Re: Tire pressure vs ride

news wrote:


======= That's a good question. The recommended tire pressure in my manual for an '06 Camry LE is 29 psi. 29 psi also appears on the sticker in the door jam.
29 psi produces a nice, smooth ride. When I pumped up the stock P205/65R15 tires by 5 psi to 34 psi, I definitely experienced a rougher ride--could easily feel the bumps. Didn't like it much at all. But my gas mileage seemed to improve from 20 mpg city driving to 24 mpg city driving, a 20 percent improvement. 20 percent seems like a lot. Is that even possible? I was expecting 5 percent at most.
Anyway, I next pumped up the tires to 38 psi, 9 psi over the recommended 29 psi. The stock Goodyear Integrity tires have a maximum cold tire inflation pressure of 44 psi, according to the sidewall, so I'm within the safety margin. Well, I don't know if it's just in my mind, but 38 psi feels smoother to me than 34 psi--I don't feel the bumps as much, and I like the ride as much as at the recommended 29 psi.
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Built_Well wrote:

I generally run our cars at about 5 psi (cold) below the maximum - the figure posted on the car is almost always too low for both fuel economy and a good safety margin. I get similar improvements in economy. As for ride, different vehicles seem to be affected differently. I suspect it may have something to do with shock/strut damping rates interacting with sidewall damping rates at various pressures, but that's only a guess. One other thing I've noticed: the increased wear I regularly get warned about never happens - we always take our tires off because of extreme age, with plenty of even tread left on them...
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mjc13<REMOVETHIS> wrote:> I generally run our cars at about 5 psi (cold) below the maximum -

======= Yeah, I guess there's suppose to be balding that occurs with over-pumped tires (loss of tread in the middle of the tire). Nice to hear that's not necessarily so.
I was thinking of going back to 34 psi from 38 (29 being recommended by the manual), but now that I hear you're not experiencing problems, I might just leave'm at 38 psi.
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I am using 31-32 psi. I never tried going more than that. Maybe I should try it too. However, many recommend lowering pressure in snow or wet roads, and its been raining in NJ for sometime now.
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EdV wrote:

Wow....
Unlike some I seem to be reading the right 'books' and running tires fully loaded with air when the vehicle weight doesn't need that is not only wrong, it is dangerous.
When I overinflate my tires by as little as 8 psi, I only end up with 3 1/2" of tread in the center of the tire touching ground. They will then spin really easy and slide out easy. The U shape this causes also lets water under the edges of the tire so it hydroplanes like mad. Very Very unsafe!
When I run proper pressure for the load, in my case 28-29 psi around town and 32 loaded for highway trips, I then get a nice 7" of the available 7.5" of tread touching the ground and no issues in the rain. My tires will take 44 psi 'if' I put that kind of load on it to 'need' that much air.
I 'also' 'use' my tires, I don't change them from age, so I actually 'see' the wear patterns.
Proper inflation allows my $225.00 each, tires to wear all the way down evenly.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's - Gone to the rust pile...
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Mike Romain wrote:

The more I think about this issue, the more it occurs to me that the automotive engineer who specified the tire pressure for my make and model of car and my tire size probably knows more about proper inflation than the wrench down the street who says to inflate to the maximum allowable pressure shown on the sidewall. So I look at the sticker behind the car door and inflate accordingly.
--
One meter, to within 0.0125% accuracy (off by just under .005 inches):
Three feet
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Very well said, I never thought of it that way. I tend to over inflate a few psi more because I imagine tire gauges have some reading inconsistencies and the fact that my tires still *looks* like it still needs some air. I'm thinking over inflation would have the lesser evil compared to a under inflated tire.
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_____________________ Me too. Only I overinflate the door pressure by 1-2lbs depending on the season and the handling of the car I'm driving. Just a tip on your tires *looking* like they need more air: Radials tend to look a little "flatter" than equivalent sized bias plies set to the same pressure.
-CC
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Five lbs makes quite a difference in ride. I keep mine at five under max too, at around 30 lbs
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EdV wrote:

Rather than compensate for an inaccurate gauge, I decided to slog around until I got a gauge I trust. I have a friend who's almost anal about tire pressure (yet who ignores the manufacturer's recommendations), and my gauge reads the same as his three favorite gauges.
Bought a cool new compressor from Costco a while back and just got around to trying it. The compressor is great, but the gauge on it is off by a lot; it read about 27 when I got the tire to 32PSI by my aforementioned tested gauge.
--
One meter, to within 0.0125% accuracy (off by just under .005 inches):
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clifto wrote:

