Maximum Tire pressure (a police perspective)

We've talked here before about max cold tire inflation pressure. Here's a fascinating article from Officer.com . Some police officers drive with maximum air pressure in their tires as shown on the tire sidewall, not the door sill or the owner's manual.

Here's a link to the article for some great photographs:

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Driving Under Pressure

Proper Tire Pressure Could Save Your Life

Posted: Wednesday, December 21, 2005

SGT. DAVE STORTON EVOC Contributor

Officer.com

How many officers check the tire pressure on their patrol car on a regular basis? We all seem to be great at checking that the lights and siren work, because the time to find out they don't work is not when you get a Code 3 call. Likewise, the time to find out your tire pressure is too low is not when you are in a pursuit and trying to take a corner at high speed.

What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed. This number on the sidewall lists the maximum amount of pressure you should ever put in the tire under normal driving conditions. Pursuits and Code 3 responses are not normal driving conditions. Many agencies maintain tire pressure at 35 psi since this is what is listed in the owner's manual and on the door placard. The reason the owner's manual lists 35 psi is because we get the same manual as the civilian version of the Crown Victoria. The police version, however, is fully loaded with communications equipment, a cage, and your gear. You are not looking for a soft and cushy ride, you want performance.

Myths about pressure

Let's put to rest some common misconceptions. The tires will not balloon out creating a peak in the center portion of the tread when tire pressure is above 35 psi. There is a steel belt that prevents this from happening. Also, you are not overstressing the tire with higher pressure, and the tire will not be forced off the rim with higher pressure. The picture above is Bobby Ore of Bobby Ore Motorsports driving a Ford Ranger on two wheels. The tires on the left side have 100 psi in them, and they happen to be tires and rims from a 1999 Crown Victoria! This is a dramatic example of how pressure holds the tire in shape, and how much stress a tire can handle.

Performance

If you were able to watch a tire as it travels across the ground at high speed, you would see that it deflects to one side during cornering. The faster you are going through a corner, the more tire deflection you get. As the tire deflects over onto the sidewall, you get less traction and more of a tendency to understeer or oversteer. This could spell disaster when negotiating a corner at high speed during a pursuit or a Code 3 run. Higher pressure keeps the tire from deflecting onto the sidewall as much, which keeps more of the treaded portion on the road.

A good demonstration for EVOC instructors is to have students drive a high-speed course in a vehicle with 32 to 35 psi. Then have them run the same course with 44 to 50 psi in the tires. The student will experience a marked difference in performance. Having officers experience this difference in vehicle performance is much more effective than just telling them to check their tire pressure.

Hydroplaning

When a tire rolls across a road covered with water, the tire tread channels water away so the rubber remains in contact with the road. The factors that affect hydroplaning are speed, and water depth. Conventional wisdom says that vehicles will hydroplane in as little as 1/16th of an inch of water. Not so coincidentally, legal tread depth is 1/16th of an inch.

Tire manufactures and the Association of Law Enforcement Emergency Response Trainers International (ALERT) have shown that tires have more of a tendency to hydroplane when pressure is low. This happens because the tire footprint (the portion of the tire actually in contact with the road) is larger. For those of you who water ski, think of which is easier to get up on: a fat ski or a skinny ski. More tire surface in contact with the water makes it easier to hydroplane, just as it is easier to water ski on a fat ski. Also, a soft tire can be pushed in more by the pressure of the water on the center portion of the tread. This results in less rubber in contact with the road.

Tire wear

Much better tire wear results from maintaining proper pressure. Tires with lower pressure will wear off the outside of the tread faster from the deflection of the tire during cornering, and the tires will heat up more from increased road friction. This is one of the factors that caused the failure of a certain brand of tires on Ford Explorers some years ago. In 1999 the San Jose Police Department realized a significant cost savings by increasing the pressure in the training fleet to 50 psi. They soon followed up by increasing the pressure in the patrol fleet to 44 psi. For liability reasons, most agencies are reluctant to exceed the maximum pressure listed on the tire for actual patrol vehicles, but they reap the cost saving when going to 50 psi on training vehicles.

Next time you inspect your vehicle, make sure you check your tire pressure since your ability to performance drive is significantly affected by it. You are not driving to the store to get a loaf of bread! You may be called upon to chase a dangerous criminal or respond to assist another officer in trouble. You don't wonder whether or not your gun is loaded before you hit the street; don't wonder whether your tire pressure is correct once the pursuit starts. Check your tires routinely, just as you do with all other critical equipment.

Reply to
Built_Well
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Well wait, I blew a belt in a new Michelin going to max pressure, max pressure gives less rubber on road that is less safe in stopping. and isnt Max pressure the Max, not cold, hot it can increase 3-4lb

Reply to
ransley

Interesting article....

