Norm Peterson

Let me toss this piece of information out regarding speed ratings. Standing waves in the tread. I've got a few pictures in a US Department of Commerce / National Bureau of Standards book titled 'Mechanics of Pneumatic Tires' that show extreme examples of this phenomenon. There's both radial distortion (which you might expect) and lateral distortion across the tread (that I suspect comes as a surprise to most people).

You absolutely have to [edit]avoid[/edit] operation of a tire at speeds that could develop such behavior, as fatigue failure is imminent should the distortion ever get as bad as that shown in the pictures that I'm looking at as I type this.

Structurally what is required to keep this distortion under control is a stiff enough tread and enough damping to minimize this change of shape, which arises from the 'flattening' (for lack of a better term) of the tire cross-section over the length of the contact patch. The tread seeks to return to its normal radius after being lifted from the road surface, but cannot so so instantaneously, hence the modeshape. At a certain wheel rpm you get resonance, and this would correspond to a critical speed for that particular tire. The stiffer tread areas that delay the onset of this to higher speeds turns out to be a good thing for tread contact patch stability, so improved steering response and slightly more ultimate grip are neat by-products. If you've ever read the text that has usually accompanied magazine tire tests where mfr X has been unwilling to submit tires for comparison against tires from mfr Y that have a higher speed rating, that may be part of the reason.

Norm

y2kse

iTrader: (1)

Thanks, Norm.

Ultimately, if we want to cut to the chase, the issue becomes a matter of risk versus return. In this instance, the question is: "If I decide to upsize my tires, how much risk am I willing to assume and what do I expect to gain in return?"

Now the simple fact is that if you run a tire that is out of spec for your vehicle, the tire may fail. And if it does, you run the risk of injury or death to yourself and others . . . a penalty, BTW, that installing a Y-pipe or a RSB is unlikely to incur. In return for assuming that risk, you have a much wider selection of tires to choose from than are available in the OEM tire size. You may also be able to purchase better performing tires for less money than those available in the OEM tire size. And last but not least, you can impress your friends and associates by running the tire of the moment. ("Image is everything!" . . . Andre Agassi.)

For some, assuming the risk of tire failure is worth it. For others it's not. But I suspect that many aren't even aware of the risk because they don't take the time to educate themselves. They simply assume that it's OK to mount a certain set of tires on their rims because Jeff did it or Bob did it or Phuong did it. And once they finally realize the risk that they're taking, they justify their purchase decision by denying the facts to themselves and flaming those with the audacity to point them out. ("It can't happen to me!")

My prayers go out to those who know the facts and choose to disregard them. But those who won't take the time to educate themselves deserve what they get.

BuddyWh

I can see this, and that stiffness and damping are determined by the material properties and the mechanical properties of the structural composite. I am absolutely confident tire designers are sensitive to them, especially for their top-of-line performance lines, but both of these change over the course of a tires life and they also take that into account through the loss of a tires speed rating.

Also remember that a speed rating is arrived at empirically, not analyticaly, i.e., by demonstration in a laboratory environment that at once encompasses and yet ignores all such considerations. That is to say, a tire may certify at a V rating but have such massive distortion (or other characteristics... I use a steel tire as an example) that you could never hold the vehicle on the road... but as long as it holds together for the duration it's a pass.

BuddyWh

y2kse

iTrader: (1)

I know you said that you won't let anyone convince you that running Kumho ECSTA 712s in size 235/40R17 on 7.0" wide rims is unwise. But I thought I'd take a crack at it anyway. I wrote the following e-mail to Kumho today:

"I have a 2000 Nissan Maxima with OEM 17" rims. The rims are 7.0" wide. The OEM tire size is 225/50R17.

I'd like to mount ECSTA Supra 712 tires in size 235/40R17 on my OEM rims. Is that OK? If not, why not?"

This was Kumho's response:

"Sorry, that rim is too narrow.

Very briefly, too wide stresses the lower bead area and too narrow stresses the tread area. As the profile of the tire is reduced (like 50 series to 40) it becomes less tolerant to rim width variations, hence the range of acceptable rims narrows.

