What is the smallest chainring I can use with a Nexus 8 hub?
 

Title pretty much covers it. Having a real hard time getting good information on this, and getting widely varying answers. I'd like to use it with a double crank, say a 42-30, but I'm not sure if the 30 would be putting too much torque into the hub.

If you use a double crank, you will need a both a front and rear derailleur too.

I also think the 30T chainring will over-torque the hub. Call Shimano in California and get the real scoop:

Shimano American Corp.
1 Holland
Irvine, CA 92618
(949) 951-5003

If you use a double you'll need a chain tensioner that can take up enough chain slack. The Rohloff and Paul Melvin both come to mind. An old worn out derailleur will also work if adjusted.

Also, that wouldn't put to much torque on it.

Some chain tensioners are just intended to take the slack out of the chain for a single speed or internal geared hub when used on a frame with vertical dropouts. Be sure to get one that allows the chain length to vary in use if you use a double crank.

A rear derailleur locked under the single cog with the limit screws is guaranteed to work and cheaper than most alternatives.

Because of the difference in the wrapping of the chain with a shift at the front I'd second the idea of a rear derraileur rather than a simple one wheel tensioner. Using a derrailleur instead would ensure there's adequite chain wrap at the rear.

Ashen, by doing this though you're bypassing one of the big advantages of a rear internal gear hub. And that is the simplification of the drive train. If you insist on using a shifting front you may as well go back to a bike that uses a cassette and proper setup for the derrailleur and not bother with the internal gear hub.

From my reading about IGH's they are not quite as efficient as a rear derrailleur system for mechanical drag. But that's made up for by avoiding the need for ANY chain guides or derrailleurs. Now you want to add back this ON TOP of the IGH drag. And add to the maintainence requirements by effectively having TWO full drive systems.

Seems like if the IGH you have isn't doing the job that you may as well sell it to someone that would be happy with it and move back to a full standard system.

One of the Shimano books I have said no smaller than 38 or 36 teeth. I'm looking for the exact words right now. Plenty of people (including Sheldon Brown) have ruined interally geared hubs by putting too small a chain ring up front and sending too much torque to the hub. It's a good question. Sheldon's web pages may have it.

edit: right now the shimano web pages say they offer a 33 and a 38 tooth crank for that hub.

Also if you have a coaster brake hub, you can't use a chain tensioner or a derailleur.

I think that the engineers at SRAM would disagree. Have you seen their dual drive hubs?

I don't actually have a hub/wheel yet, this is more the research phase. What I'm realizing is I really want a Rohloff, but just can't afford it. The Nexus 8 would be fine day to day, but not really low enough for touring. I'll just have to decide what to do.

And it all depends on what your rear sprocket will be and what you want to do with it. Mine came with a 19 and a 23 tooth sprocket. I set it up for road use with a 48 front, 19 rear which works great for me. It's pretty close to a 34/50 compact double with a 13x27 cassette. I figured that out by using Sheldon's gear ratio calculator.

First, go here:
http://www.sheldonbrown.com/gears/
Enter the gearing of a derailleur equipped bike that you are interested in matching. You can leave the wheel and crank sizes at 27/170 because we are only interested in the relative values here. Let's do a standard road combo of 39/53 on the chainrings and 11/25 on the sprockets. Don't worry about the gears in the middle of the cassette.

The results are (ignoring the middle numbers) :
9.7 to 3.1 9.7 represents top gear, 3.1 represents bottom gear.

Now, go to this page:
http://www.sheldonbrown.com/gears/internal.html

This page lets you calculate the same gain ratios for a Nexus 8. We'll try to match to the gearing we saw for the 39/53 11/25. If you enter 53 for the front sprocket and 18 for the rear sprocket, and set the dropdown to Nexus 8, we find that the top ratio is 9.6 and the bottom ratio is 3.1. That's pretty darn close to the target ratio.

Keep in mind these gain ratios are relative and the real numbers will be affected by wheel size and crank length, but as a method for determining if the gearing you are thinking about is close to the mark, it's a really useful tool.

I realize the steps above are kind of complicated. If you have trouble figuring it out, let me know and I'll try and help you out.

Finally, when comparing the Nexus to the Rohloff, ignore the chainrings and sprockets you see on the Rohloff. Internally, that hub is geared differently than the Nexus so the chainring/sprocket combo you would use on that is totally different than you would use on the Nexus to produce similar results at the rear wheel.

I understand and have used the gear inch calculators often. I really just wanted to know when I'd be over torquing the hub. Looks like to get the range I want, I'll have to look beyond the Nexus. Thanks everyone.