Correct tire pressure???

[CapriRacer]

You know about Murphy's Law?

Immediately after I posted that statement, I found a non-German car that also had several inflation pressures listed on the placard. I called some folks in Europe and they informed me that it is common practice over there to list an empty and a fully loaded inflation pressure.

What I haven't done yet is to canvass Asian vehicle manufacturers.

But nevertheless, I stand corrected.

 

[EK]

Inflation pressure is dependent on tire load rating and load. The mfr specifies a tire and inflation pressure that meets the load rating requirement. Typically the inflation pressure is at max load for safety reasons.

This is the most comprehensive guide on light truck inflation pressure that I have found...

http://www.goodyear.com/truck/pdf/LoadInflationBook.pdf

For light truck tires, I have found that tires get better tire life when run at inflation pressures in the table corresponding to the actual load. The traction is also better.

 

[CapriRacer]

Tireman said:

"Be careful in foillowing some of the info here. It is not correct.
Example "LT265/75R16 neds 40 psi to carry same load as P245/75R16 at 35 psi" WRONG the LT is rated at 2100 at 40 and the P245 is rated 2271 at 35.

Sorry to point this out, but I think you forgot to apply the 10% derating that P metric tires are supposed to use when used in a Light Truck application (which all LT's would be!) So technically, the LT265/75R16 needs 38 1/2 psi for the same load.

 

[Tireman]

I have seen a number of minor misconceptions in this series.
Tire load capacity is based by industry standards on the size and inflation. Not on brand or load range. "Plies" went away in the 70's so should not be used
The max calculated load is based on the the max inflation for the specified load range.
The max inflation pressure on the sidewall should NEVER be exceeded
Since some OE vehicle manufacturers shoose the smallest tire ( lowest reserve load capability) they leave zero reserve so you should never run any lower than the OE specified inflation (on the placard).
I am a tire engineer with 36 years experience and have access to the industry load tables.
Be careful in foillowing some of the info here. It is not correct.
Example "LT265/75R16 neds 40 psi to carry same load as P245/75R16 at 35 psi" WRONG the LT is rated at 2100 at 40 and the P245 is rated 2271 at 35.

 

[Tireman]

Tim H stated

quote:

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This is not neccessarly true, the 'plys' on the sidewall, while the same in amount, can vary in thickness and even with the same amount of air pressure, can vastly effect the way a car handles.

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He then suggested buying some 4 ply tires.
If anyone is offering you passenger ties based on "plies" run away as fast as you can cuz that person either doesn't know a thing about tires or is trying to sell you some 25 year old tires.

Most current radials have 1 or 2 body plies with the 1 ply tires being in general P205 or P215 and smaller.
The primary source of handling is in the tread and belt package with increased belt stiffness and stiffer tread comopound and wider tread for equal tire section generally used to give improved handling (for street tires).

 

[Mitch Alsup]

If you want to determin the proper inflation for you vehicle, your driving habits and your driving environment, there is a handy dandy little device called a probe-tipped tire pyrometer. Using this device, you can determine the proper tire inflation pressure (hot or cold) for stock tires, or any aftermarket tires you happen to put on the vehicle. As a side benefit, you can also determine if the alighment is correct and even if the factory alignment specifications are correct (once again) for your vehicle, your driving habits, and your driving environment. {This is the high tech version of the chalk across the tread mentioned near the beginning of this thread. Note also, white shoe polish works just as well}

Tire pressure: A tire is correctly inflated when the whole tread participates in delivering loads through the contact patch. When this happens, there will be an even temperature profile accross the tire. So measure the temperature at the outermost edge, the center,a nd the innermost edge. If the center is at the average temperature of the outer and inner edges, the tire is correctly inflated. If the center temperature is lower than average, the tire is under inflated; it it is higher, the tire is overinflated. Since all of this is done at operating temperatures, to get the cold temperatures, drive home and let the car sit for 4 hours or overnight then measure the pressures.

Camber: Camber is correct when the inner edge is hotter than the outer edge by about 10dF street applications or 20dF racing applications (closed wheel cars). The outer edge of the tire is cooled by the rush of air across its edge, the inner edge is not cooled in this manner, settingup the temperature difference. Also note: racing applications may desire more or less camber to achieve these operating conditions (mostly more). If you end up altering the camber settings, you may need to change the inflation pressure since you have just changed the amount of load the tire can carry and thereby its temperature which alters its pressure.

Toe: Toe is correct when there is no thrust from the combination of all 4 tires on the road in straight level driving, and when the 4 tires are similarly loaded (e.g. same temps side to side). Cars needing toe adjustments outside of factory spec probably need corner weighting to re-equalize the load each tire carries. Toe maladies show up as one tire running several degrees hotter than its neighbor on the same axel (street applications).

