Boltz.

iTrader: (42)

I understand the basics here, but when looking at two specific tires that I own and am deciding between to run I am at a loss for how this will translate into real world performance.

Both tires will go on wheels that have the same width and offset (10" et18-20). Both tires are brand new Hankook R-S3.

Tire #1 - 255/40/17 - Section width - N/A - Tread width - 10"
Tire #2 - 285/35/18 - Section width - 11.4" - Tread width - 10.1"

I understand that section width represents the measured width that tires are compared by and that it is measured from mid sidewall to mid sidewall, not by actual tread width. I understand the differences in tire diameter and their effect on acceleration and fitment. I understand that the 18" combo will add ~7lbs per corner.

Now, generally you would assume that more section width equals more grip but in the case of these two tires I am not sure if the added weight (50lb combo 18" vs 43lb combo 17") is going to equate into any extra grip. If you were to go by the tread width measurement alone, you wouldn't assume that there would be a huge difference.

I've observed that without any tires in them, the 285 do sit about 5-6" higher when all four are stacked on top of each other in a pile. With 10" wheels mounted on the 255/40/17 and the tires stretched, the difference when stacked is more like 1-2". This last bit is probably useless data, since the section width would create this extra height when stacked, but I wanted to post all the information I could

I would like to note that the 255/40/17 was designed by Hankook to be a 'cheater' tire and that the stated tread width of the 255/35/18 R-S3 is only 8.7" by comparison.


At this point, I am considering selling the 285/35/18 tires, getting some cheap 265/35/18 tires for my 18" wheels and running them on the street then swapping the 17" 255/40/17 R-S3 for the track, but, the jury is still out on this. You be the jury.

Neal@tirerack

iTrader: (25)

Some good links on this here,

Tire Specs Explained: Section Width

Tire Specs Explained: Tread Width

Boltz.

iTrader: (42)

Thanks for the links Neal. I will read those.

If you have a moment, can you comment at all on the specific question at hand? I'd love to hear what you have to say.

Thx

boomn29

iTrader: (47)

The truth is, every manufacturer measures them a bit differently. The fact you have both in your possession will really help you SEE the differences - or lack thereof.

But I do have a few questions first:

• What size rim are you going to mount these on?
• What are you using these tires for? Is overall height (effects gearing and mph) important to you in this instance?

And here's some pics I snapped of different size width comparisons (mounted):
http://boomn29.blogspot.com/2010/04/...mparisons.html

Smike

iTrader: (8)

7lbs more rotational mass. :eww:

Stacking the tires is not a good way to judge width. Effect is cumulative.

Neal has the links. SW is SW. Thats in the tire mold. And what is reported for DOT.

Tread width is not mandated or measured specifically by many manufactures.

Rim width used is going to make a big difference here. Too narrow of a rim with a wide tire will make the tire bulge over. Too wide for the tire and you run the risk of de-beading the bead from the rim under cornering forces.

You have the tires in hand. So get all the weights, measure the tread area on all, and take into account all the other factors (compound, responsiveness, diameter change).

Iowa999

iTrader: (1)

I don't see a specific question, but you seem to be asking about performance differences between tires of different sizes on a given rim, such as 285/35/18 vs 265/35/18 on an 18x10. If so, the answer depends on the tire. Some tires don't seem to "mind" being wedged on a rim that is, by general rule, not wide enough; other tires drop off in performance very steeply when wedged on a small wheel. For example, I've seen some ridiculously-wide tires on stock wheels in Stock-class autocrossing (which limits the wheel, but not the tire) and the drivers swear that it's better than a standard-sized tire. These people are often running Hoosiers. I've also seen people swear that you must obey things like the 85% rule (i.e., the wheel is 85% or more of the width of the tire) and say with equal confidence that putting a wider tire on the wheel will lower the grip. These people are often running Kumhos. No idea about Hankooks. It's an empirical question, however, so you need someone who runs Hankooks or you need to test it yourself.

