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I just ripped apart my mom's mountain bike's rear hub. One of the bearings was incredibly rough, so I replaced it, and the other one was smooth, but didn't turn very easily. Since I didn't have a replacement for it on hand, I just put it back, but I'm not sure if I should try to replace it. Is there a good rule of thumb on bearings? Does one failed bearing imply that another one is likely to go soon?

While we're at it, should I be doing anything to the freewheel other than cleaning it and re-greasing it? I've heard it's important not to put too much grease inside the freewheel, or it'll increase drag when you're coasting, so I was careful of that.

The specifics for this bike:

  • Mavic wheels
  • Shimano 9-speed freewheel
  • Full suspension mountain bike that gets fairly heavy use on XC singletrack terrain
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    It's unclear what you mean by "one bearing". If, within a single bearing assembly, there is one ball that appears pitted, all balls should be replaced. You should never (except in emergency situations) replace some of the balls in a bearing and not others. But if, say, the front hub bearings have gone bad and the hub needs replacing, that says nothing about the condition of the rear hub or other bearings on the bike. – Daniel R Hicks Jun 7 '15 at 0:26
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According to Lennard Zinn, you should replace all of the bearings even if only one is obviously bad. That way you ensure round, smooth, consistent bearings. I've been using his repair manuals for years and he's pretty much never steered me wrong.

[Edited to repair the link]

2

To be sure, Sealed bearings or caged loose bearings? If they are loose, I usually do the "finger test" on the(cleaned) cone when I remove the bearings. Then again on the cups. If there is pitting on either, change it and the bearings out. Most cups are pressed in hubs, so if those get pitted you may be looking at a new hub/wheel. Sealed bearings I change when they are running rough. That simple. These are wearable parts for a reason. Changing them when they aren't acting right keeps you from breaking more expensive parts.

  • Good point referencing the cost difference. New bearings cost about the same as a coffee. New hubs cost 10-50 times as much, and you still need to relace the rim. Welcome to SE Bicycles too. – Criggie Apr 14 '17 at 22:16
  • Thanks man, it's funny, I've been a wrench for a long time, and I work in the industry, and found this site from looking up an answer to a problem installing a thermostat in my house. – CardMechanic Apr 17 '17 at 14:29
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You should replace all the bearings when you do a hub overhaul, regardless if the bearings look smooth or not. Also making sure that the bearings are adjusted correctly - not to loose and not too tight is very important.... in my opinion...

  • It would be helpful if you explained why it's necessary to replace all the bearings, rather than just the ones that seem worn. What difference would it make? – David Richerby Apr 14 '17 at 23:31
  • This answer duplicates what User313 said in 2011, 6 years ago. – Criggie Apr 18 '17 at 7:04

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