I can only say this. When I put my 33x9.5" BFG 'mud terrain' tires up to the max on the sidewall, or 50 psi, the amount of tread 'touching' the ground is a strip 3" wide in the middle. This is determined by covering the tread in chalk and driving forward straight.

It will then do 360's faster than you can blink in the rain hard like that....

The measured total width of my tread is 7.5".

When I then go to 'proper' pressure or 30 psi, I then have 7" of my tread touching the ground.

I do not drive at high speed, nor do I take corners like a cop though...

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail
Reply to
Mike Romain

That info may be correct for a CV as it is a fairly heavy vehicle, but it's entirely possible that a smaller, lighter vehicle would respond better to *lower* tire pressure. In fact, the article contradicts itself:

doesn't "less rubber in contact with the road" imply reduced dry weather performance?

As always, I'd take the advice given with a grain of salt. The conclusions reached may be valid *for the specific vehicles under discussion* but certainly can't be extrapolated to the vehicular population as a whole.

nate

Reply to
N8N

Another thing, it will prematurly wear out in the center, besides not being as safe in handling.

Reply to
ransley

I purchased an M35 a year and a half ago. Surprisingly Infiniti, in their "infinite" wisdom decided to install Goodyear RSA tires on these cars. These tires have been blasted left and right by every owner I've spoken with. They're pure crap. During the first month of ownership, on one occassion, I had to drive from Florida to the Washington area, at 55 to avoid vibration. The problem was taken care of by the dealer and I haven't had any issues since, but here's the kicker... The door jamb says to use 33psi cold......the owners manual says to use 35psi cold. The tires say max pressure of 40..... I've used 38 as a norm and the tire wear is pretty even.. Since it's a regular practice for dealer to use a low air pressure setting in the tires to give a soft ride, I can't believe anything except what I find good for me.. The one problem with these w i d e low profile tires is that any change in temperature, especially around the change of seasons, can significantly change your tire pressure, to the tune of up to 5psi. Along with safety issues and the price of 18" tires, it would be very rewarding for you to pay attention to the tire pressures.

Reply to
BobJ

tire pressure is some kind of black art. I can see a cop car wanting max pressure, because they are often driven fully loaded...

Here's my real world experiences:

2001 TA. Manual says 30psi. Unless you're normally driving above 100mph, and then you need to run 38psi. I love that part of the manual. :) 1990 1/2 ton. "Big" 265/75R16 all seasons. "Normal" is 45psi. Pump 'em up to 65 for towing one time, and the truck rides better. Even empty. When I was autoXing, we'd pump up the tires to 35-40 to keep the tire from folding over. Drag racing - my drag radials run around 18-20 pounds. Dirt racing - the guys with bias ply's run like 12 pounds, I run radials and the best grip is around 18-20 pounds cold. the RF goes up to 35-38 by the end of 200 laps.

Ray

Reply to
ray

More reading:

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Reply to
John Kunkel

Bullshit. Over-inflate any tire and it will wear the center first.

Reply to
Mike

Bullshit ! I have used steel belted radials since they came out and have NEVER seen this happen. If you over-inflate the tires they wear in the center, under-inflated they wear on the edges. The steel belts are not solid steel, they are a woven steel mesh and are able to stretch and flex with the rubber in the tire. I have seen customers come in complaining of a harsh ride and the tire pressures have been in the 90 - 120 PSI range and the tread was worn off in the center of the tire, never on the edges. Your misleading information may cost someone their life if they are dumb enough to believe you!

Reply to
Mike

Oh BS, prove it to your dumb ass self, Get water on your driveway, drive throught it and measure the tread width, go pump up your tires

20 lb and do it again, you will have less tread on the ground with more pressure, what touches is what wears.
Reply to
ransley

Aside from riding like an empty lumber truck, wearing out the tire center prematurely, risking a blowout and giving imprecise and jittery handling I see no problem at all running at the maximum sidewall pressure. Gawd what misinformation.

Reply to
John S.

John S. wrote: Gawd what misinformation. ======== Consider the source. I've seen Highway Patrol Crown Victorias zooming by at well over 100 mph on the freeways, plus they mention extra weight of special equipment. Not uncommon to add pressure for high speed use and extra weight. An extra 5 psi for speeds over 100 mph would bring 35 to 40, so they're only adding another 4 psi for additional weight and firmer handling for the police vehicle. The advice to check pressure regularly is good. The mistake is posting to the Camry group. From 35 psi to 44 in a Police Crown Vic is quite different from running 44 psi in the rear tires of a Camry. Roughly 3,000 lbs., split

60/40, F/R, puts only 600 lbs. load on a rear tire, so the tires would be dangerously overinflated in the one case but could be fine in the other specialized use. Factory inflation on the Generation 3 Camry, 4 cyl. is 26 psi. 28/26. F/R unladen seems best overall. Factory says 29 F/R for 4 passengers with luggage. (Those used the 14 inch 70 series tires). Because the tires carry maximum load of 1312 lbs. at 44 psi, doesn't mean one should use that cold inflation pressure to carry less than half that standard load.
Reply to
Daniel

Got a link to that article ?

