Let’s start with the basics. when you turn the steering wheel the wheels turn, we can all agree on that. In other words, the steering wheel gives you control over your tires. Obviously….but there’s a little asterisk there. Some fine print.

The steering wheel does not give you direct control over the entire tire. When you’re turning you’re obviously not able to grab the tire, you can’t just take the outer surface in its entirety and then change its direction of travel. Your steering wheel is not directly connected to the tire. Instead your steering wheel directly operates a small thin rod which is attached to the steering knuckle which is attached to a bearing and a hub which is then attached to the wheel and the tire is then installed on the wheel.

I know you’re probably wondering why I’m wasting your time with something so obvious? Well that’s, because it’s extremely important to understand that because you do not have perfect direct control of the tire in its entirety the only thing you can actually do with the steering wheel is to deform the tire and hope that deformation leads to the vehicle going in the desired direction.

Now if you have ever been in a car and gone through a corner at some speed you have probably felt how there’s a force acting on your body that’s getting you to lean in the opposite direction of the corner. Obviously this is centrifugal force, the same force that makes this happen…also wants to run you off the road.

So if centrifugal force wants to push you off the road, you point the tires in the other direction to go through the corner. But, there’s a catch. When you’re traveling at normal road speeds, the direction where you’re tire is pointing at any moment in time….is not the direction where your car is going. By turning the steering wheel we only turn the wheel and then the wheel deforms the tire. The problem is that the wheel can’t deform all of the tire equally. The contact patch, or the part of the tire that is in contact with the road can’t be twisted by steering inputs. This is because the contact patch is temporarily stuck to the road by the friction between the tire and the road surface. So what your steering inputs do instead is that they twist the tire around the contact patch.

So when you make steering inputs the direction in which your tire and wheel point changes but the direction of the tread in the contact patch stays the same. Now during the very brief point of time that it takes the tire to roll through this contact patch, the tire is actually heading in the direction of the contact patch and not in the direction in which you pointed the tire and the wheel with your steering inputs.

But because a tire is rolling forward it means that the treads which you deformed or deflected with your steering inputs will all eventually end up in the contact patch too. But the catch is that once they land and become the contact patch, the fact that they are deflected means that they hit the surface offset and pointing in a different direction from the previous contact patch.

This also means that if we observe the tire at any point during the corner, the tire is never actually heading in the direction in which you point it with the steering inputs. The tire is always heading in the direction of the contact patch because it’s contacting the road and rolling through only the current contact patch. All the other contact patches are in the air. Now the difference in direction or the angle between the direction in which you point the tire and the direction in which the tire is heading during that moment is called the slip angle.

Now the name is a bit confusing and it’s important to understand that the “slip” in the slip angle does not mean that the tire is slipping all over the place or that the vehicle is drifting. It refers to the fact that each tread element or subsequent contact patch of the tire gets slipped laterally ahead of the previous one. Essentially it’s the slip between the direction in which the tire is pointing and the direction in which it’s actually going in that moment. Despite what some YouTube videos may try to convince you, slip angle is not a special driving technique used by cartoon characters and legendary racing drivers.

Slip angle is not optional, it’s mandatory because without a slip angle it would be impossible to corner. And that’s because as each tread element gets slipped laterally ahead of the last one we generate a force. This force is known as cornering force. If slip angle equals zero then cornering force also equals zero. If slip angle is zero then you are going perfectly straight. The nice thing is that we can easily observe the cornering force.

A special thank you to my patrons:
Daniel
Pepe
Brian Alvarez
Peter Della Flora
Dave Westwood
Joe C
Zwoa Meda Beda
Toma Marini
Cole Philips

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