Yet another tire thread, big is bad?

[RoadWarrior222]

Hi folks,

Had my nose in a book about handling recently. One thing that jumped out at me was that on a particular size rim, the widest tire you can put on it, might not grip as well as an "optimum" size tire for that rim. They had a little curve graph done for a formula racer for a couple of rim sizes, but it wasn't labelled and there's not really any further details given. It makes sense though that too wide a tire won't stay flat enough when aired up, and will crown, or that too narrow a tire won't work well either.

However, I'm completely lacking in information as to the degree of this difference, and I've had a hard time filleting anything useful out of the internet on it. Also I'd like to know whether the recommended rim widths for particular tires are the range in which it will stay on, or represent the peak grip part of the curve. In the former case, it would seem that just because they mount up, doesn't mean they're good, they could be enough down the grip curve for that rim to mean that you're getting a poor match.

While trying to find some info, I also found some lists of tire dimensions that show that the tread width for a given section width is narrower the higher the aspect ratio. Also the tread width doesn't seem to go up as much as the section width for each step up. So apparently you might 5 inches of tread width on a 70 series, 5.3 on a 65 series and 5.8 on a 60 series all with the same section width. So it would be apparent that going from a 14" size to the same section width in a 15" size is an upgrade anyway, because it seems to get more tread width than going up a section size on the same wheel.

So does anyone have any hard info that lets you calculate best section size and aspect ratio for a given rim size? And that lets you see how much worse or better other sizes are?


I was dead set on getting 225/60R15 for my pumpers to put on the mini, but changed my mind to 215/65R15... but now I'm wondering if I really should have 205/65R15 on them, because 215s might actually give less grip on a 6" rim. Basically I'm looking how to figure out how I'll get the best lateral grip and traction. The way the actual tread widths don't seem to go up as one expects also seems to say that I'd get less than about 3% more tread on the ground (provided it's flat and makes good contact) per increase in section, vs the 5% you'd expect. So if the loss of grip due to tread crowning is more than 3% from fitting it on a slightly narrower than ideal rim, then I wouldn't be getting anywhere if I put them on. On the other hand, just going from 205/70R14 to 205/65R15 appears to offer a 10% gain in tread area, due to tread widths being larger on lower aspect ratios, also if 205/70 is less than ideal on the 5.5 steelie I get the extra gain of getting nearer the peak on the grip vs width curve for the 6" rim than I was on the 5.5" wide rim.


Now howabout empirical experience, has anyone been unhappy with the results going from a 205 to a 215 or 225 on a 6" wide rim?

thanks,

RW222

[2.216VTurbo]

I put some 225's on my Stock 6" S Centurians years ago, great ride because of the large sidewall 'bubble' or bulge, but they squirmed around on those rims like they were live squid:(. It was kind of an unsettling feeling:yuck: Now on the Rampage RT I have 195/55/15's on 6.5" SL rims. Talk about direct turn in response! Not harsh because of the slightly taller sidewall from the 55's but just good roadfeel. The sidewall is like super strait with almost no bulge;)

[turbovanman²]

I won't be much help as I'll throw another wrench in the mix. Tire brands also vary enough to cause a difference. This is an example so don't take it to gospel.

Say you buy Goodyears X tire, 215/65R15 and Toyo's Y tire, same size. BOTH will have different widths, sidewall heights etc and even if they are similiar both will handle differently. Some tires can be as much as 1-2 inchs narrower from brand to brand or vice versa. You say, how can this be, because I guess loopholes, so like Alan's experience, his particular tire handled like crap.

Another real example, I had some Goodyear Eagle LS's on the van, then put some Nankang EX500's on, same size-P225/55R16 but visually, huge difference. The NK's were alot wider and aprox 1 inch taller.

For the van, I would put 215's on, I had that size on with Pizza wheels, handled well, 205's are I feel, too skinny. Even my current size works well, stable and no rollover on a 6 inch wheel.

[shadow88]

^^^
Very good points there! Yet another wrench if you will.... The tire brands will offer different tread designs and compounds. IN MY OPINION - Those will have an even further affect than how much tire hangs over the edges of the rims or not.

[contraption22]

I felt this difference when I went from OE 6" wide wheels to 7" wheels with the same 205/50/17's on my SRT-4. Turrning response greatly improved.

[GLHNSLHT2]

For a Minivan I don't think there's enough difference in grip from section width to really matter is there? My 205/60's on my new yorker are cheasy all season siped generic tires that grip good enough I don't want to corner any harder for fear of the door flying open and me flying off the couch someone bolted in as a seat. But really you could make up the difference in section width just based on the brand and type of tire you do buy. It's all personal preference I like the tire to stick out just past the rim so if I hit a pebble I don't wack the rim.

[Vigo]

The issue the OP brought up is serious stuff.

Ive posted a few times how i went from high performance all season 205/55 to summer ('ultra high performance') 225/50 on a 6 inch rim and it ruined the driving experience. It made the car very inconsistent on turn in.

As for absolute grip, honestly.. its not the biggest issue. There is a minor grip difference when you put too wide of a tire on a rim, but its not about crowning, or about how it fits the wheel when its sitting still. Its really all about how much it deforms when you have a big side load on it.

But the bigger issues than ultimate grip are transient stability and steering response.. I dunno if it makes sense to differentiate the two or not. Steering response is basically the lag time between you making a steering input and the car actually changing direction. Having a vague feeling due to a longer steering response basically means you are 'driving blind' on corner entry because you dont have a DIRECT cause and effect feedback through the wheel that teaches you what works and what doesnt. Basically without good steering response its hard to learn the best way to take a turn because you basically make a shot in the dark on turn in and then just keep playing catch up correcting it mid turn.

The transient stability thing is sort of like steering response but it helps to think of it as going from one direction to another, as opposed to going from straight to turning. I guess you could think of it as like sweeping a broom back and forth on the floor. The taller the sidewall, and the more oversize the tire width is for the rim, the more the tread/tire will shift from side to side relative to the rim. Sort of like a long bristled broom compared to a short one. Or the way a worn out wiper blade will flop over whenever it changes direction on the windshield. That time when it is deforming going from one direction to the other destabilizes the car. sort of the rubberband effect. Theres a period of tension or stretching until it snaps and changes direction, and when it snaps you usually lose traction.
I dunno how many of you have driven hard on a bunch of different rims, but theres a phenomenon where if you have stockish tires on a car, you can get it to oversteer by making a quick direction change instead of a smooth one, and then you put short sidewall, properly fitting tires on and the same thing causes it to understeer. Thats basically due to the rubberband thing with the sidewall i just mentioned, and it will happen with an overwide tire as easily as one with a really tall sidewall because they both have the tread moving side to side a lot relative to the rim.

Also, section widths are measured from the widest point of the tire, which with a tall sidewall is usually in the middle of the sidewall, and with a short sidewall its generally the same width at the tread and at the sidewall. So the fact that the section width number for shorter sidewalls is more representative of the tread width is purely coincidental, because either way they are measured from the 'widest point' as measured on a recommended rim width.

[Polygon]

^Beat me to it. That pretty much sums it up. :thumb: