Hyperinflating tires
 

I'm starting a discussion thread here - and I'm hoping to move a discussion that got started when I introduced myself:

http://www.ecomodder.com/forum/showt...ires-2522.html

So hopefully this will be a bit more visible and a bit more "on topic".

First, let's define some terms:

Underinflation: Using less inflation pressure than is listed on the vehicle placard (assuming the placard tire size is being used.)

Overinflation: Using more inflation pressure than is listed on the vehicle placard (assuming the placard tire size is being used.)

Hyperinflation: Using more inflation pressure than the maximum listed on the tire sidewall.

Vehicle placard: The sticker on the vehicle that lists the original tire size and the proper inflation pressure for that size.

Agree?

Any other terms we need to define?

Welcome to the site. What is your back ground ?

credentials... lets see some.

I'll agree with your definitions. I've read numerous articles that all warn of the dangers of overinflation, yet NONE of them define what overinflation is. Are they referring to above mfg spec or above max sidewall? There is no way to tell.

I'd have to agree with the others though. If you are going to tell us how it is then your going to need to back it up with some factual evidence.

I've worked for one of the major tire manufacturers for over 30 years. Been in manufacturing, design, testing, was the technical liason to Ford, and my current assignment is in an area where I interact with the outside world. I'm the technical advisor to the 800 number as well as serve on several committes with the various tire technical organizations, such as the Rubber Manufacturers Association, the Tire and Rim Association, and the Tire Industry Association.

My main reason for posting here concerns hyperinflation - inflating tires over the maximum posted on the sidewall of the tire.

I think it might be best if we confine the discussion to passenger car tires and avoid the confusion with Light Truck tires (as defined by the letters "LT" in the tire size) and Truck Tires (over the road, big rig stuff)

And before I start, it should be obvious that I have access to a lot of information. but a lot of it is confined to the normal operating enevlope of a passenger car tire (Inflation pressure in the 30's, speeds below 70 mph, and moderate loads.) Tire manufacturers aren't going to fund studies and testing that doesn't lead to more sales, so the results of some studies won't be published because of that.

Not to mention that a lot of fundamentals have been lost to time - and I think we're going to find this a very real limitation in this area.

Anyway, enough for now, I've got work to do!

I will agree with the definitions there.

I personally would never inflate my tires past the limit posted on the tire. I always assume that the manufacturer has tested these tires to a point that the number on the outside stands for some point at which some of the tested tires began to fail after use. I actually go even beyond that and will adjust my tires based on my driving for the day. I have found that my tires will go up by a factor of 'x' based on what kind of driving I'm doing, more so for highway driving then around town driving, so I keep them at the max listed on the sidewall unless I'm going on the highway where the temperature will cause the 'x' factor of expansion to occur in which case I release pressure enough so that it stays within safe limits while they are heated up.

I agree that some of us go over the sidewall limit and that it is technically unsafe. But how unsafe is it. Is it like crossing the road unsafe or base jumping unsafe? My point is that although we are exceeding the safe working load, it is by a (probably) small margin and should pose no problem. I'm happy with that personally but obviously its a personal thing. What i really want to know is how the traction varies with pressure, all the way to sidewall.
I'm thinking it goes up because if it dramatically went down, we would be told about it everywhere. Instead, silence. Now i often joke about being able to find anything on the internet but the silence on this subject is phenomenal!
Anyway, it will be good to hear more from you CapriRacer. You sound like a knowlegeable bloke and have a lot to offer here.

ollie

Hello,

For me, I think it is unsafe to inflate my car over 39-40psi. The sidewall maximum pressure of my tires (Yokohama Avid TRZ 185/60TR15) is 44psi -- but I find that I start losing wet (and dry) traction if I go above 39psi.

I believe the recommended pressure is only 32psi, and it seems like anything between 36 and 39 is virtually the same rolling resistance; judding by my coasting distances and MPG.

This is well put. If coasting distances (with a degree of confidence) prove that X psi is no better than Y psi, then you have a valid conclusion to keep it lower. Still, in my application, I haven't found any valid sources to persuade me not to run pressures over sidewall max.

