Tire pressure

From Trackpedia

This is basically the pressure inside your tires. There are usually two tire pressures that are important, cold and hot. You measure cold before driving the car at all. Hot should be measured as fast as possible when you bring the car off the track.

Typically on hot days, you set the cold pressure in the paddock to the correct setting. This depends on the car, tire type, weather and you. When you finish a session then you release pressure down to the maximum hot pressure you want to see. Further sessions that day usually also require more air to be released as the air and track temperatures rise and you typically get faster through out the day.

Tire pressure can have a huge impact on the handling of the car and how long the tires will last. Incorrect tire pressures can even cause the tire to fail or blow out. You might be surprise to learn that an overinflated tire is less likely to explode in comparison to an underinflated tire which is in fact least preffered in almost all respects! Tires are the number one priority in car checkup, be it a roadcar or racecar. No other car component (yes, not even the brakes) are so involved in accidents. You will be amazed by the effect on cornering and braking stability and well as braking distances! A tire grips the ground with a patch smaller than a shoe.

You will be surprised to hear what this little shoe does: It carries the car's weight; it dampens bumps; it accelerates when you give it gas/shift up, decelerates as you lift-off/shift down/brake, turns when you steer; It deals with different surfaces, conditions, forces, speeds and temperatures, It rolls during cornering, it drains water on wet roads (in 60mph, that about a gallon per second!). That's alot of effort in a day/hour/minute!! Remember, your engine does not speed you up, just as your brakes do not stop you, the tires do both! Bad tires are equal to faulty brakes and make the latter work harder!

In this regard, you might have heared the advice to put the good tires in the back axle. This is based on the claim that a blowout or hydroplaning effect on the rear tires would result in completly incontrolable oversteer, unlike the predictable and controlable understeer during a front tire blowout or hydroplaning. However, there are issues supporting the opposite claim of placing good tires in front, like a dramatically decreased braking distance. Our answer is to not compromise: The good tires go on all four wheels! It is impossible to compromise on anything less than that, particularly given the relativelly cheap expense on tires.

Note: Never have different tires or tires with different levels of wear on the same axle, this is a sure fire way of adversely affecting your handling. yre compounds

Tire compounds

The compound of a tyre describes the blend of materials used in the construction. Tires are full of nature's goodness - rubber, carbon and oil and it's the proportions of these which determine the properties of the finished tire.

Choosing a tire compound is a matter of compromise, and you need to think about the type of events and / or road use you're planning on putting your four wheeled machine through.

Hard: Harder compounds are useful because they wear relatively slowly, but provide less grip then softer varieties. We don't need to go into the science here, but it involves..... If you're planning on using your car for road use and a few track days, then perhaps the softest compounds aren't for you. If you're getting 10,000 miles between replacements then you've hit roughly the right balance.

Soft: Soft compound rubber provides the best grip in the dry, heats up to operating temperature fast but wears quickly. Soft compounds are also used in winter tyres as this allows them to have a lower optimum operating temperature. Chose a soft compound if you're car is a dedicated track machine, or if you have the luxury of a road and a track set of wheels. For longer events such as 24 hour races, it might be worth opting for a slightly harder compound so you don't need to pull into the pits every 20 minutes.

Tire patterns

Slicks: Threadless tires, providing ultimate dry grip, but no wet grip. In times, semi-slicks are used in specific conditions. Both types are illegal for road use.
Wet Tires: Deep threaded tires with a pattern made particularly for the sake of water draining to avoid hydroplaining.
Intermediate: Recommended when weather may change. This will allow you to drive legally on the road and provide a decent compromise in wet and dry conditions
Directionall-Thread: The tread pattern on these tires optimised for one direction of rotation. You can recognise directional tyres by the arrow shaped grooves pointing in the direction of wheel rotation. The primary advantage is the increased ability to clear water and resist aquaplaning, especially at higher speeds. In dry conditions, the added benefit of directional versus standard tread patterns is much reduced.
Asymmetrical thread: Asymmetric tyres have a different pattern on the inside and outside and are usually only used in high performance vehicles. The inner side tends to have more grooves to help with water dispersal, while the outer edge has stiffer tread blocks with less tread patterning to provide enhanced cornering performance. Asymmetric tyres must only be used in one direction or they can actually compromise handling.

