SYO237
SyTy Registry
I thought I would pass along this article written by a friend who races here locally with the SCCA and is a driving instructor. Article is on aligning torque, basically the force put on the tires to want to straighten back out when under load in a turn while rubber is being chewed off them.
Its a good read and informative to anyone wanting to race their truck.
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How many times have you found yourself in this situation – front tires grinding, heading for the outside cones, rubber dust billowing off the tires? What in the world did you do wrong? Many people would say you’re going too fast, but I say that you ignored the signals presented to you by aligning torque.
What is aligning torque? Also known as self-aligning torque, it is a force generated by tires as cornering forces increase that wants to straighten themselves out; i.e. return to center. You feel this in your fingertips as an increase in the effort required to turn the steering wheel as cornering loads build. Power steering somewhat masks this effect, but it still exists and can be felt in the steering wheel.
How can we use the information presented to our fingertips to improve our race performance? First, it is important to understand the relationship between slip angle and traction. All tires, whether soft race compounds or all-weather hard compounds, build cornering forces as slip angles increase in a relationship illustrated in the graph below. Depending on the tire, these forces will peak somewhere between 6 and 10 degrees of slip. If the slip angle increases further, the tire will generate less cornering force, even as much as a 20% reduction. Less cornering force means less speed, as well as a huge increase in the wear rate of the tire. The driver pictured above probably burned off 1/32” of rubber on that run, and collected a cone or two for his efforts as well. Also notice that the street tire has a somewhat broader peak in its traction curve, meaning that it is a bit more forgiving of ham-fisted steering inputs than the racing tire. The race tire, with its greatly increased traction limits, is also more sensitive than the street tire. This makes controlling the slip angle of the tires even more important.
Now we can relate this information back to the subject at hand, aligning torque. The graph below illustrates that the self-aligning torque increases with slip angle analogously to the increase in grip. In fact, the torque peaks at the exact same moment as the peak traction. This means we can feel when we have reached the peak traction capabilities of the tire by how much force it takes to turn the steering wheel. When we reach the maximum resistance to turning the steering wheel, we have also reached the peak performance of the tire in cornering speed.
So how can we bring this back to the high-pressure, action-packed environment of the autocross course to improve our performance? Again, it comes down to smoothness and feel. At the entrance to the turn, slowly increase the steering angle while feeling the cornering forces build in your fingertips. When you reach the highest cornering speed possible, the force required to turn the wheel will be at its highest as well. However, none of us are perfect, and we’ve all entered a turn too quickly on occasion. At that point, the slip angle of the tires will go beyond the peak traction and fall into the downhill range. At that same point, the self-aligning torque will also start decreasing and steering effort will drop dramatically. In other words, the steering will become pinky-finger light; you could dial in full lock with little effort. In fact, that is the natural tendency – to keep increasing the steering input because the car isn’t turning as much as you want it to. That is the exact reaction displayed by our pictured driver; “Oh my god, I’m heading for that cone wall – I’d better turn more!”
What should he have done to fix this mistake? Well, as soon as he felt the steering effort drop off, he should have taken two actions. First, back off of the throttle just a hair; maybe a ½” at most. This transfers the vehicle’s weight forward and restores traction to the front tires. Secondly, and this is the counter-intuitive part, back off the steering input just a bit. The reason for this is that you are already past the peak traction of the tires on the slip angle chart and you want it to return to the top of the traction curve. This seemingly simple action takes quite a bit of mental effort, as it really goes against your natural tendencies.
When traction is restored to the front tires, you will feel good resistance in your fingertips again. At that point, particularly on long-duration sweepers and 180° turns, maintain that steering angle. If you need to adjust your line slightly, do it with throttle inputs – back off slightly to tighten your line, increase the throttle slightly to widen it, but keep the steering steady. Your aim is to keep that slip angle right at the peak of traction, around 6-8° of slip. It’s all in the fingertips. Listen to an expert driver’s exhaust as they round a long 180. You will invariably hear the small adjustments in throttle all the way around the corner – WAA waa WAA waa. But do you see tire smoke? Almost never; they have learned that not only is that the fast way around the corner, it also minimizes tire wear to cut down your tire bill at the end of the season.
At the exit of the corner, gradually increase your throttle opening as the steering angle decreases. Pretend as if there is a string tied between your hand and your foot; as the steering straightens out, you can apply more and more throttle until, when the steering is straight ahead, you’re back to full throttle.
