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Loose-ball bottom bracket adjustment

Old 04-25-14, 06:53 AM
  #1  
jyl
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Loose-ball bottom bracket adjustment

Let's suppose, just hypothetically, that a particular loose-ball bottom bracket just doesn't seem to want to hit the sweet spot where it rotates with absolutely perfect smoothness AND the cranks have absolutely zero perceptible play. Would you adjust (a) very slightly tight, with a tiny bit of roughness when rotating, or (b) very slightly loose, with a tiny bit of play detectable when pushing a pedal to the right or left, or (c) keep re-adjusting because every BB can always find the sweet spot?
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Old 04-25-14, 07:00 AM
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(b)
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Old 04-25-14, 07:03 AM
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I would replace the balls as some of the are not round or undersized. Also look carefully at the cones because something is wrong. Roger
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Old 04-25-14, 07:05 AM
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Answer--- B or C. Never "preload" ball bearing, it will destroy the bearing.
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Old 04-25-14, 07:06 AM
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Originally Posted by brooklyn_bike View Post
(b)
Concur.
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Old 04-25-14, 07:29 AM
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Tightening the lock ring will always loosen the preload slightly as it draws outward on the adjustable cup. Finding the sweet spot is knowing how much to over tighten before tightening the lock ring.
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Old 04-25-14, 07:34 AM
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I like to adjust loose or caged bearing headsets and BBs with as little mass attached to them as possible. No cranks on the BB spindle. If it feels right "naked", both smooth and tight without play, don't worry how it feels with the cranks attached. Of course the cranks have to be tightly attached without play to the spindle, but that is a different matter. If you can't get it right with just the bare spindle as determined by the feel in your fingers, then the suggestion to replace the balls and check out the cups and races should be followed.
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Old 04-25-14, 07:35 AM
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Originally Posted by Dan Burkhart View Post
Tightening the lock ring will always loosen the preload slightly as it draws outward on the adjustable cup. Finding the sweet spot is knowing how much to over tighten before tightening the lock ring.
Absolutely true. And true for similar headset bearing assemblies, too.
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Old 04-25-14, 08:41 AM
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Good suggestions above, but if they don't work it may mean that the bottom bracket shell needs facing. If the shell faces aren't both perpendicular to the spindle axis the bearings will exhibit both play and tightness that can't be adjusted out.
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Old 04-25-14, 12:43 PM
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Originally Posted by rydabent View Post
Answer--- B or C. Never "preload" ball bearing, it will destroy the bearing.
Wrong. The bearings are designed to run in a preload. Bottom Bracket Bearing adjustment by Jobst Brandt
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Old 04-25-14, 01:47 PM
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The right answer is A

Angular contact bearings need preload
to work correctly. Slightly loose is significantly worse than slightly tight because of something we might call axle drop.

Look at this image as you read the explanation below.


In a correctly adjusted zero play bearing, the balls are always in contact with the inner and outer races. However with angular contact play allows a smaller part of the cone (or lager part of the cup) to be used. That means that there won't be full contact all the way around. The balls begin to lose contact on either side of the bottom, so the axle can drop between adjacent balls when one isn't directly below.

That sets up two problems. The first is vibration because the axle has to rise each time a ball passes directly below, then drop when supported by a pair of balls on either side. The second problem is the dropped axle wedges the balls apart forcing them into loaded contact with each other, or the spacer part of the retainer. Since balls are moving in the opposite direction where they'd touch, you'd be introducing ball on ball sliding friction and wear.

So shoot for the sweet spot, and if you can't hit it dead on a hair tight is a pass. Don't let that give you license to over load the bearing by making it more than slightly preloaded.

Lastly, the sweet spot is easy to hit, and if you're having issues, it might be that you have one ball too many in there.
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Last edited by FBinNY; 04-25-14 at 01:51 PM.
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Old 04-25-14, 02:14 PM
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Originally Posted by FBinNY View Post
The right answer is A

Angular contact bearings need preload
to work correctly. Slightly loose is significantly worse than slightly tight because of something we might call axle drop.
Counterintuitive and true.


Originally Posted by FBinNY View Post
. . . In a correctly adjusted zero play bearing, the balls are always in contact with the inner and outer races. However with angular contact play allows a smaller part of the cone (or lager part of the cup) to be used. That means that there won't be full contact all the way around. The balls begin to lose contact on either side of the bottom, so the axle can drop between adjacent balls when one isn't directly below.

That sets up two problems. The first is vibration because the axle has to rise each time a ball passes directly below, then drop when supported by a pair of balls on either side. The second problem is the dropped axle wedges the balls apart forcing them into loaded contact with each other, or the spacer part of the retainer. Since balls are moving in the opposite direction where they'd touch, you'd be introducing ball on ball sliding friction and wear. . .
Or, to restate, a little preload ensures that the primary load is always carried by multiple balls. Only one or two at a time carry the load in a loose bearing. And the afore mentioned vibration usually sets up dynamic loads in excess of design limits.

Bike bearings seem to tolerate looseness well because they are so overbuilt, but it accelerates wear.
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Old 04-25-14, 04:07 PM
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Originally Posted by rpenmanparker View Post
Absolutely true. And true for similar headset bearing assemblies, too.
Opposite for most (all?) headsets, hubs and pedals. The issue is which way the lockring or nut pushes. In most cases the locknut would push the part ahead of it (ie. the cone or race) forward tightening any adjustment.

The BB is a unique exception because the lockring bears against the BB shell and pulls the cup outward as it's tightened loosening the adjustment.

BTW- I try to measure or detect play at the end of a lever arm, so with BBs I attach the right crank. Then I look for play at the end of the arm, while it's lined up with the chainstay. The small gap gives me a very sensitive frame of reference to detect play. I shoot for the loosest adjustment with zero detectable play, looking for minimal preload.

