First, start measuring everything in cm, not inches. If you don't have a metric tape measure then multiply by 2.54.

As a general rule your mountain bike frame is usually 10-12 cm smaller than what you would ride on a road bike.

It's explained a bit on this site.

http://www.coloradocyclist.com/bikefit/

Definitely the medium. If you had a short torso I'd say the small, as the biggest difference between the two will be the reach. You'll need the extra room of the medium, on the small frame you'll probably hit your knees on the bars or stem when you're out of the saddle. Also, mountain bikes are set up more like compact road frames with a short seat tube, so the medium won't be too "tall" for you.

What kind of bike are you getting?

Thanks Cerveloguy,

Actually, I tried that... I used both the frame size formula and the inseam formula... The average falls right in the middle (approx. 17.5)

So i'm a bit torn... I feel I should lean towards the 18.5 because of my long torso... However, I have a buddy who's an excellent mountain biker who errs on the side of small.

Dongustav,

Thanks a bunch... You're right, when I simulate the standover height, I still have a good amount of clearance.

The bike i'm getting is a Giant NRS... I'm taking a run at a few XTERRA races this year as well as getting back to my roots and entering in a few cross country races for training.

Thanks again.

where are you on the east coast? I'm in CT & have raced xterras for the past three years, so maybe I'll see you out there.

-Don

Don,

I'm actually up here in the frozen tundra of Canada... (New Brunswick bordering on Maine to be specific). I was actually looking at the east coast sched. a few weeks ago... I'm going to have to take a second look and see if I can get down there for at least a race. I have a busy summer that ends with XTERRA Nevada (where my girlfriend is from) But between weddings and a trip to Europe between races are going to make scheduling a trip difficult... Maybe a long weekend.

If you have any good suggestions for some of the northeast states... Let me know and i'll PM you if I make it down.

Cheers

the northeast race schedule looks like this:

Mayy 22 - King of the Hill Xterra - Lebanon, NJ

June 12 -- Garnet Hill ORT -- North River, NY

July 3 -- Xterra East Championship - Richmond, VA

July 17 -- Capitol City Dirt Tri -- Hartford, CT

August 7 -- Grafton Lakes Xterra -- Grafton, NY

August 21 -- Farmington Valley Off-Road Triathlon -- Farmington, CT

August 28 -- Schiff Scout Xterra -- Wading River, NY (long island)

The 4 in bold would be closest for you, though if you come to CT for one, I'd recommend Farmington over Hartford.