Except these people all have a say in the tire pressure: Marketing. "Dubs" for everyone! The people who mandate that the OEM tires last through the warranty period. The company lawyers who don't want a Ford/Firestone repeat. The fuel economy guys. The handling department. The NVH department.
No, I'm not smarter than the car company, but I'm also not bound by the same rules they are.
I like to use the cold air intake (intake lid) on my Trans Am. Aftermarket ones are worth 10hp. How can GM do such a lousy job? In this case, it's induction noise - the GM piece has a big honkin' resonator on it and the aftermarket ones don't. More air coming in more noise. Also, the GM one probably cost them $15 or less, mine was $100. (FWIW, I also didn't want to cut up a factory piece when installing nitrous, and I can confirm the intake lid is worth a tenth at the strip.)
It's all a tradeoff.
Ray
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wrote:

Indeed; the "story" on the FordFirestoneFiasco was that lower than optimal pressure was specified specifically to reduce tire grip, in order to prevent getting enough side Gs to tip the beast over. Unfortunately, when that came up against a run of tires on the low side of the safety margin for abuse, due to management problems at the factory, they blew.
"For example, the documents show that Ford lowered its recommended tire pressure on the 15-inch Firestone P235 ATX tires to improve the Explorer's stability after engineering simulations showed it failing J-turn emergency-avoidance maneuvers in 1989. The lower tire pressure was considered a critical component of the Explorer's safe handling, enough so that Ford weighed adding a warning sticker inside the vehicle alerting occupants that a tire inflation level of 26 pounds per square inch was "required" to help prevent "loss of control, rollover and serious injury." The warning labels were never added, however. A Ford spokesman said Tuesday that the company made design and suspension modifications in the final months before the Explorer's February 1990 launch that obviated the need. Ford considered and rejected several major design changes, including widening the Explorer by 2 inches to lower its center of gravity. Instead, Ford lowered the vehicle by one-half inch and stiffened its front suspension springs. "The engineering team felt they had achieved their goal of becoming a safety leader, so there was not the need for that warning label," said Ford spokesman Jon Harmon. Nevertheless, Ford's decision to lower the recommended tire pressure on its Explorer tires to 26 pounds per square inch, from 30 pounds per square inch recommended by Firestone, is expected to figure prominently when a fourth congressional hearing over the Firestone tire crisis begins tomorrow in Washington. -Role of Ford Explorer Design Is Studied In Connection With Firestone Tire Suits THE WALL STREET JOURNAL September 20, 2000
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I'd use the recommended pressure. No less. No more. Yeah, Fords/ Firestones were just too bad, unfortunately.
Michelin on tire life and air pressure: http://www.michelinman.com/tire-care/tire-saving-tips / http://www.michelinman.com/tire-care/tire-basics /

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I'd use specified pressure. No more, no less.
Michelin tire life and air pressure: http://www.michelinman.com/tire-care/tire-saving-tips /

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They say that to protect themselves against lawsuits, since changing the air pressure can have an adverse affect on rollover tendency of a vehicle, especially SUV's and trucks. Trust me, there are many rollover accidents and deaths each year, and some of those are with Michelin tires. If Michelin started recommending tire pressures different from the vehicle manufacturer, they would be sued for hundreds of millions of dollars each year as a result of the accidents and deaths.
With regard to their rotation diagram, it is clearly wrong since you never want to cross tire from one side to the other. It causes a horribly harsh ride when you do that.
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On 10/27/07 12:45 AM, in article WfAUi.46758$ snipped-for-privacy@bignews8.bellsouth.net,

Huh? Total BS.
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clifto wrote:

Has anyone at all said to inflate to the maximum? Was the tire you're using now on the market when your car was designed? Do you think that all makes and models of tire were tested by this hypothetical engineer...?
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On Sun, 28 Oct 2007 10:13:27 GMT, "mjc13<REMOVETHIS>"

Well, some people do say to blindly "Inflate to the tire sidewall maximum", but thankfully it's getting rarer as knowledge is diffused. That maximum pressure is very rarely going to get you the optimum ride, traction and tread life, usually it will make things much worse.
If the car still has the exact OEM size and type tires on it, then the car maker recommendations on the door sticker are still valid. But people often switch to different size, speed rating or load rating tires and sometimes rim size is also changed, and then you need to (metaphorically) throw the car sticker out the window and go with the tire maker's Load & Pressure Chart as the primary reference.
The one thing that is important about the manufacturer label is any front/rear pressure biases, usually related to weight distribution in the vehicle - the tires at the heavier end of the car are usually kept higher. If you are figuring pressure for new tires place the same ratio of bias between the axles, even if the overall pressures are higher or lower because of the different tire type.
--<< Bruce >>--
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On Sun, 28 Oct 2007 12:15:27 -0700, Bruce L. Bergman

The sticker the car manufacturer puts on the door regarding tire pressure is only for the tires that originally came on the car. When those tires are replaced, you should go with the tire manufacturer's recommendation on tire pressure. The weight-bias tire pressures between front and back might be different also. You can ball-park the pressure based on the max pressure on the sidewall or you can take the car to a grain elevator/truck scale and weigh the front and back axles. Then look up the recommended tire pressure for that weight. The tire dealer should have the pressure chart.
Jack
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