cubacherek

Long story short, I have twin 9 year old daughters around 4’-11” 90lbs. We ride easy single track, gravel paths and streets. They are both somewhat aggressive on single track. They have out grown they 20” bikes and we are looking at new/used bikes. Any suggestions on 24” (wheel size) bikes vs 26” 13”-15” frame bikes. Is there any advantages or disadvantages to these sizes?

Lanovran

24" is the largest of what will typically be considered a true "kids' bike," whereas the 26" wheeled bikes you mention will probably be adult bikes. The latter will usually (though not always) have better quality frames and components, and ride more efficiently than a kid's bike. If you were to bring your girls into my shop, we'd all recommend the adult bike, and most likely the 13" frame size (maybe the 15" depending on their growth rate and bike-handling skills). If they're already at 4'11", they would quickly outgrow a 24" bike, and you'd be right back in the same situation before too long. The larger bikes, however, will offer more room to grow, giving the bikes greater longevity for your girls.

When you're in the adult bike category, wheel size is more an indicator of how the bike will ride/handle, and/or for what sort of terrain it's best suited, while the frame size is the more important determining factor for general fit. There are some who might disagree, but with the right frame size and geometry, wheel size shouldn't really matter at all for fit. Road bikes and more pavement-friendly hybrids will typically have 29"/700c wheels, while 26" wheels will usually be reserved for more quick-handling, technical-trail kinds of mountain bikes, or for some cruisers and such. Given the kind of riding you're doing, I might recommend something along the lines of a mountain-esque hybrid, like the Trek Dual Sport/Neko family of bikes.

In general, you'll want to get your daughters onto a couple of different bikes to try them on for size. Let them compare the 24" to the adult-size bikes, and see if you can take some for a spin. I'm willing to bet they'll be amazed at the difference from their little kids' bikes. In any case, best of luck to you in finding the right rides for your twins!

cubacherek

Thanks for the input.

VegasTriker

More important than "will they outgrow the bike" is the question does the bike fit your child well enough to ride it safely? Can she reach the ground with her feet flat on the ground if the seat is all the way down in the seat tube? I have tried to talk parents out of buying too big a bike when they think the child "will grow into it". It is harder for a young child to ride a bike that doesn't fit and it makes it much harder to start and stop safely. If you are buying used bikes and keep them in reasonable shape, you can pass them on to someone else and recoup at least part of the purchase price. Buy one that fits.

TrojanHorse

I got 26" bikes for my kids when they were about 7 or so, and shorter than your daughters. Last year I "upgraded" my son to a 44cm road bike and my daughter still prefers her old 26" bike. 13" frame, she's 5'7" and it's a skosh too small, so that leaves you with quite a bit of room to grow. She's 13 now, if it matters.

I'd make sure the frame has a bosses for a bottle mount too. My daughter's does not, my son's does (same size!)

sailor2

This is actually not a big deal in my experience. While nice to have, it's cheap to fix if not present.
I used hose clamps on two of the kids bikes without bosses.
There are also decent handlebars bottle mounts available.