Tire Pressure Monitor

TireChek Economical, Accurate and Reliable System

This concept was invented and Engineered by March 1995. This presentation was first placed on the Internet in January 1999.

TireChek is a unique new invention that is a low cost system for monitoring vehicle tire pressures. If any of the vehicle's tires loses a small amount of air such that its pressure becomes ONE PSI low, that tire's appropriate under-dashboard mounted red indicator light will flash a warning. The drawing below shows a Deluxe version that also includes yellow indicators for ONE-HALF PSI low earlier warning!

Most modern tires have steel, radial-belted construction. This design structure makes the sidewalls very stiff. Even if a tire is as much as SEVEN PSI low, it is hard to tell "by eye" that it needs air. TireChek would alert you WELL before a tire ever got that low, and WAY before any safety danger could exist!

The TireChek principle of operation allows for amazing performance! TireChek can be configured for INCREDIBLE sensitivity to loss of pressure in a tire. A specialized version of TireChek could monitor racecar tires for EXTREMELY small changes in tire pressure! A pressure loss of as little as 0.014 PSI could be detected and recorded by the pit crew from telemetry from the car! They could then closely monitor that tire's pressure to see if it continued to lose air. This extremely early warning could represent the difference between life and death at 200 mph if a low tire is about to go away. More than that, at the speeds of racecars, fairly small changes in tire pressures can severely affect the handling of the vehicle, so this very precise information could assist a crew and driver in improving competitiveness.

Do you realize that if your car's tires are only 5 pounds (PSI) low, that your gas mileage drops by around 10%? You normally buy about 600 gallons of gasoline per year to drive an average of 12,000 miles, and you may need to buy an extra 60 gallons of gas with low tires.

For an average car, that's about $240 per year in extra gasoline!

In addition, did you know that the same slightly low tire will wear out several % sooner? That's another $100 or more out of your pocket; four times that much if you replace ALL your tires when the first one wears out!

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More importantly, that same tire is several % more likely to fail and blow out while you or a family member is driving. Then there would certainly be the towing and repair charges. There would be the inconvenience. What about the potential of an accident? What about being stranded at night in an unfamiliar neighborhood?

(the tire industry seems to have either not ever done such simple and obvious testing or has kept the results secret, and the only numbers that we have found seem to be uncertain, and so we have placed estimates in the paragraphs above, until we can obtain figures that we know are reliable.)

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Wouldn't it be GREAT if there was an inexpensive monitoring system for your car that would continuously be alert for any quick OR SLOW air pressure loss in any of the tires on any of your vehicles?


We have invented TireChek!

TireChek is not yet available on the market. In the early Spring of 2000, we contracted with a company in Florida to manufacture the TireChek systems. There are not any technical problems, but that company has had delays in getting production under way. Whenever they start manufacturing the units, we will let you know on this page! We first thought they would be in production by now, but we have now removed our reference to a specific availability date. Sorry!

The principle of that company had severe medical difficulties and passed on, and that small company seems to have then ceased to exist. It appears that it will be up to some different company to now manufacture the TireChek devices for the market.

The initial selling price, including all necessary parts, is expected to be $99.
For a handy person, less than an hour's work should install TireChek.

For less handy people, we expect that many auto repair shops will soon be carrying TireChek but they would likely charge a little more because of their cost of installation.

This system was first invented in March 1995, and the electronics was designed and some crude prototypes were tested in that year. It was then determined that TireChek would be able to be sold for $99 to the retail market. At the time, we were not aware of any other efforts to monitor tire pressures. Around three years later, German luxury autos started having an extra-cost-option of around $3,000 to monitor their tire pressures! Their approach was extremely different than ours, on inserting a sophisticated electronics (computer) INSIDE each tire, which would include pressure sensors and temperature sensors and a radio transmitter. They use their device to calculate the effects of the high temperature and to adjust the pressure reading accordingly, and then to send that information in a radio signal to a separate computer inside the car, which then determines whether they need to display a warning light or buzzer regarding low tire pressure. It is no wonder that their system is so expensive! But they seem to have overlooked that electronic devices tend to FAIL when exposed to extreme heat (where the temperature INSIDE a tire during a highway drive is often around 240°F, extremely shortening the lifetime of electronic devices). They also seem to have overlooked that extreme vibration (where tires experience as much as 50 Gs acceleration when they hit a rock or a pothole) where electronic devices tend to also fail prematurely. And finally, they seem to choose to overlook the fact that their electronics INSIDE the tires require BATTERIES in order to work, and batteries also suffer from great heat and vibration, as well as eventually needing to be replaced. With such exotic electronic devices inside the tires, do you really think that the high school kid working at the local tire store will know WHAT it is, or how to fix it, or even how to replace its exotic battery? And will that kid know to transfer those expensive devices from one set of tires to another when you buy new tires? Probably not.