Tire choice

It's best practice to fit the newest tires to the axle which needs the most grip. If you have a front wheel drive car this will usually be the front axle unless your car has a tendency to oversteer, in which case the rear wheels could probably benefit from the most grip. Rear wheel drive cars should generally have the newest tyres at the back to prevent oversteer and minimise wheelspin under power. Whether you're using race tires, high performance tires, or regular street tires, the goal of adjusting tire pressures, as with all other adjustable suspension components, is to maximize the contact patch, and thus the handling grip of the car. Generally, adjustable shocks, anti-roll bars, replaceable springs, wheel alignment, and other adjustable suspension features are used to provide the majority of adjustment to the car's handling balance and grip. Tire pressure is typically used as a fine tuning adjustment.

However, the majority of weekend track drivers are using relatively stock cars, and have only their tire pressures to adjust. Whether it's your only tuning method, or the last refining step, you need to understand what is happening to the tire during racing, and what the variables are to be effective in adjusting tire pressure. We're going to present this in the form of two phases in determining the correct tire pressures to race at.

First is the large scale setting of what the tire operates best at. Considering the wide variety of tires and cars used at any weekend hobby event, we can expect a wide range of optimum tire pressures to be used at fully heated racing temperatures. Somewhere within the 30-40 psig range is likely for most cars, but that's still a very broad range. If 40 psig is the max, then 30-40 psig represents a 25% window, and nothing in racing is as vague as a 25% window. So first, you have to determine where in this 10 psig band your tires perform best.

The second phase is the fine tuning realm of within 2 psig of the optimum setting. This small adjustable range can be played with based on track conditions to tweak for the best performance at any given time on the track.

We want to first step you through a practical approach to your first few race day's tire pressure settings, then step you through a system to determine the general range of pressure the car handles best at, then finally through some tips for fine tuning. First, however, there's some basic background stuff to cover.

Measurement

The first rule in taking tire temperatures is that they must be taken when the car comes in from a full race-speed lap. Taking temperatures after a cool down lap is useless. If necessary, the driver should come in a lap early (before the checkered flag) to ensure the tires are race hot. If the driver waits for the checkered flag, chances are he'll get stuck behind slower traffic. The track will be under a no passing rule during cool down, and the tires will cool too much.

Tire temperatures are taken in three places on the tire. First on an outer tread block on a street tire, or about 1" in from the outside edge of a racing slick. Second, in the middle of the tire. Third, on an inner tread block or 1" in from the inside edge of the tire. Often on high performance treaded tires, you'll need to measure the second tread block in from the edge. The outer shoulder block will not retain enough heat for a meaningful reading.

The temperature probe should be inserted all the way in. Whenever taking temps of a treaded tire, be sure to center the probe in the tread block. The edges will cool faster, so it is important to have as much rubber surrounding the probe as possible. Wiggle the probe a little when first pressing it in to ensure it is well seated.

The temperature reading will fluctuate for several seconds. Do not wait for it to be perfectly stable. As soon as it settles to within a couple of degrees F, take the reading and move on. You shouldn't have to wait more than 5 seconds for each reading.

Always measure each tire in the same pattern. Whether it's inside, center, outside, or the other way around doesn't matter. Just do it the same way every time.

Be consistent in the tire sequence also. Generally, you'll want to measure the hottest side first to be sure they're not overly heated. For example, if the entry to the pits is after a series of long right turns, measure the left tires first. Whatever the sequence, be sure to repeat it with every reading.

Point of origin

or your first racing experience, or your first experiences with a new tire, you'll need to first determine the realm of the gross tire pressure setting. Assuming there's no one to tell you how your tire and car combination are best set up, you'll need to experiment to know whether the car handles better in the 30-34 psig range, the 33-36 psig range, or the 36-40 psig range. Once you have a feel for this, you can fine tune to within 1/2 to 1 psig for optimum handling balance and performance.

There are two points from which to set tire pressures. First, you have to adjust the cold (ambient) pressures before the car takes to the track for the first time, or after it has completely cooled off between sessions. After the tire has reached full racing temperature, it is possible to fine tune from the fully heated point, but it will also be important to know when the tire has cooled back down what it's cold temperature is.

We know the tires are going to heat up when they're driven on. The friction between the rubber and the road will generate heat. A lot of heat. This heat is going to transfer to the gaseous air in the tire, and cause it to expand which leads to an increase in the tire pressure. It happens to work out that an increase of about 10 degrees F causes about 1 psig increased tire pressure.

