Emotional pressure to find speed will be reduced if you manage your tire pressure with a plan. Every set up out there relies on the contact patch and getting the desired pressure at the point the rubber meets the road. Several important factors need to be considered for optimal tire pressure.
To achieve precise tire pressure readings you must have an accurate tire pressure gauge. Starting with the right gauge is paramount. Nearly all gauges are more accurate in the center of the range. For example; a 30 PSI gauge is going to be most accurate between 10 and 20 PSI. It will still work at 5 or 25 PSI but the percentage of error increases. For a 60 pound gauge you will get good results between 20 and 40 PSI. Just as before it will work fine at 15 or 45 PSI but the accuracy percentage goes down.
Racing Tire Gauges with a Glow in The Dark dial face are a big advantage when it gets dark. There is a difference in glow coatings and better quality gauges have coatings that glow longer. You can recharge a Glow in The Dark Gauge quickly by placing it near any light source.
Since accuracy is best in the center of the range, you should choose a gauge that fits the pressure ranges for your type of racing. Gauges found for $10.00 dollars at the local auto parts store are designed for passenger cars and their percentage of error is too high for racing purposes. If your passenger car has 30 PSI instead of 32 PSI it is really not too big of a deal but on a racecar 2 pounds would be the difference between winning and kissing your sister. Choose a quality gauge that has less than a 2% accuracy tolerance. Obviously, better accuracy allows your team to slice information for repeatable performance.
Larger tire pressure gauges such as this 4" gauge offer more accuracy. Larger gears allow for precision machining and the larger dial creates more resolution and better viewing angle. Rubber bumpers should always be included with racing tire gauges.
Digital Gauges
A quality digital gauge can give you better resolution yet you can be fooled by the digital display on a low quality version. Low quality digital gauges may post a number on the display but the accuracy must be supported by quality sensors for the display reading to be reliable. Digital gauges should have a backlight for easy night time viewing and the numbers should be large and easy to read. Digital, by itself, does not guarantee that the gauge will be more accurate. You may find a practical improvement as digital gauges remove the variable of viewing angle. Analog gauges can add to accuracy percentage error due to the viewing angle - the individual user’s interpretation of the needle verses the printed hash mark can cause a variation in results. A quality analog gauge will have minimal viewing angle error.
Digital Gauges can provide more accuracy but digital alone isn't the only factor. Quality racing gauges like the one shown is very accurate. Gauges with sensors designed for passenger car use are inexpensive but their low quality sensors are less than the standard required for racing.
Paying more for one digital gauge verses another does not guarantee accuracy but it can be a factor. There can be an accuracy correlation to gauge head size. If the display is very small then that could be a clue that the electronics contain low cost sensors. Digital sensors intended for passenger car use simply do not provide the resolution needed for racing. If a digital gauge rounds to half pound or even full pound you most likely find improved accuracy and quality with a gauge that reads in 1/4 pound or better digital increments.
Analog Gauges
For analog tire pressure gauges you will find many options. Gauge faces that have Glow in The Dark coatings are a big help when it gets dark. There is a difference in coating quality so take note of the manufactures that utilize longer lasting Glow coatings.
Analog gauge head size is a factor in assessing quality. In general, a 4” gauge will be more accurate than a 2” gauge. Larger mechanisms typically have more precision as the larger gears have the mass for easier machining. The longer throw on a 4” pressure mechanism offers smooth and steady needle movement resulting in improved accuracy as compared to a 2” version. With a larger gauge face the needle is easier to read and interpretation error is reduced as the larger circumference provides an expanded scale and improved visual perception of the enhanced scale graduations.
Liquid Filled Analog Gauges
There is a myth that liquid filled gauges are better. The mere presence of liquid does not ensure better quality. Liquid filled gauges may or not be good quality but the liquid alone is not the factor that guarantees accuracy. Liquid filled gauges work great to reduce needle vibration. If a gauge were to be mounted on a machine that vibrates then the liquid would help to reduce needle shake or bounce. Since tire pressure gauges are not used in a setting where vibration is an issue, the liquid serves only as a gauge damper. The liquid does absorb shock as the needle movement is controlled during gauge inflation. This dampening effect is desirable. Quality analog gauges have internal dampening systems without using liquid. Liquid is one way to provide dampening but dampening methods that do not utilize liquid are equally if not more effective.
