09-21-2016, 12:13 AM
Join Date: Nov 2015
Total Cats: 10
DIY alignment observation
I’ve been playing around with my alignment lately. As anyone who has fiddled with their alignment knows, you can use the pythagorean theorem to calculate your toe or camber pretty easily by comparing your wheel to a straight reference. I’ve been using the string method and a carpenter's
square as my references. I found it annoying to have to calculate the angle every time i made measurements, and it got me thinking: what is the optimal hypotenuse length (e.g., the diameter of your wheel, or length of your alignment stick) to get a 10:1 ratio between changes in distance
between your reference & wheel, and changes to your alignment in degrees?
It turns out the optimal length for an alignment stick is just about exactly 573mm, or 22 9/16”. With an alignment stick this length, you can move the top of your stick inboard 1mm from the reference, and it will lower your camber by .1°. Here’s where it gets funky: the OD’s of our more common wheel/tire combos are very close to this number:
195/50/15 on 15x7’s = 576mm
205/50/15 on 15x8 = 586mm
225/45/15 on 15x9 = 583mm
All three of these will get you within a gnat’s *** of .1° change in alignment for every 1mm of
movement at the outside edge of the tire.
Now obviously this doesn’t take into consideration sidewall deflection, tire inflation, tire wear, or
any number of other factors.
I don’t know if this is common knowledge, but I thought it was worth sharing.