Between the extreme cost of that optional method of monitoring tire pressures, and the fact that they have never turned out to be particularly accurate in their results, and these matters having to do with failures due to heat or vibration or teen age kids, the chance that that approach would make sense on any large scale seems ludicrous.

However, the US Federal Government passed a Law that MANDATED that tire pressure monitoring systems be installed in EVERY NEW VEHICLE manufactured. I believe the first "absolute deadline" was January 1, 2001. The manufacturers had no possible way of complying with that law, so it got delayed into 2003. And then delayed again and again and again.

The auto manufacturers do NOT have any credible way to manufacture a tire pressure monitoring system which does not cost astounding amounts of money (which would greatly increase the cost of every vehicle to the customers). They also have no way to assure that the electronics would not quickly fail in millions of vehicles (due to heat or vibration or failed batteries), which the manufacturers would then have to pay to replace (billions of extra dollars that they cannot afford), and which would not actually fix much except to require later follow-up repairs over and over. So they really have NO credible way to comply with that Law that the US Federal Government passed around 2000. It turns out that our TireChek, which we invented and refined and tested in 1995, is likely the ONLY economical system which can accomplish that need!

In an assembly line, the cost to add TireChek to each vehicle should be well UNDER $2 per vehicle, and possibly even less than that!

Installation of the TireChek System is fast and easy. As mentioned above, a person who is reasonably familiar with vehicles should be able to install everything in less than an hour. In an auto repair shop, a team of three technicians should be able to install the entire system in about five minutes on any normal car!

Once the installation is complete, each of the tire pressures are checked with a good gauge, and if necessary, inflated to correct levels. The vehicle is then driven away! Within a minute or two, one or more of the TireChek warning indicators will light. This is actually due to the great accuracy and precision of the TireChek system! Knowing that the tires are at the desired pressures, the CALIBRATE button (in the middle of the display) is then pressed. That's it! (Pressing CALIBRATE teaches TireChek that each of the tires is at the pressure YOU desire.) The entire system is installed and calibrated that quickly! You will never have to press CALIBRATE again unless you replace a tire or if you add or intentionally remove air from a tire (or you disconnect or replace the vehicle's battery).

During installation, the wheels (usually) do not even have to be removed from the vehicle! The tires do NOT have to be dismounted and remounted, nor do they have to be re-balanced!

During a normal day in the life of a vehicle tire, the pressure changes A LOT! Just the difference from the temperatures of night and day can change tire pressure by several PSI (pounds per square inch). Then, when the vehicle is actually driven, the friction with the road, flexing of the tire body when it rotates and a variety of other forces build up a lot of heat in the tire. Tire temperatures can exceed 240°F! This great range of tire temperatures also cause great changes in tire pressure. (That's why they always tell you to check tire pressure BEFORE driving).

(Remember the Ideal Gas Law from Chemistry in High School? PV = nRT? For this situation, V, n, and R are relatively constant, so the pressure [P] is nearly proportional to the [absolute] temperature [T]. In winter, tires that are at 0°F [459 absolute] are at say, 30 PSI [45 psia]. After driving at high speed for a while, those tires are at 200°F [659 absolute], so the pressure has increased to [64 psia] 49 PSI! Isn't that interesting?) If you measured them then, and got a reading of 40 PSI, you might think that you should remove some air, when in reality, the tire was 9 PSI low already!

These continuous changes in pressure can really complicate the situation of a driver monitoring them. As discussed above, there are several VERY expensive tire pressure monitoring systems on the market that actually install pressure-sensors and transmitters inside each tire, to measure real pressures in each tire. The information on that pressure is then sent by radio signals to a receiver inside the vehicle. Unfortunately, a sophisticated computer is then necessary to correlate that measured pressure with the day's temperature and the tire's temperature, to be able to determine if the measured pressure is within the proper range. (For example, in the case mentioned just above, the system would need to understand that 37 PSI was actually an under-inflation condition, because of the temperature!) Such high-tech systems are necessarily VERY expensive, because they include a wide variety of expensive individual components. And, regarding the future, think about who could possibly maintain or repair that exotic system if (when?) it fails, and how much THAT would cost! And, because of all the external variables that can affect tire pressures, such systems often cannot be set as precisely as they are capable of. In addition, since those competing systems generally install complex electronic devices INSIDE the tires, those devices are subject to the tremendous vibration of that environment, the VERY high temperatures which occur in tires (which are detrimental to electronic devices), and their sensors and transmitters are powered by tiny batteries that are also inside the tires and will eventually fail. Wanna take a guess at what a local tire dealership will charge you to take the tire apart (to get to the sensor/transmitter) and then for their mechanics to stare at this exotic transmitter device, trying to figure out where the battery is? And how many days your vehicle would be out of commission while they try to find replacement parts (or, more likely, whole sensor/transmitters)?