Boltz.

iTrader: (42)

Not comparing a 285 vs a 265 on the same wheel. Talking about 285/35/18 on a 18x10 compared to a 255/40/17 on a 17x10. The 285's (without wheels in either set) stack much higher than the 255 as you would expect. When I try to measure the tread width, it appears that the 285's are possibly close to an inch wider per tire as expected. What is my REAL contact patch going to be though, I have no idea, so my measuring in that regard may be useless. NOW when I look at the manufacturer's rating for a second opinion, since I cannot determine my actual TW contact patch, they rate the tires having the same TW.



The rotational mass is a big factor here. On the other hand, the strength of the 18" wheel is much greater than the 17", which is worth some weight. At the same time I am adding 2 piece rotors and coilovers on all four corners which will help with the weight issue. Obviously, its still a 7lb difference, but you understand what I'm getting at I believe.

I understand that SW is SW, but SW doesn't equal TW, which is where my curiosity stems from. If one tire is heavier than another and has a greater SW but they both have equal TW, would one not always go with the lighter (skinnier SW) tire?

The 255's are slightly stretched on the 17x10. The rim width range on the 17" is 8.5-10". The same range on the 18" is 9.5-11". I know that the slightly stretched 17" tire might suffer from less sidewall flex on a 10" wheel than a 9" wheel because of the stretch and you could guess that it might have less flex than the 285 on the 18x10 because the 285 will likely sit square on the 18x10, BUT then again the profile (sidewall) on the 18" is smaller anyhow and should flex less than the 17" if they had the same stretch or lack thereof. That was a mouthful, hope it made sense.

I think that the rim width, being within the range won't suffer drastically from de-beading or rolling over on itself.

You mention to take into account all of the other factors....

Compound - Same tires, same compound
Responsiveness - Already laid out my concerns and confusion there. 17" have taller sidewall but are also stretched to stiffen them. 18" won't be stretched, but have a shorter sidewall.
Gearing - Don't care at this point, the car has enough power for me

The 17's are 17x10 +18
The 18's are 18x10 +20

Tires will be used for street duty, drag racing, HPDE's, possibly autocross and hopefully a competitive event or two for fun. Acceleration difference between the tires doesn't matter (in terms of circumference), not concerned about running out of gear in first and both will give me enough gear in fourth for drag racing.

_______________

I understand this is highly theoretical and there are a lot of factors contributing to the whole equation and that it is hard to speculate real world performance. I guess the bottom line is what is going to feel better (assuming that in this case that would lead to faster lap times) around a road course, like Autobahn or Road America.

With all of the information I have provided I am thinking that I have possibly let the real question astray by adding a second variable to the equation. The second variable being stretch vs no stretch. Then I made it convoluted by adding a third variable, increased rotational mass. Bah.

How much should I bank on the MFG's TW rating? If the TW is rated the same, does that mean the effective contact patch would be the same, ceteris peribus?

Hope this post adds some information so that you all can understand what the heck I'm talking about. <-EDIT; Don't mean that in a cocky way, just trying to make sure that what I am typing is on the same level as what I am thinking.

Boltz.

iTrader: (42)

I guess I should just literally post some questions instead of posting thoughts and hoping for discussion.

-How much weight should I put into the manufacturers tread width rating? Also, if applicable, how does it apply to effective contact patch (understanding that has a lot to do with vehicle weight and tire pressure)

-Making the assumption in this case that WIDER is better, just because a tire is taller when stacked on its side, does it mean it has a greater contact patch and more traction? Assume you are comparing the same tire type.

-If the tire seems to have a greater TW than the other and are the same tire, made by the same manufacturer, why would they rate them with a nearly identical TW?


I guess I just was thinking 285>255 and everything was fine and dandy, then when I read the MFG's TW rating I was

Thx

boomn29

iTrader: (47)

Seems like you have all the information and theories figured out.

Like I mentioned earlier, everybody measures/rates things differently. Even from the same company. I have in my possession 2 tires from the same manufacturer; same compound; different sizes. And the one with the smaller number is actually wider (albeit shorter). Figure that one out. My guess is some SCCA team needed a 'cheater' tire like you mentioned.

If this isn't for competition, and if your car isn't maxed out in other categories, you're really just splitting hairs. Go with what's cheaper and be done with it. Or do what I've done and run a lot of different setups.
Truth be told, an extra 1/2" of contact patch is really hard to equate down to laptime savings. Changing tread compounds (street vs r-comp vs dot-slick vs slick) would be the bigger laptime drop; that much I can confirm.