The problem we have is the different kinds of tires on all kinds of vehicles.

If people want to play it safe, test in an open parking lot -- similar to Autocross (or even attend one). Try the same lot when it rains -- with different pressures. You can learn quite a bit about vehicle dynamics that way.

For me, there doesn't seem to be any difference with wet vs. dry traction -- but then again, I haven't "officially" tested it -- just parking-lot stuff. Oversteer is more pronounced in wet weather, but I'm driving slower, and IMO, helps the car's handling dynamics (front-heavy: tends to push in corners).

RH77

Capriracer can confirm this for sure, but I believe the sidewall max psi is for when the tire is cold. The pressure will go up while driving fast but I'd assume that has been considered when the engineers set the max psi rating.
ie, no problems doing 75mph in death valley in the summer at max rated load...

I've been doing some autocross lately with my 95 Neon, and with all season tires, P185/65R14 which are relatively narrow high profile tire common on most cars, I get the best times at about 46psi cold in the front tires, and 35psi cold in the back. Autocross is basically 80 seconds of extreme accident avoidence manuevers at relatively low speeds, up to 40mph in our area. So I would say that running higher psi is definitely safer for city driving. You turn the steering wheel and your car turns quicker and more predictably, there is less deflection from the wheel rim to the contact patch on the tire.

As for absolute braking performance in a straight line, I bet 10psi is best for the rear tires and maybe 25 psi in the fronts. That would give you 4 big fat contact patches. Not good for anything else like turning or going through potholes though....

No tire or tire pressure is close to optimal for all conditions. By inflating your tires up to or slightly beyond the max on the sidewall you might be increasing your stopping distance by 2% on clean dry pavement, you might be reducing your risk of aquaplaning by 2% and you are able to turn 4% better while saving 4% more fuel... Or not, unless you have team of engineers test your tires on your car on your roads you'll never know for sure.

For me, I'm comfortable running 40psi in all tires on both my Neon and Tracker, this is 5psi over the sidewall max for the Neon and the Tracker. I don't drive 100mph for hours in the desert and I can see that tire is very much in contact all the way across the tread so I think this is good comprimise. YMMV.
Ian

And the apparent difference between LT and P sized tires when it comes to inflation are? Far as I know an LT normally has a stiffer sidewall along with a load range rating. For example, on the Heep I have 30x9.5x15 BFG AT/KO Load Range C rated for 50 PSI and about ~2000lbs load.

This is why I want to stay in the passenger car tire arena.

LT tires are designed to be inflated up to 50, 65, 80, 95 psi, depending on the load range, where passenger car tires are designed to be inflated up to 35 psi (assuming similar usage)

And - no - the sidewall isn't designed to be stiffer, it just is because of the added reinforcement to allow higher inflation pressures.

And to answer IndyIan - all inflation pressures are cold pressures except when they are specifically are calling out pressure buildup.

OK Capri, we seem to be warming up to you now ... so how about some scientific information?

It's what we are waiting for ... Just as an FYI I have been running 50psi cold, Nitrofill in my tires since new, about 2 months now. I have seen better FE, better handling and better coasting ability. Also less hydroplaning in the rain, that data however was @ 50mph, a speed where hydroplaning hasn't been an issue with this vehicle before. Sidewall max rating for my goodyears is 44psi.

Also I have reduced my vehicles overall weight by about 300lbs.

I for one would like to see the topic stay on P series tires. It would be a good topic for later for us SUV and truck owners.

Label me a skeptic still...

Working for several years as the lot boy at a Toyota dealer, I noticed that the cars all came with the tires overinflated. Not just a few pounds, they were typically all jacked up to 60psi. I wouldn't want to leave them like that for driving around, but it gives me a feeling of some confidence that maintaining the max sidewall rating of 44 won't overstress the tire.

Hello,

I think this is the right thread for this question: who can shed some light on inflating tires with "pure" nitrogen -- as opposed to ~78% nitrogen, ~21% oxygen, ~1% argon (really?), ~0.038% carbon dioxide; aka air?

