Almost every mtn bike I've ridden feels too large, ie - I feel too spread out on the bike.

My road bike is a 55 cm with a 55 cm top tube.

How do I determine what size mtn bike I should be riding?

Thanks

By trying out smaller sizes until you find one you like. Alot of company webpages have sizing charts on which are helpful.

If, like me, you are between sizes on most sites, I would say go to lower size if you ain't planning on growing and want something chuckable, or higher size if you want to blast along for a long time over smoother stuff/up hills.

test ride as much as you can before buying

Subtract 12 cm from your road bike size to get the MTB size in cm and then convert that to inches. My road bike is 55 cm. Subtract 12 = 43 cm. Its 17 inches. My MTB size is 18" and I ride a 17.5" frame which is the next size. That would be a medium with a 32" inch inseam.

Everyone I talk to about their own MTB sizing is so different. I think because there's so many varied ways to "do" mountain biking. Some want all-day endurance fit, some want air-born balance, some want climbing or downhill efficiency.

Some want slightly smaller sizing than the charts, some want slightly larger... and different mountain bike brands have their own take on this too.

If you already know what your doing, I'd say go with what feels right.

Personally I look for frames with low BB heights, short chain stays, a bit longer top tubes so that I can use really short stems which I like the feel of.

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The information you provide isn't enough in any case. What type of riding you do, what kind of mountain bike, more specifics about your body and riding position preferences? In any case all I can say is try smaller bikes if they feel too large for you. Are you fitting yourself to frames with particular seat and stem/bar setups?

You might want to check out stack & reach type sizing, gaining popularity these days, rather than focusing too much on seat tube (or even top tube) measurements.

That works for me. I ride a 56 cm road and 17.5" MTB, which fits me great.

mountain bikes arent sized as specificaly as road bikes because the rider is out of the saddle a lot he is not as disciplined as a roadie(in body position) he must make physical adjustments that depend on the trail that lies ahead so just give yourself 2-3 in clearance over the top tube and def ride the bike, if its too small your lower back will hurt and you will regret your purchase always remember it is easier to build a bike up but you cant make it smaller

I ride the same size road bike as you and fit a 16 or 17 inch mountain bike usually. Some brands I am right on the line between a small and medium and neither is quite right.

How tall are you? I'm looking to get a bike but would order the frame, I'm 5' 11" with a 32" inseam, I ride a 56 road bike, I'm thinking of getting an 18" frame, but would really like to be as informed as possible before I order one.

Just pick something comfortable.

Agreed. Usually XC (cross country) bikes are more stretched out (top tube) and have aggressive geometry.
As for the top tube standover, you need at least 2 inches clearance for bail out emergencies.

the only way to know if it is comfortable is to take it out on the trail.
some shops have bikes you can rent and see if the size is right or not otherwise some
companies will bring a truckload of demos to different places across the country
if you can find the company with the bike you want and get to a demo to try out a few sizes
you may have a better bet at getting a good fit.

i ride downhill should i am 5.8" should i get a 16'' or 17''

I'm 5'10 and my hardtail is a 15" bike. Sizing has sooooo much to do with what you want to do with the bike that giving out sizing specs over the internet is stupid. If you're main goal is to climb like a goat then you're gonna want something way different that someone who's big fun comes from the downhill.

Like mtnbiker 66 says, depends a lot on how you want the bike to handle. Also not all bikes are measured equally on seat tube length nor is seat tube length alone enough to tell...

ok i want it for downhill freeride stuff including some big hit stuff the bike im looking at is the iron horse sunday and the only size left on the good deal i found is 17'' and im wondering if that will work

^ See if your LBS has that bike and ride it... the only way you'll ever know for sure.

Theoretically, I'd say 17" is better than 16" for your height. If you find the cockpit of the 17 slightly too big, you can always use a shorter stem (which is better for DH/FR). If you went for the 16 and found it too small, using a longer stem is generally not a good idea for DH/FR.

I'm 5'10" and I've gone with 18.5" or 19" in the past with about a 40mm stem. The chart says I should be a 17" but I found those a bit cramped (I've got a long-ish torso I guess).

Only you can make the call, bsben...

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If all he's gonna be doing is FR/DH riding. Then a smaller bike is going to be better suited. The Girlscout is 5'9" and his FR rig is a 15 1/2 inch Stinky.At 5'8" id say don't go any bigger that a 16.A true FR or DH bike is gonna come with a short blocky strong stem anyway.