Long gone are the days of...
Long gone are the days of hiking the left front wheel in a Dirt Late Model. Most of the time today we see dirt cars racing with all four tires on the track. This car is pointed mostly straight ahead but still running on three tires. Why do that when using four tires will represent an increase of 33 percent in traction?
I've gotten emails telling me that some readers don't fully understand how to put all of the information we present into a package that will help them improve their car and give them the opportunity to win. I understand that all of this can be somewhat confusing if you don't know the ultimate goals.
We ran this information by you in the July '09 issue and decided we should present it again with a new title and approach. If you read this before, read it again and try to get the point that all we do is toward making the tires happy. If you've not seen this before, I can tell you that what is presented here is some of the most important and meaningful information you'll ever get anywhere.
When we begin to understand how various settings and procedures affect the amount of traction in our race cars, we can better prepare the car for racing. What we need is traction control, not the electronic kind, but an approach to our design and setup of the car that will increase traction. The following is an attempt at helping you understand the most basic influences that affect any race car's performance and limit how fast it will go through the turns. Knowing those will help you build more traction.
We can see that as the number...
We can see that as the number of pounds of load the tire supports increases, the units of traction do not increase at the same rate. The dashed line would represent a linear equal increase in traction to the increase in load supported by the tire. In reality, the solid line more closely represents the true picture. At 100 pounds of load, the units of traction are 1.0. If we increase the load to 600 pounds, the units of traction only increase to 4.4 instead of six times the load, which would be 6.0.
With more traction, we can brake harder, go faster through the turns and accelerate harder and faster off the turns. Everything we do with race car setup, outside engine prep, aero tweaking, and driveline technology, is connected with trying to develop more traction.
Traction enhancing technology has grown in recent times. We have collectively learned what the tires want and somewhat how to give them the opportunity to maintain grip with the racing surface as much as the laws of physics will allow. Let's face it, there are limits to everything in this physical world, so we go in search of finding the ultimate limit. We try to learn to recognize when we get to the limit so we can stop looking lest we go backwards.
The principle of stopping when you're ahead is true in developing a good handling package and remains true when developing the best traction package. Know when you've gone far enough. The 99 percent rule applies here. If you have the fastest car in the field, any change you make has a 99 percent chance of slowing you down.
As the tire pressure is reduced...
As the tire pressure is reduced from optimum, the pressure on the middle portion of the tire is reduced resulting in less traction. More loading occurs at the outside edges of the tire, but the overall grip level is less.
The word package is an important one, because we might well be using several different approaches at the same time to enhance traction. They rarely interfere with each other and each one will add a little to the package. Collectively, they can add up to a marked improvement in available traction while under power.
Tires Are The Key
It has been said many times before by most authors of racing technology that everything we do related to chassis setup comes down to making the tires work harder. Tires are the ultimate connection between the car and the racing surface. Far more racers have been disqualified for illegal tires than any other infringement. It's because improvement to grip represents the most gain in performance.
The concept of tire performance is always at the forefront when trying to understand ways to improve the overall performance in our race car. It is again at the very top of the list when we discuss traction. Listed here are seven areas of influence that directly affect the amount of traction that a set of dirt or asphalt tires will develop:
Increasing the amount of vertical loading (weight) on the tire increases traction, but in a non-linear way. That is to say that as we increase loading on a tire, it will gain traction, but not in exact multiples. If a tire has "X" amount of traction with 400 pounds on it, the traction will be less than double as we apply 800 pounds of loading to it. The amount of traction will be less than 2 times X.
A similar situation occurs...
A similar situation occurs as we over-inflate the tire. The outer edges of the tire lose pressure to the racing surface which results in less traction. At optimum pressure, the entire width of the tire contact patch will exert equal pressure on the racing surface.
The size and cross sectional loading of the contact patch helps determine how much traction we will have for a particular tire. An added effect related to the contact patch and traction involves grooving and siping with dirt tires and will be discussed later on.
Reducing the air pressure will usually increase the size of the tire contract patch which would seem to enhance traction, but excessively low or high pressures may reduce the loading on portions of the tire so that the total loading of the tire is reduced and we end up with less available traction for that tire. There is an optimum operating air pressure for each tire that will offer maximum contact patch area and equal loading across the width of the patch.
Camber also affects the size and cross sectional loading of the contact patch. The correct camber angle compensates for the deflection of the tire sidewalls as the lateral force is applied when we turn the car. More or less camber than what would be ideal means that one side of the tire will support more load than the other and this also reduces traction.