I run nitrogen in my tires as my work offers it ... the only benefit I can see is that it will control the expansion of air since there is none, also since the nitrogen molecules are larger than air, general seepage of air through side walls valve stem leaks etc .. is slightly reduced. I still have my wheels capped off every week though ..

Expansion is reduced, but still exists (nitrogen follows the ideal gas law).

Probably the biggest difference you'll see in expansion comes becasue there is less water vapor than there is in the air you get from a typical compressor setup.

Good luck finding THE answer, as such a thing does not exists. I'd go with RH77's advice.

Capri, I can't wait to see your take on hyperinflation. You already stated you had access to a lot of information under normal operating conditions, but this is hardly useful in regards to hyperinflation.

If I was a tire manufacturer, I'm pretty sure I would fund studies to test the limits of my tires for liability purposes.

Ok so to anyone, I would like to get arguments against running at 50 psi a tire which is rated for 44 psi. What I would also like, is that for every argument you make, you explain the basis of that argument and/or provide a credible source.

I will just chime in here to say I love the term hyperinflation, and I'm going to start using it, :)

Looking forward to hearing more!

Hey Ya'll,

I refuse to believe that a tire manufacturer would put a max inflation number on the side of a tire that was higher than it was safe to inflate it to. If it says Max 35 PSI, then that tire better be able to handle that pressure even with high heat conditions or inaccurate guages causing the tire to exceed that number. If it could not handle that then they would be facing lawsuits when the tires failed.

They would obviously design the tire to handle even more pressure than listed on the side to have a margin of safety to take into consideration heat and tire guage variables. This is why people get away with exceeding those max numbers, but exceeding those numbers brings the tire closer to the edge of it's safety margin. The more hyperinflated tires are closer to the point of failure wheather it is a safety or actual tire failure, but only the manufacturers would know how close.

If tire X fails at a certain PSI then they would want to make sure that no one inflates their tires beyond that pressure. They would do this by putting a number on the side of the tire that has a margin of safety big enough for them to feel like tire failure lawsuits would be unlikely. The tire companies would know how big this margin of safety is, but they would not necessarily want the consumer to know because they would be more interested in maximizing profits than maximizing tire wear or fuel economy.

Capriracer, would you agree to these statements? and when can we talk about truck tires?

Later,

Allan Greenblazer

CapriRacer -

I would add :

Maxinflation: Inflating to the maximum listed on the tire sidewall.

I *deliberately* got LRR tires rated at 51 PSI so I could go "pressure to the max" and not worry about violating the sidewall max.

This is a site I reference to others because I think it has a *lot* of good info on tire technology :

The wheel and tyre Bible
http://www.carbibles.com/tyre_bible.html

CarloSW2

CapriRacer -

I posted this thread in hopes you would comment :

My Continental Contact-Pro Tires
http://ecomodder.com/forum/showthrea...ires-2642.html

CarloSW2

I like that definition.

I apologize for being silent for the last 3 days, but I’ve been looking for information concerning tire inflation vs various tire properties. I spent a lot of time going through a 700 page publication called “The Pneumatic Tire”. The book is published by NHTSA (National Highway and Traffic Safety Administration) . It is a series of summaries written by various authorities on the subject of tires and tire testing.

So far I’ve only skimmed the articles, looking for graphs and charts that might be helpful, and while I found a lot of stuff that is interesting, I haven’t found anything that deals directly with the subject at hand. I suspect this is because inflation pressure is usually a given in most studies.

I will have to go over the articles again in more depth to see if there are some equations that might include inflation pressure as a term – but wasn’t one of the items studied.

But I suspect that the effect inflation pressure has on various properties was studied long ago and I may be forced into doing searches the old fashioned way – in a library.



Funny you should ask a question this way. I did find something in my searching I didn’t expect:

http://www.geocities.com/barrystiretech/SNforRubber.JPG


This an S-N curve for rubber. (Stress - Number of cycles) S-N curves describe the way in which materials behave when subjected to cyclical loading. What these curves say is that the more stress a material experiences, the less cycles it can endure before failure.

S-N curves are very similar for pretty much all structural materials – in that number of cycles before failure is very highly dependent on the stress that the material experiences.

