I've made some more measurements and I now feel reasonably happy to post the results.
Here is a picture of how I made the measurements and it really is very simple and very cheap to do.
I fastened a Bosch laser level (cost about $30) to the brake disk using a velcro tie strap. The side of the laser level has a flat machined surface ideal for this purpose. This laser "draws" a line rather than a dot and you can see the red line on the piece of aluminium on the floor. I drew two lines on the aluminium perpendicular to the laser line 30" apart (2 x wheel diameter).
I then used the trolley jack to set the suspension at normal ride height, having first disconnected the lower shock absorber mount. Where the laser line crossed the two previously drawn lines I used a fine scribe to mark the exact position. The procedure was repeated with the suspension at 1" and 2" above and below the normal position.
I now had five scribe marks on each drawn line. I then used a set of vernier calipers to accurately measure the distance between each corresponding pair of scribe marks and subtracted one from the other to get the deviation (toe-in or toe-out) then divided by 2 to get the deviation for a 15" wheel (remember, the scribe marks are 30" apart).
Here is a chart showing two sets of results, the second one after a very minor suspension adjustment.
With care, I believe it is possible to measure to within 0.002" using this method.
I have taken multiple measurements of the same suspension setup and I am happy that the results are repeatable.
Of course, the further apart the guide lines, the more accurate the results. I had to rotate the wheel slightly each time in order for the laser line to reach the ends of the 30" scale, but this doesn't effect the results at all.
I hope this helps someone.
Paul
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