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While it isn't very long (only a paragraph on under and overinflating), I thought having a link to it might give more credibility to what we tell people the dangers and risks are of overinflating. Tirerack is one of the main players for bying tires on the internet. They are not a green site in anyway.

They also have a few other tech docs on tires and inflation that might be of interest.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1&currentpage=33

Interesting how while discussing underinflation, they mention the word "failure". But they don't mention it while discussion overinflation.

Just read that myself last evening, Tom :)

Terry

For anyone that rides a bicycle on a regular basis, we all know how tire inflation, or lack of it, influences the amount of effort needed to keep the bike moving. I recently left my tires inflated to 90psi instead of 100psi and the effect was quite noticeable. With lower psi, it takes more effort to keep the bike moving and I get tired quicker. The legs don't lie. ;) There is a direct correlation to automobile tire inflation. The lower the psi, the harder it is for the power train to move the vehicle, which means more fuel consumed.:)

cool read, but i was hoping i find out something i didn't know :P

I don't think there is much about FE on the net that would be groundbreaking. Well, except on this site!

heh

The interesting thing about it is the fact it backs up what we say on a site that 's goal is not to promote FE but just sell tires.

One thing I find amusing is over on the Prius forums where some
people have been running their stock Integrity tires at Toyota's
recommended pressure [35 or thereabouts] and observed lots of
edge wear, others have been running design pressure [44] and
had nice even wear across the tread and have gotten 50,000 miles
or so out of decidedly inferior tires ... and a lot of other people
reading all this and STILL not believing that they should run
the higher pressures.
.
_H*

I don't think I will ever go with manufacturer recomendations on tires from now on. I know what to expect with the higher pressures and I'm okay with it. Heck, with all the stuff on the net about lower pressures I'd be afraid to run at factory recomended pressures.

Most everyone pumps air in their tires when they get gas. This usualy involves driving to the station and from what I read even a little driving can raise the psi in the tires so they may read a few lbs low so people will pump them up to manufacturers recomendation. The problem is that when the tire cools it will be a few lbs short anyway. I suspect a majority of people are running around with underinflated tires.

Lots of these people are the same people that say higher pressures are dangerous not realizing they are at more risk then we are and they don't even know it.

http://www.nhtsa.dot.gov/cars/rules/rulings/TPMS_FMVSS_No138/part5.6.html

i thought that was interesting, at lower speed hydroplanning isn't a big deal unless the water was above ground level. The higher the speed (above 50 mph) and the more under-inflated the tire is, then the lower the tire-to-road contact and the higher is the chance of hydroplaning

So, at 60ps the hydroplane speed is 80.17 mph. Man not even close to the spead I would be going.

As to the effect of tire pressure to hydroplaning, imagine an ice skate blade versus the bottom of your shoe on ice. The skate can make a turn but your shoes can only take you straight. An over inflated tire is more like a poker chip, hard riding but more maneuverable, it cuts through the water a little better. I also like the idea that by limiting my speed to 55 Max and overinflating my tires, I'm much less inclined to lose traction in the rain. But I'm not so confident of this winter's snow. Any experience there?

I've been driving on snow with "over" pressured tires and haven't had any more issues than previous years at lower pressures. I'm not willing to lower the pressures now to compare over the short term. ;)

I've been driving on snow with "over" pressured tires and haven't had any more issues than previous years at lower pressures. I'm not willing to lower the pressures now to compare over the short term. ;)

I know exactly how you feel. I'm not willing to lower the 76 psi in mine either.:p

Has any document addressed the discrepancy between the pressure recommended by the auto manufacturers, (usually much lower) and those recommended by the tire makers?

