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Question about tire-pressure for your car.


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#1 of 37 OFFLINE   Ted Lee

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Posted October 24 2001 - 02:35 PM

hi all - all measurements taken when tires are cold here's the deal. my 94 accord says 32psi in the door jamb. yet my tires say 44psi max. when i run my car at 32psi it feels terrible...kinda sluggish or something. when i inflate it to about 36psi it feels much better. am i doing any damage to the car or tires? i guess i'm worried about premature/bad wear on the tires. tia ------------------ "The ship of death has a new captain." - nosferatu (1922)
 

#2 of 37 OFFLINE   Bill Catherall

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Posted October 24 2001 - 02:38 PM

How would a door jam know what kind of tires you're using? Posted Image Go by what the tires say.

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#3 of 37 OFFLINE   Anthony Hom

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Posted October 24 2001 - 03:39 PM

I hope that last comment was a joke. The max tire pressure on the side of a tire is the maximum pressure your tire can go before it blows off your wheels! That may sound a little exxagerated, but it's closer to the truth. The reason the pressure on the door is lower is based on calculations. All tires are very general, few are made for specific cars. The majority of cars out there get the tires that will inflate below the max limit. this is based on the weight of the car and the number of people, avg size of load, etc. They think will go into the car. That could be why you like driving with 36 PSI. If you drive by yourself, with no extra load, it may ride rougher, but if you had a full complement of passengers it won't feel so bad. So in general, the car companies choose a tire that they want to use for a vehicle. then based on weight of the car and max. load, they compute the tire pressure. If it comes too close to the max pressure, they use a bigger tire. If not enough pressure, then they could go to a smaller tire.

#4 of 37 OFFLINE   Bill Catherall

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Posted October 24 2001 - 04:31 PM

[quote]

The max tire pressure on the side of a tire is the maximum pressure your tire can go before it blows off your wheels!

[quote]
No it wasn't a joke. And the max pressure is the pressure you should not exceed when filling. It isn't the burst pressure. The burst pressure is much higher. The engineers use a "factor of safety" when engineering the tires. 44psi max would be their recommended maximum pressure.

Were those Ford Explorer owners going by the psi from the door jam or from the tire? According to Firestone, if the wheels were inflated to their recommended pressure (which is higher than what Ford was recommending) then those tires wouldn't have blown.

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#5 of 37 OFFLINE   Mike Lenthol

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Posted October 24 2001 - 04:59 PM

I inflate to about 5 psi (when hot) under the maximum. Tried doing a 3 gas tank test long long time ago, running like 20 psi, which is when it barely looked like they needed more air, and 39 psi. Higher psi improved mpg about 2-4. Rough ride and too much tire squeal but good mpg Posted Image

#6 of 37 OFFLINE   Todd Hochard

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Posted October 24 2001 - 07:39 PM

Inflating a tire signficantly above the car's (not the tire's) recommended pressure can cause a loss of traction, and a premature wear of the center of the tread. However, most car's pressures are a bit below what might be considered optimum for a given tire/wheel/car combo. Often, these numbers are artifically lowered to improve ride quality (reduce bump harshness), or "improve" traction (as in the case of the Explorer). Running the tires 3-4 psi above suggested should not cause a problem, unless your wheel is a bit narrow for tire width. You might even pick up 1/2mpg or so. In fact, if you routinely run the car at high speed in the summer, the tires should be 3-4psi above, to reduce internal heat buildup (the ultimate demise of the Explorer tires). Running them at the max rated pressure will most certainly shorten the life of the tire, and possibly your shocks. Under no circumstances would I ever run below 30psi on most passenger vehicles. The exception would be the large tires on the limited 4Runners, etc. Ford recommended 24psi for the tires. Ridiculously low (I think Firestone said 28-30 was more reasonable- I agree), and a blatant attempt to try to cover a "flawed" chassis design. Firestone seriously got screwed, IMO, and I think Ford ought to be called on it. I also think that the drivers of the vehicles hold some responsibility. I'd bet over half the tires that went were below 24psi. I see half-inflated tires on the road everyday. Todd [Edited last by Todd Hochard on October 24, 2001 at 10:48 PM]
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#7 of 37 OFFLINE   Denward

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Posted October 24 2001 - 07:57 PM

