7" vs 8"... with same tire

[dentist90]

Question for the tire construction guys. My car has stock tires and wheels, 16x6" front w/ 205/55 Yokohama ES100, and 16x7 rear w/ 225/50 ES100.
One thing that drives me nuts, especially now that new rear tires have been placed, is the amount of sidewall flex I appear to have. Response during transitions is very unsecure feeling. The butt end waggles way too much and it all appears to be due to sidewall flex. When I push sideways on the bumper the only movement visible is the sidewalls flexing. The grip is excellent once the tires have 'loaded up', but when you transition from right to left turn (eg, slalom) there is just so much yaw you don't feel like you can precisely point the car. I'm not talking about body roll. I have checked the (stock) swaybars and the bushings are firm.
Question is: How much will putting 8" wide wheels on the rear tires help with this? There is presently a fair bit of tire bulge and I can't help thinking that flattening the sidewall profile will help with the flex issue. This will also widen the rear track almost 1", will it not? Not sure if this is such a big deal as the suspension mounting points don't widen with the tire contact.
Anyone gone with 16x8" on stock tires (on an SC or Carrera) have an opinion?

[911s55]

A 225 on a 7" is a bit chubby especially on the bottom.

Going to an 8" will give you a firmer and more vertical sidewall that stays under the wheel.

I have 205's on 15x7' and the sidewall is very square and compliant.

The 225 on 7's would work at higher speed, track vs. auto cross as the higher wheel speed stands the tire up. If for street or slower events you would do better with an 8".

[dentist90]

Part of my plan is to move the 7"s up to the front for the 205's. I've played with rear air pressures, and all it seems to do is prematurely wear the center rib when I take them up to 38psi. I am hoping wider wheels will provide a firmer ride. I'm sure a better tire would make a big difference, but I want to control the variables here... just change the rim width first.
Worth it?

[450knotOffice]

How new are the rear tires? I ask because when I got new rear tires on my '84 a few years ago, the back end of the car was all over the place - very unstable during left-right (and vice versa) transitions. It was very unsettling to me. After some time the stability improved, but not completely. Later, after talking to Tyson Schmidt, I found out that he NEVER replaces only the two rears or two fronts. He always replaces the whole set. Why? Because the rubber on the older tires on the car will have different grip and flex characteristics than the new rubber, which will destabilize the car, in his opinion. Eventually, I replaced all four tires and have had ZERO stability issues. By the way, the ES 100's are known for their stiff sidewall, so you shouldn't be getting much sidewall flex from those tires.

I have 7's in front and 8's in back, btw. However, while I was having those wheels refurbished by Al Reed a few years ago, he loaned me a set of 6's and 7's with 205's and 225's. The car felt fine with those tires (crucially, they were a matched set of Dunlop SP Sport 8000's front and back which I assume were the same age front and back).

If your fronts are a lot older than your rears, and your rears are really new, that might well be your issue, as it was for me.

Also, you might want to have your rear alignment settings checked - specifically toe. If your car has zero toe to a little toe out, the back end will feel unstable. You want just a little toe in in back, which adds stability to the rear end.

[jorian]

I run 7's and 8's with 205's and 225's respectively. I use the Yoko Advan AO48 R's. They have very little flex, handle extremely well but are noisy and a bit slippery in the wet.

[Bill Verburg]

OEM manufacturers spec their wheels on the narrow side because
1) its cheaper
2) it gives a more comfortable ride, which the vast majority of buyers prefer.

every tire has a range of widths spec'd by the tire engineers, using the widest wheel w/i the spec range will give the best performance but worst ride.

For an ES 100 225/50x16 the spec range is 6 - 8"

[dentist90]

I would gratefully trade ride comfort for handling response.
I anticipate that there will be a little more tread squirm going from near-bald tires to full tread new ones. As I go thru 2 to 3 sets of rear tires for every front pair, replacing all 4 each time would be cost prohibitive. Plus I'd be throwing away front tires with 60% of their tread remaining.
I think Bill gave me the answer I was hoping for... trade comfort for performance. Plus it will fill the fender better. As Es100s are no longer in production I will be looking for a different tire next time. My fronts will be due by then too.
Things felt really good when I had bald Yoko520's on the back, then when I changed to 1st set of ES100's the rear felt like it swayed way too much. As the ES100's wore down things improved, but when I just put new replacement ES100's it came right back. So if a wider wheel may help stabilize the tire I think I'll just have to cough up.
Thanks for your input guys!

[burgermeister]

+1 on 450knotoffice's comments ...

[74threeliter]

After reading this comment above:
{
A 225 on a 7" is a bit chubby especially on the bottom.

