Join Date: May 2014
Bronco Info: 78 351/NP435, lifted 4", 33s. 79 in progress, built 400, low gear C6, lots more to come.
Info: scrub radius vs. SAI vs. tire size vs. backspace vs. offset
I've been doing some measuring and calculating on my 79 Bronco to try to express the affects of tire height and placement on scrub radius. Ultimately, I would get my measured steering axis inclination from an alignment shop, but there's no need to pay for that until I actually need alignment work. So these results are only to approximate the same handling with tire and wheel changes.
I calculated a 10.847 degree steering axis inclination on my Bronco (which does not allot for whatever my camber angle is, so again is still approximate). This gives me a 6.022" scrub radius with my near-bald 33x12.50 Maxxis Buckshots on 15x8 MT Classic IIs.
I found that with the same size wheels, every 2" in tire diameter increase would need a negative change of 4-5 mm offset, or 3/16" less backspace to maintain the same scrub radius. The change in offset is the indicator to see how much scrub radius is affected.
I am reducing my negative offset by 9.5 mm, which allots a change from 33s to about 36 or 37s to keep the same driving characteristics I have now. The trick is if that would fit without rub. I'm staying with 33s for now, but going from 15x8 to 15x10 adds 5/8 to the backspace and brings the outside edge out 1 3/8. Overall, the tread centerline will come out 3/8". This will slightly improve my radius arm clearance, which isn't a problem anyway, but I think will allow 35s with minimal control change due to scrub radius once I get the lift kit installed.
I understand that much less backspace might be needed as tire size grows to clear the radius arms, but hopefully this gives an idea of how much (or how little) the original scrub-radius and handling characteristics can be affected with tire and wheel changes.
...or is this all pointless? One thing I DON'T know is the actual characteristics of factory wheels. I am interested in measured dimensions to apply to my process.