I think this curve is of interest because some discussion has centered around the phenomenon that tires will accept a lot of inflation pressure before exploding. This would be a one-time loading. Just for reference, a passenger car tire would experience 30 million revolutions in 50,000 miles (on the S-N curve that would be 3 x 10^7) If I read the curve correct, at 30M cycles, the stress at failure is only 20 to 25% of that for a single cycle.

Put another way, when a tire engineer is designing a tire, he would choose materials that appear to be grossly over designed. He’s doing that to account for this S-N curve.

That probably also means that hyperinflating tires would result in a fatigue failure many cycles down the road - due to the overall stress in the tire caused by the inflation pressure.


So to answer Greenblazer's question - ah ..... well ...... ah ....... I can't answer it in the form it is in.

The tire is designed around this fatique problem and the bursting pressure of a tire is kind of an incidental property. I haven't seem many tires that have been subject to a "pressure to burst" test, but there are a few videos on Youtube - and there it looks like either the rim or the bead is failing - and most tires fail in the are of the belt package - a completely different area.

But what is going on is that the tire standards organizations have delineated a set of operating conditions (pressure, load, etc.) - and for passenger car tires the operating range is between 1000 and 2500 # (or so), 26 to 36 psi with a provision to allow for higher pressures for special situations, such as high speed operation, and speeds up to the speed rating of the tire (with appropriate adjustments in load and pressure), etc.

So when the tire manufacturers put a maximum pressure on the sidewall of the tire, they are really telling you that this is the limitation they've designed around. So if you exceed those limitations, expect problems since this is an area that the tire manufacturers have not tested in and it may or may not be risky (OK, I think we all know that it is risky, the question is how large is the risk.)

Put another way - the tire manufacturers have warned everyone to stay away from those pressures, so the user assumes liability.

But I can tell you that when tire engineers design tires for higher pressures, they use stronger materials - larger ply cords, more ends per inch, more plies, different materials, etc.) Confounded in this is that uusually the load carrying capacity also goes up, and that's the way a tire is actually used.





Anytime! I just wanted to make it clear that this discussion is about passenger car tires - and other types of tires (truck, aircraft, racing) are different and I didn't want folks to get confused.

So if you want to talk about truck tires, just make it clear. BTW, there are several many different types of trucks - and the tires for each group are indeed different (and delineated so in some way), so please be aware of that when you post.

If I have this right, the concern is the car following the grooves in the pavement. We tire engineers call that "groove wander" - although the term "tramlining" is also used (I prefer the former - a bit more descriptive, I think.)

What is happening is that the grooves in the tire are lining up with the grooves in the pavement and when edge meets edge, the tire tends to be follow the edge in the pavement. This phenomenon is going to be affected by the number of grooves in the tire (and the number of grooves in the pavement) and the amount of footprint pressure for each groove.

Obviously over inflating the tires is going to put more pressure on the center grooves - and those grooves will dominate the way the tire behaves. Less pressure will tend to engage the rest of the grooves and decrease the effect.

2 problems: While there are standards about pavement grooving - spacing, width - there are so many non-standard pavement grooving, it is difficult for a tire manufacturer to space the grooves in a tire to account for all of them.

Grooves in a tire are necessary to allow a pathway for the water when the tire rolls through the water - which is what the grooves in the pavement are also doing. In designing a tire, well defined grooves work better that grooves that are interrupted by the tread pattern. Plus the edges of a tread element are areas were the wear will be more rapid that in the center of the element. "Heel and toe" wear is a good examples of this at work. Well defined grooves tend to be more resistant to irregular wear, especially diagonal wear.


In your case, I would try decreasing the pressure and see if that doesn't help.

Um Ok... Well CapriRacer,

I do appreciate you responding, but it's a bit of a technical response for me to wrap my mind around. I read it through a couple of times, and I think you are saying that an overinflated tire might or will fail sooner than a properly inflated tire, but not untill it has been stressed past the failure point by lots of use. In other words a new tire might hold higher psi than printed for a while, but with age and use could fail.

