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Ok guys here goes i'll try to answer a few of the questions asked here & a couple that have popped up via pm at the same time 
i'll also pm folk seperately with more detail as required
Firstly the pic of the badly bent frame ...... imo thats from accident damage not from any particular weakness in the frame itself, from the pics i'd say it would not straighten simply by pulling it on a jig......again only my opinion but that would need the front tubes replacing as well as the headstock pulling (not as difficult as its sounds if you have the right kit)
Welding.... personally i use mig as thats what i have to hand & what i'm pretty good with, i would prefer to tig or bronze weld my frames if i'm honest but finances (& skill) dont allow that, i always drill an air hole in any new tube for gasses to escape
Distortion during welding...... as i said ive done quite a few frames over the years some i've done on my jig & some ive done on the floor of the shed, quite a few of those shed floor jobs have later been on the jig to be checked & none have been twisted out of alignment
The main reason for this is i always fit either a swingarm or an acurately measured piece of tube between the swingarm pivot points & bolt it up as if it was a swingarm, if you dont do this then fit brace D the frame wont squeeze in a little to the arm as it does on a stock bike & you have problems
I always like to fit some bolted together (but empty) bottom engine cases when doing any sort of frame work to help prevent twisting but as its not always possible a simple engine sized jig made from 1" box is as good & also helps to transfer any heat away from engine mounts etc you can also use heat sinks but ive never needed to on a simple bracing job
Another trick to help prevent distortion is to space or alternate your welds for example on the horizontal bottom part of brace D i would weld the back half of both ends of the tube first then let it cool before working on the other half (again at both ends) this means the heat generated thats is trying to bend the tube out of shape on one end is ofsett by the same amount of heat doing the same at the other end, you will be hopping from one side of the frame to the other admittedly but you wont be able to mig all the way around any of these tubes in 1 hit anyway so every little trick helps
Brace C the most important one imo ..... for this brace i tend to use a piece of either 2.5" or 3" x1" thin walled box erw (1.8mm wall thickness if memory serves me well) the shape gives it its strenth in the required plane ! fitting this one can be tricky as because of where the rearmost part of the stock tubes bend down they also bend out, this makes the brace angled slightly from the vertical, i advise cutting the brace & tack welding it on then test fitting the tank before fully welding it
Once happy i tack it securely in a good few places then carefully weld the inside by the do an inch leave an inch method on the inside before letting it cool then going back & doing the bits you've missed + the ends & letting it cool before welding the outside in one continuous run, again with this i would be working on both sides at once as mentioned above
I hope thats all of some use to you all
thanks tone
i'll also pm folk seperately with more detail as requiredFirstly the pic of the badly bent frame ...... imo thats from accident damage not from any particular weakness in the frame itself, from the pics i'd say it would not straighten simply by pulling it on a jig......again only my opinion but that would need the front tubes replacing as well as the headstock pulling (not as difficult as its sounds if you have the right kit)
Welding.... personally i use mig as thats what i have to hand & what i'm pretty good with, i would prefer to tig or bronze weld my frames if i'm honest but finances (& skill) dont allow that, i always drill an air hole in any new tube for gasses to escape
Distortion during welding...... as i said ive done quite a few frames over the years some i've done on my jig & some ive done on the floor of the shed, quite a few of those shed floor jobs have later been on the jig to be checked & none have been twisted out of alignment
The main reason for this is i always fit either a swingarm or an acurately measured piece of tube between the swingarm pivot points & bolt it up as if it was a swingarm, if you dont do this then fit brace D the frame wont squeeze in a little to the arm as it does on a stock bike & you have problems
I always like to fit some bolted together (but empty) bottom engine cases when doing any sort of frame work to help prevent twisting but as its not always possible a simple engine sized jig made from 1" box is as good & also helps to transfer any heat away from engine mounts etc you can also use heat sinks but ive never needed to on a simple bracing job
Another trick to help prevent distortion is to space or alternate your welds for example on the horizontal bottom part of brace D i would weld the back half of both ends of the tube first then let it cool before working on the other half (again at both ends) this means the heat generated thats is trying to bend the tube out of shape on one end is ofsett by the same amount of heat doing the same at the other end, you will be hopping from one side of the frame to the other admittedly but you wont be able to mig all the way around any of these tubes in 1 hit anyway so every little trick helps
Brace C the most important one imo ..... for this brace i tend to use a piece of either 2.5" or 3" x1" thin walled box erw (1.8mm wall thickness if memory serves me well) the shape gives it its strenth in the required plane ! fitting this one can be tricky as because of where the rearmost part of the stock tubes bend down they also bend out, this makes the brace angled slightly from the vertical, i advise cutting the brace & tack welding it on then test fitting the tank before fully welding it
Once happy i tack it securely in a good few places then carefully weld the inside by the do an inch leave an inch method on the inside before letting it cool then going back & doing the bits you've missed + the ends & letting it cool before welding the outside in one continuous run, again with this i would be working on both sides at once as mentioned above
I hope thats all of some use to you all
thanks tone
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