"more" illegal?
necessary?
"more" dangerous?

I heard this was a big no no and something they really enforce, and something that will get your ride pulled off the road pretty quickly.
Would you use your frame for an air tank?  

Well i built a tube chassis (2x4 tube steel) and i thought about using the chassis for the air tanks but a few different people told me that with the amount of air pressure that i need to use round pipe because it is an even pressure all the around and with tube steel there is more pressure on the sides so it will ballon.  Never really thought about the dangerous side of it but i thought it might be a good idea for a show truck.

Josh  

...and it seems like this would leave you with fewer options for frame mods and general maintenance could be problematic. you can't exactly FIX it on the go... ya stay clear of that route    

Off roaders have used their large tube bumpers as air tanks for years. I agree on the square vs round tube theory though. Hadn't thought of that one! Seamless round is what I'd use and have a *** good welder, weld the ends.

Maybe you could incorporate a large tube crossmember into your frame design.  

This may be dumb, I don't know, but, when you pump air, you pump water, even with water traps, there's still water being pumped... you can pretty eaily drain an air tank... if it's the frame, do you just say screw it, or how do you handle this? either way, you've got water in thar now.  

How about a sweet drag session on that fresh tank, i mean frame  

Drain it regularly! Duh! Good one RED. lol
Which brings up another viable point. Make sure it's not one of the lowest points of contact on your frame. A good drag session could ruin this idea PRONTO!  

well i'm just sayin....
and that could be ONE problem in a good drag session hahaha  

no cop would ever know your frame is the air tank.

you can make just the rear section that doesn't drag on the ground the air tank. A six foot section of 4x8 tubing would work out to be about 3-plus gallons of air.

might want to make two or three passes with the welder to ensure the seams are air tight and pressure test afterwards.

go for it monica.  

sounds like too much work for 3 gallons of air. eh you first hahah  

no way! that tube would have to be at least .250 wall to keep from ballooning like 66Sub said.
and since my frame is rectangle and not round tubing i'm not doin it.  

Okay nowhere in the California vehicle code does it say that you cannot use your frame as storage. Using your frame for an air tank is an excellent option if you’re limited on space. You can get anywhere from a gallon of air to about 12 gallons of air for most minitrucks, depending on how much of the frame you use for storage. Full size trucks will obviously get more storage because of area.  

what about something like this? There's no way they'd deem a pressurized frame as safe (assuming they knew).

Vehicle Not Equipped or Unsafe
24002.  (a) It is unlawful to operate any vehicle or combination of vehicles which is in an unsafe condition, or which is not safely loaded, and which presents an immediate safety hazard.

(b) It is unlawful to operate any vehicle or combination of vehicles which is not equipped as provided in this code.

Amended Ch. 696, Stats. 1992. Effective September 15, 1992.  

Bah... you could nail any one of our trucks with that one... lol  

I am building a ranger with a frame rail tank and the best way to go that I have found is this.  Back half the frame. Right behind the cab I will go straight up about 22" ( depends on what size tire you are going to run) This is the portion of the frame/tank you will want to run the compressors to also a drain at the bottom.  This way the front of the back half acts as a water trap.  Also by being off the ground 22" or whatever your ends up being you can drag away.  When you weld anything to it like x-members always weld a 1/4" plate to the rail then weld the x-member to the plate.  I have built a dozen frames this way w/o any probs.