TireChek does not require any exotic electronics to be inside the tire (or actually anything at all inside the tire), and our system is FAR less subject to all of those kinds of problems and failures. And even if something would ever fail, repair should be very simple and easy and inexpensive, and at no time would the vehicle need to be out of commission!

TireChek is able to sense VERY small changes in tire pressure. The theoretical design enables the possibility of detecting changes as small as 0.014 PSI! That's 1/70 PSI low! (Actually, theoretically, even less than that!)

In the real world, this extreme accuracy is impractical. When you drive around a corner, centrifugal force on the vehicle causes a weight transfer to the 'outer' wheels, which instantaneously fractionally increases the pressure in those tires. When a driver sits down, he/she adds a human body weight to the vehicle. This actually very slightly raises the pressures in ALL four tires, with the actual pressure rise in the two driver's side tires a little more than in the passenger side tires, and by different amounts! Of course, THIS effect depends on the weight of the driver! Other systems are not sensitive enough to have to be concerned about such minute changes, but TireChek is so sensitive that we found it necessary to consider such things! Because of many variable like this, we have initially chosen a detection threshold of about 3 PSI for normal cars (0.2 PSI for racing cars, because they have far fewer variables that can affect the readings) to minimize false alarms. TireChek systems with the optional yellow indicators sense about 1/3 that, or 1 PSI. These yellow indicator lights are likely to occasionally light due to the kinds of things mentioned above. The TireChek system is a continuous monitoring system, though, and such false indications soon clear themselves unless an actual loss of pressure has occurred.

No one else can accomplish this kind of sensitivity, even in VERY EXPENSIVE computerized systems. We monitor the tire pressures in a unique way (now in the Patenting process). We are then able to get our great sensitivity to air loss by continuously and instantaneously monitoring and then COMPARING the pressures of the two tires on the vehicle. They normally experience the same day/night temperature variations. They normally heat up similarly while you drive. This allows us to set our sensitivity to be WAY better than other systems! Maybe someday they will let us teach them this wonderful improvement!

If either of the two starts to lose any air, EVEN AT AN EXTREMELY SLOW RATE, our TireChek will alert you, well before it would be obvious visually or even on a tire pressure gauge. Many drivers check their tires visually. Modern stiff-walled radial tires can be much more than 7 PSI low and will still appear normal visually! Even when checked with a tire gauge, inaccuracies can occur, since some inexpensive tire gauges can be inaccurate by several PSI.

In case you're thinking this means the system will be so fussy that you have to be super precise about filling your tires, be assured! (That IS actually necessary in some of those expensive competing systems!) Once you have aired your tires to the proper pressures you desire, you just touch the CALIBRATE button on the TireChek panel if and when any red warning lights appear. The TireChek system would then understand that each of the tires are now at the pressures YOU wanted them to be at. Even if you intentionally fill them to different pressures! Even if you have unmatched tires on the axle! (That is not recommended, by the way!)

In the event of a very slow leak, TireChek would alert you as soon as any of the tires was low enough to match the threshold PSI difference that that version of TireChek was configured with! All of the advantages mentioned above that are related to maintaining tire pressure would be yours!

As a bonus benefit, in the event that a tire picked up a road hazard, and represented a true and immediate safety hazard, you would almost immediately know that there was a problem. You would then likely have plenty of time to slow down to a safer speed. You would even know WHICH tire was having the problem, to try to be alert to anything unexpected that tire might do. There's even a good chance that you might have enough time (at that slower, safer speed) to get to a tire repair center well before the pressure got down to a dangerous level.

Finally, in the very remote possibility that a tire had a hidden defect, where it was subject to heating up unusually rapidly during driving, the TireChek Monitor would alert you to have your tires checked.

TireChek! Something you SHOULD have on your car, truck, RV and trailer!

(Following is a technical discussion on one of the two central concepts of the TireChek invention.)

Design Theory

Current competing tire pressure-monitoring systems use sensors that directly measure or sense the actual pressure in each tire, which is then sent to a central data processor for analysis.