The chemical makeup of the compound of the rubber helps to determine how much traction is available from a tire. A softer tire will provide more traction, but the maximum amount of traction that can be utilized over a long period of time concerns how the tire holds up to heat and wear.
If we could look down on the...
If we could look down on the tire contact patch during cornering, we would want to see an even pattern across the width of the tire much like this sketch. The tire is exerting equal pressure and maximum contact patch.
A tire that is a little harder may sometimes hold up better and be faster toward the end of the race when the tires have built up a lot of heat and are well worn after a number of laps. The way a softer tire grips better is in the way it conforms to the track surface irregularities. A harder tire will ride over the small dips and depressions in the asphalt or dirt while a more compliant tire will fill those areas to create more surface contact area that is riding on the track.
There is a fine line between being soft enough to fill the gaps and being too soft and causing the rubber to peel off and cause excessive wear. Racing tire companies are constantly experimenting with compounds to make the tire stick better while still maintaining a decent life span.
Angle of Attack
The amount of traction available from a tire can actually be enhanced simply by increasing its angle of attack relative to the direction of the car, but only up to a point. From straight ahead, we can turn the wheel and with each degree of angle of deviation from the direction of travel, the traction in the tire increases.
There is a point we reach where the gain is reduced and we approach the limit of attack angle that the tire can handle. Once that point is reached, going beyond causes a sudden loss of grip and traction falls off drastically. The tire will then slide across the track surface. This principle is true of all four of our tires whether they are the front or rear tires. We will provide more on this subject later.
We have talked a lot about trying to achieve a balance in your setup, but what does that mean exactly? Balance in the dynamic sense is when you have both ends of the car working together and trying to do the same thing. In a cornering situation, doing the same thing all comes down to wanting to roll to the same angle. All of what a suspension system is influenced by comes down to a simple, but comprehensive final result, a roll angle that the system is trying to end up at.
It is the matching of the desires of the front and rear suspensions that causes the correct transfer of loads during cornering. What we get out of that matching of roll angles is the best loading of all of the tires in combination that will yield the most traction, period. Here's why.
If the RF tire has too much...
If the RF tire has too much negative camber set into it, at mid-turn, the contact patch might well look like the pattern shown here and the tire would have less traction. It is important to set the proper camber so that in the turns, the tire will have the largest footprint or contact patch area.
An opposing pair of tires (tires on the same axle, at the same end of the car) will develop maximum traction when they are equally loaded. That is a generally true statement that has been made many times in the past in countless publications. A car that is not as fast as others, but neutral in handling, probably does not have a dynamically balanced setup where both ends are working together.
An example of this is when the rear suspension is trying to roll to a greater angle than the front. Excess load will be transferred from the left front to the right front causing more unequal loading of the front tires and therefore less traction.
The car would be tight under this scenario, but to make it neutral in handling, we reduce the crossweight, which unloads the LR and RF making both ends less equally loaded and therefore resulting in less traction for both ends. The car will be neutral in handling, but slower through the turns.
The shape of the track for both dirt and asphalt can influence the available traction in several different ways. We need to know a little about how the track is banked, how the banking angle is changing entering and coming off the corners and how the radius of the turn might be changing.
A highly banked racetrack is very forgiving when it comes to needing traction for accelerating off the corners. There is so much added loading on the tires from the mechanical downforce of the banking and associated lateral forces, that many times the tires are loaded to the extent that we can't break them loose under normal conditions with a balanced setup. The tracks where we worry about getting off the corners are the ones that are flatter and with less surface grip.
Hard tires, or ones that have...
Hard tires, or ones that have been used and retain less overall rubber, will not conform to the track surface irregularities well. With less of the tire making contact with the track surface, less traction will result.
Also, the severity of change in banking angle of the racing surface in the transitions of the track where we are braking into and accelerating off the corners can cause changes to the pitch angle of the chassis that works to unload one or more tires. This can cause more unequal loading and thereby reduce traction.
A track that goes from high banking to low banking fairly quickly can cause the left rear tire to unload quickly making the car loose. There are two ways this can happen. One is when the outside edge of the track drops in elevation and the right front tire follows the drop-off, load will be reduced at the left rear tire causing loss of traction in that tire if the shock rebound setting is too high.
The other problem occurs when the inside edge of the track rises up to match the elevation of the outside edge of the track. As the left front tire rises up, the LF and RR pair of tires becomes more loaded momentarily causing loss of loading in the opposing pair of tires. The loss of crossweight percent (RF to LR) makes the car lose traction in the rear.
