/repair_help/howfix_hub.shtml
12-04-02, 04:18 AM
Hmmm, good point roadbuzz - I should have thought of that :)
Calvin - I have heard the following schools of thought on the adjustment of wheel bearings:
1) adjustment with slight play
2) adjust to remove play, but stay as free running as possible
3) adjust with slight tightness i.e. slight pre load of the bearing
Now, I will stress here that I am no expert, but it seems to me that option 1 means that the bearing is slack, and so the axle can rise and drop under load as each ball runs beneath it. This means that the load is not spread so evenly over the bearing surfaces.
In option 2, this situation is mostly corrected. Option 3 is my choice, and is a common engineering practice. I adjust so the axle can still be turned (off the bike) with the fingers. This is a more complicated situation on a bike with quick release hubs, because the axle is compressed when the skewer lever is tightened - in fact the wheel bearing may have play when off the bike, but this disappears when fitted. So in this case, a little fiddling is necessary to get things "just-so" after a bearing service.
Sorry if this was all a bit long winded - I'd genuinely be interested to hear other opinions.
12-04-02, 07:36 AM
As as general rule, bearings should be adjusted so they are as loose a possible, but without play, in the their working position. For a QR type hub, there should be play when out of the bike, but not play when mountend in the bike. A useful and educational test is to use spacers to act as dropouts. Clamp a the skewer on the spacers and then turn the axle with your fingers. You will be able to feel quite a bit of difference between the skewer being open and closed.
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