Nope. Front wheels.

What's your definition of porperly inflated ? If the tires were properly inflated they would wear evenly regarless of which axle they are installed on. Wear on the front edges is not due to steel belted radial design, it has more to due with front end geometry and tire design. Front whell drive and four wheel drive vehicles seem to have more of a problem with this.

The person who's tires are already under-inflated that believes the steel belt tale and lets more air out to get even tread wear. Remember the Ford/Firestone fiasco ?

Reply to
Mike

..

_____________

You know, The WAR IN IRAQ is a less contentious debate than cold tire pressure!

That's 'effin SAD, if you want my honest opinion.

GM, Ford, Toyota, Chrysler, BMW, and all the others do a lot of test driving of their vehicles to come up with the recommended pressures they list on the door pillar or under the trunk lid. And in some cases, as was with my 1981 Buick Century of which I owned two during my college years, the recommended pressure DOES = the maximum allowed pressure on the tire sidewall.

But in the cases where the auto mfg's recommended pressure is, say, 30 F/R or 32F / 34R, then I would think someone is pretty 'effin STUPID to put 45lbs. psi of air in them just because "that's what the TIRE says"!

I like to WATCH basketball, NOT feel like I'm driving one or a passenger in one - thank you very effin' much!

-Chris-"1lb psi cold above door-jamb sticker" - Coaster

Reply to
ChrisCoaster

Pressure is pressure no matter the car. The Crown Vic has much larger tires to compensate for the much larger car, but 45 psi is still too hard for safety.

Reply to
John S.

As someone who has worked on cars professionally for the last 25 years I have to wonder were you get your information from. I have never seen a properly inflated tire wear in the center, NEVER, drive tire or not ! Also, when I say properly inflated I don't mean going by the door jamb sticker. The manufacturer may set the recomended pressure a little low for a better ride and this will cause the tires to wear on the edges. Inflate the tire to proper pressure and the edge wear stops.

- and

Not with properly inflated tires unless you like to corner fast.

On RWD cars, this results in the rear tires wearing in the center

Bullshit ! A properly inflated tire WILL NOT wear in the center! I have seen thousands of tires on all kinds of vehicles and the ONLY cause of centere tread wear in OVERINFLATION !

Front tire edge wear is common on some vehicles due to suspension geometry but MOST vehicles will not show egde wear in normal driving unless the are underinflated.

Thar's why you rotate your tire on a regular basis.

Wrong again. If you read the vehicle owners manual or the tire manufacturers websites you will notice that for sustained high speed driving they recommend you INCREASE the tire pressure. LOW tire pressure causes the tire to flex, which causes heat buildup and can soften the tire and cause it to fail. An overinflated tire will wear in the center but WILL NOT blow out.

+

Not sure what you mean by "appearance" ? If you mean looking at the sidewall to visually determine if the tire is properly inflated then I agree. If you mean by looking at tread wear then I don't agree, and neither do the tire manufacturers. Got to any tire manufacturers website and most will have pictures that show uneven wear on tires due to improper inflation.

Reply to
Mike

You do this on your daily driver ?

Ah, I now see were the problem is ! You are talking about race car tire wear and we are talking about street car tire wear. Of coarse hard cornering will heat up the outer edges of a tire. This will happen on a race car AND street driven vehicles. I like to corner hard and always have edge wear on my front tires even if I up the inflation by a few psi.

Reply to
Mike

This article has been updated. This new article addresses many of the concerns stated here:

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Reply to
dave5150

So basically the updated article simply reiterates, using mathematical form ulas, what the original said.

I have personal eyewitness(or seat-witness) testimony of a friend who compl ained of "something wrong" with her car's ride or its alignment. She said it was very hard to steer accurately and was very bouncy and shake y.

I checked her pressures after about one hour sitting in our driveway. One tire was 50, the others, 47-48psi. Max. cold placard on her tires states 5

1psi. Specified cold sticker on the driver's B-pillar: 30psi F&R.

I lowered all four to exactly 38psi(knowing that they would cool off by nex t morning and would go down). She said her car "never felt this good or wa s so easy to handle". The steering was taut and at same time responsive.

She returned in two days to conduct business with my wife - she sells cookw are - and I checked her pressures after she'd been parked here a while. 35 all around. I left them there, but told her to tell her mechanic to put no more than 36pounds hot in those things!

Maximum cold tire placard does have some utility: Constant use with full l oad of passengers and/or cargo comes to mind, or towing vehicles/vessels. But for most solo driving, or with one/two passenger, max cold on the tire placard could translate to driving a basketball!

I keep my personal vehicle and my wife's at 'plus-two': 2psi over b-pillar door sticker. No complaints from either of us. :)

Reply to
thekmanrocks

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