1" is pretty neutral and a good starting point. I only use back spacing for fine tuning at the track if a major adjustment is needed as the track conditions change. Yes the contact patch comes into play but not like you think... think of where your roll center is at, attach an imaginary lever to that point and think of pivoting it across the center of your contact patch of the tire, say on the right rear for example. The theory here is in weight transfer of the roll center to the outer point on that imaginary lever. If you have the chassis on scales with 1" back spacing, pt your foot on your right rear bumper and push it down an inch... note the weight and cross percentages change. Now place the 4" offset wheel on the right rear.... the contact point of the imaginary lever from your roll center to the center of the contact patch is longer giving more leverage... hence applying more leverage as the set-up transfers to the right wheel quicker (as a longer lever would). That would loosen the chassis by doing this on that corner. With that 4" wheel on the right rear, chassis on the scales, do the same experiment as before, push the right rear bumper down 1 inch and notice the scale numbers and their change. Obviously offsetting the right rear 3 inches out with a back space variable will loosen the chassis per weight transfer there according to the chassis roll center to contact patch leverage (weight distribution difference as opposed to the 1 inch back space). The backspacing can be changed on each corner of the car thinking of the lever on the same principals. When you get to the front of the car, offsetting the left front out will loosen the chassis as did the right rear offset but at different rates (of coarse) depending on the balance point of the chassis. Although it is on the opposite corner of the chassis and the effect is the same, backspace changes per corner of the car effect it's handling characteristic as the criss-cross counter part (front to rear, left to right) are the same.. the handling characteristic changes per weight transfer on different parts of the track. Corner entry looseness or tightness as well as corner exit characteristics change when the roll center leverage (weight transfer) per corner is changed.
In Legends, in order to alter offsets, your chassis would have to have a base set-up in it that the stance was narrow to allow the changes to meet overall chassis width during tech after a feature event and offset changes being made. Many spring weight changes or adjustments can result in the same effects and be less hassle to change as well as not having to carry extra rims and tires to the track to make these changes. Like I said in the beginning... 1" offset all the way around is a neutral setting and rather easier to work with along with spring adjustments than fighting drastic changes imposed on odd offset rim combos when chasing set-up at the track by adjusting springs. Offset rims will work for the adjustments but you normally have to start the night with a narrow wheel stance to begin with for the evenings events. If you use your scales as a tool and not just to check weight numbers and percentages with the car setting on them, you better get used to seeing what the chassis does with offsets on the chassis while on the scales and effects by manipulating weight changes on the corners like jacking on one corner (say the left front) to simulate corner and weighting down the right to see these changes while in the shop. Know where you are with the offset rim changes beforehand because if you just slap different offset rims on to go to the track and try... you're opening a can of worms you may not want to fight. JMHO. Easier for the majority of 'joe racers' to start with a neutral rim offset (1 inch) and adjust chassis from there by tweaking on the springs rather than try to figure out the ultimate rim offset and serious differences in poundage effect per spring tweak. There is advantages to be found with offsets per roll center influence on the chassis though.... starting with neutral offsets is easiest (feel like I talked in a circle here)
~Gimpster~
