Temperature And Tires

[tireman9]

<p>On this and other RV forums, I see mention of tire temperature and the use of IR "guns" to keep track of tire temperature.</p>
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<p>Using hand held "guns" on conductive metal objects such as wheels, hubs, brakes and engine components is OK but using them to learn what the critical temperature is, on tires, just a waste of time. The main reason this is not a good idea is that rubber is really a good insulator so the hot zone of tires which is buried internal to the structure as the heat is generated internal to cords and at the high strain (movement) locations between the steel at the belt edges. IR guns can only measure the surface temperature of the tire so you are not getting accurate reading of the critical location.</p>
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<p>The other problem with using the guns is repeatability. Locating the precise point of measurement if you are going to do a comparison of the same tire at two different times is critical. Even the angle you hold the gun can affect the reading.</p>
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<p>There is also the problem of why heat is bad for tires. A tire does not fail because of the average temperature of the tire but because of the hottest temperature at a specific location internal to the tire structure.</p>
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<p>Here is a sample graphic from Finite Element computer model showing the hottest area in red and coolest in blue.</p>
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<p><img alt="o5rr6t.jpg" src="http://i42.tinypic.com/o5rr6t.jpg" /></p>
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<p>If you look near the shoulder (just to the left of the red zone) you see very high changes in temperature with minor changes in location of temperature measurement. Since this is a computer simulation it is basic in that it does not introduce the variations in rubber thickness ( insulation) due to sidewall decoration or tread pattern. These variations simply add to the difficulty in getting accurate measurement in real tires.</p>
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<p>When I used laboratory quality IR thermograph equipment costing tens of thousands I was able to measure significant temperature differences over distances as small as 0.10" this is smaller than to normal target area offered by the Harbor Freight IR gun. This means that unless you measure at the identical location on a tire at +/- 0.05" location you will not get repeatable temperatures.</p>
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<p>The other variable is time and distance traveled since you were driving at your controlled speed. Based on my real life experience I would suggest that you need to drop from your constant highway speed of 60mph to 0 within 20 seconds and then take the tire temperature at the same time from stop (say 20 seconds +/- 5) or the temperature reading would not be providing you the information you would need to make a valid A-B comparison for load or inflation adjustment.</p>
<p>When I was working Indy Car tire tests we would shoot for collecting the 12 tread temperatures using a needle probe always in the same sequence at 3 to 5 second intervals (less than 60 seconds total with 45 seconds the goal for all 12 measurements starting from the second the car stopped moving.)</p>
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<p>You might as well just use your hand to judge if the tire is hot or really hot.</p>
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<p>I have two posts on my blog on the topic of IR guns</p>
<p><a href="http://www.rvtiresafety.com/2012/05/what-do-you-think-about-temperature.html">This one</a> has additional temperature pictures.</p>
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<p><a href="http://www.rvtiresafety.com/2012/05/what-do-you-think-of-ir-guns-part-2.html">This post</a> has data showing a direct comparison between internal TPMS, IR gun and a professional racing pyrometer as used by a major tire company race tire engineer.</p>
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<p><a href="http://www.rvtiresafety.com/2011/05/tire-temperature-pressure-hot-topic.html">This post </a>covers "Gas Law" and the mathimatical relationship between temperature and pressure and has a picture of my race car.</p>
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<p><strong>Bottom Line</strong></p>
<p>Does this mean you can't use your HF $29 IR gun?  Not at all. Just keep in mind that we are talking about trying to improve the safety of your travels. I am not sure if it a good practice to use a "toy" when making safety related decisions.</p>
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[jfxg48]

Thanks, Tireman...

Apparently, aiming an IR gun at the sidewall is about the most useless thing you can do! Makes me feel smart! It looks like the inner rubber along the outer tread band is where the heat builds. So what are we to do? Is there any way to get a valid sense of critical tire temps? I recall a post of yours a while back mentioning 157 degrees as a critical point. At the time I was measuring sidewall temps of about 130 with an IR gun, thinking I was fine. Is there any reliable correlation?

Another: when my TST system measures temperature, what is it measuring? Air temp of the air in the filler tube? Temp of the tube metal itself?