A track that has a decreasing radius in the latter portion of one of the turns can cause a car to develop a loose condition at that point. Usually, older tracks that were originally dirt and then paved retain a straight front stretch and a rounded out back "straightaway." This "D" shape causes Turns 1 and 4 to be a smaller radius than Turns 2 and 3 for that reason. So, Turn 4 is difficult to accelerate off of due to the decreasing radius.
Remember we said that traction increases for a set of opposing tires when we increase the angle of attack (simply put, this is when we turn the steering wheel more). If the car is neutral in and through the middle of the turns, then as we approach the tightest portion of the turn past midway, where the radius is less, we need to turn the steering wheel more and that produces more front traction than rear traction.
The balance we enjoyed through the middle of the turn is now upset and the car becomes loose just when we are getting back to the throttle. This causes loss of rear traction. We will study ways to compensate for this later.
The Racing Surface
New tires are faster not only...
New tires are faster not only because they have better rubber compound qualities over used tires, but also because they have a thicker layer of rubber and that helps to provide a cushion to help fill the gaps in the track surface. As we wear off rubber naturally, the layer of cushion decreases and the tire rides more on the ridges in the track rather than on all of the surface area. Tires that are treated, either legally or not, will conform to the track surface irregularities better and provide more traction. You can overdue softening of the tires to the point of causing excess wear over the same period of time.
The surface we race on largely determines the amount of traction available and we will look at dirt and asphalt tracks separately. On dirt tracks, the amount of moisture dictates the amount of grip the track gives us for the most part. Bumps, grooves, banking angles, and the overall radius all help determine how much grip is available for traction in the corners. The setup related to shocks, springs, and front and rear geometry help determine how much traction will be available for a certain set of conditions.
On asphalt tracks, and even some "dirt" tracks that have been oiled to the point of almost being asphalt, the surface is more consistent and other than holes or bumps and rises in the surface, we can expect the grip to be the same over the course of the event. Flatter banking and older asphalt dictates the need for more traction control efforts.
Now that we have some kind of understanding of just what affects traction in our race car's tires, we need to think out our setups so that we can use that information to enhance the overall amount of traction in our tires.
Much of what we present month to month deals with the components and methods we can use to promote balance in our setups. As you have read above, when we select the correct spring rates, front and rear roll center locations, shock settings, and load distribution, we will then enjoy the most traction our car is capable of.
Engine Torque Promotes Equal Loading
Some tracks and sanctioning...
Some tracks and sanctioning bodies allow the use of tire treatment which softens the rubber compound. This can be a way to limit cost, allowing a team to run otherwise uncompetitive hard, old tires. Other tracks have rules that don't allow this practice, but mostly look the other way and the teams must soak their tires in order to be competitive. Promoters should define and enforce tire rules either way.
There is one effect that helps promote traction that every stock car has, but few realize, it's the effect of engine torque. When we get back on the throttle, the torque from the rotation of the engine, through the driveshaft, tries to rotate the whole rear end in a counter clockwise direction when viewed from the rear. This action, or force, loads the left rear tire as well as the right front. When those two corners are more loaded, the crossweight percent goes up and the car gets tighter. Also, if the RR tire was supporting more weight than the LR tire, then with this effect, the two rear tires would be more equally loaded providing more traction.
A question often asked is why the car does not get loose immediately when we gas it up if the rear tires are already providing all of their available traction keeping the car off the wall. The introduction of power would cause the tires to lose traction if it were not for the added affect of the engine torque. There is no way to enhance this effect and the magnitude is dependent on the amount of torque the engine develops at a given rpm verses the width of the rear tires. The wider the rear track width, the less effect torque will have on adding load to the LR.
As a driver turns the steering...
As a driver turns the steering wheel, the front tires develop an angle of attack relative to the direction of travel of the car. The more the wheel is turned, the greater the angle of attack. With more angle of attack, the front end gains traction up to a point. This is how a driver can compensate for a tight car. Steering angle and steering input are ways we determine how neutral a setup is.
When the steering angle becomes...
When the steering angle becomes excessive, the tire gives up most of it's available traction resulting in a severe push. Either this happens with a very tight setup, or the excess steering angle overtakes the rear in front traction and the car will go loose. This is called the 'tight-loose" syndrome.
Many current dirt tracks as...
Many current dirt tracks as well as some asphalt tracks that used to be dirt have developed a "D" shape. This is caused by having a wall along the grandstand side only and as the track gets raced on and groomed, the back side away from the grandstands gets pushed out. This makes Turns 1 and 4 tighter than Turns 2 and 3. More steering is required for the tighter turns and in Turn 4 it is usually very difficult to get traction under power as opposed to exiting Turn 2.