{IR reading temperature guages are, first order, OK. But since tires cool rapidly on their skins where the IR guns read the temp, you have to be very fast to get an accurate reading. The probe-tipped pyrometer reaches under the rubber surface to ge the actual operating temperatures}

 

[CapriRacer]

Mitch,

I was wondering what results who've obtained using the temperature method. In particular, what pressure did you determine compared to the placard?

 

[Mitch Alsup]

CapriRacer Wrote:
Mitch,

I was wondering what results who've obtained using the temperature method. In particular, what pressure did you determine compared to the placard?

I reply:

For the car I have done extensive work upon (Ferrari F55B) it was more of getting the suspension aligned properly to the correct operating behavior of the car. (There were several things wrong with the suspension when I got the car--and all were fixed without replacing any suspension parts). When I got the car it would oversteer (basically) all the time! Lots of fun, but you could never relax while driving it.

I found a combination of tire pressures that calmed the car to a reasonable degree. RF 33 PSI, LF 31 PSI, RR 31 PSI, LR 33-34 PSI. It still oversteered, but no longer pulled to the right under throttle and left under brakes, and was generally driveable.

But to be fair, the factory placard in my car has 3 different tire combinations and three different air pressure recommendations, everything from 29/32 to 31/31 depending upon the maker of the (1995) tires. So, I set out to find that combination of {ride heights, suspension alignments, and tire pressures} that would result in long life with sticky tires under both street and track applications.

I ended up setting up the suspension so that the proper cold tire pressures are 32 PSI (nitrogen) front and rear. Track applications with (wet) air are 29 PSI (overnight in garage) cold (31 track cool); and allways 40 PSI hot off the track.

Since this car has adjustible ride heights, and since adjusting rear ride height controls the roll couple distribution, I can adjust oversteer/understeer with rear ride height rather than tire pressure. So, I diddled with it (raised front, lowered rear) until it felt right at 32 PSI all 4 corners. At the track, camber was set for a nice temperature profile across the tread; and toe set for dead on center resonse. (coincidentally I get a nice temperature profile on the street also.) I am currently getting 15K+ miles on a set of Bridgestone S03s (and 13 track days--I only get 5 track days on a set (8) of brake pads). Many people won't think of this as good milage, but if you knew how I drive at the track you would be amazed that I get 1/2 of this milage (many slower track drivers don't)

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For drivers who are quite sensitive to their cars' feedback, there is a procedure where you drive a single road many times and can determine the correct overall tire pressures and the correct front to rear tire pressures.

The basic procedure is to fill up the tires to an obviously overly high pressure (say 40 PSI) and then drive a road over and over slowly letting air out of the tire(s) until the best results are obtained.

The first order of business is finding a road where you can drive at a consistently at speed many many times (a nice untraveled country road works great.) It is best if this procedure is delayed until you get all 4 tires replaced so you are starting on a set of tires that are not worn improperly. It is also paramount that you use a dial air pressure guage so you can read better than 0.5 PSI changes in air pressure and make adjustments at this level of accuracy.

First: make 5 passes over this (at leat 5 mile strech) recording you notes on an actual sheet of paper! How did the car feel under acceleration, braking, turning left, turning right, on-center feel?

Then if the car feels 'loose' let some air out of the front tires, if it feels 'tight', let some air out of the rear tires. Both left and right are adjusted in 2 PSI increments (downward) until the balance is about correct, Then in 1 PSI increments.

Now that balance is about correct, it is time to find the point of best traction--and here sensitivity is paramount. Drop the pressures in 1 PSI increments on all 4 corners until you feel the traction start to go down. Then find a filling station and put 2 PSI more in each tire. This will put you very close to the optimal tire pressures--ASSUMING the suspension is in order, and aligned properly.

Now go home and the next morning, measure the (now cold) tire pressures.

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But the real point to all of this is that the driver can find the proper tire pressures for his vehicle, the way he drives, in the conditions underwhich he/she drives! No one set of tire pressures is correct for all (pick random tire size) 225/50ZR16 on a V6 Toyota Camry. It will vary from brand to brand and even on batch to batch of the exact same serial number from the tire moulds! (not much but it does vary).

The factory numbers are simply a starting point (a safe starting point) but almost invariably on the soft side trading tire life and tire grip for ride. If this is not the optimal choice for you and your driving conditions, at last now you have a procedure to follow to find what is better for you under your driving conditions.

 

[CapriRacer]

Mitch,

My only concern here is that there is a load carrying capacity issue when you deal with a tire's inflation pressure, and I'm afraid that many unsophisticated readers will not understand that the "temperature method" (or the "chalk method" or the "pressure buildup method") should not be taken unilaterally. This information has to be viewed as part of a bigger picture.

For example, racecars are set up using the temperature method. But race tires generally aren't used long enough to have tire durability issues, but when they do, it's usually because of an excessive temperature issue.

When dealing with street tires, long term durability has to be considered. And proper inflation pressure is part of that consideration. I would hate it if one of our readers had an accident (or worse) because of what he thought he read on this web site.