According to the Car Talk guys (and who can argue with them?), you should take the pressure when the tires are cold and go by the automaker's recommendation on the door jamb. Having said that, I like to inflate about 2 psi above the recommendation because it makes my Saab feel much more agile. Mike, I have no doubt that your MPG test results are real, but 20 psi is surely seriously underinflated so it's really not a fair test. By the time a tire LOOKS low on air, it's FLAT. Ted, if you're really overinflating, you'll notice uneven tread wear with the center of the tire going bald before the edges. I think most people don't check their pressure often enough and get bald edges. If you keep it at 4 psi over recommended, I wouldn't worry. ------------------
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#8 of 37 OFFLINE   Jay H

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Posted October 25 2001 - 03:28 AM

My car's doorjam recommendations are 29 front and 31 rear. But I found that after I replaced the original Bridgestone Turanzas with Dunlop SP8000s (same size) that the tires would rub on the sidewalls when going over dips in the road. Hence, I've since inflated my tires up to about 38psi all around and that has helped a whole lot. The max PSI ratings on the Dunlops is 44psi so I don't see a problem with that at all. I think the Turanzas are more GT and the Dunlops are more of a HP tire and the sidewalls are alot softer than the Bridgestones so I'm getting more sidewall deflection when going into dips (not potholes). So I just put more PSI in than what I ran with the bridgestones... Jay
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#9 of 37 OFFLINE   Jeff Blair

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Posted October 25 2001 - 05:31 AM

I know in my dad's car, a Sterling, it says basicly this. If you are going to be running under 100mph inflate to 32, over 100mph inflate to 34. So there isn't much of a diffrence. I know in my little sports car, I run at about 34-35. It deffently handles better at that pressure. But, I would go the diffrence between what your car says, and the tire. Just my 2 cents. Posted Image

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#10 of 37 OFFLINE   Samuel Des

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Posted October 25 2001 - 05:44 AM

Interesting thread!

[quote]

All tires are very general, few are made for specific cars. The majority of cars out there get the tires that will inflate below the max limit. this is based on the weight of the car and the number of people, avg size of load, etc. They think will go into the car.

[quote]

Does this mean that you should usually go with the tires that came with the car? I once thought that I might get Pirelli's (sp?), but it is a frivolous expense.

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#11 of 37 OFFLINE   Randy Tennison

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Posted October 25 2001 - 05:44 AM

As a former police pursuit driving instructor, and DECAT instructor (Driver Energy Conservation Awareness Training), I can honestly say that you should inflate the tires to the sidewall pressure (cold). No less, no more.

When driving, you want all of the tire you can get in contact with the road. This is what gives you traction. Many people think that lowering the pressure of the tires will put more tire in contact with the road. Actually, the opposite is true. Running your tires at a lower pressure will actually cause the center of the tire rise up, and you will be driving on the outsides of the tire, with less tire touching the road surface(we used to do a demonstration of this at the driving range). The tire surface gives you traction, not the tread in the tire. The tread is there only to prevent hydroplaning on wet surfaces. In dry conditions, tires without tread (slicks) provide the best traction (thus why race drivers run tires without tread).

And absolutely, going a little above the sidewall pressure will not cause the tire to explode. The pressure in the tire will expand as the tire heats up with friction (hot air expands, remember 6th grade science). Therefore, you always inflate the tires cold (or cool). In otherwords, inflate after a very short drive, or a significant cool down period after a long drive.

Many years ago, a man had just bought new radial tires. He was driving down the road, went around a corner, slid off the road, and died in the crash. The family sued the tire manufacturer for his death. The tire manufacturer investigated, and could find nothing which should have caused the wreck. They then retraced the mans steps, and discovered that he had stopped at a filling station. The filling station attendant remembered him, and said he had gotten gas, and the attendant noticed his tires looked a little flat, so he inflated them until they looked right. Radial tires always look a little flat, but he didn't know that. He had inflated them until they looked full. The investigator went to the junk yard, and discovered that the tires had over 100 psi in them (when recommended was 35 psi). They found out that the tires were so overinflated that he was driving only on about an inch of the center of the tire, and that is why he lost traction and crashed. The tires did not explode from the overinflation.

Inflate the tires to the psi which the tire experts (the manufacturer) designed them for. They know better than anyone.

