In this post let’s talk about where the “rubber meets the road”. This area is known as the "contact patch". In order to discuss the contact patch, we will also be talking about weight transfer, how the vehicles weight moves around as we drive. See, once your feet leave mother earth and sit in the seat of a vehicle, the only thing keeping you stuck to the road are your tires.
Tires are way more complex than most people give them credit for, but we’re not going to get into the construction of the tire, but rather the function of the tire on the surface of the road. As well as how you as a driver effect that function.
An average tire has a contact patch about the size of the palm of your hand. Another way to consider the contact patch size is to take a standard size sheet of paper and fold it in half and then half again. The rectangle shape you now have is about the size of the average contact patch. So as you are driving down the road, the only thing keeping you connected to the surface of the road, is a patch of rubber that totals to be about the size of a sheet of paper! Yea, keep thinking about that for too long and it just might scare you away from driving all together.
Now let’s add to the fact that this relatively small contact patch is almost constantly changing in size.
When we brake, accelerate and turn the steering wheel left and right, we are constantly moving the vehicles mass around and thereby changing the size of the contact patch on the road surface.
Here's the deal;
Cars move on three basic axis, Vertical, transverse and longitudinal. (Fig. 1)
The vertical axis is the axis the vehicle rotates on if its spinning, also called the “yaw” axis. The transverse axis comes in to play when braking and/or accelerating. And lastly, the longitudinal axis is what the car moves on when cornering.
Fig. 1
Let's say we are starting with a vehicle that has equal weight distribution across both axles. This is to say that half of the vehicles weight is supported by the front axle, and the other half by the rear axle. This set up provides four equal contact patches on the road. (Fig. 2)
Fig.2
As long as the vehicle is either sitting still or driving in a straight line at a constant speed, the four contact patches will remain equal. But lets say that we press the brake pedal or simply decelerate. When we do this the weight of the vehicle shifts to the front. You know, when the smelly little pine tree hanging from the rear-view mirror swings toward the windshield. This 'shift of weight' to the front applies more weight to the front axle therefore making the front contact patches a little bigger. Much in the same way that if you had a beach ball sitting on the ground it would have a certain portion of the ball touching the ground, but if you sat on it, more of the ball would be touching the ground. Same concept.
But, because a vehicle moves the way that it does on these axis, the weight that was added to the front, came from the rear. This means the rear axle now has less weight on it therefore the rear contact patches become a bit smaller. (Fig. 3)
Fig. 3
Obviously the above figure is an exaduration, but I think you get the point.
Here's the deal;
Cars move on three basic axis, Vertical, transverse and longitudinal. (Fig. 1)
The vertical axis is the axis the vehicle rotates on if its spinning, also called the “yaw” axis. The transverse axis comes in to play when braking and/or accelerating. And lastly, the longitudinal axis is what the car moves on when cornering.
Fig. 1
Let's say we are starting with a vehicle that has equal weight distribution across both axles. This is to say that half of the vehicles weight is supported by the front axle, and the other half by the rear axle. This set up provides four equal contact patches on the road. (Fig. 2)
Fig.2
As long as the vehicle is either sitting still or driving in a straight line at a constant speed, the four contact patches will remain equal. But lets say that we press the brake pedal or simply decelerate. When we do this the weight of the vehicle shifts to the front. You know, when the smelly little pine tree hanging from the rear-view mirror swings toward the windshield. This 'shift of weight' to the front applies more weight to the front axle therefore making the front contact patches a little bigger. Much in the same way that if you had a beach ball sitting on the ground it would have a certain portion of the ball touching the ground, but if you sat on it, more of the ball would be touching the ground. Same concept.
But, because a vehicle moves the way that it does on these axis, the weight that was added to the front, came from the rear. This means the rear axle now has less weight on it therefore the rear contact patches become a bit smaller. (Fig. 3)
Fig. 3
Obviously the above figure is an exaduration, but I think you get the point.
With the above being true, then obviously when we accelerate in a vehicle just the opposite happens. Weight moves to the rear axel and increases the rear contact patch and the weight came from the front and reduced the contact patch size on the front tires.
Likewise the same happens when you turn the steering wheel except it’s a left to right thing rather than a front to rear.
So this is how the “vehicle dynamics" effects the contact patch.
There is a way of looking at the contact patch called the “friction circle”. It’s a simple way of looking at a tires grip on the surface.
I’m all about simplicity, so I will explain this as simply as I know how. Well, simple is actually the ONLY way I know how to explain it, so its more about my ability than yours.
The circle below will represent the “limit” of the tires contact patch on a dry road. (Fig. 4) This means that if you go outside of that black circle, the tire starts to slip at a high rate. So if accelerating, it means wheel spin. If cornering it means the tire is sliding sideways. And if braking it means you have locked the tires and it’s skidding. Or nowadays, you've activated the ABS.
