Due to the position of the...
Due to the position of the valve stem, it is not always possible to get the optimal gauge position. This is not the end of the world. If you utilize the same position each time, you will minimize the hysteresis in the gauge. It is important to be as consistent as possible.
When I talk with racers or consult with a race team, I hear it over and over again: "Tires are one of the most important components on the car." I have no real argument with that statement and tend to agree with it.
I find it puzzling, though, that tires are mistreated and their information is often misread. The most common measurement of tires is air pressure, followed by stagger. It is neglect of the air pressure adjustment that I find most intriguing. For such an important component, tire pressure measurement is usually the least robust process of all the processes on the car.
The method that most race teams use to mitigate the risk associated with measuring tire pressure is to have the same person check the tires. While this is a plan, it is nonetheless a weak one. What really needs to be developed is an SOP (Standard Operating Procedure) that outlines how the process is going to be accomplished, regardless of who does it.
The tools of the trade. You...
The tools of the trade. You can have all the tires in the world, but if you don't set their pressures correctly, they will do you no good at all.
What is the best way to check the tires? We could argue about this for hours. What we will concentrate on is the tool we use for checking the tire pressure: the lowly tire pressure gauge.
There are multiple ways to check tire pressure, but in reality there is only one repeatable method, and that's with a tire pressure gauge. The selection then broadens to two different types of gauges, analog and digital. Most people assume that a digital gauge is the best tool for accomplishing this measurement. The argument is based on the fact that when people see a digital display, they instantly assume that the gauge will be more accurate than an analog gauge. While a digital gauge may be easier to read, that in and of itself does not portend any greater level of accuracy.
We need to exercise great care when differentiating the two types of gauges. Many times, a digital gauge is not a digital gauge at all-it's merely an analog gauge with a digital display. This method offers no more accuracy than a standard analog gauge. It is just easier to read.
These photos show the view...
These photos show the view slightly to the right of the gauge, from the left of the gauge, and directly into the face of the gauge.
We need to define exactly how an air pressure gauge works. There is a multiplicity of methods used to measure air pressure, but the ones we need to concern ourselves with are those most commonly used in racing today. Most air pressure gauges are based on a Bourdon tube to activate the needle or in some cases a digital potentiometer. The Bourdon tube's name was derived from Frenchman Eugene Bourdon, who developed the tube in the mid-19th century.
The Bourdon tube is closed on one end and open on the other. The open end is where the pressure is applied, and the pressure source, in this case, is a valve stem. The tube does not have a round cross section but is usually a very flat oval, and the tube is also formed into a semicircle. The closed end of the tube is attached to a section of a gear or a rack that is driving a pinion gear. On the other end of the pinion gear is the needle, the part we see as it moves on the gauge, and usually they are on the same shaft. When air pressure is applied, the Bourdon tube tries to become straight. This movement causes the gear or rack connected to the end of the tube to turn the pinion gear that is attached to the gauge needle. The greater the pressure applied to the tube, the more the tube tries to return to a straight position. Once you understand how the mechanism operates, it is really a very simple device. The execution can be a bit messy, but the concept is easy to understand.
Close inspection of the photos...
Close inspection of the photos shows almost a 0.5-psi difference in pressure from the various views.
A digital gauge is based on a strain gauge, which operates by measuring the deflection of a surface and turning that deflection into an electrical signal that is then sent through a conditioning circuit to an amplifier circuit. This amplified signal is then routed to a meter, and we read that signal as a measurement of pressure. The deflection can come from a variety of sources, but mainly a piezoelectric sensor or metal diaphragm acting on a strain gauge. The pressure causes a deformation and the strain gauge measures this deformation or distortion. It is then conditioned through a variety of electrical circuits that are displayed on a meter.
Remember, though, that a gauge is not inherently more accurate simply because it is digital. The argument has always been that due to the digital readout, there is less of a chance of misreading the gauge. That is completely true. The problem is the scale. It is very easy to place a meter on the gauge that reads to the third or fourth decimal place, but you may not be measuring to that level.
Be mindful of how you read...
Be mindful of how you read the gauge.
Using a gauge with a digital readout eliminates the parallax factor. That is, when you are reading an analog gauge, it is very easy to look at the gauge and not maintain a completely parallel relationship to the front of the gauge. Looking slightly to the left, right, above, or below the center of the gauge face can lead to misinterpretations of the actual reading. This can cause the same tire to have a different pressure when measured by multiple people measuring the pressure with the same gauge. This is a classic example of not knowing how to use the tool and inducing operator error into a measurement.
