Derbo,
This has a lot of great info, I appreciate it! I'll try and respond point by point:
1. Yes, I meant comfort from a non-quantifiable standpoint. I doubt anyone here has sat with their car and run over a rough road at a set speed and then swapped out wheel/tire combos and compared logged data from accelerometers, but hey, this place has incredible people so you never know! All your points make sense and they're basically what I assumed. I forgot to mention tire compound as another factor, but you touched on that as well. Are summer performance compounds less pliable/forgiving than all seasons or is there really no general trend i.e. there are both hard and soft compounds in all seasons and summer performance tires? And at the end of the day it sounds like you got a little improvement in comfort, but not much, correct?
2. How does a wider track from a smaller offset affect day-to-day road feel vs. during aggressive handling? Would it introduce tramlining? I'd prefer to keep my car relatively stable and comfortable since I don't plan to track the car often or even very soon.
3. I meant those questions in more of a general decrease of sidewall height, not specific numbers since it is a ratio. As far as the grip questions, I guess I should elaborate: since the contact patch shape will become more square in the rear on a 245mm tire but more rectangular on the front, will that affect stability/straightline grip vs. turn in ability, etc. Shouldn't a more square contact patch shape in the rear provide greater rear straightline traction while a wider contact patch shape in the front provide better turn in grip? Or am I thinking waaaaay beyond what I'll actually notice?
For my own personal reference, the approximate sidewall heights are ranked in descending order:
- 235/45R17 - 4.16"
- 245/40R17 - 3.86"
- 235/40R18 - 3.70"
- 245/35R18 - 3.36"
Stock:
- 225/40R18 - 3.54"
- 255/35R18 - 3.51"
From this I think the 245's on 18s is too short of a sidewall. 235s would be preferable.
4. I mentioned the rotational inertia question from my own basic understanding of physics and Stephen's mention of it in his review of his 17" Apex wheels
here. He seemed very enthusiastic but I can't tell if that's just from losing so much unsprung mass vs the actual shift in the mass' location around the rotational axis. I was just wondering if there would be appreciable difference where the mass is located in a 17" setup vs. an 18" setup but if you think it's only noticeable over 100mph, then it's really not worth considering haha
I really do like the way your car looks, but still have love for the 18s
I appreciate that! Through my education I've learned that if you have a question, it's probable that at least one other person has the same question as you so you're potentially helping out more than just yourself. How else are we all supposed to learn?
Some more questions:
- Has anyone had a negative experience from switching to a square setup? I figured a square setup would give the car a more neutral feel, although I can't really complain about the stock handling as particularly understeer-y, with the added benefit of being able to rotate tires. But maybe it's worth keeping a staggered setup. Someone sway me!
- Rotating tires: I read somewhere that because of the greater negative camber in the rear that the treadwear is not ideal to be rotated to the front and can introduce NVH through odd harmonics as well as unfavorable handling characteristics. Anyone else heard this? I figure if you rotate often enough you'd avoid this anyways.
- Has anyone run a 235 section tire? Did you notice a loss in rear grip or overall grip? My father's 540i w/ sport package (also up for sale soon if anyone's interested!) has a stock 235/45R17 setup and it rides even better than the ZHP he just purchased. It definitely is more tail happy than our cars, but I think that comes with a mixture of smaller section tires, heavier weight, and 320 ft-lbs of torque.
- What sort of issues/handling characteristics does excessively wide front tires introduce? Derbo mentions a 255/40R17 tire since it's roughly a 25" diameter wheel, but it seems pretty darn wide on the front.
- I guess I'm still nervous messing with the offset too much. A 17x8.5" ET40 wheel will be 0.7mm further away from the suspension strut in the front. The outer rim will poke out 13.4mm more than before (source), while the 18x8.5" ET45 wheel is closer to stock and will be 4.3mm closer to the suspension strut. The outer rim will poke out 8.3mm more than before (source). Am I really thinking about this too much?
Again, any help or insight is greatly appreciated!