I will have to respectfully disagree that it is a waste of time & effort to use a string. Clearly if it can be made to work to measure wheel alignment in the 1-2mm regime then it is worth the effort. I think it even better than that if done properly.
The sting method (and I use that term loosely because there are variations) is generally applied to a road ready bike except for the wheel alignment. It probably works just as well for checking frame alignment against the bare wheels alone.
So for the sake of discussion I will only contrast the string v.s. a laser for measuring straight lines as that is what will be used in whatever variation of alignment employed. Any method that can be conceived of for alignment will invariably project two straight line from either side of the rear wheel wheel to the front of the wheel. This is true for strings as well as lasers.
Of course in your description you said to get a straight edge that was really straight; but that begs the question how do you know the edge is straight. Well I confirmed the accuracy of my straight edge with a thin string. It was 1/16" at 6 foot. So one way or another your straight edge is predicated on some form of "at home" straightness measure.
So the question then becomes:
a.) how straight are the lines?
b.) how well can I position (with certainty those lines)?
For a.) everybody knows that a laser is light and is essentially a perfect coherent light source, but alas nothing is perfect and there is beam divergence. For a decent laser Beam divergence is typically 1 mrad so at 2 meters the beam is approximately 2mm is diameter. The question then is how well can that spot be located at with 1ft (one wheel) and at 7 ft (the other wheel)? Having tried this myself, trying to put a measure a laser at it's edge can be a little difficult so if you have a special fixture that is carefully factory aligned (probably with a string) then I would say that the real accuracy of being able to read a full spot is probably within a 0.25mm .
If you are sighting down a straight edge for example , it is difficult to tell when you are splitting the beam for true 1/2 and at best you are probably to a +/-1/8mRad accuracy which is +/-1/4mm at 6 foot.
In contrast how bad is the string? Well it depends allot on how you use it. The string will be quite straight and only suffer moderate catenary is you keep it thin and taught. If you use the string to measure horizontal distances then there is no error in the string as all of the catenery is in the vertical plane.
The real trick with use this string is how well (read accurately and consistently) it can be placed adjacent to a wheel or a tire . If you set the string up so it touches the wheel and then expect to site down the string to ensure it is straight, then you will likely be many mm off in your alignment and I would have to agree it is not going to be worth the effort. On the other hand if you position the string next to the wheel or tire and to make sure that the string is located against both left and right sides the same use the string strum technique.
We obviously want the string straight so that means the string can's be pressed against the wheel. It can however be very close. To insure it is not pressed against the wheel strum the line to ensure it will vibrate up and down, but not how long it continues to vibrate. Adjust the distance so that the fibers of the string cause the vibration to diminish consistently on both sides but still vibrate vertically[This is the main trick required to get accuracy with a string]. I used a very thick string that was approx 1mm in diameter (it is easy to use something much thinner down to 0.2mm). With the vibration method described, you can very easily determine position within +/-1/8 of the string diameter. Of course the hairyness of the string matters to some extent.
So for my string, it was 1mm and I was easily achieving measure positional accuracy to within +/-1/8 mm. At the other end you are typically using a caliper to measure offset between the wheel and the string once you have your strings properly strum positioned. I'm using a veneer caliper and a 1mm thick string; it is easy to achieve +/-1mm of accuracy there as well.
So my conclusion is that using a "considered" string approach, 1mm accuracy can easily be attained and that is all that is ever really required for either wheel or frame alignment. Infact, you would be hard pressed without specially machined and calibrated fixtures to even measure the straightness of a straight edge with a laser. I had both (laser(3 in fact) and string) and finally settled on measuring divergence from a string in the horizontal plane.
Jim
Comment