I found a pdf document on the Preliminary findings of the effect of tire inflation pressure on the peak and slide coefficients of friction. Its from the US Dept of Transportation, dated june 2002. I tried copying some graphs from it onto here but dont know how. I could email it to someone here if you want? There were some suprising results in there. The tests were only done on a few tires and they behaved differently at different pressures. So i wonder if all tires are a lot different to each other.
Here's an example:
Test done at 50mph in the wet.
ASTM 1136 SRTT radial coefficient at 17psi:0.8, at 35psi: 0.87
Goodyear eagle LS radial coefficient at 17psi: 0.9 at 35psi: 0.86

I dont know what the first tire is, i believe it is a test tire. Whats surprising is that the goodyear has higher grip at 17psi and it drops as you go up to 35 psi. This seems to fly in the face of conventional wisdom. I have two years on my tires and they have at least another two years in them so i have no plans to drop the pressure!!

Has any document addressed the discrepancy between the pressure recommended by the auto manufacturers, (usually much lower) and those recommended by the tire makers?

I'm not sure if there are any documents on that but I've always heard it's got more to do with giving a smooth ride than anything else. Has anyone else heard this?

I know the ride was more comfortable when I have the tires on my 06 Civic at 34psi...but the FE advantage of having them at 40+psi was enough to sacrifice the comfort

It took me 2 sets of tires on my tundra to figure out that the 26 psi toyota recommends was not doing any favors for my tire life. Once I started running the max sidewall pressure on the tire they lasted like they should.(30000 miles versus 70000miles) I always try to fill my tires on a day when the temp has dropped to around 30-35 degrees overnight. That way I can get a true cold pressure. FWIW I run 44 psi in the hch2 on all 4 corners and 35psi on the tundra.

edit: I have read that the lower psi rating on the doorsill is for comfort reasons also.

Has any document addressed the discrepancy between the pressure recommended by the auto manufacturers, (usually much lower) and those recommended by the tire makers?

While I don't have a link to show proof of why there is the difference in pressures... The reason why Automakers have different pressures listed than the tire is because the tire is made in a generic size, destined to be used in a wide range of OEM/aftermarket applications.

This tire needs to be able to hold varying amounts of load as each vehicle is a different weight.

The pressures you see listed on the side of the tire is to achieve the maximum load rating of the tire when the pressures are measured cold. The pressures listed on the B-Pilar of the vehicle are specific to the vehicle to achieve the minimum amount of air pressure required to achieve the load carrying capability of the vehicle, NVH (noise, vibration, harmonics), suspension design, and fuel economy.

Basically the numbers listed on the B-Pillar are a trade off to ensure a quite, comfortable ride at the expense of FE.

The Automakers have to balance:

Comfort (Low pressure is better)
Noise (Lower pressure is usually better)
Wet & Dry Handling (Higher pressure is better)
Rolling Resistance (Higher Pressure is better)
Stability (Balance front & rear pressures to vehicle weight)
Safety (Balance between High and Low Pressure)

With all of these tradeoffs, rolling resistance usually gets lower priority because it costs a lot more to improve shocks & struts for better comfort and handling.

Also, they take into account who the potential buyer is for the car. A Lincoln buyer is more concerned about comfort than a BMW buyer. Each car has its own personality, and it is difficult to change the culture at the car company to modify the "personality."

With new CAFE standards, expect to see less comfort and better rolling resistance.

The recommened cold psi settings are most likely for comfort. Levels above the recommendation would probably lead to a more harsh ride. I've seen on my tires (summitomo htr) that it likes being pumped up to 40lbs. The wear is very even, but the ride is more harsh. My Civic manual states that pumping up the tires up around 5lbs is recommended in certain cases but comfort would suffer.

FWIW, there was a question about what an ASTM tire is. ASTM was American Society for Testing and Materials. It's a reference tire.

Maybe CapriRacer will check in.

There's a whole lot of linear thinking going on, which gets repeated as if a little is good, more must be better.

That is, since you get better performance in Auto Cross at a few PSI over placard, therefore braking and cornering must be even better at a few tens of PSI over placard. This is an extrapolation folly.

