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Bearings; Why have engineers done this?
Topic Started: Nov 19 2012, 09:51 AM (194 Views)
Chadders
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Mud Connoisseur
Someting has occured to me this morning.

Why is it that we have taken the bearings out of the inside of bottom brackets to now use outboard cups yet in headsets we have gone the opposite way and moved the cups inside the headtube a la cartridge systems?

Stiffness is proportional to length and thickness of tube. A head tube is stiffened by moving the cups inboard and lengthening the tube. Why not just make the BB shell wider and thicker instead of weakening the structure by having the cups hanging off the bike?

Ideas anyone?
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Bunnyflop
Mudking
er, not a flipping clue but it sounds v silly to me.... :D :D
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darth vader
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Mudking
I thought that larger diameter bearings spread the load better hence wear out less quickley
hence the use in BBs wear the whole rides weight is going through the bearing


less stress on head set ? hence smaller? dont know really
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belugabob
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Mudking
Probably a standards thing, what with most frames having the same size bb shell.
Isn't what you describe what that new fangled bb30 standard is all about?

UPDATE: Some more info here
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woodsman
Bog Monster
Marketing,the next new 'it must be better' thing.

I've gone back to square taper on my Enigma build, a shiney titanium axle BB, made in Hampshire by Royce. Cost a king's ransom but it the smoothest ever and really noticable when riding.
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Ming the Merciless
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Overlord of all Known Sheds!
Press fit BB's are sh@te! Very susceptible to water ingress and also last nowhere near as long as a hope external BB. You also need a good drift tool and press to remove and refitr them. CRAP, CRAP, CRAP. I'd have a Hope external relube-able BB over any other BB I've had to suffer.
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darth vader
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Mudking
I find this hard to believe


Ming using a press and drift tool?

surely a Hammer is the tool for the job :D :D

what has happened to Ming ? Wash your hands from any press tool infection and get a hammer ( or as they say in the Army 'Engineering tool fine adjusting')
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Tez
Fairly Muddied
So I have a press fit bb30 and have to agree it is ''sh@te''. Mine is new and after a couple of wet rides its dead! My knowledge of all bike stuff is as bad as my bb, can I change it to something longer lasting or am I stuck with it?

Tez.

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guy
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Mudking
The issues are different.

On a bottom bracket if you have the bearings mounted outboard you can have a larger diameter axle within a standard bottom bracket shell which is where you get the additional strength. This is a big issue as you are putting a lot of force through the cranks and into the bottom bracket when you stamp on the pedals. The overall width of the bottom bracket, she'll and cranks is limited by the Q-factor width of you pedal stroke as you dont want you pedals too close together or too far apart as this can be either uncomfortable or innefficient.

On your headset it's different. The forces are still very large but they are mainly vertical not latteral. You also have the issue of welding the steerer tube to the down and head tubes but the height of this area has an impact on how your bike handles: an large external headset would just add to this height and you would note able to do this for all frame sizes as it would spoil the handling of a smaller frame . Fnally there also have this issue that the lower race takes more force (from impacts through the forks) than the upper (through the handlebars) hence tapered head tubes.


Cor, with all that BS I should have been an engineer! ;)
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Ming the Merciless
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Overlord of all Known Sheds!
Not much you can do with a shitmano pressfit bb, apparently the Hope external BB bearings (not in the bb carrier) may fit.

I'm getting into the habit of turning the bike upside down and taking the seat post out so any trapped moisture can escape. It truly is a crap design as there is no way you can get at the bearings for any TLC. Hopefully somebody will come up with a better sealed or relube-able bearing.
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Chadders
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Mud Connoisseur
I think I like Woodsmans answer best. I have never had an external bottom bracket feel as smooth or last as long as a cartridge type.

So it begs the question why not just expand the BB to take a bigger cartridge?
Personally I blame the weight weenies for this one because weight is the only disadvantge I can see.
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