Fig. 4
Now, If its raining and the road is wet, the “limit” becomes smaller because the surface is more slippery. (the blue line Fig. 5)
Fig. 5
And of course if you were driving on snow or ice, that circle would be even smaller yet.
So lets say that we are driving on a dry asphalt surface and we end up in a moment where we have to break pretty hard. Maybe we weren't paying attention and we were caught by surprise by a stopped vehicle in front of us. So we apply the brakes hard. (Fig. 6)
Fig. 6
As you can see I added the directional forces that are applied to the contact patch. Yes, I do know my right from my left. Again I am showing forces applied. So for example, when you turn the wheel to the right, the force on the contact patch pushes left.
You can see that in figure six we are braking hard so it pushes the tire close to the limit of its ability, but still inside of the tires capability. All is good, no skidding. No ABS. No lock up.
Now let’s say that we realize that we are closing in on the car in front of us too fast and we are not going to be able to stop before making impact. This means we are going to have to swerve to avoid. We find an opening to the left and we turn the wheel. (Fig. 7)
Fig. 7
Again, you can see that the red line shows that the forces to the left are still within the limits of the tires grip., therefore we successfully avoid the incident without any loss of traction right? …………..WRONG!
When you vector the two lines, they meet OUTSIDE of the limit of grip. This is because you are utilizing both forces on the tire at the same time. (Fig. 8)
Fig. 8
So, as I said, you see that when you vector the lines, the two points meet OUTSIDE of the tires limit of grip and now you have a tire that has lost traction. A tire can only do 100% of one thing. This means you can use the brakes all the way to the limit or you can accelerate to the limit and you can corner to the limit. But of you are braking at the limit and then you try to corner at the limit, the tire is not going to be able to handle both of those inputs at 100% and it will lose traction.
Does this mean that we were not able to avoid the incident? Not necessarily, it simply means that the tire is sliding and now you have to "manage" it. You have to get the tire to function back within its limits. You can do this by reducing your braking or reducing the steering input or both. And, provided you have the time to do this, you could still avoid the incident.
Also, these days we now have vehicles with systems that can recognize when you have gotten yourself beyond the tires limit and they can assist you in getting the vehicle on your intended path. Some of these systems are known as; traction control, stability control and ABS. In the situation in this example, the systems would have come into play. (Fig. 9)
Fig. 9
The yellow line shows that the systems would operate to keep you near the limit to try and maximize both of the requests you have made, within the tires ability. Provided you have ABS, all you really need to do is stay on the brake and steer the vehicle, the systems will do the rest.
So that’s a quick and dirty way of showing you how inputs to the vehicles control systems affect the contact patch. In a more complicated but realistic picture you would see that a real contact patch is not a circle, it’s more of an oval type shape. (Fig. 10) below is a picture taken by a camera mounted under a glass plate as a vehicle drove over the plate. This shows an actual contact patch. The patch and its size will also vary dependent on tire pressure.
Fig. 10
The reality is, the example I have discussed, no matter how detailed (or simplified) is only for one specific moment in time. Its what the tire is capable of at that moment, at that time, on that specific piece of asphalt, at that speed, with that steering angle. As the vehicle moves, the tire (contact patch) is constantly changing the surface that it is in contact with. Maybe it started out on a perfectly clean dry spot of asphalt, but then transitioned to a spot that had a little sand or oil on it. Whatever the case may be, its an ever changing scenario.
A lot of race cars have data acquisition on them and its really cool to look at data and look at the plot points from the G-meter. They use these meters to show where the car can be faster and where it had more or less grip. It can also show the driver where he can gain time/speed by utilizing the vehicle a bit more to its limits in some areas. And where he/she may be losing time/speed due to going beyond the tires grip level in some places. It’s a cool science, and although you may not be into racing and you feel you may never take a tire to it’s "limit of grip". The reality is, the tire has no idea if it has gotten to its limit or gone beyond its limit because its on a race track and the driver is trying to get the most out of his/her contact patch in an effort to go fast, or if its on a highway trying to avoid a head on collision with another vehicle. All the tire knows is that it has been taken to or beyond it’s limit and it will react accordingly. It’s now up to you as a driver to manage the rest.
What?......................you mean they didn’t explain that to you in Driver’s Ed?..................... I’m shocked!
Now give you're sixteen year old the keys and send him/her on there way. Scary isn't it? It's no wonder the leading cause of death for teens is auto accidents.
Likewise the same happens when you turn the steering wheel except it’s a left to right thing rather than a front to rear.
So this is how the “vehicle dynamics" effects the contact patch.
There is a way of looking at the contact patch called the “friction circle”. It’s a simple way of looking at a tires grip on the surface.
I’m all about simplicity, so I will explain this as simply as I know how. Well, simple is actually the ONLY way I know how to explain it, so its more about my ability than yours.