Who knew that racing was such a lesson in physics? Now that you do, you can go out and improve your autocross times and have more fun in the process. Enjoy!
Its a good read and informative to anyone wanting to race their truck.
-----------------------------------------------------------------------------------------------------------------------
How many times have you found yourself in this situation – front tires grinding, heading for the outside cones, rubber dust billowing off the tires? What in the world did you do wrong? Many people would say you’re going too fast, but I say that you ignored the signals presented to you by aligning torque.
What is aligning torque? Also known as self-aligning torque, it is a force generated by tires as cornering forces increase that wants to straighten themselves out; i.e. return to center. You feel this in your fingertips as an increase in the effort required to turn the steering wheel as cornering loads build. Power steering somewhat masks this effect, but it still exists and can be felt in the steering wheel.
How can we use the information presented to our fingertips to improve our race performance? First, it is important to understand the relationship between slip angle and traction. All tires, whether soft race compounds or all-weather hard compounds, build cornering forces as slip angles increase in a relationship illustrated in the graph below. Depending on the tire, these forces will peak somewhere between 6 and 10 degrees of slip. If the slip angle increases further, the tire will generate less cornering force, even as much as a 20% reduction. Less cornering force means less speed, as well as a huge increase in the wear rate of the tire. The driver pictured above probably burned off 1/32” of rubber on that run, and collected a cone or two for his efforts as well. Also notice that the street tire has a somewhat broader peak in its traction curve, meaning that it is a bit more forgiving of ham-fisted steering inputs than the racing tire. The race tire, with its greatly increased traction limits, is also more sensitive than the street tire. This makes controlling the slip angle of the tires even more important.
Now we can relate this information back to the subject at hand, aligning torque. The graph below illustrates that the self-aligning torque increases with slip angle analogously to the increase in grip. In fact, the torque peaks at the exact same moment as the peak traction. This means we can feel when we have reached the peak traction capabilities of the tire by how much force it takes to turn the steering wheel. When we reach the maximum resistance to turning the steering wheel, we have also reached the peak performance of the tire in cornering speed.
So how can we bring this back to the high-pressure, action-packed environment of the autocross course to improve our performance? Again, it comes down to smoothness and feel. At the entrance to the turn, slowly increase the steering angle while feeling the cornering forces build in your fingertips. When you reach the highest cornering speed possible, the force required to turn the wheel will be at its highest as well. However, none of us are perfect, and we’ve all entered a turn too quickly on occasion. At that point, the slip angle of the tires will go beyond the peak traction and fall into the downhill range. At that same point, the self-aligning torque will also start decreasing and steering effort will drop dramatically. In other words, the steering will become pinky-finger light; you could dial in full lock with little effort. In fact, that is the natural tendency – to keep increasing the steering input because the car isn’t turning as much as you want it to. That is the exact reaction displayed by our pictured driver; “Oh my god, I’m heading for that cone wall – I’d better turn more!”
What should he have done to fix this mistake? Well, as soon as he felt the steering effort drop off, he should have taken two actions. First, back off of the throttle just a hair; maybe a ½” at most. This transfers the vehicle’s weight forward and restores traction to the front tires. Secondly, and this is the counter-intuitive part, back off the steering input just a bit. The reason for this is that you are already past the peak traction of the tires on the slip angle chart and you want it to return to the top of the traction curve. This seemingly simple action takes quite a bit of mental effort, as it really goes against your natural tendencies.
When traction is restored to the front tires, you will feel good resistance in your fingertips again. At that point, particularly on long-duration sweepers and 180° turns, maintain that steering angle. If you need to adjust your line slightly, do it with throttle inputs – back off slightly to tighten your line, increase the throttle slightly to widen it, but keep the steering steady. Your aim is to keep that slip angle right at the peak of traction, around 6-8° of slip. It’s all in the fingertips. Listen to an expert driver’s exhaust as they round a long 180. You will invariably hear the small adjustments in throttle all the way around the corner – WAA waa WAA waa. But do you see tire smoke? Almost never; they have learned that not only is that the fast way around the corner, it also minimizes tire wear to cut down your tire bill at the end of the season.
At the exit of the corner, gradually increase your throttle opening as the steering angle decreases. Pretend as if there is a string tied between your hand and your foot; as the steering straightens out, you can apply more and more throttle until, when the steering is straight ahead, you’re back to full throttle.
Who knew that racing was such a lesson in physics? Now that you do, you can go out and improve your autocross times and have more fun in the process. Enjoy!