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PostPosted: Sun Feb 17, 2013 2:19 am 
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Joined: Tue May 03, 2005 2:20 am
Posts: 5793
Location: Belgium
Hi,

Quote:
But the argument is that loaded friction is different than unloaded (I'm a sceptic


IMHO at the low range of revs most bicycle components that are running high friction unloaded is still going to be high friction loaded.
Loaded friction may be less (probably will be) than unloaded friction though if and when that friction is caused by too much lube or too tight initial tolerances of the bearing, for instance. As in no bedding in has occurred yet.

OTOH a bearing that spins freely unloaded may exhibit higher friction when loaded for the same reasons but in reverse. I.e a lack of lubrication and/or not so tight tolerances allow a bearing to spin freely unloaded.
When loaded however this bearing's low level of lubrification may now well increase friction as the contact area with the races increases significantly especially when the load is not a simple radial one but also a lateral one.

Ideally you'd like a cup and cone bearing that, no matter what load or what angle, is going to act as a single line contact bearing with an extremely thin contact point/line at any angle.

Deep groove bearings work in a similar manner, alternating conctact surface direction depending on radial load but when running perfectly horizontal the balls see much more contact area than a mere single line contact. They're surface area of contact is roughly twice when new but when used is approaching more and more the shape of the grooved race.
Hence the contact line is growing wider which invariably increases friction.

Ciao, :)

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Last edited by fdegrove on Sun Feb 17, 2013 1:22 pm, edited 1 time in total.

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PostPosted: Sun Feb 17, 2013 7:01 am 
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Joined: Sun Feb 10, 2013 5:11 pm
Posts: 84
Rick wrote:

I have to rebuild my BB30 bracket 4 times per year to get rid if the clicks and squeeks



There is just no way. Really? Do you guys really have to overhaul your BB30 bottom brackets this much? I am asking because I don't have one. BB30 can't be that bad. Can it?


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Posted: Sun Feb 17, 2013 7:01 am 


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PostPosted: Sun Feb 17, 2013 6:20 pm 
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Joined: Sun Nov 14, 2010 10:25 pm
Posts: 476
on the mtb it was time to change bearings after 1 year.
on the road bike I'm still on the original set of bearings after almost 3 years. I did flush them out and repacked them with grease some time ago though, but they showed very little wear on the races.

If your having to change the bearings 4 times a year, something is wrong. possible allignment issue of the axle or something.

btw: If you want a good bb30 bearing try SKF.

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PostPosted: Sun Feb 17, 2013 6:43 pm 
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in the industry
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Joined: Sun Jul 29, 2012 2:59 pm
Posts: 634
Location: Ruidoso, NM
djconnel wrote:
But the argument is that loaded friction is different than unloaded (I'm a sceptic).


With a larger ID bearing the friction of the seal, grease, and bearings acts over a greater lever. If everything is otherwise the same, the drag of larger ID bearings will be greater.

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PostPosted: Mon Feb 18, 2013 1:54 pm 
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Joined: Fri Dec 15, 2006 7:45 pm
Posts: 3334
Location: Natovi Landing
[quote="djconnel"] But the argument is that loaded friction is different than unloaded (I'm a sceptic). quote]

+1. I get the point that this is true in many other higher load/rpm applications, but it's lazily trotted out as self-evidently true for cycling. No data on this.

I start from the point that if it feels good with minimal load (turning in the stand) it should feel better under more load.


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PostPosted: Tue Feb 19, 2013 1:25 am 
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Tubbie Guru

Joined: Tue May 03, 2005 2:20 am
Posts: 5793
Location: Belgium
Hi,

WMW wrote:
djconnel wrote:
But the argument is that loaded friction is different than unloaded (I'm a sceptic).


With a larger ID bearing the friction of the seal, grease, and bearings acts over a greater lever. If everything is otherwise the same, the drag of larger ID bearings will be greater.


For what is used on road bikes the difference in OD is never going to be more than a couple of mm which, all else being kept equal, should not make much of a difference unless we're talking miliwatt range.

All of these industrial bearings are originally engineered for high loads/high revs., they don't care about contacting seals as these don't bother them for too long. These seals are broken in fast at such high revs. after which they no longer cause drag.
Bicycle bearings run at at low revs and low loads and should be designed accordingly. They are not. A Few exceptions notwithstanding.

Even the best available bearings are still using contacting seals, are still lubricated with a lube that's not drag free.
Anno 2013 one would have hoped for better and moreover that this would have been implemented throughout the entire bike.
Sadly, it is not. Worse still, some aereas benefit from fine tech whilst others are completely ignored essentialy cancelling out the benefit of new tech in first place..... Go figure...

A couple examples:

Most pedals run on an industrial bearing and a bushing. How do you lube this correctly so it does run drag free without sacrificing the bushing? You can't.
What then is the point of running a crank on oil lubed high grade ceramic bearings?

Same for the freewheel. Coasting or not this is bound to be the achilles heel and mother of fricton. Again a combination of a bushing (of little impact once it's loaded) and a set (or at least one) of bearings that are not up to speed.
Again, little point in running top ceramic bearings in that rear hub when all is going to be dragged down by a set of steel bearings that, once again, are going to take like forever to run in anyhow?
How about chains? Tyres?

All the tech is there but no one really uses it to the full. Pity...

Ciao, ;)

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