-mr. bill

Hi Bill:

___The following will help as well although a rehash of what has been linked in the past ...

http://www.nhtsa.dot.gov/cars/rules/rulings/TPMS-FMVSS-No138-2005/part2.html

GoodYear tire study on how MAX Sidewall helps braking distances in most instances ...

http://www.officer.com/web/online/Editorial-and-Features/Driving-Under-Pressure/19$27281

MAX Sidewall helps handling and emergency maneuvers.

___Good Luck

___Wayne

Hi Bill:

___The following will help as well although a rehash of what has been linked in the past ...

http://www.nhtsa.dot.gov/cars/rules/rulings/TPMS-FMVSS-No138-2005/part2.html

GoodYear tire study on how MAX Sidewall helps braking distances in most instances ...

MAX Sidewall for the GoodYear Integrity 215/70R15 is 44.
MAX Sidewall for the GoodYear Wrangler 235/75R15 is 51.

The data only goes to 35 psi.

Please try again.

-mr. bill

Hi Bill:

___Good catch. Go back to the first link ...

http://www.officer.com/web/online/Ed...ssure/19$27281

___And please provide some of your own your own data for everyone’s benefit as well :)

___Good Luck

___Wayne

That's the second link.

I can tell you that in Auto X with my car, people run below MAX Sidewall. Oddly, that would be the pressure with the best performance tradeoff. Higher than that performance drops *AND* rotation (getting the back out) is way too abrupt.

I've looked for objective data that says that Auto X folks get better performance running max sidewall and above.

There's a simple reason I haven't posted it. I haven't found it.


One thing that is different than Auto X with your police example - Crown Vics in police config are pushing (if not crossing) 5,000 pounds. Keeping those sidewalls from rolling must be a bear. One thing that is the same - no road hazards where the cones are set up.

-mr. bill

Mr. Bill,

You might enjoy this article I just found while looking for a pressure vs. traction graph.

I have seen this graph before, but can not find it. Basically, traction will go up as pressure rises, until peak traction. Then traction drops off drastically with higher pressure.

The reason is that for cornering, the car relies on the flex of the entire tire. With too much pressure, only the tread rubber is flexing and it does not provide as much traction. With not enough pressure, the tire will roll off of the tread and ride on the sidewall.

Long Article on Autocross Pressure (http://www.autocross.us/forums/lofiversion/index.php?t46.html)

Yeah, the description of the shape of the graph is key:

"(Illus. resembling a mountain with a rounded peak.)"

Visualize that. Taking the data between 25-and-35 psi has you going up the side of the mountain. The extrapolation folly is expecting that the mountain has no peak. The danger is the mountain top is just several psi above placard.

-mr. bill

That's the second link.

I can tell you that in Auto X with my car, people run below MAX Sidewall. Oddly, that would be the pressure with the best performance tradeoff. Higher than that performance drops *AND* rotation (getting the back out) is way too abrupt.

I've looked for objective data that says that Auto X folks get better performance running max sidewall and above.

There's a simple reason I haven't posted it. I haven't found it.


One thing that is different than Auto X with your police example - Crown Vics in police config are pushing (if not crossing) 5,000 pounds. Keeping those sidewalls from rolling must be a bear. One thing that is the same - no road hazards where the cones are set up.

-mr. bill

How about letting us know what auto x drivers are finding best.

Is there a correlation between what works best in the auto x world and handling/braking for emergency stops? If so please provide the data. The data has been primarily related to braking, my guess is auto x is more focused on control while not braking.

From personal experience my truck went from considerable understeer to fairly neutral after going from placard to max sidewall several years ago. I have a few daily commute corners that I push the limits of my tires and speeds have gone up significantly with max sidewall pressure.

I doubt anyone thinks there is an unlimited positive effect with pressure increase, the data that has been provided shows nothing but positive going from 29-30 (what my placard recommends) to 35. To assume the curve crashes beyond 35 for the typical commuter doesn't reflect my experience.

You're kinda being a naysayer without any information.

looking for a pressure vs. traction graph.

