Knowledge about 24" straight bar frame?

Why not just fit up some 24" wheels to an old school MTB or Road bike frame?

Early MTB's and 10 speed racers have that straight, horizontal geometry.

I put 26" wheels on a 70's road bike frame that used 27" originally and it looks and rides better now. Another plus is the modern front end with suspension forks and better V brakes.

From my tank building experience it is much easier to start with a true straightbar frame as opposed to a "curved" straightbar frame. It is just easier to start with a flat tank bottom than a curved tank bottom. Gives a good starting point to build up. The more curves the more challenging the tank construction. Also, if you plan to build the tank yourself then almost any straightbar frame is fine. If you plan to source the tank elsewhere then find the tank first and then get the frame that fits the tank. I don't know of anyone supporting the 24" frames with available tanks.

The 24" DX frames from the 50's are good and bad. They're balloon tire frames, so have plenty of clearance for wheels. The space between the two bars is pretty small though. And, as Pat indicates, the second bar is not straight, it has a slight curve which will complicate the tank, but not impossible.

Tornado's are middleweights, they're what we call a double straight bar. It looks like they have a little more room for a tank, but these are middle weights, less room for tires, but I built up a 26" middle weight and had plenty of tire space. I used a '62 typhoon. They make a 24" version too. Typhon and Tornado are similar, but I think the pocket is larger on the Tornado. Check flea bay...

If you build up a 26" board track style, you'll find it to be a very compact ride. Plus, you can use 24" thick brick tires, they fit super nice on a balloon frame, but chain clearance will keep you up at night...

Have fun.