On a track like Autobahn; a solid alignment and some practice will yield faster times w/o a doubt.

Iowa999

iTrader: (1)

It's a starting point, but it is not a rating, per se, but is a claim that depends on a variety of factors, so it isn't close to definitely and universally true. Thus, it gives you an idea of the size of the contact patch, but that;s all.

Not really. If you're talking about a stack of unmounted tires, then it can be very misleading. If we're talking about a stack of mounted tires, then you're closer, but the stack's height is really a measure of sidewall width, not tread width, as you know. I would never use stack height for anything except a measure of how much room they will take to store. Measure the treadwidth of mounted tires for the information you want.

Because there are no rules on treadwidth claims. And they might be lazy. I really don't take those numbers very seriously. Some manufacturers have been suspected (got that lawyers? I said suspected, not known) to flat out lie. Falken, for example, run wider than their stated widths. Whether this has anything to do with sanctioning rules that place limited on the width of tires as stated on the tires and Falken's attempt to get more people to run their tires is pure speculation (got that lawyers? I said speculation, not fact).

I need to go eat dinner. Then I'll get to the apples vs oranges ... er, I mean the 18s vs 17s question.

03whitegsr

iTrader: (8)

I think you are just looking at one specific size that like you mentioned is a "cheater" tire. The class they targeted had a maximum width of 255s, so they simply took a larger width and slapped a smaller size on the side of it.

The Hoosier 255/35/18 is almost identical in width to the 275/35/18, only difference is diameter.

In most cases, the larger the tire size, the larger the tire width. That 255 is just a fake size IMO. It's what I'd run though as it's nice for the gearing and still runs nice and wide.

Iowa999

iTrader: (1)

17s vs 18s.

Yes, all else being equal, the 18s will have a higher rotational moment, meaning that they will be harder to accelerate (and brake, which is, of course, also acceleration). They will also absorb less of the small bumps that the sidewalls help deal with. But they will let your brakes breathe better and they will be more responsive laterally. My conclusion: better for road racing and maybe better for autocrossing (especially if you left-foot brake).

The 17s would be better on the street and drag racing, albeit for different reasons. 17s are better on the street for bump-absorption and rim protection; they are better for drag racing because of the lower moment and the sidewall will give a bit more on launch. (I actually ran 16" snow tires when drag racing a 325 foot-pound DSM, but that was at a track where they had so much crap laid down at the line that it would literally rip the shoes off your feet if you got out of the car.)

Upshot: I would run the 18s for road racing and set the suspension up with the 18s in mind. (And if you don't want my advice, look at what Chris Wirth, Sean Caron, Mark Daddio, and John Mueller are doing ... all are on 18s.) I would use the 17s for street and drag racing.

Boltz.

iTrader: (42)

I guess the take away here is if you know the tire was made as a cheater tire, don't trust the mfg's rating on its size.

It seems the 18's will be a much better route, I also like the fact that they will allow the brakes to cool better. Although it makes perfect sense, I've never thought of it in that way before. I always just considered the rule of thumb to be 'run the wheels just as big as they need to be to clear the brakes'. The 285's will also be better as I've got a decent bit of room (about 3 fingers) between my fenders and the 255's (on 17x10 +18's). Now the hard part is going to be dealing with their height.

03whitegsr

iTrader: (8)

Not sure if tire size limits is an issue in what you are doing but if a wider tire comes with a weight penalty, you may want to stay on the narrow tire.


This has been my mentality with SCCA Street Mod. With a 2.0L motor and 275 wide tires, you can run the car down to 2620 pounds. This is ~76.3 pounds/inch of tire width in the front. To match that grip ratio at the 2820 pound minimum means a 295-305 wide tires which means aftermarket fenders and lots of clearance issues. Even then, I have a feeling that 200 pounds less weight is going to be more beneficial in the end then a 305 tire.

Iowa999

iTrader: (1)

You know, sentences like that make up for entire threads about 18x12s or 335 rears and 275 fronts. Thanks for writing it (and the rest of the post).