[grbrown]

An alternative to a fragile light tube (never heard that before, but brilliant!) is a suitable plank or strip of wood (often not very straight), or strip of metal. One local MoT station has a plank with small blocks fitted under it, to raise it enough to contact both tyres at two points. Looks like a dachsund bench!

You can check wheel alignment by sight, without any tools, with a little practice. An alternative is a length of string. Tie it to a spoke in the rear wheel, then wrap it around the tyre and pull it to the front of the bike, so it passes the edges of both tyres at two points on each, below the line of the frame.

You can then adjust pointing of the front wheel to its optimum and, with the string just touching the rear tyre at two points, see where it lies relative to the two points on the front tyre. With a long enough length of string you can do this both sides at the same time. If necessary adjust the rear wheel in small increments until both sides of the front tyre are the same.

Note on bikes with fat tyres on both ends that the front is often slightly wider than the rear, which sounds odd but is so (16 inch Avon Venoms are 136mm wide at the front, 133mm at the rear).