Tire Gauge Care
Regardless of the type of gauge you use, it pays to take proper care of your precision tool. All racing gauges, whether digital or analog, should have a rubber bumper for protection. Ideally, you would never drop a tire gauge. Dropping tire gauges even one time can cause accuracy error potentially voiding the manufactures warranty. If you drop a tire gauge and that does not a rubber bumper then the shock is transmitted directly through the analog mechanism or digital sensor. Permanent damage can be the result. Rubber bumpers on tire gauges of any type are a must.
Using tire gauges properly is as important as selecting the right gauge. Over pressuring the gauge can and will damage your gauge. Let’s say a racer purchased a 30 PSI gauge to be in the center of the range for racing tires. On the way to the track the racer’s trailer gets a flat and our racer uses their racing gauge to check the trailer tires. The subsequent 60 PSI pegs the needle. In an instant it is more than likely that permanent damage has occurred to the racing gauge and the over inflation has destroyed the calibration. Care must be taken to not over load analog or digital gauges even one time. Tire gauges are precision instruments. Racing tire pressure gauges lead a hard life and the rough treatment introduces a decline in accuracy. Storing gauges safely during transport and on race night will provide for better long term performance.
Calibration
Many gauges have a fixed calibration and can not be calibrated in the field. Fixed calibration is held nicely for long periods of time if gauges are not dropped and are used within the required range.
If you have a gauge that can be calibrated you need to take it to a certified testing house or send it back to the factory for periodic calibration. Neither of these options is ideal. For racing purposes the most practical method is to simply purchase a second gauge and store it for the sole purpose of checking the calibration of your main gauge. Upon purchase – compare the two new gauges on and verify that they obtain the same reading on the same tire. If there happens to be a small difference simply record the difference and periodically verify that the gauge you use matches your master gauge and the original comparison. Use the master calibration gauge only for testing. Keeping a test gauge as a master is the most practical way to verify the accuracy of the gauge you rely on. You can compare your master gauge against you track gauge weekly as part of your set up routine.
If you use two gauges on one tire and both gauges give you the same result it is likely that they are accurate. While possible, t is unlikely that two gauges would be off by the same amount.
Adjustment
You can use tire pressure and your accurate pressure readings to adjust your car. For Bias Ply tires the air pressure can be adjusted to help your car get through the turns. Thinking out the adjustment options can help you maximize practice time or provide handling adjustability during pit stop races.
Using an accurate tire gauge will help you match your contact patch for adjustability. Thinking about how air pressure affects contact patch size, stagger and sway bar load will give you more options to find the fastest set up.
It has been my experience that you can not stretch bias ply tires so it is important to buy tires that are the right size to begin with. Crews sometimes over inflate tires in an attempt stretch them. Measurements taken right after a tire is over inflated can show a larger circumference. The reality is that as soon as the tire gets hot the tire tends to return to the factory size. Consistency in your pressures at each corner on the car is critical too. Adding 5 pounds above your standard pressure to the RR in an effort to meet your stagger numbers is not recommended.
With Bias Ply tires I recommend staying within 2lbs of your standard per tire to dial in stagger. If you can’t get to your desired stagger within a 2 PSI window then your pressure and spring rate changes will be so dramatic that having the right stagger will not overcome the pressure induced spring rate changes. Over inflated tires create too much heat and premature wear. Check your tire sizes after mounting them and if they are not the right size then ask your supplier for another set. I am more likely to drop the left side tire pressures to help with stagger adjustments verses over inflating the right – it is a balance but over inflated tires do heat up in the center. Over inflation can cause over heating and stagger variations so it should be avoided.
To help your car handle here are some Bias Ply pressure tips
Car is loose everywhere:
Add Pressure to the RF which loads the sway bar for more cross weight. Drop Pressure to the RR which reduces stagger, adds cross weight, and makes the RR footprint larger for more grip.
Car is loose off:
Reduce pressure at the RR which makes the RR footprint larger for more grip, adds cross weight, and reduces stagger.
Car is tight in the Center and Loose off:
Drop the LR tire pressure. Drop the LF Pressure. The added rear stagger will help the car turn in the center. The larger LR footprint will help on exit. With both left side pressures being lowered the cross weight change will be minimal. The larger LF footprint will create more grip in the center helping the car to turn. Cars that turn better in the center have a better angle for the exit so often curing the center automatically improves exit issues.
Air pressure on bias ply tires is fine tuning tool and the adjustments work best when the car is already handling well. Fine tuning can be achieved and pit stop adjustments are more beneficial if your tire pressure gauge is accurate. Proper care and selection of tire gauges is the key to producing race winning accuracy.
Go Forward – Move Ahead
Jeff Butcher
Courtesy of JOES Racing Products
2/5/10
