JBinMD wrote:
I think a wheel's stated width is the width between the beads, but the backspace is measured from the overall outside width, which is usually about an inch wider total or 1/2" per side. For instance, an 8" wheel usually has a total width of about 9".
You are entirely correct. On a 15x8, 0 offset happens at 4.5" BS, not 4" BS. It is a mistake to not account for the wheel lip when converting between BS and offset... Perhaps not one of dire consequences, but a mistake none the less.
cbaily wrote:
As it is now a 5" backspace fits but JUST barely but limits steering angle greatly by contacting the upper control arm mount.
FYI: They do make narrower angle ball joint holders, that could alleviate some of your steering angle interference @5" BS. [url=https://www.amazon.com/Allstar-Performance-ALL56292-Joint-Holder/dp/B007WLYPTC[/url]
cbaily wrote:
Buy 2 inch drop spindles allowing me to run a greater backspaced wheel without contact of the BJ and control arm but obviously would mess with front ride height. I might be able to get away with it by throwing a 2" longer swegde tube in the suspension.
I like this one with the 1-2 inch longer push rod, assuming you don't have enough adjustment in the rod ends. One thing to consider is that in doing so, you'll actually be making for an even larger scrub radius to overcome.
cbaily wrote:
I Know its somewhat opinionated but what would be a good scrub to shoot for?
Mazda marketeering claimed that the NA Miata had zero scrub radius, but much like 50/50 weight distribution claims, claims like that typically rely on an inherent level of imprecision that waters down the definition and rationale. Then NB Miata, which is effectively the same suspension, went to a 5mm lower offset wheel, giving it a small amount of additional positive static scrub. Pretty much every other 'great handling' RWD car I'm aware of also runs a small amount of positive static scrub. Consider that depending on which direction the lateral forces are occurring, zero static scrub also alternates between positive and negative dynamic scrub when taking corners at speed. This seems to coincide with the nominal positive static scrub radius is generally being used on sports and race cars. So I'd generally keep it as small as reasonable, but for this reason would also tend to err on the positive side. Based on what I noted about the effect of drop spindles on scrub radius, then even the 7" BS should actually end up pretty much there anyway... Unfortunately though, I doubt you'll be able to find 7" BS on 15x8 wheels.