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I'm curious: why are some hub shells much larger diameter than others? Is it an aesthetic consideration? Or are there cost/functionality tradeoffs? Take for example this Dura-Ace hub -- pretty skinny. Same for a classic Campy record hub. Compare to this much fatter Tiagra hub, or a Phil Wood front hub. I'm just talking standard hubs, not internal gears, powertaps, dynamos, etc. [Addition after DR Hicks answer: nor I am I talking about hub flanges, but rather the barrel of the hub shell.]

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Well, interestingly enough, the Dura-Ace and Campy hubs appear to be large flange (comparing flange diameter to axle diameter). The Tiagra is small flange. Part of the apparent difference is probably just illusion, but there clearly is some "real" difference as well. Phil Wood hubs are by definition built like tank, so that's kind of out of the spectrum. (But when you compare prices you see part of it -- the Dura-Ace is about 5 times the price of the Tiagra.) –  Daniel R Hicks Nov 30 '12 at 12:25
    
Marking you as accepted for now, but interested to see if there are any other theories out there . . . –  joseph_morris Nov 30 '12 at 18:48
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2 Answers 2

up vote 4 down vote accepted

A smaller diameter hub is lighter (and likely cheaper).

A larger diameter hub has several advantages:

  1. The bearings can be larger
  2. Since the spoke flange has a larger diameter, it also has a larger circumference, meaning that spoke holes are farther apart. This reduces the stress on the spoke flange with heavily-loaded bikes, and also makes higher spoke counts practical.
  3. Related to 2, the spokes, for a given "cross", leave the hub at a more oblique angle, meaning that the force of the spoke is pulling more tangentially and less radially. This again reduces stress on the spoke flange and also increases the rigidity of the wheel against torque.

A non-obvious disadvantage of the larger hub, when it gets MUCH larger, is that, for 3 or 4 cross, the spoke arrives at the rim at a definite angle. Unless the rim had been drilled at a matching angle, this increases the stress on the spoke where it enters the nipple, increasing the chance of spoke failure. But this is mostly a problem with geared hubs.

It's pretty much tradition to have larger diameter hubs on touring bikes and tandems. Not sure to what extent this is a practical measure vs "expectations" -- the larger flange looks like its built to be rugged, while the small flange looks "light on its feet".

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I got the impression he was comparing the main body of the hub axle rather than the spoke flange. So the more expensive hubs have narrower axle bodies, but the cheaper ones are wider –  Mac Nov 30 '12 at 3:33
    
Correct, sorry I was asking about hub barrel, not flange. I suppose it could be bigger bearings, but why wouldn't designers of expensive road hubs (Campy record, Dura Ace) want to have larger bearings also? –  joseph_morris Nov 30 '12 at 5:48
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I suspect the reason may be aesthetic. As you've noticed, the cheaper/entry level hubs have large barrels and the more expensive versions have narrower barrels. It must be hard to upsell a customer to a more expensive hub... there's not much to differentiate them apart from the looks.

Personally, the narrow barrels look sleek and fast to me and I might be prepared to drop a few more $$ on them if I'm putting them on a fast road bike.

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