You Think You Know But You Don't - Slip Angle Explained in a Way You Will Understand
330,153
Published 2023-02-26
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.
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All Comments (21)
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Another fantastic and simple explanation. Once again you prove how knowledgeable you are. “If you can’t explain it in easy to understand terms, you don’t understand what you are talking about”. Well done!!!
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You're still the GOAT man, this my absolute favourite channel for car related educational stuff. Your videos never disappoint.
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My boy getting to 700k today for sure . Congrats in advance!
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I came to a similar conclusion at some point in the past when I tried to understand how one can steer slightly into a strong side-wind while continuing to drive straight on a highway. My thought was that the tread blocks were effectively “walking” sideways. Thank you for the much more engineering-minded explanation! Kudos, as always!
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This was great. Slip angles are always a tough topic to explain. Next, could you tackle the topic of contact patch aspect ratios? Wide tires have wide but short contact patches. This results in each piece of the tire's tread being part of the contact patch for a shorter time. Consequently, for any given slip angle, each point on the tire's tread will experience less lateral displacement before it finally exits the contact patch. This can contribute to the tire tolerating a larger slip angle. But it also reduces feedback at the adhesion limit. When the tire begins to lose adhesion, the lost of grip starts at the rear of the contact patch where the lateral displacement of the tread is greatest and the rubber's restorative force is highest. As the rubber's restorative force (the force pulling it back to it's natural position) exceeds the friction that binds it to the road, it begins to release that grip on the road. Since it happens first at the rear of the contact patch, and then progresses forward, it effectively shortens the contact patch and shifts the centerpoint of the contact patch forward. This movement of the contact patch produces an effective change in the self centering force (because the contact patch is closer to the point where the pivot axis of the wheel intersects the ground), usually causing the steering to feel lighter as grip deteriorates, because the change in camber reduces the centering force produced by the steering system. In a tire with a long, skinny contact patch, this progression is more gradual, making it easier to detect and react to. In a tire with a wide, short contact patch, the loss of grip starts at the rear by has shorter distance to spread before reaching the front of the contact patch. This not only makes the loss of grip more sudden, but it also reduces the degree to which the contact patch centerpoint can move. Thus, wide tires cannot produce as much change in steering centering force at the limits of adhesion, compared to narrow tires. It'd be great to see some of these topics covered in your enjoyable, understandable, well-illustrated style. Edit: For some reason I mentioned camber when I was thinking about the contact patch and the self centering effect of caster.
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thank you for the videos. although I consider myself a life long motor head (I am 61 years old), I have learned more engineering behind the drive in your videos than I have anywhere else.
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Warning: this video has spoilers
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I've been a rev-head for 70 years, and I've learned more in the last couple of years than the previous 68, thanks to the high-info content of your channel, and a few others. Thanks for the intensive research, and amazing graphics!
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I imagine the grip graph is a lot smoother for cross-ply tyres - hence a lot more dancing around the limit of grip.
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This guy, my tuner, and my usual mechanic are pretty much the only people I listen to regarding cars.
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12:03 now this explanation might lead you to the wrong assumption that you always should apply rotation to the steering wheel linear with like 5-6 degrees per contact patch (just referring to the used numbers) but theres much more to it. a car has very different grip levels through the process of cornering. earlier in the video there was the clip of the dodge viper acr extreme aero going through brünnchen at the nürburgring. in that clip you can see it pretty well, the corner has a kind of a bump at the entry and you saw the cars front do a little dip afterwards. right in the moment after the bump when the cars front comes down again and compresses the front springs and dumps its weight back into the tires, you can apply much more steering angle at once (again because of the momentarily higher grip levels) and then take even more speed into yhe corner because you dont need to apply so much more angle to the steering wheel at the remaining part of the turn. the most important thing in racing is to control the balance and forces on each tire all the time and use it to position the car into a turn, keep it straight and controlled while exiting and using all of the available grip all the time. at the viper shown in the clip, a typical front engine rwd car you will be using a trick as well, which is downshifting in the right moment to get an additional braking effect on the rear tires just when the clutch dumps back after the downshift, by the engine brake and the higher resistance coming from the now faster rotating engine, gearbox and drivetrain. by that you can shift some of the cars weight to the front when the clutch engages and use that additional grip i explained with the bump at the corner entry and shift it back and use the rear tires grip as much as possible once the clutch releases again and get back the balance over the car and try to spread the load equally to each tire. this is mostly used in f/r cars because the engine in the front rather makes the car understeer and with the driveshaft in the rear you can counter that by throwing it into the small moment of oversteer. mid engine cars can use that as well but theyre naturally already more oversteery and snappy and more sensitive anyway.
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One of my favorite channels on cars. I've learned so much about first principles watching your videos. Looking forward to more!
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I appreciate that you also added the explanation in the video description. Sometimes I don't have the time to watch a whole video on a topic and would rather read on it, but most times the site will just direct me to a video. You're doing an excellent work here and I hope your channel grows further.
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One remark: centrifugal force doesn't exist in reality. It just a simplification. The truly acting forces while cornering is centripetal force(which tries to force car to turn) and it's counterforce - inertia(which tries to make car go straight). Not a big difference in terms, but huge in understanding real turning physics. Despite this anyway this video is still one of the best popular explanation of car turning on youtube )
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This is an incredible channel, thank you, I learned so much.
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I like to think of tires as "feet on a wheel". For each foot length, that is all the tire you have. It's helped me avoid at least a few scrapes. Great video!
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Imho the go to channel on youtube about in depth and understandable car dynamics!
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Insane video. I feel 10x smarter after watching this.
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wonderful explanation, as always. I discovered your channel a few weeks ago and I'm watching little by little all your videos and I really appreciate them. Complimenti davvero, sei un grande, continua così 👍