Going to an 8" will give you a firmer and more vertical sidewall that stays under the wheel.
}

it leads me to believe why my 7/9's w/ 225/245 with brand new rears makes my rear want to come around and spin me on a higher speed turn.

[Bill Verburg]

Originally Posted by 74threeliter

After reading this comment above:
{
A 225 on a 7" is a bit chubby especially on the bottom.

Going to an 8" will give you a firmer and more vertical sidewall that stays under the wheel.
}

it leads me to believe why my 7/9's w/ 225/245 with brand new rears makes my rear want to come around and spin me on a higher speed turn.

The wider wheel stretches and supports the sidewall better than the narrower, This makes the contribution to total spring rate at a corner by the tire greater. It does not change the shape or size of the contact patch. From my notes from back when I was using RE71s in 225/50x16 when mounted on an 8x16 wheel the tire is strethed into a measured 248/36 w/ a carcass width of 9 3/4" and overall height of 24 3/4"

Most 245/45x16 have a spec wheel of 7.5 - 9" , again the 245/45 on a 9 will be better supported and increase traction in the back. The same sort of stretching occurs here too, a 245/45 when mounted on a 9 becomes a 260/34 w/ 10 1/4" carcass width and 24" overall height. Every tire brand is going to be slightly different from the others so you experience may vary a bit.

From my previous post a 225/50 wants a 6-8" A 7 is right in the middle of the range(neutral) if you use an 8 on the front then you will increase traction in the front.

Of course other things can be adjusted as well, tire pressures, spring rates, sway bar rates, weights, alignment etc.

The difference between and old and new tire can also significantly affect the balance of f/r traction. That's why it's usually recommended to fit the same brand and age tire f/r(i.e. new tires f/r) and to fit new tires after ~5 yrs no matter how much tread is left. W/ age tires get harder and slicker, the less they are used the faster the aging process.

[Wil Ferch]

I've felt this tendency too...if your car has 40-50K miles and you "think" your OEM shocks are OK..it may be partially this. May need new shocks anyway....this helped me when this surfaced around this time.

New tires will also have this trait...for me it doesn't go away untill the rears have 500-700 miles on them.

Also....38 psi is probably way too high for the rear. Factory specs of 29/36 are also for comfort and trending the car to always-understeer. Try 31/34 cold.

[74threeliter]

Point taken... I put 205's on 7's and with the 245's on 9's and the warm and fuzzy feeling has come back to the faith in my cars suspension. I am keeping the 225's on 7's as an extra set of rears NOT fronts. much thanks for the replies not to hijack the original question, which to choose, 8" or 7" with same tire?

[jorian]

I always get 205's for the 7's and I have run 245's on 8's but far prefer 225's. I also run 30/32 PSI cold. I have new Bilstein Sports all round and big torsions.

[G50targa]

Side wall flex was a concern when I picked out new tires for my Carrera recently. I went with Yokohama S.Drives as I heard that those tires have stiff side walls. Also, there is a notch where the tire attaches to the rim which makes the side wall stand up a bit (if that makes any sense..), which might aid in side wall stiffness.

[Bill Verburg]

I'd like to know where you guys get your info on sidewall stiffness, i've certaintly never seen that info published anywhere.

"I heard" just doesn't cut it.

The only sure way to get a stiffer sidewall is to get a shorter sidwall from a lower tire profile. The heirarchy of sidewall stiffness will go like this 225/40x15 > 225/45x16 > 225/50x16 > 225/55x16 > 225/60x16

The S-drive is a very nice tire, mostly because of it's silica based compound. The notch is called a rim protector, it's purpose is inherent in it's description

[RoninLB]

"The wider wheel stretches and supports the sidewall better than the narrower, This makes the contribution to total spring rate at a corner by the tire greater. "




thx Bill



i've been figuring the S-03 Bridgestone replacement had a stiffer sidewall. It feels like I increased tire psi 5# on all 4.


I went from 4 6x16 Fuchs w/205x55x16 S-03s to 4 7x16 951 Fuchs w/205x55x16 050A PolePosition.

So now the 050A PP replacements probably has same sidewall but the stiffer feel is because of a wider wheel.

[DanielDudley]

I'm just some guy, but sometimes overinflating a tire will get you the same result as underinflating. Start out with a tire pressure a few pounds less than manufacturers reccomended, and work your way up.

A 225 on a seven is not out of spec., but I have been told more tha a few times that you need to run less air when you run a bigger tire on a smaller rim, or when you go up a size. It has something to do with the PSI and the larger contact patch.

You want the maximum contact patch, and when you get that, you will have a flat patch that is less likely to roll under than a rounded patch. Might be worth a shot.