Not all US bikes used A/M, most early stuff was just modifed or in the case of the astro came out like that, but having said that Most Aftermarket frames were Chromoly so they were lighter and stronger.
Angles where only changed to make smaller circles, or built more upright for TT and jumps most geometry never strayed far, it was the forks and arse end that took up a lot of setting up. Many frames where just modified Standards like nortons and triumph it wasnt till the late 60's the after market frames really took off. Over time the guys who played with frames rather than having to go through the exercise of cutting lugs and bits off and welding bits in for strenghten just found it easier to just start from scratch, plus they where able to make it how they wanted and so much lighter with the 6series tube and 4130Cro-mo, as usual as these guys started winning more and more so the next guy would ask for a set of lowered pegs, or can you build me a frame too and on it went until you have 5-6 guys all building and selling frames. IF you draw a line through the swing arm in the picture below you can see the the whole chassis is lower and streched even through it is compact, the centre of gravity is the focus, as is in the bottom half and rear of the frame not the head angles, just changing the head angle will not give you all of those peices of the puzzle you really need. if you look at it compared to my MX 360 in the back ground (even though the fork are turned in ) there isnt a lot off differance in the head angles, there really not that far apart .
i think my tracky Rakes around 25 to 26 degrees but then variable a degree for every inch you move the forks upwards and Low trail numbers of between 3 to 4 inches making the bike easy to point into turns, wheel base is 54-57 inches, but having said that it is as light as shit and would be half the weight of a normal bike. So all those combo's make the frame fast, not just a head angle.
IF you look at the more commercial frames that were sold into shops and well promoted like a champion, if you speak to people like Neil keen he will tell you " champions were almost copies of the Yamaha dt1 frames, just "cleaned up a bit. Doug schwerma, in the beginning , knew nothing about geometry and freely admitted it, but was a brilliant promoter of his product. Neil says the champions handle bad, ( he sold them as well,) and if you knew how to ride a tri. or bsa or a trackmaster they were poor, but we've all seen guys who didn't, win a lot of races with champion. " they were just well promoted and sold over the counter, rider feedback came afterwards, where as other frames you took them your motor and told them how you wanted it to run in or out.
Ray and neils originally geometry was based on the truimph, they were making trackmasters for Racing themselves and sold others off paying for there racing, neil made his first infamous DT1 Frames for the new unstamped ( pre production) Yamaha motors for the Japs out of old truimph bends spliced together, (see pic of my first batch DT1 frame its got way too much headroom) and yet the set up worked still because of how he positioned the motor and how low it was also the swing arm is critical. Others like Boss custom built them for racers and himself, and by the time Terry knight left Doug to set up his business after they closed under a cloud, Aftermarket frames had really become the standard and the geometry had been well founded and they were just tweeked from there. Redline frame geometry was fairly standard too, until they sold up in the early 70's and Mike took the name only and went straight to Japan to build BMX frames.
What frames do you have now ??
Bultaco and similar units always worked very well as really do most standard frames, the only standard frames i have heard that needed to go to the effort of raking is the TT. But IF you stuff around with a frame angle you wanna know what your doing as it will throw everything else out, so if you mess with the front, you change the rear and there is only so far you can push the rear wheel forward so then you have to increase the heights and on it goes.
I wouldnt waste you time changing anything, the other problem you have is unless you ride the same track forever what you do on a short track wont work on a long track as well, also short trackers were only meant to go one way like a speedway bike so were kinda talking about 2 differant frame set ups anyway.
i think you would be better off changing angles if needed through taking the forks through the triples (adding degress) and looking at the swing arm and where the wheel base can be altered. you can also make changes in the shocker body hieght ie drop to 12.5 " shocks and you totally change all the angle again. IF you look closely at the trackmaster below notice where the motr sits in relation to the swing arm, it looks a lot lower and the swing arm is straight not going up, you would have to recreate all those angle not just the head area, down low is where its all happening on these frames, and it would be very hard to recreate that.
IF i have ever learned one thing told to me, it was that Everything you do changes somthing else, if you cant get the back out the problems at the other end ( the fronts not turning in) dont mess at where you think the problem is, as its usually caused by somthing on the other end. Try that with kids what ever they say its doing change the opposite, because thats where the problem starts, somone once talked about cause and effect its very true.
How does all this relate to US frames, well what im saying is you probally have all you need right here already to build a great Australian dirt tracker, however if you want a good looking bike and like the look of them then styling wise you cant go past the Flat track frames, all the other stuff is just that. Are you talking about pre 77 stuff, after that the factorys started gettting it right and you find most frames where standard, even now most FT's are standard YZ, RMZ frames with the shocks dumped and that swing arm geometry area worked on.
Having said that if you looking to build up a v twin or other big bore from a road based unit like a harley, RS , tz etc then you should look into buying in a track frame as you would benefit from the ready made knowledge thats already set up in the frame like a missle or c&J, it would be too much effort otherwise as you would also need to mess about with the seat positions, rakes, pivots points etc. you would really need to know what you where doing otherwise you would have a very expensive deco lamp holder in $$$ cromo.
What have you got and what are you looking to build ? that is probally the question i should have asked rather then wasting all your time reading this . sorry.