The circle below will represent the “limit” of the tires contact patch on a dry road. (Fig. 4) This means that if you go outside of that black circle, the tire starts to slip at a high rate. So if accelerating, it means wheel spin. If cornering it means the tire is sliding sideways. And if braking it means you have locked the tires and it’s skidding. Or nowadays, you've activated the ABS.
Fig. 4
Now, If its raining and the road is wet, the “limit” becomes smaller because the surface is more slippery. (the blue line Fig. 5)
Fig. 5
And of course if you were driving on snow or ice, that circle would be even smaller yet.
So lets say that we are driving on a dry asphalt surface and we end up in a moment where we have to break pretty hard. Maybe we weren't paying attention and we were caught by surprise by a stopped vehicle in front of us. So we apply the brakes hard. (Fig. 6)
Fig. 6
As you can see I added the directional forces that are applied to the contact patch. Yes, I do know my right from my left. Again I am showing forces applied. So for example, when you turn the wheel to the right, the force on the contact patch pushes left.
You can see that in figure six we are braking hard so it pushes the tire close to the limit of its ability, but still inside of the tires capability. All is good, no skidding. No ABS. No lock up.
Now let’s say that we realize that we are closing in on the car in front of us too fast and we are not going to be able to stop before making impact. This means we are going to have to swerve to avoid. We find an opening to the left and we turn the wheel. (Fig. 7)
Fig. 7
Again, you can see that the red line shows that the forces to the left are still within the limits of the tires grip., therefore we successfully avoid the incident without any loss of traction right? …………..WRONG!
When you vector the two lines, they meet OUTSIDE of the limit of grip. This is because you are utilizing both forces on the tire at the same time. (Fig. 8)
Fig. 8
So, as I said, you see that when you vector the lines, the two points meet OUTSIDE of the tires limit of grip and now you have a tire that has lost traction. A tire can only do 100% of one thing. This means you can use the brakes all the way to the limit or you can accelerate to the limit and you can corner to the limit. But of you are braking at the limit and then you try to corner at the limit, the tire is not going to be able to handle both of those inputs at 100% and it will lose traction.
Does this mean that we were not able to avoid the incident? Not necessarily, it simply means that the tire is sliding and now you have to "manage" it. You have to get the tire to function back within its limits. You can do this by reducing your braking or reducing the steering input or both. And, provided you have the time to do this, you could still avoid the incident.
Also, these days we now have vehicles with systems that can recognize when you have gotten yourself beyond the tires limit and they can assist you in getting the vehicle on your intended path. Some of these systems are known as; traction control, stability control and ABS. In the situation in this example, the systems would have come into play. (Fig. 9)
Fig. 9
The yellow line shows that the systems would operate to keep you near the limit to try and maximize both of the requests you have made, within the tires ability. Provided you have ABS, all you really need to do is stay on the brake and steer the vehicle, the systems will do the rest.
So that’s a quick and dirty way of showing you how inputs to the vehicles control systems affect the contact patch. In a more complicated but realistic picture you would see that a real contact patch is not a circle, it’s more of an oval type shape. (Fig. 10) below is a picture taken by a camera mounted under a glass plate as a vehicle drove over the plate. This shows an actual contact patch. The patch and its size will also vary dependent on tire pressure.
Fig. 10
The reality is, the example I have discussed, no matter how detailed (or simplified) is only for one specific moment in time. Its what the tire is capable of at that moment, at that time, on that specific piece of asphalt, at that speed, with that steering angle. As the vehicle moves, the tire (contact patch) is constantly changing the surface that it is in contact with. Maybe it started out on a perfectly clean dry spot of asphalt, but then transitioned to a spot that had a little sand or oil on it. Whatever the case may be, its an ever changing scenario.
A lot of race cars have data acquisition on them and its really cool to look at data and look at the plot points from the G-meter. They use these meters to show where the car can be faster and where it had more or less grip. It can also show the driver where he can gain time/speed by utilizing the vehicle a bit more to its limits in some areas. And where he/she may be losing time/speed due to going beyond the tires grip level in some places. It’s a cool science, and although you may not be into racing and you feel you may never take a tire to it’s "limit of grip". The reality is, the tire has no idea if it has gotten to its limit or gone beyond its limit because its on a race track and the driver is trying to get the most out of his/her contact patch in an effort to go fast, or if its on a highway trying to avoid a head on collision with another vehicle. All the tire knows is that it has been taken to or beyond it’s limit and it will react accordingly. It’s now up to you as a driver to manage the rest.
What?......................you mean they didn’t explain that to you in Driver’s Ed?..................... I’m shocked!
Now give you're sixteen year old the keys and send him/her on there way. Scary isn't it? It's no wonder the leading cause of death for teens is auto accidents.
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