CSC MEETING 10.19.12 Air Down Clinic

[LandCruiserPhil]

CSC Monthly Meeting

On 10.19.2012 I will be holding an Air Down Clinic starting at 7:30

Overview
Why air down
How to air down
How low can you go
How low do you go
How to air up



Live demonstrations
I will be letting air out of my tire
I will be putting back in my tire
I will duplicate the above while on top of a foreign object

 

[Blues Dawg]

Boy - and I wasn't going to make it! I may have to rearrange my schedule...

 

[LandCruiserPhil]

How much air should I be running on the street?

Another commonly asked question on tire pressure. Below is the best answer I have found given the variables with tire size, vehicle weight, and tire load rating.


I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.
The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).
The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.
If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.
To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressure in the tire. It should be higher than we cold but no more than 10% higher.
Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR. In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.
On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.
So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.

 

[murf]

Tires are a great topic (I myself working for Goodyear tire for 10 years) and are one of the most neglected items on many peoples vehicles (general populace). Dear ol' Dad always said there are two things you Must have good your rig to be safe: Brakes and Tires.

Yes, always err on the side of overinflation vs underinflation like Phil says, its all about rolling resistance and the resulting heat generation. Thats why when tires get a nail and run low for miles the result is a extreme overheating condition and the tire "blows out" as a result of too much heat generation from running so low.

Again like Phil points out, one would also need to consider weight of the vehicle vs inflation. I had a CJ7 (gasp!) back in the day with 10" wide wheels and 331250's. The back of the vehicle was so light that there wasnt enough weight to get the tire to lay flat. (also happens on pickups alot). This results in only the center section of the rear contacting the road. Ive seen many tires (esp on those that arent rotated) that have twice as much wear on the center than on the outside. This would also give the opposite effect on outside wear on overloaded tires where the middle is thick and the edges are worn down.

One trick is to get your rear tires good and wet then drive over a dry parking lot to see your contact patch is by where the dust sticks to the wet tire; then air down to where you finally start getting the majority of the tire face touching the surface as shown on the tire where it picks up the dust. The rear inflation on my Jeep was almost half of what I ran in the front tires. All the front heavy vehicles in my families fleet run more in front and less in back (front wheel drive vehicles, pickups, etc) The LC not so much being so heavy all around.

I frequently chk my pressures, rotate my tires on schedule, and keep an eye on my tires for cuts and nails.

Now that were going from summer to fall temps, tire pressures can drop measureably. Last week we got a multitude of calls here at the dealership about tire lights coming on due to the change in temps.

My .02 FWIW. Good topic

 

[Cee Fish]

...Now that were going from summer to fall temps, tire pressures can drop measureably. Last week we got a multitude of calls here at the dealership about tire lights coming on due to the change in temps....

My .02 FWIW. Good topic

Good point Eric, every 10 degree change in temp results in ~1lb of air pressure (more if you run oversize tires).

 

[NLXTACY]

Phil you should record it for the class next time

 

[BrotherRob]

Phil you should record it for the class next time

Put it on a DVD and sell it.

 

[sdnative]

Record it and put it on youtube? Would love to see it.

Good info, I'll have to test mine. I usually just fill it to 45 and forget it.

A recent thread on treadwrights got me thinking. The site says airing down will void any warranty. So does airing down damage the tire?

 

[BrotherRob]

Record it and put it on youtube? Would love to see it.

Good info, I'll have to test mine. I usually just fill it to 45 and forget it.

A recent thread on treadwrights got me thinking. The site says airing down will void any warranty. So does airing down damage the tire?

On the highway yes. Offroad, it's more likely to prevent damage.

 

[Tools R Us]

...
A recent thread on treadwrights got me thinking. The site says airing down will void any warranty.

They are retreads, used tires, shows how much faith they have in them.

So does airing down damage the tire?

It's about speed, the best air pressure is speed dependent. For around town type driving, slightly lower is better, for extended highway speed driving slightly higher. At low speed, low air pressures are not going to harm the tire, but even a short distance at highway speed can overheat/damage tires.

 

[LandCruiserPhil]

Important detail I failed to cover after reviewing my notes:

The length of the contact patch provides the flotation and the width of the contact patch effects the rolling resistance. The balance of the two for the type of terrain traveled is what you are after. A lot more information available if you look for skinny verses wide tire articles.

As for air pressure and tire damage is will always come back to heat generation being the damaging factor.

 

[Otter]

Prior to the run yesterday, LandcruiserPhil and I experimented with the air pressure in my tires. I'm running 33x12.50x15s, and was recommended to run them at 38psi by the installer.

Before leaving the meeting spot (Sprouts at 94th and Shea), we aired my tires down to 25psi on the driver side, and 29 psi on the passenger side. We rechecked the pressure at Baja. The driver side was up to 27 or so and the passenger side went up to 31psi. The recommendation is to be within 10%, so we set mine at 30psi. The ride was much improved on the way to Mt. Ord.

The plus is now I air down to 15psi and takes almost no time to air back up ;p

Thanks for the help Phil.

 

[1leglance]

an old trick I have used for years is the chalk & parking lot test...

Go to a parking lot where you can do straights about 100ft long
Take a stick or 2 of the fat sidewalk chalk kids use (get at the dollar store)
Draw lines across your tire tread...I do about 5-7 of them
Drive in a straight line
Now look at the wear of the marks....
Gone in the middle equals over inflation
Gone on the edges equals underinflation

I do this empty and then fully loaded for camping with the tank full and family on board (only takes a few minutes.

Once you know these numbers (and they will vary front to rear and side to side I write them on a bit of medical tape that is attached to my air hose. That way it reminds me what to air up each tire to.

Oh and I am running a 1970 Suburban with 255/85/16 BFG M/T's ....tall & skinny for the win

 

[Rudster]

Phil,

Thanks for the air down clinic...good job

 

[Rudster]

more pics

 

[Tools R Us]

....tall & skinny for the win

What do they win?

I like my tall and skinny tires, but some would argue that 33" tires are not tall, especially on a BMW?