motorcycle gas tank, questions

iam in need of honing my metal skills, ive decided to make a gas tank or two for a motorcycle, my question is, what is the average tubing width on the backbone of a motorcycle or should i just leave the bottom whole and cut out the tunnel when the width of the backbone is figured out?

will

 

About two inches, up to a little over three for late BSAs and Triumphs.
Leave room for wiring and rubber mounts.
Most people start with a tunnel and flat bottom as a separate structure, make the mounts to fit it to the frame, braze in the petcock bosses and baffles, then make the top with sides.
The old giant Hoske and Heinrich tanks for BMWs and Zundapps were make out of about thirty pieces and had an unbelievable amount of filler in them.
I have had good results ith 18 ga. for the tunnel and bottom, and 22 ga for the tops.
Gas is heavy and tanks with a deep tunnel will flex and crack, make bosses on the bottom to hold a strap acrooss the two halves.
If it's for a chopper you don't have to make it big or worry too much about strength...they are just for profiling and don't get ridden seriously. You could almost cast a chopper tank from solid bondo I guess

 

I'd leave it open till the dackbone of the cycle was determined. That way you could also figure your tank mounts then also.

 

cool, thanks for the info, i started making the form tonight, gonna hammer form it then braze it together, making it outa four peice, not including the bottom, then brazing them all together then if i sell or give it to someone they can make the tunnel and bottom.

 

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If it's for a chopper you don't have to make it big or worry too much about strength...they are just for profiling and don't get ridden seriously. You could almost cast a chopper tank from solid bondo I guess

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You've got to be kidding. Thats like saying no one drives a hot rod seriously.