Let me simplify and ask again, is it, or is it not safe to run tires at MAX sidewall PSI in your educated opinion? A simple less technical answer would be appreciated.

I don't mean to sound snarky if I am comming accross that way, text can be funny like that. I don't have the technical background to quite grasp what you are saying.

Thanks for your information and participation in this forum.

Later,

Allan Greenblazer

P.S. I will be away until Sunday Night.

You got it, he is saying that increased pressures reduce the fatigue life, but there is still a huge safety factor there. So the tread will be long gone from the tire before you get to that fatigue failure. If he simplified it in the first place he'd probably catch crap for not backing up his claims and providing technical data/charts.

As for inflating to max sidewall pressure, I think it will be a while till we have more data to support or debunk that. And I feel in the end it will come down to "sometimes", depending on the vehicle/cargo weight, tire load range (this is where where having LT tires will become more of a factor), etc.

I for one don't think max sidewall pressure is always a good idea. Like I said above, it's a "sometimes". My dad used to have a Chevy K2500 (4x4 3/4 ton work truck model). He had me take it in for new tires, they were load range E (same as what came on it stock). The tire shop decided they needed the full 85 psi called out on the sidewall. The ride was horrible. A fairly quick stop in town had the front tires audibly fighting for traction. It was really bad on a washboard road. I stopped in front of a buddy's house and layed a little rubber with a quick stab of the gas. What was revealed was two burnout's probably around 2.5 inches wide each, from 285 series tires (probably around 9" tread width).
Too much pressure + too little weight = crowned tread and very small contact patch

I had a simular experience with a stripped out Toyota pickup I had, weighed in well under 3000 lbs. Built it up with load range E Swampers becasue I had a set of free 16" wheels. Tire shop decided they needed 60 psi after mounting. A single tire's load rating was greater than what my whole truck weighed! When I aired down for wheeling there wasn't any visable buldge in the sidewall untill around 6-7 psi! A little too much tire for the truck!

Allow me to shorten:


I don't think we know how large the "safety" factor is - too many variables. More to the point is that there is an element of statistics involved - tail end of the curve type stuff - that would influence the point at which a failure would occur due to fatigue. As you point out, there's a lot of variables. Things such as speed, road smoothness, etc. would all influence where that point is.

Just as a reminder, when the tire pressure is increased, the wear rate is also decreased, meaning the tire will wear slower - more cycles before removal. This is also going to complicate the situation.



If you watch videos of dragsters, you'll see that the black patch left by the tires is narrower than the full width of the footprint in contact with the ground. This phenomenon also exhibits itself in wear - tire wear on driven tires tends to be more in the center than on the shoulders. You can't see this in FWD cars because the driven tires are also the steering tires - and steering tires tend to wear on the shoulders. But on RWD sedans, for example, the rear tires will wear in the centers and the fronts will wear on the shoulders. Needless to say, this would be aggravated in an unloaded pickup truck where the rear tire pressure is specified for a load in the bed.

re fatigue failure:

according to wikipedia "fatigue is the progressive and localized structural damage that occurs when a material is subjected to cyclic loading".

Now what is cyclic loading? In the case at hand, one could probably argue that it happens at every rotation of the wheel at the contact patch, and probably even every time there is a tire warm up cycle. Constant higher pressure does not look like cyclic loading to me, but I might be way off.

From my understanding of it, hysteresis plays the biggest role in tire fatigue. This seems to be demonstrated by the fact that tires which have been run at low pressure for extended periods of time are prone to cord failure and tread separation. So reducing sidewall hysteresis by means of increased pressure would appear like a good way to reduce the occurrence of fatigue failure.

I always thought that, on dragsters, centrifugal force acting on the spinning tire was the largest reason that the tire has a narrower contact patch while spinning than while stationary. The same would be true of passenger car tires except to a much lesser degree due to a much slower spin speed.

The issue isn't the pressure. The issue is the stress in the tire - and it changes as an individual section of the tire revolves through the contact patch. That's the cycle.

That would br true up to a point. But we are discussing way beyond that point.