Thanks for considering Kumho tires, Frank S"

If you'd like to send me your e-mail address, I'd be happy to forward the original message and Kumho's response to you so you can verify them for yourself. You can reach me at bld522@yahoo.com.

doublea

iTrader: (7)

Oops, I dont know why I wrote that euhm...

P.S: Sometimes mistake create opportunities for others...

AA

y2kse

iTrader: (1)

Agreed, doublea. But I am concerned about you running those tires on your 7.0" wide rims. PLEASE consider replacing them now. Your life is worth more than a set of tires.

y2kse

nddst

LOL

doublea

iTrader: (7)

I just spend 800$ so I'm going to stick with it for now unles I have a problem. Just so you remember, when I originally place the order it was for 235-45-17 but I ended up having 235-40-17, and because the garage didn't follow my word upon installation it was too late to return it, so I had no other choice than keeping it unless I have a problem, I'll be carefull anyway and if there is something wrong over time, I'll let you know so we can share commun experience, but for now my car rock.


I consider this case over, meanwhile if you guys want to continue debate over something you have no control feel free to do so, I respect your idea and opinions.

Cheers

AA

y2kse

iTrader: (1)

Good luck to you then, doublea. And don't be concerned about the debate. I'm well aware that I'm powerless over what others decide to do. All I can do is carry the message . . .

BTW, there's another thread going right now concerning the Michelin A/S. I put up a post there that might have some relevance. I've copied my post here for reference:

--------------------------------------------------------------------------------

I'm going to go out on a limb . . . but not too far. Let's compare the OEM 215/55R16 Toyos to the 225/45R17 Potenza S-03s and see how they stack up.

According the tire size calculator at http://www.miata.net/garage/tirecalc.html, the speedometer error between the 215/55R16s and the 225/45R17s is only 1.4% . . . well within the 3.0% guideline. In addition, they both have the same load rating . . . 91. So in terms of upsizing from the 215/55R16s, the 225/45R17s look pretty darn good.

So what about replacing the 225/50R17 RE-92s with 225/45R17 S-03s. Well there's where things get a little sticky. The minimum rim width for the 225/45R17 is 7.0", so you're OK there. But the speedometer error between those two tires is 3.4% . . . slightly over the accepted maximum of 3.0%. In addition, the 225/50R17 RE-92 has a load rating of 93 while the 225/45R17 S-03 has a load rating of 91. And the general rule is that you shouldn't reduce the load rating when selecting new tires.

Now I suspect that among you, the brighter bulbs in the chandelier are going to look at this scenario and ask, what the hell is up with that? Why should it be all right to upsize to 225/45R17s from 215/55R16s and not be all right to downsize to 225/45R17s from 225/50R17s on the SAME DAMN CAR? Thus we enter into something of a gray area when it comes to making a decision regarding tires.

My own PERSONAL view is that it's probably OK to run 225/45R17s whether you're upsizing from 215/55R16s or downsizing from 225/50R17s. Just keep an eye on the load rating for the 225/45R17s you're planning to purchase and make sure that it doesn't drop below 91.

For those of you who currently run 225/50R17 RE-92s and share my view, I've just opened up a world of tire possibilities. For example, Kumho ECSTA Supra 712s in size 225/45R17 carry a load rating of 91. They also cost $100 apiece at Tirerack.com.

Enjoy the ride.

Norm Peterson

Let me add something for the benefit of those who might do the 225/45 replacement. Let the record show that I'm leaning toward eventually doing this swap myself (and have been so inclined since somewhat before this thread got started).

The capacity of a Load Index 93 tire is 1433 lbs and the capacity of a Load Index 91 tire is 1356 lbs. So you should consider increasing your inflation pressures a little. You should probably go up by at least as much as the 5.7% difference in capacity (about 2 psi) in order to maintain the same margin tire capacity vs load. At this point I'll note that not all tire makes/models seem to carry Load Index designations, so you might have to do your own math with other than 1356 lbs capacity for the new tire. Check the max load capacity on the tire sidewall to be sure.

As strictly a guess, +3 or +4 psi should do it for a true 91 Index tire (if I find anything that suggests otherwise I will post it - I do have some load vs inflation pressure tables somewhere, just not here).

Anticipating questions as to why 32 psi with the 91 Index 215/55-16 tire is OK (presumably 32 psi is the recommended inflation for the 16's - please correct me if I'm wrong), I'm going to suggest that the extra capacity of the "sportier" 17" tire is with the expectation of harder driving in cars equipped with the 17's, which translates into more severe tire loading.

Norm

y2kse

iTrader: (1)

Good post, Norm. I have a few comments.

First, the minimum acceptable load rating according to page 10-9 of the owners manual is 91. Most of the 225/45R17 tires I've investigated (although not all of them) have a 91 load rating. So as long as you keep the 225/45R17s properly inflated, load rating shouldn't become a problem.

Overinflating the tires to artificially achieve a load rating higher than 91 may not be a good idea, however. First, the 225/45R17 tire with proper inflation is going to produce a harsher ride than the 225/50R17 to begin with. Overinflating the 225/45R17 is only going to make the ride even harsher. More important, however, is the fact that overinflation causes the centers of the tires to wear faster than the edges. It also reduces traction because the amount of tire contacting the road decreases.

I don't know how critical overinflating the tires by only 3 or 4 psi would be, but I think it's unnecessary. My recommendation would be to simply run the 225/45R17s properly inflated and let it go at that.

Comments?

Norm Peterson

Harshness from shorter profiles and higher pressures is to be expected. File that under not getting something for nothing. In my case a slightly more stiff-legged ride is not a problem.

I'll have to think some more regarding the balancing of increased center-tread wear due to higher pressure against increased shoulder wear due to slightly higher loading relative to the smaller tire size if the pressure is left OE. I'm working from the basic premise that tire deflections are also a function of load, tire volume, and inflation pressure. Perhaps this should be tied to your actual vehicle loading and on how hard you drive. If we're going to re-spec our cars we ought to try to consider all that goes into these decisions, though in narrowing the focus to suit an audience of one rather than 100,000+ we can skew the basis for our decision differently than did Nissan.

I'll try to track down those load vs pressure tables I mentioned earlier over the weekend and see what the load capacity correlation between different tire sizes and inflation pressures really is. I'm not convinced that I should make much sacrifice in that area.

Norm

BuddyWh

If you are overinflating compared to the maximum pressure as imprinted on the sidewall, it is very critical! Don't do it... very dangerous.

If overinflating relative to the pressure indicated on the data plate, that should be safe unless it's already close to your tire's max rated pressure. It is quite common to adjust the data plate pressure when tire wear is incorrect, adjusting handling, or carrying loads close to GVWR. Just do not ever exceed the max pressure imprinted on the sidewall and, of course, remember pressure will rise 3-4 lbs in a hot tire so account for that.

In short, data plate pressure should be considered a minimum operating pressure and the tires rated pressure the max; stay within that range and you'll be safe with a properly load rated tire on the vehicle! If not... well... you best have done the engineering analysis yourself.

BuddyWh

y2kse

iTrader: (1)

Cool. I'll be looking forward to hearing back from you on that.

Norm Peterson

Not at all. It's one of the shortcomings of bb discussions that on occasion hasty interpretations and/or less than optimum phrasing of text can occur. I realize that my comment probably raised this issue, so as clarification:

I was just talking about adjusting the 32 psi Nissan-recommended pressure upward based on the relative rated tire capacities. By Monday I should have better info than a little judgement applied to the ratio of rated capacity.

Maximum pressure on most of these tires seems to be either 44 or 51 psi, so there's quite a bit of 'headroom' for a slight bump in pressure when you're only starting at 32.

Norm

BuddyWh

Thanks for the clarification... it is important because others following may be even more thrown off than I.

But this leads to another question... I have always operated under the premise the load rating given for a tire was at the max pressure (the one on the sidewall). That is why, when loading close to GVWR or GAWR you increase the pressure (from data plate) to return sidewall bulging and tread contact to "normal", being careful not to exceed sidewall pressure. You have not "pushed" the load rating of the tire... you just achieved optimal operating geometry making the tire better able to handle the load without heating excessively. Of course, for this to work safely... your tire load rating x 2 (two tires on an axle) has to be MORE than the GAWR of the vehicle. Actually "much" more to account for impact loading, moments, surges, whatever is easiest on your engineering sensibilies.

So, GAWR/GVWR and tire load rating specification are inseparably linked, i.e., the Max's tire load rating is given as 91 in order to preserve the GAWR of the vehicle. This leads to the conclusion it is "safe" to use a tire with a lower load rating than Nissan recommends but you MUST de-rate the vehicle GVWR/GAWR commensurately. That will take a little analysis, but easy enough for most people so inclined... and have the equation!

But I've never seen all this written, it just makes damn good sense even after 20 years experience. But most importantly, I have never heard of anyone suggesting you can push the load rating of a tire anywhere but "down".

At any rate, and to cut this off... I also am very interested in the table you refer to and the context in which it is presented.

BuddyWh

Norm Peterson

I tracked down a copy of those load & inflation tables over the weekend. Here's what I've found and figured out so far. These tables are quite old, but the trends are reasonably linear, so as a basis for comparison they should be OK. Just because profiles now are 50-ish (vs 70-ish in my tables) and that we're now using Load Index instead of alphabetic size descriptions doesn't invalidate the physics behind it all.

The weight sticker on my 20AE gives 4310 lbs GVW, with the front/rear 2313/2013 (no, I don't know why the math is off those few lbs). At any rate, that's 1157 lbs/front tire and 1007 lbs/rear tire. That defines the general range over which we're interested, from a little over 1000 lbs/tire up to the maximum, which I've assumed occurs at the 44 psi maximum.

I've constructed a table that indicates how the capacity of a Load Index 91 tire probably lines up with that of a Load Index 93 tire. Therefore, use it at your own risk. The capacity of a L.I. 93 tire at any given inflation pressure is equalled by the capacity of a L.I. 91 tire inflated between 2 and 4 psi higher. What that tells me is that either the 91 tire needs a bit more pressure or that a 93 tire can get by with a bit less. So if a L.I of 91 is sufficient, the 93 represents an "oversize" (unless the GVW is higher with the 93's). That's about as far as I'm willing to extrapolate things - and here's where some input from somebody with the 215/55-16 tires would really be useful. What pressures are suggested for the 215/55-16 equipped cars? And what's the GVW info?

psi . . L.I.91 L.I.93
Datum-4 950 1010
Datum-3 980 1040
Datum-2 1010 1070
Datum-1 1040 1100
Datum . 1070 1130
Datum+1 1095 1160
Datum+2 1120 1190
Datum+3 1145 1215
Datum+4 1170 1240
Datum+5 1195 xxxx
Datum+6 xxxx 1300
Datum+7 1245 xxxx
Datum+8 1270 1350
xxxxxxxx 1295 xxxx
xxxxxxxx xxxx 1400
xxxxxxxx 1340 xxxx
Max. . . 1360 1440


Norm

y2kse

iTrader: (1)

OK Norm. Let me make sure I understand your information.

The GVWR of my wife's 2K GXE and my 2K SE are both 2365 in the fronts and 1988 in the rears. That breaks down to 1183 lbs per tire in front and 994 in the rears.

According to the table you present, the minimum acceptable tire pressure for a 91 load rated tire on our vehicles is 37 psi (corresponding to 1195 pounds per tire) on the fronts and 30 psi (corresponding to 1010 pounds per tire) on the rears. Is that correct?

Norm Peterson

I think the use of that table should be limited to the delta-pressure going between the sizes. Too many assumptions are involved to directly use the numbers. I'm still trying to find something more current. I'm going to edit that post and its table to minimize that interpretation.

I noticed a similar disparity and have (so far) chalked it mostly up to Nissan specifying a mean value (which also introduces a little more understeer). If memory serves, our Mazda 626-V6 pressure sticker provides different pressures front to rear and also gives sets of pressures for two different loading conditions.

Norm

BuddyWh

My 2k1 GXE has load GVWR and GAWR's exactly like yours, as I recall. I have, as I recall, Avid V4's 215-55/16's. Checking the Yokohama web site I find the tire has an L.I. of 91 and a max load rating of 1366 lbs at 35 PSI, cold. That would make these tires safely preserve the front GAWR of 2313 Lbs with over 400 lbs margin. If I recall, the data plate pressure is 32/32 PSI, for "normal" operating of course.

I don't know the max pressure on the sidewall and it isn't rated in the data tables anyway so I would be very hesitant about increasing load rating any by raising pressure to, say, 44lbs. And at any rate, this leads to the following:

I noted an interesting disparity in your table: it shows a max load for an LI 91 tire as 1145 lbs at 35psi (well below the front GAWR of my Max) whereas my Avid's spec'd max load is 1366 lbs. Hmmmm... how to account for that... maybe, the table is constructed assuming the pressure will climb to 44 hot thus comparing to your table's L.I. of 1360 lbs... pretty darn close now. But that is pure speculation as we don't really know how to use it.

In practice, I consider the L.I. useless. It is impossible to use for an analysis at the curb side as I'm loading a pile of bricks in the bed of my truck trying to figure if I've overloaded... so why worry about it for my car. The "max load" and GAWR are the important numbers, I can work head math on them and they are accessible. When buying a tire, I'll continue as I always have: make sure that twice the "max load" of the tire, always clearly marked on the sidewall, is 200-300 lbs more than the greatest GAWR.

And an interesting thing I noticed on Yokohama's web site... the rim width given for my tire is "approximately" 6.5-7.5 inches. Pretty lousey way to spec. and hardly compliant to ANSI Y14.5. What do they mean... well, maybe they're saying you could put the tire on a 6" (approximately 6.5) or an 8" (approximately 7.5) rim? If they meant 7.5 max why not say 7.5 max? Dunno... just dunno.

BuddyWh

Norm Peterson

Yet another piece of confusion. My 225/50's make no reference to 35 psi on their sidewalls. Obviously we're all much better off if the rated loads in the L.I. tables are taken at a specified 35 psi instead of at whatever max inflation pressure is imprinted on the tire.

Given that such is the case, the reserve capacity at 32 psi would be approximately the difference between 2313/2 and 1366-3*40/2 or 1157 lbs vs 1306 lbs. Still plenty of margin, at 150 lbs per tire or 13%. (Whether it's actually printed 1366 or follows the L.I. tables at 1356 is an insignificant 10 lbs and under 1%.) And as you also suggested, it may be the rated load at the hot inflation pressure that's important, since that would represent normal operation for endurance considerations.

It's that ~40 lbs change in capacity per 2 psi of inflation pressure for a tire of this general size and range of inflation pressures that's the most reliable information that I can extract from those old tables. And estimating load capacity at pressures other than those where the tire is rated is what started this whole branch of the discussion.


Before I forget - thanks to y2kse and BuddyWh for the info.


Norm

sleepermax

iTrader: (5)

Guys this is good info, but a bit over my head. Let me ask you this: is it safe to assume your tires are properly inflated if you experience even tire wear? Of course this is really not useful for your purposes (i.e. pre-determining proper PSI), and it would only be apparent after several thousand miles of wear and of consistently maintaining the same cold tire pressure.

y2kse

iTrader: (1)

Yup. It's over my head as well. But it's also really cool to see just how far the technical side of this can go.

The questions can easily compound. Here's one, for example. The 17" tires call for the same air pressure all around. But the 16" tires call for more pressure in the front tires than the rear tires. Why is that?