Youth Mountain Bike fitment

I know there are some serious bike enthusiasts here, so I'm hoping you can help me. My son is 12 and, when I measured him 5 minutes ago, was 5' 1/4", up 1/4 inch from a few days ago. His inseam is about 27", measured from bare feet to his crotch in his underwear.

His current bike is a 20" tire BMX bike. He isn't doing any real trail riding, just going around the neighborhood and 5 mile jaunts on the paved greenway, and it is working fine for that.

Looking at an online calculator, it seems that a 15" post height is what I am looking for at this moment, but giving how fast he is growing, I want to get more than a few months out of it. I've also been scouring the local Craigslist, and if any measurements are given at all, most of the things I am seeing are way bigger than that, like 19" or it seems they are giving the wheel measurements instead. Compounding the issue, some of the bikes that do list both measurements have varied sizes: like 18" frames with 24" tires and 18" frames with 26" tires, that kind of thing.

The bottom line is I have no idea how to narrow the field and determine which bikes we should even go look at to test fit. We don't need a high performance bike, and he has shown no interest in modification at the moment, but I also want something that's decent.

Here are the ones I am looking at:

http://raleigh.craigslist.org/bik/5536882248.html

http://raleigh.craigslist.org/bik/5516525911.html

http://raleigh.craigslist.org/bik/5540123341.html

http://raleigh.craigslist.org/bik/5525089852.html

http://raleigh.craigslist.org/bik/5532395919.html

http://raleigh.craigslist.org/bik/5524524867.html

http://raleigh.craigslist.org/bik/5543663949.html

http://raleigh.craigslist.org/bik/5542413937.html

Bonus for reading this far: Vintage Schwinn with banana seat:

http://raleigh.craigslist.org/bik/5505932341.html

Of the bikes you listed, the Trek and the GT have the best potential for growing with him. At five feet plus and growing, he's already outstripped most of the 24 inch wheel kids mountain bikes. If you want a bike that will fit him just as poorly at adult height as it does now, a 24" wheel Street BMX bike would work. As far as an actual rideable fit, the favorite fit method for most kids at our bike shop was to set saddle height where they could barely get the balls of both feet on the ground at the same time while sitting in the saddle. It's a higher seat position than many non-riders think they want, and a bit lower than most racers know they need. It is however a good compromise for kids, as they tend to be more aggressive riders doing short distances, so ultimate efficiency isn't important and that's a decent saddle height for moderate jumps, climbing without having your knees up in your chest, and dismounting swiftly when your youthful confidence overpowers your skill level.

He certainly sounds like he is ready for his first adult sized wheel bike, and in the interest of his street cred, I'd definitely counsel against the girls bike with the pink shock fork...

Frame size will determine the fit and stand over height.

The frame size and wheel size can both affect the reach to the bars and other geometry for fit. As mentioned above 24" wheels will probably result in a cramped fit now, or in the near future.

The wheel size on a mountain bike will have an affect on speed and ride quality over rough terrain.

I would suggest looking at 15" or 16" frames (may also be listed as small). That will get you in the ballpark, but he will really need to sit on it to see if the reach and rise are comfortable.

As a point of reference, I'm 5'7" and have a 29 inch inseam.

I just bought a mountain bike with a 16" frame and 27.5" wheels. I would not want to go any larger in the frame size because the stand over would be too high.

http://raleigh.craigslist.org/bik/5516525911.html

this one is both a girls bike and actually a Wal Mart bike that retails for 150ish. Wouldn't call it a great deal.

I didn't notice that was a girl's bike, but I see it now.

Is the Giant not worth it? I figured the Giant, GT and Trek were the best of the bunch. There are literally thousands of postings in that section of the Craigslist at the moment, so I'm sure I'll find something soon.

Seeing what size this is:

http://raleigh.craigslist.org/bik/5536344945.html

This looks almost vintage:

http://raleigh.craigslist.org/bik/5526590309.html

This says 15" but seems taller in front.

http://raleigh.craigslist.org/bik/5542748838.html

The Giant is as old or older than the bike that looks vintage to you. It has a long stem on it, so there's a good chance that even if the frame and stand over height are good, he'll be uncomfortably stretched out to reach the handlebars. The "vintage" Trek has two red flags. The chain is rusted solid, so to be sure it's safe for riding, you're in about $20 bucks for a low end new chain, and new tires if the back one has been left flat for as long as the chain has been left without lube. Also,almost all the bikes in the range you are looking at will have a one piece thread-on freewheel, and those are even more intolerant of abuse and bad storage than hubs and pedal assemblies. Odds are good if the chain is that rusty, and the tires are flat, you're looking at $100 plus to get the bike rideable safely, especially if you take it to a bike shop instead of doing it all yourself..

Newer mountain bikes will look taller in front, as "compact" frame geometry has taken over in the last fifteen years. The contact points-saddle, pedals, handkebars- will all fit the same, but the frame will look smaller at the middle of the diamond shape. You will notice that the seat post clamp is usually a few inches above the top tube of the bike frame on this style of bike.

Go to your local bicycle shop. They will educate you in the differences and allow your child to test fit several variations. Once you have a better idea of what suites your needs, go search used.

I remember women's bikes having a different frame geometry than men's bikes, but it seems that the new frame style kind of changes that.

How is this a women's bike?

http://raleigh.craigslist.org/bik/5529753003.html

(edit: it seems it is a men's bike, based on the Trek Website)