My apologies. I misread your post to be honest. I am just curious how you adjust the backspacing accordingly to the scrub radius.
Short answer: you mock it up or diagram it out as best you can. I usually do mockups and then diagram it, because I'm good at getting things wrong the first time around.
Better answer: scrub radius is the difference between the contact point of the kingpin axis and the center point of the tire contact patch; as the tire grows in diameter, the kingpin axis' contact point moves further and further outboard, because triangles. If the center of the contact patch of the tire doesn't move outward as well - i.e. no change in backspacing - the scrub radius will increase... because triangles, again. Doesn't matter if you have a wide or narrow tire, in that case; there will still be a change in scrub, and the center point of the contact patch will be effectively identical between tire widths.
And all of that may mean exactly nothing; I'm too new to Toyota steering geometry to even pretend that I know anything about it.
Last edited: