Confused by wheel width benefits

[raredesign]

I always thought a wider patch was better, given the compound and other characteristics were comparable or better.

Then I came across an article from tunneruniversity.com (not sure if this is a personal blog not worth its salt), but here is a quote:

The thinner wheels will therefore be better (and of course there are extremes in both cases) for straight line acceleration and braking. The wider ones, because of the direction of the forces on the tire tread in the corners, will be better for cornering at the expense of some straight line acceleration.

Say what?

Anyway, I've been looking into the difference in wheel/tire diameter and width differences to see if I want to move from my 255/18's to a 315/18...or a smaller rear such as 315/15. I think I have scrapped the latter since I don't plan to drag much if at all, and I want the better street handling and reliability.

Oh the controversy!

 

[stkjock]

yep, makes perfect sense.

you want to run as narrow a tire as you can, more wheel and tire = more weight = slower acceleration.

Contact patch is more than just tire/wheel width.

Look at drag cars - skinnies up front and tall - wide tires out back


for road course it's different as the lateral forces are much more important

 

[dark steed]

I think he was talking about the straight line braking being better on a narrower tire part. At least that is what got my attention, as the fronts do the bulk of the braking...

 

[csamsh]

Disclaimer- that article might be about drag racing, in which case my comments probably don't apply.

Anyway.

You know what that article has NONE of? Data.

This is some engineer who has never been with 14 miles of a track, and probably can't threshhold brake to save his life.

 

[Stephen31201]

^^^ what he said

 

[raredesign]

I wouldn't say this is a technically more scientific discussion as there is still no data like csamsh said, but at least it seems to be in the context of scientific approach, and by those with such a mindset.
https://www.physicsforums.com/threads/wider-tire-giving-better-traction-debate.330790/

Thinking about this more, as well as looking at some photos, it seems even that when comparing a 20" with thin sidewall to a smaller rim with a thicker sidewall, given a tire with the same circumference, the smaller rim and thicker sidewall appear to give a larger contact patch longitudinally (or with the length of the car), as the flex causes the tire to fold on itself.

So that leads me to my next question: What is the traction experience in those who have gone to 20" from 17 or 18". I realize this is not necessarily a controlled test either, since you may have changed tire type that gives more traction due to the compound.

I really like the look of a 20" that fills the wheel-well gap, but staying at an 18 and increasing sidewall may be my best straight line traction solution as I don't plan on drifting lol. Well, hopefully not.

Edit: Since we are on the topic; has anyone noticed a drastic change in gearing with a 20" that makes you regret the decision? If I am going to be pushing 700rwhp by spring, I am not sure if that will be noticeable in daily street driving, or if light to light traffic would make it less appealing than a smaller tire. Again, if my sidewall causes the circumference to be the same, I guess it balances out anyway.

 

[stkjock]

for straight line - smaller diameter is better, less weight (in general) and that weight is closer to the center.

additionally, the bigger sidewall allows for more flex and increased traction.

Why are "drag packs" generally sold with 15" rear set ups............................. better traction and accleration

 

[raredesign]

for straight line - smaller diameter is better, less weight (in general) and that weight is closer to the center.

additionally, the bigger sidewall allows for more flex and increased traction.

Why are "drag packs" generally sold with 15" rear set ups............................. better traction and accleration

Ah good points. Glad to see I was thinking in the right direction. Perhaps I see some higher sidewall 18" NT555's in my future for daily street. I have Michelin Pilot Super Sport's right now, and at close to stock horsepower I don't slip at all. I think that will be a different story after this build though.

 

[Norm Peterson]

While those links are probably well-intended, I don't think either of them gets around to answering what I think is your question.

The short answer is that with 700 whp and at least in 1st gear it's probably not going to matter much.

Somewhat longer, all else equal, and let's keep this in terms of dry pavement and appropriate temperatures . . . a long, narrow contact patch favors the longitudinal traction demands. That'd be acceleration (rear tires only here) and braking (mainly front tires). Conversely, a short wide CP favors the lateral (cornering).

Sidewall height affects the ability of a tire to absorb the shock of a hard launch - particularly with a manual transmission. Taller is better here, and I also suspect that wheels relatively narrow for the tire size may be better as well. The point being that the more flexibility you can give to the sidewalls the better they will be at absorbing shock. Just don't get carried away, as the car will get "woozy-feeling" at the big end from all that flexibility. Even though you may be going perfectly straight, there can be forces that try to move the car around laterally at either or both of its ends, and however much "cornering stiffness" your wheel and tire package has is what makes this manageable (or not). Tire pressure is part of this, and I'm sure you've heard what happens with a really quick car when its driver experiments with a way-too-low rear tire pressure.

"Woozy-feeling" is pretty much what you're trying to eliminate in a corner-carving car - laterally it's a lack of precision - so for that you'd want to push the above compromises the other way with relatively wider wheels and shorter sidewalls. About here is where one of the things mentioned in one of those links is definitely useful - the further you push your compromises in this direction the more important it becomes to make your car compatible. For example, a wide tire isn't giving you all it can if your car's camber in a corner can't keep its CP somewhere near flat on the ground, and the same goes for an IRS car on acceleration when the rear squats.


Incidentally, don't put a whole lot of stock in any article that talks about contact patch loading in terms of corner weight divided by inflation pressure. Contact patch psi is highly nonuniform and can easily vary by 50% of the lowest actual number (this is one of the factors that you're tweaking with inflation pressure adjustments at the strip/autocross/road course, to maximize overall grip). In years past, even greater variance was entirely likely.


Norm

 

[raredesign]

Thanks Norm

 

[Norm Peterson]

You'll notice that I added a little since your above post.


Norm

 

[KonaBlueBryar]

ill add that moving from 19X8.5s with a 255/40/19 to some 18 inch gt500 wheels with 285/40/18 was a big mistake. maybe some people like the ride, but I'm used to having a smaller sidewall. I can feel the sidewall of the 18s flex when I take corners hard, and I'm not a fan of that. it makes me feel like I have body roll again and that was a good reason I lowered my car in the first place lol. I cant wait till I install my 20x10.5s with a 295/30/20 and regain that "stiff" sidewall feeling. I guess you could say I'm a hater of 18 wheels with big tires lol

 

[Norm Peterson]

The 18" GT500 wheels are 9.5" wide, which is the minimum recommended width for those 285/40's. At minimum width, you absolutely will have a 'softer' setup. For both steering response and ride (and by implication from that softer ride, slightly more roll).


Norm

 

[Pentalab]

The 18" GT500 wheels are 9.5" wide, which is the minimum recommended width for those 285/40's. At minimum width, you absolutely will have a 'softer' setup. For both steering response and ride (and by implication from that softer ride, slightly more roll).


Norm

Norm is correct of course. A 285-40-18 is fine..as long as a 10" rim is used. I tried a similar stunt, replacing my front 255-45-18 tires ( that were on 9" rims, = 1/2" over..and worked good)...with 275-40-18 tires. A 9" is the min width rim for a 275-40-18..and does not work too well. ( plus it ends up being like only 270mm wide anyway). New 10" wide front rims just arrived. The 275-40-18"s will come off the 9" rims..and onto the 10" rims. Being a 1/2" over, they will end up being close to 280mm.

The GT-500 rims are identical on all 4 corners, 9.5" wide, with a 45mm offset. IMO, they are too narrow for a 285-40-18.... but a bit wide for the front 255-45-18 ( they are 1" over). IMO, a 1/2" over is good, but 1" over is a bit much.

Being a 1/2" under on the rears is one thing, but being 1/2" under on the fronts is bad news. The turn in etc, is all screwed up, it ends up a sloppy mess for handling.

Ideally, a 285-40-18 would be on a 10.5" rim. Good luck with your 295's on 10.5" rims.

 

[KatoS197]

Norm is correct of course. A 285-40-18 is fine..as long as a 10" rim is used. I tried a similar stunt, replacing my front 255-45-18 tires ( that were on 9" rims, = 1/2" over..and worked good)...with 275-40-18 tires. A 9" is the min width rim for a 275-40-18..and does not work too well. ( plus it ends up being like only 270mm wide anyway). New 10" wide front rims just arrived. The 275-40-18"s will come off the 9" rims..and onto the 10" rims. Being a 1/2" over, they will end up being close to 280mm.

The GT-500 rims are identical on all 4 corners, 9.5" wide, with a 45mm offset. IMO, they are too narrow for a 285-40-18.... but a bit wide for the front 255-45-18 ( they are 1" over). IMO, a 1/2" over is good, but 1" over is a bit much.

