and have to ask this question. Do the vehicle manufacturer's door-sticker(or trunk sticker) tire pressure recommendations always, sometimes, or never result in the condition of the tire on the right-hand side("PROPER INFLATION") of the diagram?
If so, is there a method one could use with their own tires, and a chalk line or other marking method, to determine if their treads are meeting the road as in that example?
And, would the tire pressure that result in even wear of the chalk line on the tire in all likelihood be higher or lower than the car manufacturer pressure, or even equal to the maximum inflation pressure on the tire itself?
And finally, if a chalk test is done on tires, should it be done cold or after only enough driving to get to a source of air, or if the tires are a little warmer?
Thanks for your input on this and any other method you may have to determine equal tread contact.
"ChrisCoaster" wrote in message news: snipped-for-privacy@l43g2000hsh.googlegroups.com... On Sep 9, 5:47 pm, ChrisCoaster wrote:
________________ That's it!
The chalk test is probably a waste of time. I know some people swear it is a great idea. If you always drove in a straight line at low speed on flat roads I suppose it might even be valid. But what happens at high speed, in a curve, on a cambered road?
Tire pressure recommendations are not an exact science. Vehicle manufacturers are trying to balance safety, cost, ride comfort, handling, noise, tire life, fuel economy (meeting CAFE rules is a priority), changes in the tire as they age/wear, etc. And they have to do this for a wide variety of drivers with vastly different driving styles and preferences, driving in radically different climates, driving with radically different loads, driving at radically different average speeds, and driving on many different types/qualities of roads. Anybody who thinks driving slowly over a chalk line is going to provide the "right" answer for tire inflation probably has an unrealistically simple view of the world. Fortunately modern radial tires are amazingly forgiving when it comes to variations in tire pressures. Significant variations in tire pressure have much less effect on tread wear than was the case for bias ply tires. Radial tire construction tends to keep the tread relatively flat over a broad range of loads and pressures. You can still under inflate a tire enough to wear the edges, over inflate it enough to wear the center, but the change in pressure necessary to cause extreme variations in wear is much greater than used to be the case. The vehicle manufacturer's pressure recommendations cannot be ideal for all people under all conditions, but they are acceptable for most people under most conditions. If you are driving at higher speeds for long distances, or with the vehicle heavily loaded, then it would be reasonable to go with a slightly higher pressures than recommended (but never in excess of the maximum pressure listed on the tire).
If you want to do the chalk test, I can't see how it would hurt anything. Most likely you will decide to add more air to the tires if you use it. I'd run the test with the tire "warm" but you need to adjust the pressure when they are cold....
Personally my tires last so long these days that I haven't replaced them because of tread wear in years. It seems that after 50,000 or so miles the tires become "bumpy," or I've had to have a couple repaired and I decide I need new tires. At least for me tread depth is not the limiting factor on tire life.
My advice is to start out with the manufacturer's recommended pressure. If you want a stiffer ride add a few pounds of pressure to the tires. Keep your eye on the tire's wear pattern (maybe even check the tread depth across the face of the tire occasionally) and adjust the pressure downward if it appears the center is wearing out faster than the edges. If only one outside edge is wearing, then you should have your alignment checked (or modify your driving style). If the tires are uniformly wearing faster on both outside edges, then maybe a little higher pressure is in order (but never exceed the tire's maximum rated pressure). I would not recommend going lower than the pressure recommended by the vehicle manufacturers. However, the truth is almost all vehicle manufacturer's pressure recommendations are far in excess of the "safe" pressures specified by the load inflation table for a particular tire size and load. Vehicle manufacturer's tend to go with the highest practical pressure recommendation in order to maximize fuel economy. Raising the pressure by 4 or 5 psi over the ideal pressure for ride , handling, and safety may only increase fuel economy by a few tenths of a mile per gallon, but given the pressure to meet CAFE targets, must manufacturers are willing to make the trade off in ride, handling, and safety in order to get that marginal increase in fuel economy.
__________________________ Whew! Ed you have proved with that tome both my esoteric off the wall questions and a cerebral conversation can exist here in the usenet/ google/whatever newsreader you use - world!
Actually Ed, one site recommending the chalkline test instructs you to roll slowly forward, about the length of your own car. Then, get out and see where the most wear is after a few rotations of the tires. WHAT?!
If I chalkline my tires I'm gonna find a level paved surface and get up to at least 20mph for 100feet or more! Are you kidding? ROLL the tires slowly over a couple a revolutions? That would be fine if we all drove like that.
Keeping my car's tires(mind you, these are after-market but original size Yokohama H-rated Avids) at 30 or 31psi has resulted in noticeable shoulder wear - even wear to both shoulders of all four tires, meaning my alignment is good - but noticeable just the same. That is what GM recommends for the 2005 - 2007 Malibu with size P-205/65-R15 tires.
I think I'll try 32 or 33psi and see what happens.
Again, thanks for your well thought out reply - THERE IS HOPE AFTER ALL! :)
On Sep 11, 5:09 pm, "C. E. White" wrote: However, the truth
_________________________ Really? I don't know about that. Like I said, the sticker on the Malibu's doorjamb recommends 30psi for the size of tire in my other reply. Both the factory originals and these Yokohamas I put on August
2007 have 35psi listed as their maximum inflation pressure. Doesn't sound like GM is over specing the tire pressure to me.
The 35 psi maximum pressure for the tire has nothing to do with the right pressure for a particular application. There are industry standard tire pressure versus load tables for most tire sizes. Take a look at
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. A P205/65R15 is rated to carry the following loads at the following pressures: Pressure Rated Load
26 psi 1213 lb
29 psi 1279 lb
32 psi 1334 lb
35 psi 1400 lb
(these rating are for passenger car use, if the tires are mounted on a light truck, load ratings should be reduced by 10%).
A 2005 Malibu has a maximum loaded weight of somewhere around 4100 lb. Assuming a 60% front / 40% rear load division, the maximum weight on one front tire is around 1,230 lb. This corresponds to pressure of around 28 psi. At the recommended inflation pressure of 30 psi, the tires are able to safety support around 1,300 lb each. This is around
5% greater than required for a Malibu loaded to the max. For a Malibu normally loaded (say 3,600 lb), the margin is even greater (more than
15% - at least 5 psi). The load inflation tables for the tire already includes a safety factor. But they do not necessarily reflect high speed driving. For the US, the speed limits are so low that as long as you are at least close to the limit, you don't have to worry. But, if you are going to do a lot of 100 mph driving, higher pressure would be advisable.
I have noticed that among US manufacturers that GM tend to run closer to the recommend pressure for the max load than Ford or Chrysler. GM tends to provide smaller tires as standard and run them at higher pressures than the others. Not sure why.
As someone else pointed out, the diagram is pretty much irrelevant for radial tires.
As it was once explained to me by a tire guy: A tire is designed to run with a certain contact patch, which you get when it's carrying max. rated load at max. rated pressure. If it's carrying, say, 80% of max. rated load, then the ideal pressure is 80% of the max. rating, which gives you the same contact patch.
On cars with standard size tires, I generally stay with the recommended pressure, or maybe a bit over for one reason or another. On trucks, especially with larger than standard tires, I use the above as a guideline, and it's worked well for many years.
Which in my case, is what I'm doing. I'm keeping 32lbs because
30psi(General Motors) results in rapid, but even, shoulder wear on all four tires, but mostly in the fronts. Car rolls longer, fuel economy is ^, and handling is a little tighter.
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