steel frame preferable. 21" is huge but if you could ride it fine when it was in one piece, use it. i got a 24" that is just silly...but hey... it rides smoother! longer distances make it slightly more flexible/springy which does wonders for vibration! tiny frames vibrate more.
less clearance between seat and the top bar can make a grown man cry
helps if the tubes are skinny, rather than these massive oversized tube frames that need mounting modifications. dont drill holes through the frame! use clamps etc...
make your son buy new parts. show some spine and teach discipline. im sure you worked to pay for it in the first place... well, beanpole can now return the favour.
v-brakes(linear pull) or cantilevers are good enough. some may argue but if a v-brake can lock the wheel with one finger...what more do you NEED?
the engines...can be anything from frustrating to a marvel of modern mass manufacturing... all down to luck apparently. personally, i say skyhawk makes the best engine. and again, i only use the 48's. convenience, mostly. except the skyhawk has different internals, be warned! i only have one for that reason mainly. everything else has just been generic HT 48cc china engines...
a handlaced wheel is far superior to any machine laced wheel, but you will pay! ignore that advice! just use the one that fits. do you want all those gears? havent had cables on my shifters since motorizing, just locked them off in an easy pedalling gear for when the engine has its moments... and you can always push if the worst happens.
yes, you can get a special hub with the sprocket mount on the other side... the rag joint works fine for most people but. how much money do you want to throw at it? less than 250....hmmmmm. possibly already laced, even, but a wheel isnt that hard to relace. takes less than an hour. need correct length spokes for relaces if any dimensions are different, such as hub flange diameter or the distance between flanges...
weakest link in the rear wheel, only thing that gives me grief, is the tensioner. cus if that slips and catches a spoke... at best, you walk home! unexpected lockups at any speed are nasty :wacko: best to, once its all decided on and in final position...screw through it into the frame or tack weld it on...