This requires both pressure sensors and radio transmitters to be installed INSIDE each tire. It also therefore means that a battery must also be installed in there to power the transmitter.

It gets more complicated. The pressure of a tire is tremendously dependent on the tire temperature. That's why they always tell you to measure tire pressures COLD, before driving very far. The air in a tire that is pressurized to 32 PSIG when cold will obey the Ideal Gas Law and rise in pressure as the tire heats up. Since extended highway driving can get the tires up to over 200°F, those tires can 'naturally' get up around 50 PSIG pressure readings after several hours of highway driving. This means that a temperature sensor must also be installed inside the tire.

Each tire regularly sends radio signals to a computer inside the car, which passes along the digital data on pressure and temperature at that moment. The computer then has to calculate, based on the tire temperature, the proper pressure reading that SHOULD BE in the tire due to tire heating. Then it compares the ACTUAL pressure it has sensed with that intended pressure. If the tire is below that amount by a threshold amount, a warning is given to the driver.

Doesn't this seem awfully complex? And, you can probably see why some such systems have been sold for around $3,000. But, have you thought about the fact that electronics devices are a little 'delicate'? Inside a tire, there is enormous vibration and extreme heat. And, what about having to replace those batteries in a few years? The tires would have to be taken off the car and taken apart to replace them. And, your normal tire place might not have a technician that has ever even SEEN that particular system, much less know how to work on it or even change the battery!

And, during installation in the first place, the wheels have to be taken off the vehicle, the tires dismounted from the wheels, the sensor/transmitters installed (firmly!), the tires re-mounted, re-balanced, and replaced on the vehicle. This is essentially a big production!

Since almost no one will be trained to repair such a complex system (in those nasty vibrating, hot environments), how much do you think a repair person would later charge you to replace the batteries or to repair or replace a failed sensor? Since he knows that you cannot go anywhere else, he could charge virtually anything he wishes! And you don't even have a paddle!!

TireChek is entirely different from this.

Consider if you were NOT given ANY measuring device or sensing equipment. How would you KNOW when a tire had low pressure? The answer is obviously that if the pressure was low enough, it would appear "flat".

What does that actually mean? The technical reason a tire looks flat is this. That tire supports a certain number of pounds of load, say 900, or around one-fourth of the total weight of a vehicle and passengers. If that tire contains air at 30 PSI, it would be necessary for there to be 900 / 30 or 30 square inches of area in contact with the road. If the tread is 5 inches wide, the tire's footprint would therefore normally be about 6 inches long. (30 square inches of footprint times 30 pounds PER square inch equals 900 pounds). (These numbers are actually approximate, because there are other factors involved. For example, the stiffness of the sidewalls of tires would actually support a little weight even if the tire had no air. But the general point we are making is quite true, and obvious.)

If that tire lost air such that its internal pressure was just 20 PSI, the load is still 900 pounds. This means that there must now be 900 / 20 or 45 square inches of area in contact with the road. The tread width is still 5 inches, so the footprint MUST now be about 9 inches long. (45 square inches of footprint times 20 pounds PER square inch equals 900 pounds).

A symptom of both of these situations, the desired and the "flat" is that the bottom of the otherwise round tire appears to be flattened. In the desired case, the flattened area is 6 inches long, and in the "flat" case, the flattened area is 9 inches long. So, when we say that we see that a tire is "flat", we are actually saying that the flattened area is larger than we think it should be.

The argument just presented is true. The numbers are not precisely accurate, because the sidewalls of a tire have some structural rigidity, which contributes to supporting the wheel's load. In low profile tires with steel radial belts and stiff sidewalls, this contribution can be substantial, but all practical pneumatic tires support most of the load by the pressurized air contained within it. This comment just means that the numbers presented above are approximate for real tires.

This explains WHY it happens. Now HOW does it occur? Simple geometry shows that the only was the footprint could get longer is by having the effective radius of the tire become less. Draw two identical diameter circles on a sheet of paper, and draw a straight line to "slice off" a thin sliver of the circle on one and a different line to slice off a wider sliver on the other. The flattened area of the circles represents the footprints. It is easy to see that the distance between the center of the circle and the line is different for the two. The flattened area is closer to the center of the circle for the drawing with the larger "footprint".

That's why a vehicle appears to have a corner of the vehicle physically low when the tire is entirely flat or nearly so. With the "effective" radius of the tire being less, the axle shaft centerline of that wheel is actually lower, nearer the road. You can easily see that the effective radius of the tire is geometrically directly related to the size of the footprint, which is directly related to the pressure inside the tire.

