I didn't mean to suggest that changing wheel/tire size would alter the geometry. Perhaps I should have clarified that the rider would have to sacrifice optimal geometry either initially, or after the rider has outgrown the frame....depending on whether the bike purchased was a good fit to start, or purchased with a good fit in mind down the road.Absolutely, if you can make a bike fit a growing child for a longer time, that's great.
The point that people seem to be missing here is that changing wheels does not change geometry(at least not crucial fit related geometry). It has nothing to do with sacrificing optimum fit or handling. It simply doesn't help.
Obviously stack and reach don't change if you swap wheels.
If you are unfamiliar with the stack and reach system give google a shout.
If you want to feel it to believe it try this:
Put your kid's bike in a trainer. Try riding it. It's too small right?
Now, put some boards or blocks under the trainer and front wheel and try riding it now. Does it fit any better? Of course it doesn't, everything you touch is still the same.
Yet, raising the bike of the floor is the exact same geometry change you would get by switching to a larger wheel.
Perhaps the reason this misconception is so persistent is an error of causality:
We see bikes with small wheels which fit small kids and we see bikes with big wheels which fit big people, so we think: "big wheels must make a bike fit bigger people"
The mistake of course is that it's not the big wheels that make the bike fit bigger people.
If you want maximum 'growth range' out of a bike, start with a bike with a very low bottom bracket and very short cranks. Then select the bike with the biggest stack and reach that still has sufficient standover.
Then, throw on a narrow handle bar, super short and slammed stem.
After the child grows, you raise the saddle, using a longer seatpost if necessary, use a longer stem with all the spacers underneath, and perhaps a wider bar.
These changes will actually make the bike FITbetter, unlike a wheel change. As a side benefit they are about 15% of the cost of a set of wheels and tires.
I understand the concepts here, but it doesn't change the underlying issue for me, and that's money. Being able to throw a set of 26's to help with rolling speed and clearance would, at least theoretically, buy some time...albeit without a great fit.
That bolded part is very beneficial information, and I'll use this as I try and hone in on the right bike for my kids...but I'd still like to find something with just a little more versatility. Specialized is marketing a Grom bike that changes wheel sizes, but it's about $500 bucks out of my price range.
It looks like I'll have to pay closer attention to BB and crank sizes as well...