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| TIRE BASICS 2 |
| CORNERING POWER AND TRACTION CAPACITY |
| Let's work with the curve from the Tire Basics 1 section: that we have a vertical load of 500 lbs. |
| We'll take it a step further; say we have a 2,000 lb car with 50/50 front to rear weight ratio, which means we have 1,000 lbs of vertical load on each wheel pair, front and rear. This gives us a static load of 500 lbs per tire. And under ideal conditions, as the traction curve showed, that tire can generate a force of 700 lbs. |
| As stated previously, the amount of force (measured at 1.4g's) is the tire's co-efficient of friction. Which means the tire can accelerate, decelerate, and corner with a force of 1.4g's. |
| This force, when cornering (lateral) is known as 'Cornering Power'. This force, when accelerating (throttle) or decelerating (braking) is known as 'Traction Capacity'. |
| One important thing to note, however, is that although our tire can generate 1.4g's of force in any direction, due to lateral load transfer during cornering, the TIRE WILL NOT corner at 1.4 g's. |
| The car will, theoretically, accelerate in a straight line and put 1.4g's of force to the ground, but that's because the weight is evenly distributed on both driven wheels. If you recall from the Tire Basics 1 section, the increase in vertical load makes the tire less efficient. The combination of increased vertical load on the outside wheels during a turn, and decreased vertical load on the inside wheels, reduces the overall cornering power of the pair. |
| SLIP ANGLE AND THE TIRE CURVE |
| The tire curve is pretty cool to know. It tells you a lot about what you should expect when putting together a build for your cars in Forza Motorsport 3. Depending upon the class of car you're building for, and assuming we're treated to somewhat realistic tire behavior, you may be running anything from an aggressive street tire to a full blown, super soft race slick. Each tire has very different behavioral characteristics as illustrated by the tire curve. (Not to be confused with the traction curve). |
| First of all, what the hell is a slip angle? Without going too crazy with details, as it was explained to me at the track, it's the angular difference between the direction the rim is facing, and the actual path of the rolling tire. It has to do with the fact that the tire is elastic and twists. So the contact patch, because it bears all the weight of the vehicle, will resist when you turn the steering wheel. The end result is that the contact patch will not turn as far as the rim does. |
| What we do need to know, in order to make sense of the tire curve (good info for your builds), is that slip angle increases with the tire's co-efficient of friction. And the higher the co-efficient, the more cornering power we have, to a limited extent. |
| The graph below represents the relationship between slip angle and the coefficient of friction. |
| THE RED LINE IS THE CURVE OF A RACE TIRE |
| THE BLUE LINE IS THE CURVE OF A STREET TIRE |
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| As you can see from the graph, the race tire's coefficient is much steeper, which is by design when compared to the street tire. Race tires are designed to increase the coefficient almost linearly in respect to increasing slip angles. This allows the race car driver to develop cornering power quickly as he enters a turn. |
| At the top of the curve, all tire curves are designed to flatten out somewhat. This gives the driver a comfortable margin of error to keep the car at the limit of tire adhesion as the slip angle increases. |
| The biggest difference between the street tire and race tire is there's a much larger flat spot, and hence more margin for error. This is by design as well. The street tire is much safer and more forgiving for the unskilled driver. As you can see, at the edge of adhesion with the street tire, it has a range from about 8 degrees to 16 degrees of slip angle before things get ugly. |
| With the race tire, the car is able to generate force much more quickly, and at much higher levels. But the operating range at the limit of adhesion is much smaller and far less forgiving at increased levels of force. The race tire is much more capable in the hands of a skilled driver, but things can get ugly in a hurry once the slip angle increases past the limit of adhesion. |
| The street tire, on the other hand, will give more feedback (tire squeal) as it is being pushed to the limit, allowing the driver ample time to make the correction necessary to bring the tire back into a safe operating range. The race tire is less forgiving, and breaks loose much sooner and with less warning. |
| THE TRACTION CIRCLE |
| Tuning aside, we enjoy driving as well. Understanding the traction circle can help us become better drivers. As we increase our understanding of the nature and importance of tires, we can apply what we learn in our tunes as well as how we handle our cars on the track. The traction circle holds valuable insight that will help us improve in all areas. |
| From what we've learned so far, we've reasoned that for a given load on a tire, that tire will generate a finite amount of force. That force can be applied to cornering, accelerating, and braking. But what about when you trail brake? You corner AND brake at the same time. And what about in a carousel turn? You corner AND accelerate at the same time. So we reason that a tire will have force applied to it in more than one direction at the same time. |
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| Let's say our tire has a coefficient of friction that can generate a total of 1.4g's of force. The graph above shows that for any given tire load used for acceleration, braking, or cornering, that less force is available for another direction. The circumference of the circle itself represents the absolute limit of tire adhesion. |
| According to VECTOR 'F1' (for force) we can see that if the tire is accelerating at a rate of 1g of forward thrust, that at the limit of tire adhesion, there is 1.0g of cornering force available for turning right. |
| If we look at VECTOR 'F2', we see that if we are using all the available tire force for braking, there is no available force left over for turning. So if we brake really hard and try to turn here, the wheels will turn but the car will continue to plow straight ahead. |
| It must also be noted that while under deceleration only, the tire will brake with a force of 1.4g's, but in the case of VECTOR 'F1', while both cornering and accelerating, the tire generates a total force of 2g's. |
| Applying this information to the way we drive, it holds insight into why our cars behave the way they do. If you've ever wondered why your car tends to understeer when entering a corner and oversteer (wheelspin) on corner exit, the answers lie in the traction circle. When braking hard, the available force is being used to slow the car, so there's less traction left to turn the car. On turn exit, a lot of cornering force is being used to keep the car stable through the turn, so there's little traction left for accelerating. This is most common in slow speed corners. We're trying to corner as fast as possible, which hopefully, is at or near the limit of adhesion. But because the corner is slow, we have an abundance of available engine power at the wheels that the tires can't handle. This is why when we're not smooth on the throttle on the way out, we light up the tires and spin them. Once the car takes its set and we've got the wheels straight, we have more available tire force for acceleration and we can really get on it. |
