Mountain Biking - What size frame

I want to get into mountain biking I am currently a roadie riding a 58cm specialized frame. I was wondering what sizing I should be looking for in a mountain bike with that being said. I am 6 foot tall as well. If anyone can make a suggestion that would be a big help thanks.

Your height isn't the critical factor when sizing a bike. Your inseam meas. is what is needed. check out www.wrenchscience.com

L8R

I've always wondered about the downside of getting a frame that's too small...does it have something to do with the seatpost?

a TOO small frame size is dependent on many different things.

The critical measurements are the length of the seat-tube, the length of the top-tube and the length of the chain-stays.

Most riders are initially sized in relation to the seat-tube. Your inseam is measured and you are placed on a frame that matches your leg length. Then the top-tube is taken into consideration. Your reach to the handlebars can be adjusted by lengthening the stem, or moving the seat back or both.

For example, for a person like me who has very short legs for my 6'0" height, I prefer a Medium sized frame (or a 55 cm road bike). The cockpit (top-tube length) sometimes feels to short and thus I usually use a longer stem for my XC bike. Now, on my DJ bike and my FR bike, I prefer the shorter cockpit and have a shortish stem on both these bikes without ill affects.

I could also go with a laid-back seatpost or an offset seatpost and achieve a similar result.

Now if you are speaking of having too much seatpost exposed, this isn't really an issue if a quality post is used.

L8R

Different frame size = different cockpit length. Got it. Thanks.

I want to get into mountain biking I am currently a roadie riding a 58cm specialized frame. I was wondering what sizing I should be looking for in a mountain bike with that being said. I am 6 foot tall as well. If anyone can make a suggestion that would be a big help thanks.

No simple answer. Too many frame styles and configurations make frame sizes meaningless. Different and "unorthodoxed" frame styles ensures dissimilar measurement points. On a road bike with typical triangular geometries, a bottom bracket to top tube along the seat post is a somewhat reliable measurement (although not exact).

Mountain bikes, especially full suspenion are notorious for having undefinable measuring points. Plus the sag needs to be considered.

Bottom line, you gotta swing your legs over a bike.

FWIW: Me:road bike 54cm (~21 in), mountain bike (Fisher Cake 2DLX) = 17.5 in.

Nowdays, I hear mountain bikers talking in more qualitative terms, like small, medium , large x-large frame sizes.

21" center of BB to top of seat tube, 24.25" top tube, 110mm stem.

That's a good starting point for a 6fter.