Wheel alignment adjustment by using a string

I have tried to align front and rear wheel, but I have some problem I don’t understand.

I use a string going from rear wheel front side and I turn front wheel so that the string goes 20mm from front wheel in front and back of the tyre. I do this on the left side and result is this 20mm front and 20mm back. Then I measure right side and result is 20mm front and 25mm back. This is ok, wheels are no aligned.

But then I adjust rear wheel. I have done it several times. I keep another side of the rear wheel shaft same and adjust 1-2mm other side. And I have same result all the time. I turn front wheel so that left results are 20mm and 20mm. Right results are 20mm front and something between 24 to 26mm on the back of the front wheel. I don’t understand that. It is like rear wheel is not adjusted. My brain is saying that at some point results should be 20/20mm left and 20/20mm right but it is impossible to found out that point. Maybe I am stupid?

I am confident that my measurement tehcnic is ok and I have made this several times today (like 10 times). String is quite easy and fast to use. Ofcourse some 1-2mm error is possible but every time there is that 5mm difference in same place. Last winter I had to put 1mm washer into the swing pivot and afterwards I measured this error also then. Now I started this excercise again as I changed rear tyre today. The bike handling is ok so no problem there, but I would like to understand why this system is not working???

Originally Posted by Mannerheim

I do this on the left side and result is this 20mm front and 20mm back. Then I measure right side and result is 20mm front and 25mm back. This is ok, wheels are no aligned.

You are saying that when the left side of the front wheel is parallel to the left side of the rear wheel, the right sides are not parallel. Either:
a) you have very strange tyres on which the left & right sides are not parallel; or
b you are doing something wrong.

My guess is the string is pulling into a slot in the side of the tread, or fouling on something...

I use a piece of 25mm square aluminium tube, much easier than string.

^ maybe they are parallel, but wheels are not going same line?
I don’t know, maybe I yesterday made some mistake anyhow. Today I managed to found out that I can adjust wheel so that they are parallel, but left side front wheel had 20mm gap and right side as much as 38mm. First thought is that other wheel is 9mm ”in wrong line”…
I tried to measure length from swing pivot to back wheel shaft that it is same on the left side than on the right side, but I don’t have correct tools for that.
Now I have had enough this measurement and adjustment. It is what it is. Probably I will continue next winter and will buy some aluminium tube. Maybe there is something wrong in the bike (=extra washer I had to put into the swing pivot because of wrong dimension swing).