Bicycle Mechanics - Converting a 21-speed MTB to internal hub gearing - what is involved?

My early 90's Schwinn MTB has a pretty low end Shimano 200GX 3x7 drivetrain that skips all the time, shifts poorly, and is mostly useless for me on the mean, flat streets of Chicago. From the first few times I rode it I knew I wanted to gut the drivetrain, and was planning on going with a single or double in front with a modern Deore or LX derailleur in back. That was before this past weekend, when I had the pleasure of going to Rapid Transit Cycles and test-riding a Dahon Curve D3. I didn't like D3 so much... but I really, really liked the shifting action of the Sturmey-Archer 3-speed hub.

So, tentative change of plans. In the spring, financial situation permitting, I'd like to go with a single in front and an 8-speed Sturmey-Archer in the back. Simple, graceful, and more than enough for this city.

Is this conversion possible?

If so... I have a decent bike multi-tool (http://www.parktool.com/images/products/productimages/spr_MTB-3open_2005729_47573.jpg), some hex keys, a POS crescent wrench, and a general lack of fear about fiddling with my bike. What else would I need, besides the actual S-A hub?

You'll need to get that hub rebuilt to a new rim & spokes, if it's not the EXACT same dimensions as the old hub, which I seriously doubt. Pretty much need to get the hub laced as a new wheel.

Make sure your frame spacing is compatible with the new hub.

I'm probably forgetting something too, so I'll leave that to the others.

What about the chain? Does a geared hub take a different sort of chain than a 7-speed cassette would, or could I just shorten my existing chain?

Would I have to hang some kind of tensioner or something from the derailleur hanger, or would it just be empty?

The Sturmey Archer 8 rear hub is unusual in that direct drive is gear 1 making it appealing for small-wheel bikes since they typically have trouble achieve higher gears. So it is tricky to get appropriate gearing for a full-sized bike in that you will probably need a small chainring.

The OLD on the hub is quite flexible. I think that by removing and adding spacers you can get anything from 115 to 135 ... http://sheldonbrown.com/harris/sturmey-archer-hubs.html

I believe that it generally comes with a 1/8" wide cog on the rear. But you can get replacement cogs that are 3/32" wide which should be compatible with the chain you have.

Personally, I think that you will have an easier time with a Shimano or SRAM internal hub since the direct gear is typically in the middle of the gear range. The OLDs on most of the hubs are 126mm or 130mm.

Whether you need a tensioner is dependent on the rear dropouts of your bike. If they are horizontal, then you can generally just shift the wheel back to achieve the necessary tension on the chain. Check out some pages on doing a fixed gear conversion to get a better idea of the problem.

The Sturmey Archer 8 rear hub is unusual in that direct drive is gear 1 making it appealing for small-wheel bikes since they typically have trouble achieve higher gears. So it is tricky to get appropriate gearing for a full-sized bike in that you will probably need a small chainring.

The OLD on the hub is quite flexible. I think that by removing and adding spacers you can get anything from 115 to 135 ... http://sheldonbrown.com/harris/sturmey-archer-hubs.html

I believe that it generally comes with a 1/8" wide cog on the rear. But you can get replacement cogs that are 3/32" wide which should be compatible with the chain you have.

Personally, I think that you will have an easier time with a Shimano or SRAM internal hub since the direct gear is typically in the middle of the gear range. The OLDs on most of the hubs are 126mm or 130mm.

Whether you need a tensioner is dependent on the rear dropouts of your bike. If they are horizontal, then you can generally just shift the wheel back to achieve the necessary tension on the chain. Check out some pages on doing a fixed gear conversion to get a better idea of the problem.

My understanding is that the Sturmey 8 sprocket is not interchangeable with any other sprockets, so you won't be able to change to a 3/32 cog. So you need a 1/8 chain. Since you are stuck with the 25T, that adds an explanation point to invisblehand's ratio issue.

The SRAM's are a little wider than the counterpart Shimanos, 130-132 for SRAM 7s vs. 126-130 depending on whether you go with the wider roller/drum brake models.

Maybe consider a new cassette (12-21 or 13-23) and a new chain?

I think that you only need a chain tensioner if you have more than one chainring, if not you'll have a fixed length chain...

Regards