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Temperature And Tires<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> <p> </p> <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> <p> </p> <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> <p> </p> <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> <p> </p> <p>Here is a sample graphic from Finite Element computer model showing the hottest area in red and coolest in blue.</p> <p> </p> <p><img alt="o5rr6t.jpg" src="http://i42.tinypic.com/o5rr6t.jpg" /></p> <p> </p> <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> <p> </p> <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> <p> </p> <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> <p> </p> <p>You might as well just use your hand to judge if the tire is hot or really hot.</p> <p> </p> <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> <p> </p> <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> <p> </p> <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> <p> </p> <p> </p> <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> <p> </p> <p> </p>
Using an Infrared Thermometer
Briarhopper posted a topic in ChassisOccasionally there is reference in discussions about using an infrared thermometer (spot radiometer) to check the temperature of something; tires, wheels, transmission housings, etc. An IR thermometer is a great tool and can help diagnose a problem or prevent a failure, but there are several things that affect infrared readings. Two things are very important and fairly easy to control: distance and surface. Distance is how far the thermometer is from the target. Any decent IR thermometer will have instructions and in those instructions it will give spot size or distance/spot ratio, which is the size of the area the instrument reads at a given distance (or distances). My spot radiometer has a ratio of 8:1 which means at a distance of 8” it reads a spot of 1” diameter, 16”:2”, 32”:4”. At 8 feet it reads a 1 foot diameter spot. The important note here is that it is not reading the tiny dot made by the laser pointer. Too far away and the spot being read may be very large and not yielding a reliable temperature of the target. More important is the surface of what is being measured. The instructions might mention emissivity, but can get fairly murky in this area. The main point here for RV folk is that bare metal or metal-like surfaces do not emit radiated energy like other surfaces. They reflect the energy from the surrounding stuff, so a reading on a chrome hub cap will be reflected temperature from the road, the sky or maybe even yourself, but not a reading of the true temp of the hub cap (chrome or aluminum wheel). Take a reading on a smooth metal surface like a clean (unpainted) oil pan, transmission pan, and it will likely read cooler or hotter than it actually is, depending on the reflected temperatures of the surrounding equipment. Tires, painted or textured surfaces generally reflect less and give better readings. Smooth, bare metal or metal coated parts (chrome) do not yield accurate readings of the target. If you want to test this for yourself, take readings on your chrome hubcap from several angles; one straight on, one from a high angle and one form a low angle. Each will likely read differently even though you have the laser dot on the same point. I uploaded an image of a chrome hub cap and wheel taken with an infrared camera. The three temperature points are all going to be relatively close in actual temperature, but each reading is influenced by the reflected energy: SP1 is off my hand, SP2 off the ground and SP3 from the sky off the top of a lug nut cover. Stay close to keep the spot size small and avoid bare shiny metal, chrome, etc. If in doubt, use a contact thermometer if safe to do so. Hope this is useful.