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#12 of 37 ONLINE   Ken_McAlinden

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Posted October 25 2001 - 06:07 AM

Vehicle manufacturers determine recommended tire pressure taking into consideration a number of factors inclusive of safety, handling, and fuel economy for a wide range of environmental conditions. If your vehicle is normally carrying large payloads or hauling a trailer, these characteristics may change. There may be more information about this in the owner's manual in addition to the certification label on the door panel. Regards, ------------------ Ken McAlinden Livonia, MI USA [Edited last by Ken_McAlinden on October 25, 2001 at 12:47 PM]
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#13 of 37 OFFLINE   Ted Lee

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Posted October 25 2001 - 07:22 AM

wow! thanks for all the info and advice. it sounds like i'm not doing any major harm at 36? the tires are definitely wearing evenly (i'm kinda anal about that). randy - i don't think my tires give a definite inflation point...i think it's a range, but i'll double-check. thanks again! ------------------ "The ship of death has a new captain." - nosferatu (1922)
 

#14 of 37 OFFLINE   Kevin Eckhardt

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Posted October 25 2001 - 07:45 AM

JayH: I would recommend that even if you're using a higher PSI than the door jamb states that you maintain the front to rear differential of 2 PSI in the tire pressures. Like doing 36 front and 38 rear. My car, like many front wheel drive cars, also has different suggested inflation amounts for the front and rear tires. Not mainting the difference in pressures can have an effect on handling at higher speeds. Kevin
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#15 of 37 OFFLINE   Scott H

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Posted October 25 2001 - 08:41 PM

First, excuse my huge post Posted Image

I withheld response earlier, as I really didn't want to start an argument with any posters, but there is some very dangerous advice here. Inflating passenger vehicle tire pressures to the maximum indicated on the sidewall of any tire is a recipe for all kinds of problems, possibly catastrophic, just as serious underinflation is. It is also directly against the instructions provided at any tire manufacturers web site, or in their reports and recommendations.

Like Randy, let me explain my experience. Besides experimentation with tire pressure and it's effects on vehicle performance aspects for many years, I have also worked with numerous state police agencies (Highway Patrol) documenting their pursuit driving tactics and instruction courses for both internal use and for television shows. I have also been through such courses myself, in addition to SCCA closed course auto racing and precision driving for television and film production. I still work closely with the CA Highway Patrol regularly, and several R&D departments and racing divisions of top auto and motorcycle manufacturers plus other professional drivers for production (stuntmen and professional race car drivers).

While inflation during course work can vary for any number of intended results, it is not the practice of any state police agency that I am aware of to maintain tire pressure on their service vehicles at the sidewall maximums. I would expect to see pressures no higher than 37 PSI cold on Impala's and Ford CV Interceptor models (which are usually equipped with one of the following tires: Firestone Firehawk PV41; General XP-2000 V4; Goodyear Eagle RS-A). Also, again in my experience, it is also not the practice of the L.A.P.D., who actively engage in more pursuits than any other law enforcement agency in the U.S. I can tell you that having been on hundreds of ride-alongs with state troopers and the many high speed grass/gravel median crossings and regular curb jumping, troopers regularly commented that inflation's on the high side result in a lot of blowouts in those situations. On many occasions we would then pin the car at the normal 130 MPH limit, indicated (Randy, if you know Trooper Curtis A. Martin, Troop 1 HQ, Rolla, MO, I rode with him on several such HS runs). A rough life for tire.


In practice, your consideration of tire pressure is for maintaining an ideal contact patch, tire temperature and structural tire integrity, and suspension and steering component performance. Mileage is a by-product. Overinflated tires drastically effect the suspension, relaying shock and contributing to tire hop. An overinflated tire maintains poor contact with the road and abuses both itself and the suspension, and negatively effects driver control, and steering in particular. In my opinion, anything over 40 PSI hot constitutes over inflation on most passenger vehicles.

An indicated sidewall maximum inflation pressure is just that, and has no relation to vehicle application. It is just a design limit not to be exceeded at any time in conjunction with the associated load limit. All passenger vehicle tires are indicated at either 35 or 44 PSI, it's just a manufacturing standard. Goodyear, like other manufacturers, does not recommend setting the tires to that pressure: http://www.goodyear....flation.html... With 59,000 miles on the tires, and measuring with a wear gauge last month, I have approximately 1/4 tread remaining. The tires are worn perfectly even. I only rotate the tires F/B never changing sides, and I've never had an alignment (but one tire has two internal plug patches in the tread Posted Image).