And another: a few weeks ago I was climbing long grades (max elev change was about 4000 ft) in 95 degree heat. Tire pressure had been set at the low elevation. My high pressure trigger is set at +20lb, I was getting alarms on 3 or 4 tires. I'm presuming this was predominantly from the heat, since my brain tells me that the absolute pressure within the tire should be unchanged with elevation. Is my brain wrong?

Thanks!

[wolfe10]

John,

Yes, theoretically, PSI will increase with elevation (actually with lower PSI outside the tire). But the change is minimal.

The most dramatic change in tire pressure is due to temperature with ROUGHLY a PSI per 10 degrees F.

[tireman9]

Thanks, Tireman...

Apparently, aiming an IR gun at the sidewall is about the most useless thing you can do! Makes me feel smart! It looks like the inner rubber along the outer tread band is where the heat builds. So what are we to do? Is there any way to get a valid sense of critical tire temps? I recall a post of yours a while back mentioning 157 degrees as a critical point. At the time I was measuring sidewall temps of about 130 with an IR gun, thinking I was fine. Is there any reliable correlation?

Another: when my TST system measures temperature, what is it measuring? Air temp of the air in the filler tube? Temp of the tube metal itself?

And another: a few weeks ago I was climbing long grades (max elev change was about 4000 ft) in 95 degree heat. Tire pressure had been set at the low elevation. My high pressure trigger is set at +20lb, I was getting alarms on 3 or 4 tires. I'm presuming this was predominantly from the heat, since my brain tells me that the absolute pressure within the tire should be unchanged with elevation. Is my brain wrong?

Thanks!

No it's not completely useless. The point I wanted to make is that the use of a hand held IR gun is are not sufficiently accurate as far as measuring the hot spot to use it as a substitute for tire pressure and knowing your actual loads.

If I saw a reading like 180F or higher I would take that as a warning that something is seriously wrong and you need to learn why the tire is so hot before moving the RV another foot. You might even want to step away from the tire till it cools down some if you saw an external sidewall temperature approaching 200F as that could be an indication that serious damage has already been done to the tire and there is a posibility of a rupture.

After the tire has cooled down (min 30 minutes) you could check the air pressure. If you saw those temps I bet you would find that you were more than 20% low which according to tire companies means you have been driving on a "flat" tire. The tire should be replaced and not driven on ANY distance if you discover you have driven on it "flat" as a sudden air loss could occur any time.

RE the question Brett correctly answered. Here is my post on that topic with a few more details and even a formula for those with a calculator.

Bottom Lime Tire Pressure Monitor Systems are jsut that. They monitor pressure. Temperature is just icing on the cake. I have done some preliminary tests on internal (air chamber) vs external (end of valve stem) TPMS but need more data before I publish the results.

[washtech]

Good Morning All:

Does anyone have any input on which tire monitoring system might be considered the Best of all offered on the market?

Thanks,

Washtech

[hermanmullins]

Just my 2 cents worth. The IR gun may tell you more then just tire temp. If you have one tire that is much hotter then the others it could mean you have a brake dragging or a bearing ready to go out. These can also elevate temp.

Herman

[tireman9]

Good Morning All:

Does anyone have any input on which tire monitoring system might be considered the Best of all offered on the market?

Thanks,

Washtech

I am sure you would like a brand of TPM as an answer but since there are a variety of different applications and what some consider most important feature may be someone elses #3.

While I do have a personal preferance, I feel it better to offer a blog post on "Best TPMS" that outlines what, in my opinion are the features for you to consider when making a purchase decision.

Hope this helps.

[tireman9]

Just my 2 cents worth. The IR gun may tell you more then just tire temp. If you have one tire that is much hotter then the others it could mean you have a brake dragging or a bearing ready to go out. These can also elevate temp.

Herman

You are correct that an IR gun can alert you to a mechanical problem such as bearing or brake problem. An IR gun works well on heat conductive material such as metal. They do not work as well on insulating rubber which is a poor conductor of heat.

If you are running low enough on pressure on one tire to get a meaningful temperature difference measurement there is a good chance you have managed to do some permanent structural damage to your tire already and simply putting air back in your tire will not "Fix It" any more than putting the Potato Salad you left out all day in the sun, will be fixed by putting it back in the refrig.

If you attend my seminar on Tire Knowledge for the RV owner at Redmond you can learn more.