how is this an 18" mtb?
 

I don't understand the sticker on this bike. it's a little round white sticker that reads: 18" but when I measure stuff, this is what I get

seat tube crank to top of tube 21"
standover 29"
top tube 22"

I'm 6' with an inseam of 34"
I was thinking I should have a bigger frame - I found this bike abandoned and have been using it off an on, but I mostly ride road bikes.
Should I be looking for a bigger frame?

My guess is 18" is the seat tube from the center of the crank to the center of where the top tube intersects it.

As Sheldon noted:

• The old standard system was to measure from the center of the bottom bracket to the very top of the seat tube.
• Some manufacturers have decided that this is too easy, so now many bikes are measured instead to the intersection of the centerline of the top tube with the centerline of the seat tube.
• Some other bikes that have seat tubes that protrude farther than normal above the top tube measure as if they were measuring to the to the top of a seat tube with normal protrusion.
• Some bikes are measured to the top edge of the top tube, even though the seat tube protrudes higher up.
• Some bikes with slanting top tubes are measured as if there were a level top tube, they use the length that the seat tube would be if it was as high as the head tube.

thanks guys, good info
so how do I compare this bike with other bikes? or determine if I need a bigger frame?

You should get a 29er

Aside from test riding any bike you're considering, you'd have to take into account all of the geometry measurements of the frame (top tube length, seat tube length, seat tube angle, head tube angle), and that's hard to measure or find out on older frames.

Measuring the top tube IMO is much more important, and keep in mind there's actual and virtual top tube lengths, and many other factors (like how the particular bike is set up with bars/stem/spacers) when comparing one bike to another, let alone the type of riding you're going to do (where things other than basic fit, like head tube angle, wheelbase, etc come into play). Go to a bunch of shops and ride a bunch of bikes, noting the dimensions and type of setup to narrow down your sweet spot is one way to do it...and looking into a 29er isn't a bad idea at your height.

buy a 29er, brah.

Many bike company sites will cite the top tube length as if it were horizontal (like your GT's), just have to guess a little bit on the seat tube lengths and how different companies arrive at their numbers.

Determine if you need a bigger frame by seeing if that stem or seatpost is out past its minimum insertion point. If either one is, consider getting a bigger frame.

OK, good suggestion. the stem and seatpost are definitely both maxed out and barely legal!

spent all day in the car in the snowstorm to get a bigger frame. looked at a couple and went with this one. the guy wanted $70 but I gave him $30 cuz it needs work. It's the same brand as what I have but much much newer!

Talera
GT All Tera
Triple Triangle
Saddle: Specialized Body Geometry Gell

top tube 23 1/2"
standover 32"
seat tube 24" (traditional measure)

Were the pointed up bar ends the key? J/k. FWIW it's probably a 22" since that's the largest listed on bikepedia for that model, and not so new considering the 1"/threaded/rigid front end.

I like bar ends - I even put drop style bar ends on the smaller red frame. I figured it wasn't too new after looking into it for a while - oh well, it's still newer than that old red one! :-) I think you're right about the size, thanks. gave it the once over and after an hour or so cleaning and lubing everything works fine except the rear derailer doesn't want to stay in the big rings - not sure id maybe there is a prpblem with the rearAlivio shifter rapidfire style combo unit

Check the low limit screw adjustment as well as the cable tension (and even the housing condition and routing). Doubt it's in the shifters themselves unless you can feel it in the shifters (does the shifter actually change it's indexed position?). Could be hanger alignment, too.

ok thanks - got a thread going in mechanics - will keep that there. go there if you want to contribute more. I just checked a few things and tried some adjustments to the tension and cable length - not quite there yet ... wish I knew thiese adjustments better. may go to bike mechanic but will try more on my own.

it was the shift BTW and the new unit solved the problem. thanks everyone. it's getting there ...