So, if a system VERY precisely monitored the height of each wheel's axle shaft centerline above the road surface, it would also monitor a tire's pressure!

Please notice several important facts. NO device needs to be placed INSIDE any tire. No transmitter in there would ever need battery replacement. System installation does NOT involve dismantling the tires and wheels, and the TireChek system can generally be installed without even removing the wheels from the vehicle! The wheels do not have to be re-balanced.

Essentially, we have found a totally different way to monitor each of the tire pressures, which does not actually require pressure sensors, but which turns out to be even more accurate and sensitive!

For the range of pressure differences we are interested in, this change of "effective" radius of a tire is really small but not insignificant. In Spring 1995, in the process of inventing this system, my testing on a Corvette with stiff-walled steel-belted radial tires included the following findings. If one of the Corvette tires is inflated to 35 PSI, its effective radius is 12.50 inches. If air is released to let the pressure drop to 31 PSI, the effective radius becomes 12.375 inches. This means that the wheel axle centerline lowered from 12.500 inches above the road to 12.375 inches above the road. This 1/8-inch difference happens to represent about 1/100 of the effective radius of the tire.

You might notice that we just pointed out that a difference of FOUR PSI caused 1/4" difference, and we earlier noted that we can detect 1/70 PSI air loss. This is obviously a sensing accuracy of AROUND ONE ONE-THOUSANDTH OF AN INCH in axle height! Impressive, huh?

It is not possible for anyone to recognize a tire being low by 1/8 inch. In general, with modern tires, a tire must be 7 to 10 PSI low before the driver notices it either by sight or by odd handling of the vehicle.

It might not even seem very possible to continuously monitor each of the wheel's axle heights, without incredibly involved distance sensing radar systems! And, even with such a system, variations of road surface heights (like potholes or grading) would keep precise accuracy very difficult to obtain.

TireChek is designed to continuously monitor the effective tire radius. We invented a way to rather simply and inexpensively monitor the axle heights of the various wheels to an accuracy that can be better than 0.001" in height difference! This amazing precision corresponds to a pressure loss sensitivity of around 0.014 PSI in a tire! In other words, with TireChek, it is not necessary to wait until a tire loses 7 or 10 PSI where it is either visible or the vehicle handling is affected. The TireChek system could give an alert if any tire was even 0.014 PSI low! And, it would indicate just which tire it was!

Now, in real life, there is little need for such incredible sensitivity. (In racecars, there IS, and the extreme sensitivity of TireChek could be used, and even enhanced further!) Such extreme sensitivity tends to result in occasional false warnings. For this reason, the normal TireChek system is set up to give a RED ALERT if any tire is 3 PSI low. The second version does the same, but also gives a YELLOW ALERT of any tire is 1 PSI low.

Since TireChek is a continuous monitoring system, if it would happen that unusual circumstances caused a warning, within one minute, new pressure analyses would either cancel the warning or confirm it. This represents a self-diagnostic for the TireChek system. A warning that persisted would always indicate a real tire pressure loss!

The actual TireChek system chooses to monitor the height of the axles of TWO wheels, for a variety of reasons. As cold tires are driven, the pressures increase as the tire temperatures increase. The vehicle body actually raises up a small fraction of an inch as the tires heat up! It turns out that TireChek is so extremely sensitive that this change would very quickly indicate a pressure variance. By choosing to continuously monitor TWO wheels, and be attentive to their DIFFERENCE of height, the effect of tire heating is eliminated from being a problem. As long as both tires heat similarly (and they will), then their radii increase similarly, and the super-sensitivity of TireChek can be successfully used, without false warnings.

Just HOW TireChek is able to monitor all of the axle heights to within 0.001", well that's central to how TireChek works! TireChek not only does this, but it does it in a system that is FAR less expensive than any competing monitoring system. TireChek does not NEED exotic pressure and temperature sensors and radio transmitters inside the tires. TireChek even has a far simpler computer as part of the system! Even better, future repairs should be very rare, and inexpensive if ever necessary!

It may have occurred to you that tandem tires might represent a problem for TireChek. That actually turns out not to be the case. The axle height of a dual or tandem tire is actually determined by the AVERAGE of the tire pressures in the two tires involved. It's another application of that "footprint" logic described earlier. If EITHER tire of a tandem would lose air, the axle would still lower, but only by about half of the distance expected with a single tire. Since TireChek has such extreme sensitivity, all that is necessary is to double the sensitivity setting for a tandem tire, to achieve a pressure loss sensitivity similar to that of TireChek configured for a single tire.

First Invented and Developed, Mar 1995,
First Published on the Web: Jan 4, 1999

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