On Fri, Sep 18, 2009 at 9:21 AM, Steve Maas <bikestuff@nonlintec.com> wrote:
> Jerome & Elizabeth Moos wrote:
>
>
> .....I've long said that the technically correct ISO BB standard would
> have been Swiss, since it was metric, and had the correct LH thread o
> on the fixed cup. But of course that decision was made based on market
> share, not technical logic. At least the English thread anointed ISO
> did happen to have the "correct" LH fixed cup as well.
>
> It's important to remember that the purpose of standards is to normalize
> what most people are doing already, not to determine what's best and try to
> force everyone to do it. That almost never works. On rare occasions when
> it's been attempted, people just ignore the standard and adhere to some kind
> of de facto standard, which is established by everyone immitating some
> common practice.
>
> French dimensions are a good example of an attempt to create a standard
> apart from common practice. We all know how well that went!
>
> As for BBs--is the ISO standard based on British dimensions the only one?
> I'm away from home and my books, so I can't check. There's no reason why
> there can't be multiple standards for some type of item.
>
> Steve Maas
> Göteborg, Sweden
>
>
> I can attest first hand to this logic of standardization. I'm writing an
ISO standard in another field, and the primary purpose is to promote global
trade by establishing a common language for design and description of
products in the relevant domain. There is a great tension between the
standard writing team and some of the reviewers. Many reviewers think that
the purpose of the standard is to show what is the limit of concievable
technology, the best engineering solution, or what their sponsors think is
best.
Back to BB engineering:
Granted, for BBs with reverse threaded BBs the pedaling torque will not be in a direction that might loosen the fitting of the BB. But there must be more going on, because not all BBs have a reversed thread, and of those that do not, not all of them loosen while in use. If a reversed thread was absolutely necessary in all potential cases, we would have to see hub locknuts using both a righty-tighty and a lefty-tighty design. However, they don't.
I'm not an ME, but I am a degreed engineer and pretty good sometimes at explaining basic physics, so lemme give this a try.
The basic principle of the bolt is what's going on. Bolts hold things on because when tightened they are stretched springs. The threads pull on the shaft of a bolt to pull the head hard against its seating surface, presumably on the part to be fixed in position. To apply pressure to that surface, the shaft must pull the bolt head. This stress in the bolt shaft (and to some degree the head) causes an elastic strain stretching the bolt by some amount. The amount of stretch is related to the necessary ammount of pressure. That in turn is based on the expected forces that could distort or break the attachment between the two parts. The greater the expected forces, or the greater the stability required in the attachment, the greater the pressure needed. For more pressure, you would want a heavier bolt, perhaps with a finer-pitch thread. Multiple bolts are also used. Safety margins are also a big part of these design decisions.
But mainly this shows the need for a torque spec. It should give a range, with the lower value representing the minimum for which the joint will have adequate stability considering a safety margin. Sometimes, like in some automotive main bearing and connecting arm bolts, the elongation is directly measured.
In BBs the cup can be made stable by tightening it enough that the chance of loosening is extremely low in the expected use over time. A secondary result to tightening a bolt is that the pressure between the threads on the bolt (or BB cup) and the threads on the nut (or BB shell) increases. As it increases the static friction increases. The force required to move the tightened cup is greater with a tighter cup. The force required to disturb or distort that joint in turn is greater - the joint is more stable and more resistant to loosening.
The torque that you should use is not infinite, because first, our muscles are limited, and mainly, the strength of metals are limited, as great as it is. It is possible to strip bolts with enough force, and similarly BB threads at least in principle. The torque spec again provides useful guidance, specifying the maximum torque that should be applied to prevent damage. A good torque wrench can be a big help, if you have a good set or torque specs for your bike.
Goteborg, huh? I like that city!
>
>
> _______________________________________________
>
--
Ken Freeman
Ann Arbor, MI USA