On the track, you're going to work the tires much more than is possible on the street. Repetitive, high cornering forces and aggressive braking are going to generate much more heat and therefore higher tire pressures. Because of this, the typical pressures you run on the street are going to be too high for the race track. During normal street driving, tire pressures will increase over the recommended factory "cold" settings about 2-4 psig. During racing, a cold tire pressure setting will increase anywhere from 6 to 10 psig. Tire pressures must be lowered from their usual street settings before going onto the track.

As with just about everything, the "correct" starting pressure "depends." There's a lot of variables. Still, you have to start somewhere, then you can test various pressure settings to find the best balance in performance. Remember, even the pros have to test several settings every race weekend to find the best performing point. There is no formula, no "right" answer that you can use every time.

So, if you're using street tires on the track, the generic, "start somewhere" point is 5 psig lower than the car manufacturer's recommended tire pressures. (Note: use the car manufacturer's recommended tire pressure as the baseline. Many service and oil change stations with poorly trained techs will pump tires up to pressure stated on the tire side wall. Hello! That's the maximum setting, not the recommended setting. Watch out for those guys!)

From a cold starting point of 5 psig less than street pressures, you can begin the trial and error cycle of making small adjustments and determining the effect. To do this, you will need to run several laps to get the tires hot to start with. Take it easy the first 3 laps or so until the tires come up to temperature. The car will be quite sloppy with the initial low pressures.

After 3 or 4 laps, the tires should feel consistent. Run several more laps to get a feel of the overall grip level and the handling balance. If you're held up by traffic, stay out until you get at least three maximum-effort laps. Then come in to take the pressures and temperatures, and report on the handling. Do not make a cool down lap before coming in as the tires will cool down significantly making the tire temperature data useless (When you park, be sure to not use the parking brake, or rest on the brake pedal as you can easily warp the brake rotors).

If you have a temperature probe, you can use the tire profile reading as the primary guide to tell you if the pressure should be increased, decreased, or left as is.

if the centers of tires are 5 or more degrees hotter than the edges, then the tire pressure should be lowered. Try about 1 psi for each 4 to 5 degrees the center is higher than the lowest edge temperature.
if the center temperature is more than a few degrees lower than the edge temperatures, then the tire pressure is too low. Try increasing it by 1 psi for each 4 to 5 degrees the center is lower than the highest edge temperature.
use the guide below for more detailed tire temperature interpretation tips.

If you do not have a temperature probe (get one!), then use this as general guide.

Generally the tire pressures should be close to equal on a car with neutral handling. If the front or rear tire pressures are significantly different, you can adjust them to a mid point between the two readings. (If the fronts are 38 psig, and the rears are 32 psig, then try adjusting them while they're still hot to all be 35 psig). Try this setting for the next session.
If the tires felt very slippery, especially if they got worse on every lap, then the tire pressure is too high. Take each tire pressure and write it down. If only the front or rear tires felt slippery (the front was slippery if the car understeered and wouldn't respond to more steering input, the back was slippery if the car wanted to oversteer and spin), then lower the pressure of just that pair by 4 psig while they're still hot. Try this setting for the next session.
If the tires felt spongy, then the tire pressures are too low. When the pressures are too low, they will have low grip and still tend to slide like a tire that is too hard, but the feeling will be different. Instead of a smooth slippery slide, the tire will scrub and have more vibration in the slide. The difference is subtle, but you'll have feel for it in the steering wheel, and from the seat in your legs and back. In this case, increase the pressure while the tires are hot by 2 psig. Try this setting for the next session.

For the first few sessions you'll want to make large pressure adjustments of 2 to 4 psig. This way in about three sessions you can note whether the car felt better in the low 30's, mid 30's, or high 30's psig range, Once you feel the car behaves better in one of these ranges, you can move on to 1 psig incremental changes to close in on the optimum setting. If after 5 or 6 laps you know the car is just not right, don't force it. Come in and make an adjustment. There's no point to driving a really poor handling car and risking an off track excursion over it.

Once you have some experience with your car, you may find that the best starting point is only 2 psig less, or as much as 6 or 8 psig less than the street pressures you run at. There is a large difference in cars, tires, and driving style that affect this. However, starting at 5 psig lower than street pressures should prevent most tires from becoming excessively hard and slippery, and from exceeding their maximum safe pressure.

