Sizing differences when transitioning from Road to MTB.
 

I usually size up perfectly for a 58cm road bike. What size mountain bike would this translate to? I am a hair under 6 feet tall.

First...convert the 58cm to inches. You live in CA, ya commie! ;)

Then add about 2 inches to the frame size. You should be able to rest your giblets on the top tube just in case you slip or something...this keeps you from bruising your feet.

Don't you mean subtract about 2 inches?

Alright, so, just from looking over the internet, a 6 footer would most likely be fitted on a 19 to 20 inch mountain bike.

58cm = 22.8 inches
So I assume subtract 2 inches...= 20.8"
So the bike I am looking at can come in size 19", and 21". I guess 19" would be the safe bet?

Nope...if you subtract 2", then you risk a stubbed toe or something. Add 2" and let your nads absorb all that body weight.

Total Noob here throws in his .02, so take the following with a grain of salt, especially since chelboed has already spoken not once but twice, and while I esteem his advice on all things MTB highly, I suspect he is pulling your leg a little, and if you take him at face value, you may risk danger to your middle leg...

I'm 5'-11 1/2" tall - when I stand up straight, which isn't often - and recently bought a 19" Specialized Stumpjumper. According to the spec sheet, the standover height in the 19" frame was right for me, and I bought it without benefit of a test drive. Now, having put some miles on the bike, I find that I'm stretched out quite a bit, and wonder if the top tube isn't a little long for me. I don't think dialling in the fit will be a problem, as I can make it up with a shorter stem and maybe riser bars instead of the straight. No biggy, just a few more dollars. I am totally stoked about the bike, and don't regret buying the 19", but maybe the 18" Stumpy would have been perfect. Now windbag will sum up:

People here all seem to say a little bit smaller frame is better than too big. I would suggest test riding a few, as the geometry between sizes and manufacturers will vary.

I'm 5'11" 1/2 too. I ride a specialized 19 inch (hardtail) frame. Which should be more or less equivalent to a Large.

Test ride a few bikes before buying. You'd be surprised how much variance there is. Top tube/stem/handlebars all make a difference to name a few.

its all rider choice and style. i ride a 58cm road bike and used to ride a 19" frame, but it always felt to big. so i traded it in on a 17" and that suited me better. of course style of riding has a lot to do with it also.

OP...I'd put you on a medium frame...somewhere around 18" unless you're built long legged/ short torso'd.

I'm 6'1.5" with approx. 34" inseam and ride an 18.5" frame. When I was doing strictly XC...I was riding 19-20" frames. I go around 23.5" on the TT length. I prefer slightly smallish frames for shredding the Gnar. The Roadie in you may prefer to be a bit more stretched out.


...and yes, I give myself a bit of marble clearance.

I have a 60cm road bike and am happiest on a 20-21" MTB frame - but that is b/c I have long arms and legs. My 19.5" Trek is a bit too small for me in terms of ETT.

Have you tried the fit calculator at competitivecyclist.com? That works quite well as a starting point.

Vandit

I think its all a matter of preference. Unless your all XC, I like a smaller frame. I ride a 56cm on my fixed roadie, and 16" on my MTB. But then all brands will fit differently, even at the "same" size, due to small geo differences. I'd say go to a LBS and get fitted if you can, and test ride different sizes of different brands. And if you want to do more XC, you could go with a larger frame, but for DJ or DS you may want something smaller.

I should be on a 58cm road bike (though my current bike is 56) and a Large (19") mtb fits me well.

Seems like 19" to 20" would be my best bet.