Bluninja - you're close! Set the laser up so the beam just kisses the widest part of the rear tyre on both sides of the arc. That way, it's running parallel with the rear tyre. (Check the tyre for "wobble" first, but you probably won't find much unless a really crap tyre fitter has done it!). The front piece of the rig goes hard up against the front tyre, you can hold it there with really thin elastic if you want. Turn the rig/ front wheel until it's parallel with the laser beam. That's why I have two score lines on the perspex to help.
The centre line of this part of the rig equates to half the difference in the widths of the front and rear tyres. This means that if front and rear are in perfect alignment, the laser beam will go down the centre line. It doesn't actually matter though as you can simply use a decent rule and measure the offset. Repeat it on the other side of the bike and if the offset is different, then you know that you will need to give the rear wheel a tweak (The front of the rear tyre needs turning towards the side with the smallest offset). Check it again when you're re-tightening the wheel nuts.
Duke - I used the string lines exactly how you suggested it. It's a pain in the arse if you have the bike on its centrestand and also if you need to rotate the rear wheel, that's why I built the laser rig. The string certainly works ok though.
Dave - Smart thinking! That should save doing full alignments all the while when adjusting chains etc. Would still do one every so often though in case of excursions up pavements, down holes in the road or, perish the thought, following the bike along the road
If it's still not clear, I'm quite happy to scan a quick sketch of the process and post it, just let me know. I hadn't intended the post to be a teach-in!!!
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