I'm 6'-1" and ride a 19.5" 4300. The frames are different, but the geometry is the same. Seat tube measurement isn't really an important factor in bike fit as much as effective top tube length is. That said, Trek 3 and 4 series frames have some odd geometry and have short top tube and wheel base measurements compared to any other manufactures frame for any given size. Your 18" may compare to other 16" bikes for cockpit dimensions, reach, etc.

If you are concerned that you have the wrong size bike, go back to your LBS and test ride other sizes and go with whichever is the most comfortable.

 

Coming from internet jockeys, we can only give generic guesses, but it would help if you gave us your stats...how tall...inseam, etc. so we can at least have a frame of reference.

 

I'm 5'9" with a 30" inseam.

A friend of mine has a 16" trek 4300 and he's a little shorter than me and he says his bike is too small but the 18" was too big for him, so I'm kinda thinking a 16" would be to small for me. I guess I'm second guessing because I told the sales guy what I wanted and they tried to sell me a 29er that was more expensive than my budget. And when I said no I wanted the trek he was less helpful than he was when he tried to push the 29er on me.

 

I'm 5'9" with a 30" inseam.

A friend of mine has a 16" trek 4300 and he's a little shorter than me and he says his bike is too small but the 18" was too big for him, so I'm kinda thinking a 16" would be to small for me. I guess I'm second guessing because I told the sales guy what I wanted and they tried to sell me a 29er that was more expensive than my budget. And when I said no I wanted the trek he was less helpful than he was when he tried to push the 29er on me.

 

Sometimes you don't have unlimited options, but in a situation like you're describing, I would be tempted to leave without buying a bike. For a larger purchase like that, I would like to get some decent help and advice, especially if I'm not completely sure of what I need.

In this day and age of bikes and parts being sold over the Internet for a large discount, the LBS needs to give enough service to make it worthwhile to shop there!

 

At your height/inseam, I'd say the 18 is a better fit than the 16".

Everybody has their own fit and preferences, but for example, I'm 5'6" and a 16" is too small for me., so I think you're fine.

Just curious, what 29er was the saleperson pushing and what size? And why did you want the trek over the 29er...price?...fit?...brand?

 

height and inseam are only vague indicators of what size frame will fit you best. if the bike shop dude sized you based solely on that, he does not know what he's doing. a 18" frame on any two company's geometry might have wildly different top tube lengths, seat tube angles, etc, and therefore fit very different. you might be used to the geometry on a 18" Diamondback frame, but that does not mean that Trek's 18" geo will fit the same way. you are comparing apples to oranges.

the real question is one of the length of the frame. get the shop to help you determine the right seat height for you. the distance from the cranks to the saddle will be the same for you on ANY size frame. once you have set that up, the question is- which size frame makes you most comfortable and in control of the handlebars? in other words, how does the reach of the bike suit you?

 

It was a specialized 29er. Not sure of the model, it was grey with no markings or graphics.

I went in knowing I wanted the trek. It was in my price range, and it liked the color scheme.

 

I had the exact same experience with a Trek 3500 about 5 months ago. I came back to mountain biking from a 17 year lay off (no suspension fork on my '91 Bridgestone MB-4) and went to my LBS to get an inexpensive name brand bike. I didn't want to spend a ton of money only to realize I wasn't going to use the bike much (there was also the issue of convincing my wife to spend more on a bike. . .). I got a new sales clerk at the LBS who pulled out a Trek 3500 and didn't tell me squat about what I was looking at despite my questions. He "fit" me for the 18" (I'm 5'9" with a fairly short inseam) by having me sit on the 16 and then the 18. I told him the 18 felt too big but he said I'd get used to it. I never did. It always felt too big and I was in constant fear that I was going to rack myself due to a too high stand-over.

That left me with a bad taste in my mouth for my LBS (I've had other "bike snob" issues with them since). I also realized that the components on the 3500 weren't going to stand up well to moderately aggresive trail riding (you get what you pay for, although I loved the black and green color scheme). I sold it for a slight loss on Craigslist and bought a slightly more expensive 17" bike online. I spent some time comparing the geometry of the bikes on paper first, though, to make sure the new bike was shorter in critical areas (but not too short).

The 17" bike feels much better. And, since I feel comfortable with the frame and have decided I will continue to ride, I've spent money upgrading the components to my own liking (it's also easier to fly my rather pricey build-up under the radar). Of course, since I didn't get the chance to try out the online bike first, there was some luck involved with the fit. I also like to do my own work on the bike, which made the online route less daunting.

What's the point? The 3500 may be the wrong bike for you in general because you are between sizes. You might want to ask your LBS to let you test ride a comparable bike from another manufacturer to see if you can find a better match. If that doesn't work, you might want to consider returning the bike and going to another shop or the online route. If you consider online, make sure to read a few of the online bike purchase threads here first, though, so you get a sense of the pros and cons.