Originally Posted by
Six jours
If the main triangle is off, it can be fixed with a vice and a length of 1" pipe. Chuck the BB shell into the vice (soft jaws are a very good idea) and eyball the frame from the front or back. The human eye is extremely good at seeing parallels. If the head tube isn't parallel with the seat tube, the unaided eye will detect it. Slide the 1" pipe into the head tube and torque it in the right direction. (You'll be amazed at how much effort this takes. If you throw your back out, you never met me.) A bit of trial and error should make it perfect.
If the rear triangle is off, it will be obvious with the string test. With the BB still clamped into the vice, simply pull the rear triangle (one stay at a time) in the right direction. If you can do the arithmetic, you should be able to fix it in one go. If not, trial and error will again get you where you need to be.
If the fork is off, you're screwed, unless you have the right tools. The fork needs to be perfect in several planes, and should really be set by someone who knows what he's doing.
Just me, but there are only 3 planes to a perfect long thin rectangle and that's all a bicycle frame and it's forks make when assembled assuming the axles and wheels have been trued. There's the vertical (front to rear) plane. The horizontal (side to side) and then there's the 90 degree planes that the axles need to be for the bike to track straight like this from the top:
Front --|-|--------|-|----|-- Rear (the first | is the intersection of the front wheel, the last | is the intersection of the rear wheel. Bottom bracket (3rd |), head (2nd |) & seat tubes (4th |) need to fall in line with those points.)
If that is straight at the front & rear axles the bike will track in a straight line.
The key is perfectly 90 degrees for straightness. A fork with more rake can offset the negligible gap of the top tube from the top since the brake pad angles are adjustable. That much gap probably means the front wheel is about that much closer, give or take to the down tube at the outer circumference of the tire for that wheel clearance.
BTW, the human eye can be tricked with depth, not everyone has 20/20 or better perfect vision in each eye and that's where a high tension string/twine/wire stretched from 2 points nailed eliminates any doubt. I'm not so certain about lasers, because that might depend upon the perfectness of any lens ? But for the most part I'd think a laser is optically, perfect straight ?