J is something to do with the rim width. I have always just run a straight edge off the wheel flange and then measured to the inner guard/leaf spring and then to the the lip on the guard.
This gives a pretty good picture for you to work out where the rim will need to be in relation to the flange. Note - the tyres can afford to be nearer the guard because of the slight bulge in the tyre means that it needs to move further before contact is made (also when the body rolls to one side, the guard has a tendency to be move away from the tyre).
Sun, 06 June 2004 07:28