Being a 1/2" under on the rears is one thing, but being 1/2" under on the fronts is bad news. The turn in etc, is all screwed up, it ends up a sloppy mess for handling.

Ideally, a 285-40-18 would be on a 10.5" rim. Good luck with your 295's on 10.5" rims.

I agree with this. I wanted to "duplicate" the GT500 wheel/tire setup for my '08 GT. Bought the wheels from AM and ordered 285/40/18 and 255/45/18 Eagle F1 Supercar tires.

The rear sidewalls seemed to bulge a little too much for my taste and the fronts looked a little stretched. For street driving it was impressive for me. I put that wheel/tire setup and BMR UCA/LCAs at the same time and the car wouldn't break loose from a stop in the rain under normal driving behavior, where previously it would slip with the same throttle input from a stop on factory wheels/tires.

 

[Norm Peterson]

I'm guessing that Ford was trying pretty hard to avoid having to stagger the wheel widths as well as the tire sizes, so they picked a width that was min-recommended for the rear and max-recommended up front. I'd heard somewhere that they were actually trying to avoid staggering even the tire sizes, but couldn't meet some corporate standard or other without doing at least that much.

Running at max-recommended works very well when cornering & handling performance is your ONLY consideration. The Camaro 1LE's rear tires (285/35-20) are fitted to 11" wide wheels. For the track day that I'm attending today, I've got 285/35's on 18x11's all around. It's still streetable, but it probably helps to be independent-minded enough to let appearance follow what works.


Norm

 

[BruceH]

I agree with this. I wanted to "duplicate" the GT500 wheel/tire setup for my '08 GT. Bought the wheels from AM and ordered 285/40/18 and 255/45/18 Eagle F1 Supercar tires.

The rear sidewalls seemed to bulge a little too much for my taste and the fronts looked a little stretched. For street driving it was impressive for me. I put that wheel/tire setup and BMR UCA/LCAs at the same time and the car wouldn't break loose from a stop in the rain under normal driving behavior, where previously it would slip with the same throttle input from a stop on factory wheels/tires.

Big disclaimer here, I'm not a corner carver but I do engage in pushing the limits on twisty roads when it's safe and there isn't anyone around. My opinion is probably not worth much on this but I'm going to share some of my experiences with 08 GT500 wheels.

First of all the tires that came with the GT500 in 08 don't work very well for anything. At least not in PNW temps. They are also pretty damn noisy.

Probably the best handling setup I had was with Hankook Evo 112 in 275/40/18 on all corners. It handled well, had good straight line grip, and they were quiet. I couldn't get over how they just didn't look right with all the wheel well space or how much a 275 front tramlines on our roads that are grooved from all the studded tire use.

If you want to use those wheels for a more straight line oriented setup then go with 295/45/18 or 305/45/18. Both sizes will work with the 9.5" rim because the 45 aspect ratio has enough sidewall. The absolute best street legal tire that still worked in the rain and colder temps was the Nitto NT05R in 305/45/18. It is a drag radial but it has a stiff enough sidewall to work well on the highway.

Putting 700rhwp to the ground is difficult. I could never get it figured out. I even went as far as 275/60/15 drag radials on the back and they would still smoke if I punched it at highway speeds. 275/60/15 are well known for getting high powered cars down the track quickly, imo they are better than the 305/45/18 I had good experiences with.

Everything becomes exponentially more difficult once you pass the mid 500rhwp level with a S197.

 

[raredesign]

I have heard a lot of good about M&H holding to the road with big numbers to the wheels. Check out the 275/45/18's they just came out with!
http://www.mandhtires.com/M-H-Drag-Radials/275-45R18#.VnRYUvmDFBc

These look to be worth a try.

Edit: Here are pictures of the same size mounted http://s940.photobucket.com/user/BlakSnake/media/Tech Stuff/100_2673.jpg.html

 

[stkjock]

I have 325/40/18s on mine and have been happy with how they grip

 

[raredesign]

I have 325/40/18s on mine and have been happy with how they grip

If you don't mind me asking, what are you running to the wheels? Do they hold up well on the road? How about if you get caught in rain? I am in Rochester, NY...and it is like "the other Seattle". If I said I was only going to drive in fair weather, I would get maybe the month of July.