Mountain Biking - mountain bike sizing

I have a question about mountain bike sizing. I'm just not sure how to compare them (if you can) to road sizing. I've been riding road bikes since the beginning of the year and have two, one is a 53cm and the other is a 54cm. I guess they are both considered mediums.

I've been looking at treks and they have sizes like 16" and 18", just don't know how to compare to road, where should I start? I'm 5'9" - 30" inseam. Any inputs would be appreciated. I can't believe after getting two bikes this year I'd want another, but I'm getting sortof hooked (lost 25lbs this year). I don't know how riding MTB would fit into my fitness plan (if I really have one), but it sure looks like fun!

Thanks in advance..:beer:

I have a 21.5" Trek and a 34" inseam. I think an 18" would work based on your measurements but a good Local Bike Shop (LBS) will be a good place to go try a few and get a good recommendation.

As far as a fitness plan - if it's fun, you'll do it more and that in itself, is a plus for your fitness.

Imagine whats gonna happen if you fly off your bike while going downhill and you land on your crossbar. Thats why you want more space. I ride the same size road bike as you and prefer a 17" bike, but not all manufacturers have equal sizing. Try before you buy.

Joe

I have 3 road bikes and I bought a Specialized Stumpjumper this spring. It is my favorite ride and single track will make parts ache that you didn't even know you had.

I asked the same question at my LBS today. I have a 56 cm Madone, they said to start with a 17.5 mountain bike, or a Medium??