"Inflation pressure does not affect grip": Autospeed article.

[dcb]

ok, so hows this sound for a quick test.

let the air out of a tire and put a strait edge across the tread on top and pump it up and watch? Not the same as under load, but will demonstrate deformation/psi trend.

I'm still not sure how big of a problem this actually is. I always get the cheap tires and don't have any handling/wear problems to report either at higher pressures (~45psi, ~2500 lb vehicle).

[CapriRacer]

Originally Posted by dcb

ok, so hows this sound for a quick test.

let the air out of a tire and put a strait edge across the tread on top and pump it up and watch? Not the same as under load, but will demonstrate deformation/psi trend.

I'm still not sure how big of a problem this actually is. I always get the cheap tires and don't have any handling/wear problems to report either at higher pressures (~45psi, ~2500 lb vehicle).

Good thinking. It's simple and while it isn't measuring exactly the same thing, it's measuring something analogous.

But I would suggest measuring the higher pressure first. From experience, we've learned that tires grow - and obviously higher pressure generate more growth. With brand new tires, it is a common industry practice to measure tires after 24 hours worth of growth - we even call it that.

We've also experienced that adding 5 psi into a tire yields less than 5 psi 24 hours later.

I do have one concern. While I haven't actually measured this, I am reasonably confident that the axles aren't centered on loaded tires. Put another way, in a tire under load, not only does the part of the tire in contact with the ground deform, but so does the portion 180° away. The "top" part of the tire is stretched ever so slightly radially- and that results in the sidewalls beuing sucked in - and that will likely distort the tread face.

I would suspect that increased inflation pressure would decrease the amount of "sag" of the axle and result in less distortion. So it is possible that the results could go either way. Personally, I think my concern is unwarranted, but if we do not get the results each of us is expecting, that might be an area worth exploring.

[gone-ot]

...the Gross Vechicle Weight Rating (GVWR) on the door sticker of my '09 Vibe is 3860 lbs, so I did a little 'number crunching' to see how close the other numbers related to that value:

1) door sticker tire pressure = 32 psi, front and rear
2) tire sidewall "maximum" air pressure = 44 psi
3) tire 'load index' value = 89, which equates to 1279 lbs at "rated" air pressure
4) so: (32psi / 44 psi)*(1279lbs / tire)*(4 tires) = 3720 lbs
5) that's 96% of GVWR, ie: (3720lbs / 3860 lbs) = 0.96
6) looks "close enuf" for decent approximation!

[CapriRacer]

Originally Posted by Old Tele man

...the Gross Vechicle Weight Rating (GVWR) on the door sticker of my '09 Vibe is 3860 lbs, so I did a little 'number crunching' to see how close the other numbers related to that value:

1) door sticker tire pressure = 32 psi, front and rear
2) tire sidewall "maximum" air pressure = 44 psi
3) tire 'load index' value = 89, which equates to 1279 lbs at maximum air pressure
4) so: (32psi / 44 psi)*(1279lbs / tire)*(4 tires) = 3720 lbs
5) that's 96% of GVWR, ie: (3720lbs / 3860 lbs) = 0.96
6) looks "close enuf" for decent approximation!

A couple of comments:

For item #4: The "rated pressure" is 35 psi, not 44 - and the load is NOT proportional to the inflation pressure.

The actual load carrying capacity of a P205/55R16 89H at 32 psi is 1157# (for a grand total of 4628#) or the tires at GVW (assuming all 4 tires carry the same load when the vehicle is fully loaded!) is 83% of the GVW.

The way it was calculated in the quote should be raised some questions as the estimated load carrying capacity of the tires would be LESS than the GVW.

[gone-ot]

...thanks for the corrections. I see now that the TRA Load Index (LI) number is specified at 35 psi (for passenger tires), not at the tires sidewall "maximum" value (44 psi for this tire).

...however, because many people here are talking about running their tires at the maximum sidewall pressure, the "approximation" is both valid and useful for "estimating" how tire pressures vary with load.

...and, yes, the relationship between the Load Index number and weight isn't linear, it's actually an exponential function that is roughly:

LI = 35*LN(lbs) - 159

[wdb]

I'm all for running tire pressures above manufacturer recommendations. I'm decidedly not in favor of running them so high that safety -- for the occupants as well as those who might encounter them on the road -- is thrown out the window. I refer to the extreme pressures - 60psi and the like - run by some folks in search of MPG improvement. There's a sensible compromise between that and manufacturer recommendations.

Originally Posted by Christ

I run Gerald (1/2 ton F150) with 50-60PSI [...] the fronts do the majority of the braking anyway. I would lose little or nearly none of my braking power (unloaded) by losing traction in the rear of the truck while braking, [...]

(Tire pressure quoted for next comment.) That all assumes that you are stopping in a straight line on a smooth road. That is not often the case in normal situations, let alone emergencies.

Originally Posted by orange4boy

More interesting tire info. Especially if you want to drive on the Autobahn.

Note that the recommendations top out at 7.5psi above manufacturer recommendations. That's going to be in the 38-44psi range for most vehicles. A long way from 60psi.

[CapriRacer]

Originally Posted by wdb

....... I refer to the extreme pressures - 60psi and the like.....

A couple more thoughts.

First believe it or not, there are F-150's where the placard pressure specifies 50 to 60 psi. But this is for an LT245/70R17, which are designed for that kind of pressure.

Some posters have mentioned honest to goodness truck tires (18 wheeler variety) being inflated to 85 psi - and of course, these types of tires are designed for use up to 100 to 120 psi.

It's really hard to tell what is going on if only the pressure is posted without any context. So be careful what you read.

[gone-ot]

...here's a TRA table showing truck tire load (weight) vs. psi:

http://www.olderogtire.com/pdf/light...ion_tables.pdf

[Christ]

Originally Posted by wdb

(Tire pressure quoted for next comment.) That all assumes that you are stopping in a straight line on a smooth road. That is not often the case in normal situations, let alone emergencies.

No, it assumes I'm driving an unloaded truck. In turns and emergency scenario braking, you lose even more of your rear braking power when driving unloaded, because you're more likely to make the rear end slide when braking heavily. This is the point of Ford's RABS (rear ABS) system, to prevent lockup of both rear tires (one solenoid controls both rear tires on the same line, PITA setup that I fully plan on removing).

IIRC, the actual braking force on several trucks is set so that only something like 30% of the usable pressure of the master cylinder is transmitted to the rear brakes, and the brakes themselves are larger in the rear so that even less PSI is transferred from the wheel cylinders to the drums to help prevent this scenario.