70 T120R Sprocket Alignment

[buzzkilr]

So I'm building a Triumph hardtail bike. Hardtail is a 4" /0 drop D Bird piece. I've seen pics of people clamping a straight edge of sorts onto their sprockets to check the spacing/ alignment of the front to rear sprockets. What I'm confused about is there's hardly any room up front for a straight edge to fit on the Triumph. What should I use? Round stock?

[buzzkilr]

Better pic

[Sky]

I just use the chain

[TriNortchopz]

The straightedge you use does not need to fully contact the front sprocket.
With the rear wheel confirmed to be straight (measure each side from the axle to known point - like the frame rear engine mount stud), clamp straightedge onto rear sprocket with the other end as close as you can get it to the engine sprocket, then eyeball it visually. I usually do this without axle spacers, then measure what length spacers you need.
Confirm center of tire in middle of the frame by running a stringline from middle of the neck, down middle of the backbone and over the tire.
Depending on width of wheel, tire may need to be offset to provide chain clearance - rear chain alignment is a higher priority.
Who installed the hardtaill? Would be worthwhile to confirm is is welded on straight too.
Got a laser? that would be helpful.
I got this as a straightedge; cheap, readily available metal u-channel - a piece of shelf support strip:

[buzzkilr]

Bolt on hardtail. I do have a laser. I'm not sure how I would use it for this operation though. I'll try what you described out this week. Thanks!

[Dougtheinternetannoyance123]

You can use the old fab trick of the "T", take a carpenters framing Square,, one of the big ones. Or a laser (We bought one from harbor freight for a big pipe and excavation project to get drop across a 300 foot field)

So lay out using the square or laser on the floor of a shop or garage where the floor is flat and smooth. Use tape, carpenters string and chalk and lay out one long line,, longer the better. Then it helps to to lay out some parallel lines, Then establish a center and run more lines cross ways. You know have a grid or established straight lines as references. Lay the bike on top of center and then use the lines on the floor to measure from using a square and calipers or rulers. There is a rule in construction that if you are off by a 1/4" at the start you will be off by a 12"/1 ft' by the end. So the longer the distance the more it shows even a small discrepancy. So the longer your reference lines and using a string or a laser it will highlight the alignment that may seem minor but is significant.

I worked for a while for a trucking company and watched old timers set up frames, and axles on trailers and tractors. A 40 foot trailer that does not track true scrubs the tires and consumes fuel. They used this same method and string. The old guy doing it would run lines out the shop 120 ft in either direction, a tire, rim or axle that might look straight would show it was not at that distance.