Quote:
Originally Posted by v3.0si
you may already know much of this, but i'm just trying to be thorough...
first the obvious - the values sent to the speedo will be higher than standard (i.e. the wheel will be spinning faster, so actual speed will be lower than recorded, and the actual trip will be shorter than recorded). also, we know for a fact that the 235 and 245 tread widths will work in the rear.
ok, now for tires -
going to a shorter side wall will increase sidewall stiffness, therefore giving the driver an enhanced feel or perceived increase in feedback. it will also require the suspension to pick up the extra give needed to accomplish a compression in the suspension overall. remember, suspension engineers not only figure shock and spring compression into their equations, but also sidewall compression (i've heard 15% - 20% of the suspension's initial compression is accomplished with a tire's sidewall!). usually i would consider going to a shorter sidewall a bad idea, but as long as they aren't runflats, it's probably just fine for this setup. a shorter sidewall will also reduce rotational mass, assuming that the non-stock wheel choice doesn't eat up the difference. this aspect is good from a suspension AND from a braking perspective. overheated brakes suck big-time! also, a smaller overall diameter is easier to accelerate from the axle's perspective, obviously increasing 0 - whatever speeds. interesting note: most racers want a very stiff suspension on their cars so there is no diving, squatting, or rolling during on track manuvers; however, the sidewalls on these race-ready rides are usually equal to, or taller than, their production brethern. whether this is a class restriction, or for performance gain, a racer will need to answer that...
front clearance -
235 vs. 245 - follow my math on this - if others on this forum are having front inner liner clearance issues with 235's over 225's on an 8.5" x et35 wheel, then a 245 tread width will as well. pulling the offset in to an 8.5" x et40 will give the same wheel clearance inside, but still move the outside of the wheel out giving more front-end track (THANKS DEKALIBER!), without having the 10mm extra tread width per side of the wheel's centerline produce inner liner clearance issues at lock. inner wheel clearance should not decrease, so choose the offsets carefully! i measured inner clearance on the stock 8.0" x et47 wheel with 225/40R18's to be 10mm. under latteral load, the wheel on the outside of the turn will straighten to zero camber, while the wheel on the inside of the turn will GAIN MORE NEGATIVE CAMBER. with a wider tire and less clearance, an inner tire sidewall-to-strut collision may occur.
great question!... had you not asked this, i would never had sat down and put it all together.  |
wow one of the most informative and helpful posts. i knew of the speedo issues and the increased harshness due to shorter sidewalls but i thought clearance would not be an issue.
ugh, now i feel like i'm just not even trying but the wheels i had in mind were 18x8.5 et42 but i guess i have no option besides the 225 if i want square? i figured 225 up front is fine but the loss in rear traction outweighs the benefits of countering understeer with a square setup.