[/URL]

One of my goals in life!:rolleyes:

ollie

Here is a study by the California govt. Transportation department. I believe LRR will be mandatory soon. Some pressure info and LRR info.

http://onlinepubs.trb.org/onlinepubs/sr/sr286.pdf

I just increased my tire pressure from 40 to 50 psi, first time. I noticed an immediate change from the scangauge, got a 4% mpg increase.

One of my goals in life!:rolleyes:

ollie

I think I remember seeing it in this book:

The Racing and High Performance Tire (http://www.amazon.com/Racing-High-Performance-Tire-Using-Balance/dp/0768012414)

Or, it may have been this one:

Bob Bondurant School on High Performance Driving (http://books.google.com/books?hl=en&id=l7sWxIjLK5EC&dq=Bob+Bondurant+School+of+High+Performance+Driving+5TH+Edition&printsec=frontcover&source=web&ots=29HtyrCBS0&sig=PIinTLBDCV7ZWnFoq0Zm-yjPQNk)

Not that it matters, since evidently I don't have data, but I wonder why both recommend +10 psi over placard or 40 psi FOR RACING ON STREET TIRES?

(I have seen the mountain top....)

-mr. bill

I would venture a guess that it is just a good starting point. It really depends on the car, suspension, tires, etc.

On my M3, the placard is 34/36 (I think). When I took it to the track and autocrossed it, I would usually go to around 38 on all four corners. You can tell a lot by looking at your tires. I know a lot of people in autocross put white chalk on their sidewall to determine how much roll they are getting. The hard core guys would use a pyrometer. I personally don't think a pyrometer will tell a whole lot on autocross unless it is a very long one. I think you are really only heating up the tread instead of the entire carcass of the tire.

But, it becomes obvious how a small change in pressure affects the temperature across the tread. Ideally, you want the same temperature all the way across, proving that the entire contact patch is working. Too little pressure and only the outside works, just the right pressure on the entire surface is working, and too much pressure, the middle and inside are working.

This is all from track or autocross where 90% of the heat is generated by cornering. On the highway, where you are going in a mostly straight line, the heat is generated just by rolling.

My rough guess would be more pressure almost always means more traction in braking (I'm sure there is a point maybe above 50-60 psi where you loose traction). The reason for this is because the weight transfer to the front tires during straight line braking allows the higher pressure to still utilize the entire contact patch.

I think in straight line acceleration, lower pressure is better. This is because the contact patch elongates and puts more rubber in contact while trying to accelerate. I know most drag cars run in the 10-20 psi range.

One last thought on this topic.

If you switch to DOT-R tires for your track days, you have to start all of your thinking over. I know that Michelin Pilot Sport Cup tires typically run in the 20-25 psi range because they are designed for those pressure with extremely stiff sidewalls etc.

So, there are different mountain peaks for different functions :D

-Chris

I've been running at 60psi for awhile. Second car now. Any time the PSI drops to 40-45 the coasting speed and MPG drops off noticeably. I'll never ride lower than that pressure.

I accept the racing data about fall off in traction above some psi, but surely the relevance to hypermiling on streets and highways is limited. The placard pressures take into account maximum design speeds, accelerations, decelerations, pot holes, etc.

Being more about conserving momentum, measured accelerations, avoiding road hazards, and DWB, the worst our tires experience is a 'death turn' which is no worse than slamming on the brakes, turning, then flooring it again.

My experience up to 60 psi on 51 max Fit tires and at 35 and 44 max sidewall on Tacoma and CRV, and all I've read here from others up to 80 to 100 psi makes it clear to me that we will not slide out in a turn and that our tires will not explode. Further, our fuel ecomomy is greatly increased and tire life greatly enhanced. After adjusting to increased NVH, the ride and handling is much better at the higher pressures, too.

Hypermilers are more likely to a) avoid driving in extreme conditions like ice and b) allow more margin for error while driving in such conditions.

When I get to cobblestone streets, I'll lower my pressures a bit, then refill. Don't want large pieces of dashboard falling off.

If race cars need low psi, that's fine with me. So do my riding mower and my rear tractor tires. None of those translate to what my cars do.