Dynamo hub fitted wrong way around - problem?

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Mick F
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Re: Dynamo hub fitted wrong way around - problem?

Post by Mick F »

What happens if you wheel (or ride) the bike backwards?
Is there a warning?
Mick F. Cornwall
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meic
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Re: Dynamo hub fitted wrong way around - problem?

Post by meic »

You would have to ride it for a long way (for going backwards that is) unless you did it to a bike which had seized since you last rode it.
As the unscrewing action would have an enormous amount of slipage and every time you ride forwards you would probably retighten things.
Yma o Hyd
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CJ
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Re: Dynamo hub fitted wrong way around - problem?

Post by CJ »

meic wrote:We should assume that as a proffesional cycle mechanic he knows that this particular hub in this situation is OK.

Unfortunately a lot of us have given up on proffesional mechanics because they often DONT know as much as they pretend.


Too true, and this is a perfect illustration of the latter case. :!:

The hub internals may well unscrew from the hub shell if this sort of wheel is used the wrong way round. They unscrew not becasue of any drag, great or small, that comes from the dynamo or its bearings, but because the threaded cylindrical interface between hubshell and internal assembly is subjected to a rotating load.

Actually the direction of loading is constant (i.e. vertical) whilst the threaded connection does the rotating, but the effect is the same. It's called precession, and it's the same effect that obliges left pedals on bicycles to have a left-hand-thread. But even though every single bicycle has two pedals, precession is a hard thing for a bicycle mechanic to get his "prodessional" brain around and so they do tend to come up with all sorts of other explanations for that threading! :roll:

So it's not too surprising that this mechanic can't understand why putting this wheel the wrong way round might be a bad idea. And I don't envy anyone the task of trying to explain precession to such a person. You can start, perhaps, by pointing him at this page by Jobst Brandt via Sheldon Brown. It attempts to explain the pedal situation in terms that a non engineer might understand. Then tell him the situation of hub internals within the shell is identical to that of a pedal in a crank. Accordingly, since it has a normal right-hand thread, it had better be positioned to screw in from the right just like a right-hand pedal.

If it's left on the wrong side of the bike then just like a wrong-threaded pedal it will sit there very happily for some time. Maybe a very long time, maybe not, it depends on how close-fitting the threads are and how tightly they were initially tightened. Sooner or later though, the internals will start to unscrew and once it really starts it won't stop.

If this happens when going down a hill it's unlikely the rider will diagnose the problem and stop the bike in a controlled manner, before the front wheel has shifted far enough off-centre to cause serious interference with the brake or frame - perhaps enough to jam the wheel and send the rider over the handlebars!
Chris Juden
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iviehoff
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Re: Dynamo hub fitted wrong way around - problem?

Post by iviehoff »

Many thanks Chris, that is what I was looking for.
JEJV
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Re: Dynamo hub fitted wrong way around - problem?

Post by JEJV »

CJ wrote:You can start, perhaps, by pointing him at this page by Jobst Brandt via Sheldon Brown It attempts to explain the pedal situation in terms that a non engineer might understand.

I found this explanation easier to understand: Re:English Bottom Bracket Set

CJ wrote:To demonstrate the effect of this rotating load...
Don't drive on the railroad track.
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