The gauge should be held in the vertical position while in use because the majority of today's gauges are designed to be used this way. Placing the gauge in any position away from vertical could place the mechanism in a load for which it was not designed. Any additional load of the gears in a direction they were not designed to function could give you an erroneous reading.
This young racer is using...
This young racer is using the correct gauge position to get the most accurate reading. The gauge is being held in the vertical position and the view is straight on. This reduces the parallax problem that can be encountered when viewing the gauge from less than straight on.
Depending on the quality of the gauge you are using, there may be some "slack" in the mechanism. This slack can cause hysteresis, which is a variation in the gauge reading caused by the mechanism of the gauge itself. The Bourdon tube and the associated mechanicals are being loaded due to the position of the gauge, and this load could cause some non-repeatable readings. This is due to the Bourdon tube being loaded in a direction it is not designed to operate. For example, you may be reading 14.5 psi on the gauge, when in reality you may actually have 14.0 psi. The whole reason you are measuring air pressure is to achieve a specific number. Using the gauge in a way it was not designed to operate could give you a reading that is less than accurate.
You must remember that an air pressure gauge is a precision instrument. It is not a wrench, a socket, or a screwdriver, and as such, it should not be stored and treated like it is a handtool. Exercise great care when storing and transporting gauges. Many gauges designed for racing applications come with a protective foam enclosure around the gauge or a molded rubber shock mount to prevent the unit from being damaged by shock or other handling damage. Just throwing a gauge in the bottom of a toolbox that is full of other tools is just asking for accuracy issues. It is very easy to chip a tooth of the gears that drive the needle if the gauge is handled improperly. This is true if you are using a digital or an analog gauge because these are delicate instruments and need to be treated with care.
This photo really shows how...
This photo really shows how delicate these gears are. It does not take much in the way of over-pressuring the gauge or dropping the gauge to damage these gears.
It is also very easy to damage the gauge if you apply more pressure than the gauge was designed to measure. If you connect a gauge with a range of 0 to 15 psi to a tire with 55 psi, you can "wrap" the gauge. This can also cause the gear teeth to slip or the Bourdon tube to become damaged, which requires replacement of the gauge. You need to exercise great care when you are dealing with this type of instrumentation. That said, the gauges that are commonly used in our sport for this type of measurement are very robust and still deliver reasonable levels of accuracy after some slight abuse. The rougher you treat these instruments, the greater the level of inaccuracy you may experience.
If you suspect that the gauge is delivering an inaccurate reading, the easiest thing to do is check the measurement against other gauges. If you find a significant difference, you may want to call the manufacturer or the dealer you purchased the gauge from and see if they offer a calibration service.
When getting an air pressure reading, make every attempt to look at the gauge directly parallel with the face of the gauge. This will reduce the parallax factor and go a long way to getting reliable, repeatable measurements. Make sure that the interface between the valve stem and the gauge is clean and free from any dirt or debris that could cause unwanted leakage during the measurement process. Make every effort to push the gauge firmly onto the valve stem with as little air leakage as possible. The obvious reason for doing this is to lose as little air from the tire as possible.
The copper-colored tube is...
The copper-colored tube is the Bourdon tube. On the lower left, it is clear to see that the open end of the tube is soldered to the brass inlet port for the gauge. When charged with air, the tube tries to straighten out, moving the white gear and driving the brass pinion gear attached to the needle on the face of the gauge-the part we look at to get our reading.
This is not an overly complex process. The problem is that many racers are often in a hurry when they are trying to make these measurements, and it is very easy to use the gauge incorrectly and get erroneous readings. The importance of these measurements to the performance of the car makes the ability to gather good data critical. We need to make sure that the misuse of the air pressure gauge does not contribute to poor measurement.
The range of the gauge is also a feature you need to be keenly aware of in the process. If you are measuring tire pressures in the 7-8-psi range, you should not select a gauge with a range of 0-100 psi. The sweet spot on the gauge is the middle. You should be using a 0-15-psi gauge for pressures in the 7-8-psi range. This is not as critical on a good digital gauge, but on an analog gauge it seems to be a very tried-and-true way of selecting and getting the accuracy you need.
The objective should be to train the team on how the tool is to be used, and possibly document the process with step-by-step instructions. Do not make the process of measuring air pressure a task specific to a man or team member, but rather a process-based task that can be accomplished by every member of the team with the same level of accuracy. As a team, we need to be able to wear a number of different hats on race day. Because we all want to reach Victory Lane, we should take the time to make sure we are not giving away anything due to poor measurement practices.