Downsizing from 18” to 17”

[totopo]

Yep...



I will disagree with you that a tall sidewall does not translate to more roll... though I'll agree it is essentially separate from the sprung weight laying down in cornering. Your suspension is based around your OEM wheel/tire size. Change nothing but the wheel/tire combo... going down on wheel size and up on the sidewall keep the overall circumference the same while retaining essentially the same tire width to... your body roll will definitely increase. I can flex a sidewall with my bare hands... but I haven't spent enough time in the gym to do the same trick with steel/aluminum wheels. Take a hard corner on a tall tire... not only does the body lay down on it's suspension, it'll squish down the taller sidewall on the outside wheels, and the tires will actually bend with the road... magnifying the effect.

Going wider definitely helps hold the tires from flexing so bad.

I have got 2 sets of rims for my Z... running the stock 225-50-R16s and I got some late model 350Z running 245-40-R18s (pretty much all I run anymore). The car definitely has more roll with the higher sidewall 16s. Same experience when I went back and forth between the 16s I had all seasons on and the 18s I had summer tires on the RSX. Far more body roll. Yes... both are wider tires too which is a huge contributor.

The article I posted was an old thing I remembered and, it is ashamed they couldn't maintain tire width constant as a control... but it's what I remembered having seen way back when.

Here's a decent vid of sidewall flex on a car in motion...



Unfortunately I can't find a comparo video really... showing something like a 16 vs an 18 with the tire having the same circumference and same width.

The actual angular roll of the car doesn't change, it is sprung and the diameter of the tire doesn't really change. What you are probably feeling is either the extra lateral translation and the car "falling over" the contact patch, or the tires are generating more grip and so you get more roll.

Sidewalls definitely flex, that is what tires do and how they generate lateral traction. But narrow tires and less sidewall don't always mean more traction like it used to I think. Not a tire expert, but these days doesn't seem to be a big factor. Kind of like camber and anti-roll bars. Things drivers love but doesn't translate to much difference in track times.

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[charleswrivers]

Its hard to come up with good comparisons that maintain the contact patch of the tire that also increases sidewall height.

Best I can tell if I'm outdriving the tires is to simply listen and look at it. Squeal and seeing discoloration at the transition between sidewall to tread shows I'm starting the wrap the tire to the point the cars is actually starting to ride on the sidewall somewhat.

I haven't seen this at all on my Si that's riding on summer rubber. I used to see it on the RSX riding it it's both taller and narrower 16s. Same on the Z.

To tell on her a little... since my wife drives the Civic more than I do... I see it on our Odyssey after she drives it sometimes. She tends to corner kind of aggressively when she's alone in a vehicle. I'll look at the van and she they've all gotten all brownish at the tread to sidewall transition... even though they're at the proper inflation. Tall sidewall for the contact patch... heavy vehicle and an overly aggressive driver.

tires are generating more grip and so you get more roll.

This is a *great* point. You can stick tires with awesome grip and it'll definitely induce that extra roll when you start taking corners. I thought I'd done something wrong way back when when I stuck good tires my first Z31 in the 90s with it's wore out suspension. It took new bushings... end links... shocks and struts and new springs that maintained the OEM height but with higher rates and *boom* it could handle like mad. On it's not just wore up but sloppy even from the factory by modern standpoint suspension, the damn thing felt like it wanted to *roll* *over* on cornering as hard as the tires would go.

The Z32s were much better but, even by modern standards with modern tires... it's a relatively poor performer. A 1990 300zx was rated at .86 on the skid pad. Our Civics? .88 on all seasons as I recall... I know my Si on the OEM summer will do .97. Cars today... riding on modern tires handle *amazing* compared to sport cars in the 90s. 80s? Forget it. You can look at a 80s road test and it's laughable how much roll a car new from the factory had. Here's an aforementioned Z31, riding on 225-50-R16s. Look at that thing lay down! That's a *new* car riding on good tires from the 80s. Imagine putting modern tires on that thing. You're practically breaking the car's suspension with so much more grip. Folks don't realize how far tires have come along over the years. A wider contact patch for most new cars helps... a lower sidewall... but the compound for the tires themselves is amazing.




I'm kind of a Z car fan(atic). Always have been. I'll admit though... I'd be hard pressed to keep up with my Civic in the twisties, even with double the horsepower and rear wheel drive to let the back hang out a little in my '94. I had new-old-stock OEM adjustables put back in it a few years back... and some it's bushings were done... but it's still essentially stock, refreshed. It'd be tough to keep up.

 

[CivicXander]

I've been considering getting 17x9 +40 paired with 265/40/17 for more width and traction. I think the combo will be overall lighter in weight too compared to the 18x8 on 235/40/18

 

[gtman]

265/40/17

That's a 10.4" wide tire. Is there such a thing as too wide? Would you need to roll the fenders or anything?

 

[CivicXander]

That's a 10.4" wide tire. Is there such a thing as too wide? Would you need to roll the fenders or anything?

I've checked all the online calculators and a 9 wide wheel with +40 offset is basically the same fitment as 18x8 +30 which is what a lot of people here are currently running, myself included. The 265 will give it a meaty look for sure but I never cared for the stretch tire look. I'm curious to find out also

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