Paul, here's a good example of the difference between a Champion with MX geometry and a proper dirt tracker. Note steeper steering rake on the black/white tracker.
While I've got these piccies up I thought it'd be the right time to show a cheap way of building one of these cool bikes. The above two bikes are built around stock frames, saving the major cost of using an aftermarket unit. The Yamaha 360 is a especially nicely done conversion with the frame deraked to 25-26 degrees and using Honda alloy "mag" wheels which are especially light. This bike could be put together really cheaply by anyone half handy with a hacksaw, portapower and TIG, MIG or oxy welder. If you're uncertain about cutting up a good MX frame, the same result can be acheived using an old DT 250/360/400 twin shocker. The aftermarket 'glass tank and seat can be purchased from a number of sources including our own Matcho Mick.
The other bike is a 350 Kawasaki Bighorn that had been converted back in the day using a rare factory flat track conversion kit. The results acheived are the same as the Yamaha. My TM/TS VMX project bike is soon to undergo a similar rebirth. This forums 'Yamaico' took us through a step by step build on his 465 Yamaha flat tracker a year or two ago. The de raking process and some great original thinking on building a Budget Dirt Tracker is shown in detail.
http://ozvmx.com/community/index.php?topic=9177.0