There's a whole lot to know about suspended multi-track vehicle dynamics. I'll sum up a few of the parameters involving wheel alignment, with some brief and simple definitions. Some will be expanded later on in this article, all of them are extensively explained on the www.Different methods each have their followers.
Too much toe-in makes one of the front tyres scrub sideways. This causes more tyre wear and, more importantly, increases rolling resistance. Too much toe-out generally does the same, with the added problem that the vehicle becomes wobbly as the weight is shifted constantly from one wheel to the other.
Good quality tyres can last 5000 to 20000 km, according to type and purpose. If the tyres wear too fast and/or unevenly, you can conclude it's time for some finetuning.
Although the deviation is small, all measuring should be done under load. This means that either the usual rider sits in the vehicle and the static measuring is done by a friendly neighbour, either a comparable weight is positioned on the seat. The center of gravity of the rider - or substituting weight - is approximately the position of the belly button.
1. Visual alignment. Bottom line: if it looks parallel, it is. Rectangular pavement is a great help. Round off to the smallest toe-in.
2. Measuring front and rear distance from wheel to wheel. You may increase precision by turning the wheels or tying longish objects to them.
3. A time consuming but very rewarding method is the coastdown method. While varying the toe-in setting, roll-out from a hill is measured (distance or time) and set out on a little graph. Top of the parabola determines the best real-world setting.
Factory settings are obviously limited to method 1. and 2. as rider weight is relatively important compared to the tiny weight of a WAW. Therefore we recommend that every rider performs the roll-out test to fine tune the wheel alignment in real world conditions.