interchangeable hubs?

On Wed, 16 Apr 2008 18:22:36 -0500, Gary Young wrote:

On Wed, 16 Apr 2008 12:47:38 +0000, _ wrote:

On Tue, 15 Apr 2008 22:22:36 -0500, Gary Young wrote:

On Wed, 16 Apr 2008 02:30:10 +0000, Ryan Cousineau wrote:

In article
,
landotter wrote:

On Apr 15, 6:01 pm, Jeff Welch wrote:
I have an old huffy 3-speed, my best guess is that it is from the
1970's. I'm just learning how to maintain and repair my own bike,
and was wondering if it is possible to convert the current Shimano
3-speed hub to one of their new Nexus 7-speed hubs. Where would I
start to figure out if it's even possible? (Tire size, frame fork,
something else?)

Sure, it's possible. A Sturmey Archer hub might be a better choice
for that frame spacing, if you don't want to spread it. Plenty of
options with and without brakes.

To elaborate, you'll almost certainly want to build a new wheel around
the Nexus hub, so just find out what the old wheel size was (you can
easily determine that from the tire markings or by taking it to a
half-competent LBS), and spec the necessary rim from there.

The Nexus 7 hubs can be built with rim, roller, or coaster brake
setups, so you can preserve the present braking setup or change it as
you prefer.

As Landotter suggests, an S-A 7-speed will fit in your frame without
changing the rear spacing, though potentially at the cost of a bit of
efficiency and niceness that the Nexus hubs have.

Does Sturmey make a 7-speed hub? If you mean the 8-speed hub, that may
not be a good choice for the OP because the gearing is better suited to
small-wheeled bikes:


Nonsense.

The only difference between a small-wheeled version and a large-wheeled
version of a successful Nexus conversion are the sizes of the chainring
and rear cog; the internal arrangements of the gearhub have no increased
suitablilty for wheels of any size. The only considerations of this
order that would be of importance are range of gearing available and
maximum torque allowed.

Take a look at Sheldon Brown's internal gear calculator:

http://sheldonbrown.com/gears/internal.html

If the OP has 26-inch wheels, a 170mm crank, and a 40-tooth chainring,
then adding the Sturmey-Archer 8-speed hub with its stock 25-tooth
sprocket would give him a range of 41.6 to 126.9 gear inches. That
strikes me as pretty high for a typical gearhub bike.

Now try the Shimano Nexus 8-speed hub with its stock 19-tooth sprocket
(all else held equal) and you get a range of 28.8 to 88.4 gear inches.

It's true that you could get the Sturmey-Archer down into the Nexus
territory with a 28-tooth chainring, but it's my understanding that such
a small ring would probably result in rapid wear of the ring and chain
(though perhaps I'm mistaken about that). Moreover, the OP, who has an
old 3-speed, probably doesn't have a crank with replaceable rings and
thus would have to buy a new crank and maybe a new bottom bracket.

This problem would be exacerbated if the OP's wheels are larger than 26
inches.

So, I don't think it's nonsense to say that the Sturmey-Archer hub is
better suited to small-wheeled bicycles (and actually I was just echoing
Sheldon Brown's judgment on that).

But it is not the HUB that is unsuited to small wheels, it's the rest of
the kit.

(And chain wear is not made worse by small sprockets until you get into the
11-12-13 tooth range; though 11's have been used on SA multi-gear hubs, and
I run a 13 on the Nexus.)