I saw on here that some 20s dont have them

I had a Denim a 18" er 20...I converted it to a chopper. No hassle from the bottom of the frame.
But it kept on breaking spokes...in an old shimano 3 speed wheel.
The Commando is a sort of boys version. No downtubes on there
Got a couple of RSWs and they have some ugly sheet metal bracing.

Also I want the 20 to be light weight. So might loose 50grams or so if I remove the tubes