Setting up your chassis..
I saw a thread on the message boards about aligning the wheel to make a bike go straight and it mentioned leveling the seat rails. A more accurate way of leveling the bike is to use the rear brake rotor, that way you eliminate the possibility of the seat rails, swingarm or axle blocks being misaligned.. After the bike is leveled by the rear wheel adjust the wheelie bars so the axle is level. Now the wheelie bars are square with the rear wheel...
The best way I have found to align the rear wheel is to snap a chalk line on the ground about 30 ft. long. Walk the bike along keeping the front wheel on the line, at the end check the rear wheel to see if it's centered on your line. If it's not, adjust your rear wheel and do it again. Mark the axle blocks and measure from this point.. After this measure from your axle to your wheelie bar wheels to make sure they are square to the rear wheel and in the center of your line..
It is important for a big tire bike to be level when leaving the line, a little trick to learn when it's level is to go to the hardware store and get a line level, make a bracket and attach it to your tank shell or top frame rail. Level your rear rotor and make sure the line level is reading level too. Now, when you go to the line before you stage glance down and see if your sitting level...After a while you won't need the level anymore..
If you do all of these things the odds are your bike will leave the line straight and true..