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Old 03-28-10, 07:55 AM   #1
Metaluna
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Effect of bearing precession on hub cones

I have a theoretical question that isn't related to any specific problem I'm having, but it occurred to me as I was repacking a hub the other day.

There are numerous threads and articles around describing what bearing precession is, especially as an explanation for why pedal and BB thread directions are reversed on opposite sides of the bike.

My question is, does this effect apply to hub cones on threaded axles? For example, looking at my 105 rear hub from the left side, the hub shell rotates counterclockwise as the bicycle moves forward. If I'm thinking about it correctly, this would result in a slight clockwise torque on the cone. With a right-hand threaded axle, I believe this would tend to pull the cone away from the locknut. Since the only thing holding the cone in place is friction against the locknut, I would think this would create a vicious cycle where that friction force is reduced, leading to more creep in the cone, leading to even less friction, etc., while simultaneously increasing pressure against the bearings (bad).

So, assuming I've got it right, is this ever a problem in real hubs? And if not, why is it a problem in pedals and BBs but not hubs? I would think it would make more sense to use a left-handed thread on the left side of the axle, and vice versa.
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Old 03-28-10, 08:18 AM   #2
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yes hub cones do become loose over time. whether its from precession or insufficient torque when adjusting the hub i dont know
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Old 03-28-10, 08:27 AM   #3
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If the locknut is tightened properly, it should never be a problem. Precession forces are very small and would never loosen a cone. Very few modern hubs even use threaded cones these days - only cheap bikes with the old 10mm threaded axles.

Some people will swear that precession forces are large enough to loosen the right side BB cup on an Italian threaded frame, but I contend that they are not. If a cup were to loosen for some other reason like the BB face not being square to the threads, or being under-torqued, those forces can eventually unscrew the cup, but not until it's fairly loose from other causes.

The opposite situation never occurs. You don't find BB cups that have tightened themselves due to precession. If you were to hand tighten a BB cup, it will never get tight, all by itself.
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Old 03-28-10, 09:52 AM   #4
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Lock nuts, the clue.
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Old 03-28-10, 09:52 AM   #5
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As DaveSSS said these forces are miniscule, and aren't a meaningful factor in why hub bearings get loose, if they do. They're most important in pedals because of the large twisting force as the pedal spindle is flrxed back and forth with respect to the rotating crank, but otherwise can be pretty much ignored.

Hubs where the bearing adjustment is well secured by a cone/keyed washer/locknut arrangement, or other secure system can go many thousands of miles without getting loose. Eventually they'll develop play because of wear, as will any bearing, but that's about it.
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Old 03-28-10, 11:13 AM   #6
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IMO left-hand threads on bikes are virtually pointless.
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Old 03-28-10, 11:23 AM   #7
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Kimmo,

I have a little experience with peddles and respectfully disagree. My kids have had issues with the peddles on their trailerbike. It seems that they were having fun peddling backwords. I didn't know it because I am usually cruising fast enough, and the trailerbike is geared low enough, that they couldn't "contribute" anyway. What they did find is that after a period of peddling backwords the pedal would fall off. Others have had this same experience with onoccupied trailerbikes. While I do understand that this probably wouldn't have happened if I had checked the torque of the pedels before the ride, I think that we would see many more people losing pedals if the left didn't have left handed threads.
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Old 03-28-10, 11:26 AM   #8
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By 'virtually' I mean completely pointless as long as the bearings aren't binding and everything's nice and tight. It could be argued the threads should be the other way round so then you know when something's wrong, cause bits fall off.

BTW it's pedals.

Last edited by Kimmo; 03-28-10 at 11:29 AM.
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Old 03-28-10, 11:30 AM   #9
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Kimmo, he got it right at least twice.
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Old 03-28-10, 11:35 AM   #10
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I'm guessing he didn't know when he was right, since he employed three different spellings.

BTW, before anyone has a go, that comment re the threads being the other way round was kinda tongue in cheek
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Old 03-28-10, 11:50 AM   #11
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Yeah, I gave it a thought, too.
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Old 03-28-10, 12:16 PM   #12
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only on some raleigh bike hubs does it matter

left side only has the single cone nut for the bearings with no lock nut.
if you insert the wheel in backwards, the hub tightens itself to destruction.
if you insert it in the correct way, the cone can't unwind because the fork dropouts and axle nut are holding it in place.
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Old 03-29-10, 12:02 AM   #13
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Quote:
Originally Posted by AEO View Post
left side only has the single cone nut for the bearings with no lock nut.
O_O

Sketchy...
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