To hit the target I apply the "Goldilocks" rule.
If I'm sure there's play, that's too loose.
If I'm sure there's no play, that's probably too tight.
If I think there might be play, but but maybe not, that's just right.
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Old 04-25-14, 04:47 PM
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I'd just replace it with a cartridge unit...
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Old 04-27-14, 07:16 AM
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david

Wrong!!! Preloading a bearing causes the races to deform, that will eventually destroy the bearing. As someone pointed out you need to slightly preload the bearing, and then when the lock ring is tighten it brings the bearing back to zero clearance or slight clearance. Simply put, a preload is a bind. Binds use up power that should be driving the bike forward. Why would you want a bind in your bearings? Preload will squeeze out all the lube also which is not good.

Last edited by rydabent; 04-27-14 at 07:24 AM.
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Old 04-27-14, 07:30 AM
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Originally Posted by rydabent View Post
. . . Wrong!!! Preloading a bearing causes the races to deform, that will eventually destroy the bearing. . .
Does placing an external load on a bearing destroy it? Please advise.
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Old 04-27-14, 07:37 AM
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Originally Posted by rydabent View Post
david

Wrong!!! Preloading a bearing causes the races to deform, that will eventually destroy the bearing. As someone pointed out you need to slightly preload the bearing, and then when the lock ring is tighten it brings the bearing back to zero clearance or slight clearance. Simply put, a preload is a bind. Binds use up power that should be driving the bike forward. Why would you want a bind in your bearings? Preload will squeeze out all the lube also which is not good.
I'm always curious about the debate on preload with any ball bearing. Overtightening for sure will cause problems, but how do we tell zero clearance vs slight preload? The way a wheel hub preload was described to me was: too loose and you can feel the bearings knocking, so tighten until they don't. (that means stop immediately, don't over do it).

Without realizing it, I use FBinNY's "Goldilocks" rule. I think you also mentioned no lube from preloading. Well, if there is axle drop, then even a preloaded bearing would probably have some tolerance on the upper bearings being looser to gather lubricant. I have to confess though the only "evidence" in my experience is from bearings that were obviously over tight and now have rough or binding races.
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Old 04-27-14, 07:38 AM
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Originally Posted by zacster View Post
I'd just replace it with a cartridge unit...
No fair changing the rules in a debate. Tends to make the whole debate moot. And what fun is that?
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Old 04-27-14, 08:02 AM
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If you agree that over tightening is bad, then it follows that any amount of overtightening or preload is bad.

Preload plus the external load results in earlier failure.
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Old 04-27-14, 08:17 AM
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Originally Posted by rydabent View Post
If you agree that over tightening is bad, then it follows that any amount of overtightening or preload is bad.

Preload plus the external load results in earlier failure.
Then it follows that any external load is "bad" even if well within design limits.
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Old 04-27-14, 08:28 AM
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Originally Posted by rydabent View Post
If you agree that over tightening is bad, then it follows that any amount of overtightening or preload is bad.

Preload plus the external load results in earlier failure.
No, that's a distortion of terms. Overfilling a glass is not the same is filling it just enough. I think were talking just enough preload so that the bearings don't slide around loosey-goosey and not applying an optimum load evenly on all bearings.
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Old 04-27-14, 09:06 AM
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Originally Posted by rydabent View Post
david

Wrong!!! Preloading a bearing causes the races to deform, that will eventually destroy the bearing. ....
I'd like to see a citation for this opinion.

OTOH - Here's what SKF (bearing maker) has to say on the subject.

The radial internal clearance of a bearing is of considerable importance if satisfactory operation is to be obtained. As a general rule, ball bearings should always have an operational clearance that is virtually zero, or there may be a slight preload.

----

Bearings have load ratings or capacities, and are designed to operate under load.

A preload is a small load well below those load ratings, and so cannot be harmful to the bearing. Obviously we're not talking about massive preloads that begin to approach the load capacity, just enough to establish that the bearing is in fact loaded.
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Old 04-27-14, 09:15 AM
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There may not be a sweet spot because most cup and cone ball bearings are not ground smoothly enough. You will feel a little roughness when there is still sometimes a little slack because the bearing surfaces and balls are not perfectly round and the bearing will be in firm contact for a fraction of a degree, then slack for a fraction of a degree.

It doesn't matter though because angular contact ball bearings need a slight preload.

They also need to be rechecked as they break in because they wear or press a smooth wear land onto the cups and cones which can open up the clearance.
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Old 04-27-14, 09:36 AM
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Originally Posted by rydabent View Post
david

Wrong!!! Preloading a bearing causes the races to deform, that will eventually destroy the bearing. As someone pointed out you need to slightly preload the bearing, and then when the lock ring is tighten it brings the bearing back to zero clearance or slight clearance. Simply put, a preload is a bind. Binds use up power that should be driving the bike forward. Why would you want a bind in your bearings? Preload will squeeze out all the lube also which is not good.
You don't have a clue as to what preload is. Read the engineer Jobst Brandt's article about it. Wheel Bearing adjustment by Jobst Brandt
Bottom Bracket Bearing adjustment by Jobst Brandt

Find me an engineering article that disputes him.
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Old 04-27-14, 10:02 AM
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Let's back up a bit - rydabent said not to preload at all, but with quick release we are somehow shortening the axle, causing tightening of the bearing. The Sheldon Brown links and Park Tool both say the wheel is slightly loose off the bike, and perfect, no rattle, when the skewers are tight. The preload is then applied when the wheel is mounted, not prior to mounting the wheel. This also is different, actually opposite when installing, than the original post asking about bottom brackets.
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