Fine Tuning

Once you know the general tire pressure range you're aiming for, you can make smaller increments to target the optimum setting.

If you're racing on a particular brand and model of tire for the first time, or you've never taken tire data before, you have some trial and error work to get through, and this will take some time. Depending on the track time available to you, it may take more than your first event to close in on the optimum pressure settings.

If you're running on a race track for the first time ever, this will lengthen how long it takes to determine the best tire pressures as your own inconsistent and incorrect driving will cloud the data you collect. However, take the right data, (use the charts we've provided, and the information in many of the recommended books), and you'll be able to quickly narrow in on the best performing pressures.

Based on the car's handling and tire temperature readings, make adjustments in small increments of 1/2 to 1 psig as needed, then run several more laps to feel the difference. Repeat this cycle as many times as needed until the best balance and maximum grip is achieved (as shown by your lap times). Use the charts we've provided (above and the Tunding Guide) to help interpret the feel and the tire temperature data into how to adjust the pressures.

Each time the car comes in, use your log sheet (see the Practice Sessions section for a downloadable log sheet) and immediately write down the tire pressures and temperatures. Make notes as to the specific handling responses in each corner of the track. Also, write down exactly the changes made on each tire.

After several sessions, you'll get to know the range within a pound or so for the tires that they perform best when hot. When they cool down, you can make note of the cold pressures, and use those pressures the next time as a starting point.

Once you have a feel for the target pressure when the tires are hot, there is some math you can use to calculate a starting point that should be within a pound or two of the best settings. How that works, requires some further understanding of the ambient air temperature and it's effects on the tire's pressure.

Adjusting For Ambient Temperature

We mentioned earlier that 10 degrees F changes the tire pressure about 1 psig. If a tire starts with a certain cold pressure in the morning, the warming of the day is effectively adding pressure to the tire over the course of that day. The temperature the tire runs at will be affected by the combination of the ambient air temperature, the track surface temperature, and the amount of friction introduced (how hard you drive).

If for your first session of the day, it's overcast, the air temp is 65 and the track temp is 70, then in the afternoon the sky is clear, the air temp is 85, and the track temp is 105, there is considerably more heat to influence the temperature of the tire surface. If you drive just as hard, the tire will be hotter, and the pressure will be higher. This change from morning to afternoon is going to be much more pronounced in some climates than others. The southwestern U.S. can see days with a 45 degree morning and a 90 degree afternoon. This will increase the starting tire pressure by 4 psig by the end of the day. This will make a world of difference in the handling of the car during each session.

To maintain the same racing pressure settings in the afternoon as achieved in the morning, you'll have to compensate for the increased pressure due to ambient temperature. While there is probably a formula to understand the effect of the relationship of the ambient temperature and the track temperature, it will be complicated by the aerodynamics around the tires, and the heat generated by the brakes. You can generally use the ambient air temperature and the 10 degrees per 1 psig relationship as a guide for adjusting tire pressures throughout the day. Take ambient air temperature readings at the start of each session, and use this to determine how to adjust the tire pressures. As the day warms, you'll have to drop tire pressures accordingly before each session.

One more item related to ambient temperature--you’ll find that if the car is parked with one side of the car facing the sun, those two tires might be 10 to 20°F warmer than the shaded side (and therefore 1 to 2 psig higher in pressure). You should cover those tires up with some simple plywood panels, or at least know not to set them to the same pressure as the cooler side of the car. Optimizing Tire Performance

The above sections provided some practical guidance to settings tire pressures. In these next sections, we'll take a little closer look at exactly what it is we're trying to do.

What exactly is the goal of tweaking tire pressure? The bottom line is that you're looking for the maximum traction possible over the largest portion of the track possible.

With a given set of tires (which have a determined rubber compound and size you can't change), the variables you can play with to achieve the maximum traction is finding the tire's best operating temperature, and optimizing the contact patch size. The discussion above in finding the best tire pressure settings is really just an indirect method of tuning the contact patch and having the tires operate at their peak temperature range.

