One of the most important adjustments you can make to your race is tire pressure. An inflation pressure change in bias ply tires adjusts circumference and with it stagger and weight distribution. Radial tires do not change size appreciably with small inflation pressure changes. However, radial stiffness, or spring rate, changes which effects understeer/oversteer balance.
Because even relatively small tire pressure changes can significantly alter a race car’s handling, it is import to accurately measure tire pressures.
Tire pressure gauges come in many sizes and shapes, ranging from the ubiquitous pencil style gauges, to high quality analog gauges and the ultimate, high-end digital gauges.
Pencil style gauges are not nearly accurate enough for racing. The same can be said for some inexpensive dial type pressure gauges.
High quality analog gauges and the more expensive digital gauges work well for racing. Analog gauges employ a curved metal tube, called a Bourdon tube, which is sealed on one end and connected to the pressure source at the other. As pressure increases, the curved tube straightens out proportionately. That motion is transferred to the gauge’s hand by a series of mechanical links and gears. The small links and fine gears make mechanical pressure gauges somewhat delicate.
“Right now, our best selling gauges are analog,” said Longacre’s Tom Glithero. “That is partly because we have so many more analog models available and partly because so many people are accustomed to using an analog gauge. Cost is also a factor, but all of that is changing. The new generation of racers that are coming in are digital people.”
Moroso’s experience is similar to Longacre’s. “Right now we only sell analog gauges,” explained Moroso’s Brint MClellan (CK Spell). “We have looked at digital gauges and may come out with one for high end [racers], but for now we have analog gauges for entry-level and professional series. The professional gauges have large, four-inch faces, adjustable zero set points and a very high accuracy of ½ percent of full scale.”
While gauge accuracy can be overemphasized. Racing analog tire pressure gauge accuracy is generally ± ½%, 1% or 2% of the full-scale reading. That accuracy specification is only for the central half of the gauge’s range. Outside that range accuracy is ±2 percent, or ±0.6 psi with a 30-psi gauge. However, the graduations on typical 30 psi gauges are in ½ psi increments, so it’s hard to read a difference of ±0.3 psi.
Since the greatest accuracy of most analog pressure gauges is around the middle of their range, you want a gauge with mid-range readings closest to the pressures you race with.
QuickCar’s Kim Goodman adds, “The bulk of what is sold in the racing industry is a one percent gauge that is reasonably accurate and will produce good repeatability. In a race application, repeatability in the gauge readings is more important [than absolute accuracy].
“If you set your race car up with your gauge reading 14 pounds, but the tire actually has 13 ½ or 14 ½ pounds of air in it, the actual pressure doesn’t really matter as long as your gauge always reads the same value for that actual pressure provided you always use the same gauge. Our gauges are built to have the highest possible repeatability for a one percent gauge.”
Long-term repeatability is important. You don’t want a gauge that slowly loses accuracy over a race season or two.
Dropping and/or banging a gauge around is the worst thing you can do to an analog gauge other than running over it with a truck. Shock of compressed air rushing into the gauge that overly accelerates mechanical parts can hurt it. Too much pressure can also cause damage. Most manufacturers have incorporated flow limiters and internal over-range stops to guard against these dangers.
Liquid filled gauges were developed to dampen shocks and vibrations that affect readings, but I don’t see an advantage over dry gauges having the same accuracy. The liquid filling performs that function very well in industrial gauges but there are problems with such gauges in racing.
The liquid expands and contracts with changing temperature which can result in inaccuracy. The most common solution to this problem is to vent the inside of the gauge to the atmosphere. However, when a vented gauge is carried around, it tends to leak liquid. Solutions to this problem including the clever sealed pressure equalization diaphragm developed by QuickCar.
Digital gauges are attractive because they have essentially no moving parts. A solid-state transducer converts air pressure into an electrical signal that can be displayed on a liquid crystal display.
“Our biggest strength is our digital gauges,” said Intercomp’s Bruce Rhoe. “Years ago, we only made digital air pressure gauges, but due to customer requests, we now offer both.
“As far as accuracy is concerned, digital gauges are much better. The electronics is not mechanical so there is no wear and tear like there is with mechanical gauges. That is not to say the analog gauges are bad, especially for racers with limited budgets, but digital gauges are superior. Once you buy a digital gauge, you will never buy another analog gauge.”
“Another advantage of digital gauges is their [large working] range and their ability to withstand being over pressured,” said Argo’s Ken Mitson. “Our RP-115 gauge will read pressures between zero and 119 psi to within a few of tenths over the entire range. And, if you [mistakenly] put it on a 200-psi bottle of air, all that will happen is that it will read “OL”, or overload. After doing that the digital gauge will still [accurately] read ten pounds of pressure in a tire.”
If you decide to invest in a digital gauge, select one that has a backlight and is sold by a company that will stand behind their gauges.
Always handle the gauge like the precision instrument it is.