26" wheels on a 650B frame?

diy

My oldest kid, needs a new bike, I'd like to get him a decent used bike for the same money as an entry-mid level hard tail. The problem is, he's currently sizing up as a small frame on most bikes. Given that he's currently growing like a weed, I was wondering how stupid it would be to put 26" wheels on a 650b Medium framed bike and perhaps go with a shorter stem and/or inline post.

There are loads of 650bs around in medium, but very few small and xsmall framed bikes.

thoughts?

.blitz

Depending on the tyres you use the bike and the BB will be 10-12 mm lower and the fork offset will be different depending on travel/sag. You might also find the tyre clearance is tight around the fork arch.

Small but not insignificant differences

The Rookie

I'm baffled by this, why put 26" wheels on a 650B framed bike, it doesn't make it any smaller, either buy a 650B in Medium - get one with (designed for) a setback seatpost and replace with an inline, that shortens the cockpit by 20mm (about one size) without affecting anything else. Or get a small 26".

diy

To be honest, I'm not sure, which is why I posed the question. On other forums some XC racers are using 26" wheels on 650bs when the course is really muddy, to allow chunkier tyres and better mud clearance. I was focusing on reducing the standover and possibly top tube length due to the more compact frame on a 27.5.

As per my OP, Small framed bikes are fairly rare.

The Rookie

Improving standover by 12.7mm (ish) is pretty meaningless, yes if you want big tyres you can put the smaller wheels on (only restrained by width available then) but frankly I wouldn't bother.