The temperature of the tire is influenced by the ambient air temperature, the surface temperature of the track, and your driving (how hard you push the car). Because you won't have the luxury of choosing an optimum rubber compound for the conditions like the pros, your main tool for achieving the maximum grip temperature is driving the car at its maximum capacity without exceeding it (and overheating the tires). This assumes the ambient temperatures are warm enough to achieve the tire temperature needed. If it's 50 degrees outside, chances are, the tires will never get hot enough to produce their maximum grip potential.

The contact patch of a given tire size is affected by the suspension geometry, and by the tire pressure. In a stock street car, you don't have any adjustability at the track in the suspension, so tire pressure is your main tool.

Optimizing Tire Temperature

While we start with setting tire pressures in their cold state, we're really interested in their pressures (and temperatures) when they're running hot.

Every tire will have a temperature range where the rubber reaches its most "sticky" point without becoming greasy, and without physically falling apart. Race tire manufacturers often have this temperature identified, but for a street tire, it's unlikely you'll find any spec sheet with this temperature data identified. Finding the best temperature range of the tire will come primarily from experience with driving on it, knowing how hard it can be pushed before overheating causes it to become slippery. When you get runs of 15 to 20 minutes or more that were fast (within a few tenths as fast as you've been able to achieve), and the tire grip stayed consistent, you've probably found the maximum grip operating temperature of the tire. Head back to the pits, and take pressure and temperature readings, and use that data as a target for that tire. For most street tires the temperature range can be expected to be about 180 to 200 degrees F before they get slippery. You may have sessions where faster laps are possible, but if, in attempting to keep that pace, the tire progressively gets slipperier, then you're driving the tires too hard and they're overheating. Optimizing the Contact Patch

We know the tires are going to heat up when they're driven on. The friction between the rubber and the road will generate heat. A lot of heat. This heat is going to transfer to the gaseous air in the tire, and cause it to expand which leads to an increase in the tire pressure. It happens to work out that an increase of about 10 degrees F causes about 1 psig increased tire pressure. During racing, a cold tire pressure setting will increase anywhere from 4 to 10 psig.

The increased pressure has an effect on changing the tire shape, and the resulting area in contact with the road. This "contact patch" is what we're trying to optimize. In fact, we're trying to maximize it.

To understand the effect of pressure on the contact patch, let's first look at a tire in a static load state (rolling straight or parked).

If the tire pressure starts out too high, the increased pressure from heat buildup will make the tire shape somewhat convex. The contact patch will be narrower with the outer edges of the tire not able to touch the road because the rounded middle has lifted the edges. This tire is said to be too "hard."

If the starting pressure is too low, even after heating up, the tire shape will tend towards being concave. The outer edges under the wheel rims will have firm contact with the road, but the middle will deflect inwards. This minimizes the contact patch in the middle. This tire is said to be too "soft."

The perfect tire will have a contact patch the full width of the tire with an even pressure across the width. This is the condition we're striving for.

We've just looked at the case of a static tire. However, when we're racing on a road course, the tires are not always in such a static state. The forces of cornering, braking, and accelerating will constantly be changing the shape of the contact patch as the rubber twists from these forces. Indeed, it is under cornering that we are typically looking for the highest levels of grip possible. All else being equal, a car that can get through corners the fastest, will be the quickest. Again, all else being equal, the car with the optimized contact patches will have the most grip and be the quickest through the corners.

While there are several suspension adjustments possible in race cars and modified street cars, we may also need to "tweak" the air pressure on an individual wheel basis to achieve the maximum contact patch under the dynamic conditions of the road course. In fact, these will be different for each track.

A harder tire may have a slight convex shape during a straight run, but may be necessary to help keep the sidewall stiff enough during cornering to prevent the tire from rolling under the wheel rim excessively. So, while in a static state, the contact patch is smaller than optimally possible, this actually increases the effective contact patch size during cornering.

These tire behaviors during cornering are what might lead us to having left side tires, right side tires, or even a single tire at a different pressure than the others. This would be done to fine tune the handling of the car through certain corners.

The final goal is actually a compromise in looking for the maximum contact patch during as much of the lap as possible. We may find that one settings is optimum for corners 3 & 7, but that another is optimum for corners 2 & 4. If corners 2 & 4 are faster than 3 & 7, then the emphasis must be placed on corners 2 & 4 as there is more to gain from them being optimized. In the end, the "correct" tire pressures are the ones that allow the fastest laps in long runs.

Referenced material

http://turnfast.com/tech_handling/handling_pressure