Anyway, submitted for your consideration Posted Image

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#16 of 37 OFFLINE   Jay H

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Posted October 26 2001 - 05:24 AM

Just to clear up something, when I do drive (I bike commute), I drive an E36 3-series BMW so it is rear wheel drive so the 2psi difference is higher in the rear. I've been slowly working up to about 37/38psi in the Dunlops cause when I was running it at 29/31 as the doorjam recommends, it would rub the rim on the slightest dip, so I have been slowly putting more PSI in the tire. Plus it's still 5PSI lower than the maximum. I'd like to get my suspension looked at soon though, if the PSI isn't helping, then I'd like to see if my shocks/suspension is going. Jay
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#17 of 37 OFFLINE   Ted Lee

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Posted October 26 2001 - 06:46 AM

thanks for the info. one other question: how much psi does a "typical" tire's pressure increase from cold to hot? i think i'm running 195 (or 205)/60/15. if it's 44max on the sidewall and i'm filling it to 35-36 (always cold) do you think i'm doing any harm? ------------------ "The ship of death has a new captain." - nosferatu (1922)
 

#18 of 37 OFFLINE   Todd Hochard

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Posted October 26 2001 - 07:38 AM

[quote]

how much psi does a "typical" tire's pressure increase from cold to hot? i think i'm running 195 (or 205)/60/15.

[quote]I've seen 2-3psi increase going from cold to hot. This on both my Acura TL-S (with 215/50/17s) and Honda Accord (with 195/60/15s). However, that's the most I've seen (on the few times I've measured), and that's typically after driving on the highway for a while.

Keep in mind that increases will vary based on a whole lot of things.

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#19 of 37 OFFLINE   Bill Catherall

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Posted October 26 2001 - 08:24 AM

Thanks for the reply Scott. It's always nice having an expert in the house. Posted Image

I suppose I should clarify my position of using the side wall pressure. What I meant by my statement was, in answering Ted's concern of exceeding the door jam's tire pressure requirements, you're not doing anything wrong as long as you don't exceed the max tire pressure as indicated on the side wall.

quote:
I can tell you that having been on hundreds of ride-alongs with state troopers and the many high speed grass/gravel median crossings and regular curb jumping, troopers regularly commented that inflation's on the high side result in a lot of blowouts in those situations.[/quote]
I don't want to get in an argument with you Scott. And I certainly respect your judgement, as you are a professional and have done many studies on this. But I never drive like that. It sounds like high pressure in the tires (as long as it's not exceeding the max) is only a problem if you're curb jumping or crossing medians. If you're driving a pursuit vehicle then tire pressure becomes a big problem. But if you're peddling along the highway, where's the danger? I hope my "ignorance" doesn't anger you (as compared to your extensive research I am indeed ignorant), but I don't see a problem in exceeding the door jam requirements as long as you don't exceed the max rating. If the car feels sluggish then pump it up. If it's too stiff and bumpy then drop it down.

Even you agree that it's not necessary to follow the vehicle manufacturer's recommendation because you have also inflated 3psi above the recommendation. The thing is most people don't have the know-how, training, or experience to test what the proper pressure is for their car. The simplest solution is to use the door jam recommendation as a minimum and the side wall recommendation as a maximum, then adjust for feel. It sounds like Dunlop's only concern about overinflation is ride stiffness and vibration. I hope I haven't frustrated you by my remarks.

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#20 of 37 ONLINE   Ken_McAlinden

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Posted October 26 2001 - 09:04 AM

[quote]

It sounds like high pressure in the tires (as long as it's not exceeding the max) is only a problem if you're curb jumping or crossing medians.

[quote]I think the point is that the amount of tire that contacts the road is a function of the tire pressure and the axle weight in addition to external factors such as road surface and driving mode. The vehicle manufacturer specs are for the combination of tire and vehicle. The tire manufacturer specs are not adjusted for vehicle interactions.



This is why, as Scott pointed out, front wheel drive vehicles will frequently require higher tire pressures for the front tires since the front axle weight is heavier with the engine and transmission both on top of it.



If you over-inflate, the portion of the tire in contact with the road becomes smaller and the point that does contact the road (center) wears harder. If you under-inflate, the outside surface and sidewall bear more load. There are trade-offs between traction/handling and fuel economy involved as well.



If you lack the expertise to know what is right for your car, you probably are not safe adjusting for "feel", either. You may like the way it feels over or under-inflated while unknowingly having a negative impact on how it performs some emergency maneuvers. I would stick to the vehicle manufacturer's recommendation unless you are routinely carrying substantial payloads, in which case, if your owner's guide does not help, you can probably get by with a few PSI increase.



Regards,



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