Bicycle Mechanics - MTB Tire Size

What's the smallest diameter tire I can put on a 26" MTB rim? The rim says 559x17. Does "17" mean the rim width is 17mm?

I typically road bike, but I have an older Rockhopper that I would like to use to pull my kid around in his trailer. It currently has really knobby 26x1.95 tires that have a lot of road resistance. I'd like to replace those tires with a smaller, smoother tire.

17MM sounds very small for a MB rim! You should probably remove the tire and actually measure it.
I run a 26x1.5" on a 20MM rim and could go to 1.25" IF I wanted to without problem.

http://sheldonbrown.com/tire-sizing.html

Near the bottom of the page

Find the ETRTO number on the rim (not the one on the tire). It should be 559 x XX. There is a page on Sheldon's site that will tell you what size tires you can use.

Link anyone?

Find the ETRTO number on the rim (not the one on the tire). It should be 559 x XX. There is a page on Sheldon's site that will tell you what size tires you can use.

Link anyone?

The OP says the rim is marked 559 x 17. That is on the narrow end of the spectrum for a mtb rim, but I do have a couple of mtb wheelsets with rims of that width (measured between the two inside walls of the rims). I've run anything from 26" x 1.0" to 26" x 2.14" tires on them, no problems. So to answer the question, "how narrow a tire can I use?", the answer is as narrow as you want for a 26" mountain bike tire, which as far as I know don't come any narrower than 1.0 inch.

Thanks for the help. I found the link to Sheldon's site regarding tire sizing. My wife has a hybrid with 1.25" tires. I didn't know if I could even go that low of diameter. I went with some 1.5" street tires that should serve me well.