Normally, the problem with hysteresis is that the heat generated changes the properties of the rubber - and that causes the failure in the form of a separation between plies. You will hardly ever see actual fabric fatigue in a tire, but if you do, it will be caused by the cord going into compression. It's sort of like pushing on a rope - the rope doesn't work very well that way.

Take a real close look at the part of the tire that is actually in contact with the ground at - oh, say - the 60 foot mark. The centrifugal forces are indeed causing the tire to grow, but the black marks being left are even smaller than the width in contact with the surface.

You are correct about the dragster tires. The latest versions deflect much less, but if you took a picture of the tire near the finish line you would see it is almost oval. The tire narrows at the top of it's rotation (furthest from the road) and then is flattened back down due to the tremendous downforce created by the rear wing.

Passenger car tires have much stiffer sidewalls and cords than racing slicks, so actual narrowing of the tires by cetrifugal force at normal speeds is negligilbe.. Only by overinflating a tire can you reduce it's contact patch.

I think you will find that drag slicks are bias ply tires. It's the belts, not the sidewalls that prevent the growth due to centrifugal forces - and if the belt isn't enough, circumferential cap plies are added.

Like this? The cords appears to be good (or at least unbroken), but all the rubber was el-gone-o.
http://i50.photobucket.com/albums/f3...8/100_1541.jpg


Happened to me, probably under-inflated. It was a company vehicle, I didn't bother checking the pressure when I took it for the week. Now I do!

Heard a very faint "tick" on the freeway. Slowed down a little, then gradually sped up to see if the rhythme of the tick correllated to speed. It did, and at close to 90mph there was this little crap-your-pants explosion. Handled it fine, despite needing to cross 3 lanes of traffic to get to the shoulder. The biggest pain was using the crappy stamped-steel lug wrench/jack handle by cell-phone light to change the thing at 5 am. I now drive this car full time, keep a real lug wench in the trunk, carry a flashlight, and check tire pressure on a weekly basis!

What I have understood so far is that the optimum tire pressure for safety and ride quality is the recommended pressure. :eek: How unsafe hyperinflation is is still a major question.

That's true.

However, the optimum tire pressure for fuel economy and for treadwear is higher. How much higher and how to balance the two sides is the question.

CapriRacer -

(thank you again for your feedback)

I was experiencing the groove wander when the tires were set to 42 PSI, so I don't think it will make a difference. I predicted this behavior before I bought the tire when I looked at the tread :

http://img135.imageshack.us/img135/9...isonlg6.th.jpg

The Continental tread reminded me a lot of my old motorcycle tires (circa 1980's), which also had the "clean/flat" tread grooves and suffered from massive groove wander. On the new Continentals, I think the tread grooves are also very wide. If I had known this in my motorcycle daze, I would have gotten a tire with the diagonal grooves you describe :

http://img229.imageshack.us/img229/7...tireog8.th.jpg

I agree that the tire behavior is dependent on the groove of the road. There are some "groove zones" on the freeway that do not effect me. It always depends. It's like a "harmonic frequency" thing. In the back of my mind I wonder if these tires were designed for "European Highways" that may not have the same kind of "groovy" freeway design patterns that we do.

Part of being a Hypermiler is "driving the road ahead". This means following the 3+ second rule and looking for escape routes from possible accident scenarios. Despite the bad traffic my LA driving conditions are very mild. I don't have to deal with snow, mud, or torrential rain the way the rest of the country does.

Overall I am very happy with the tires. The GPS correction means I always get a 2% bonus whenever I fill-in my gaslog. That's happy stuff for me, :) .

CarloSW2

I have noticed that in order for me to keep my tires @ 50psi I have to stay on top of them more often. At 44 psi I was checking my tires once a month with no problems. Now that I am up to 50psi I am checking them weekly and noticing a drop in pressure each time. The drop varies, 2-3psi in the rear, as much as 8psi in the fronts. I am going to blame the valve stems since they are the cheasy rubber ones you get at the tire store, but otherwise the wheels have no punctures ... and the FE gains I got are indisputable.

But yeah, I may rename my Vibe the Radio Flyer, because it does ride rougher, but coming from low riders, it is very tolerable.