In this video I discuss the relationship between grip and slip fore and aft – also known as percentage of slip. Knowing your tire’s optimal slip under acceleration and braking will help maximize your traction.
Percentage Of Slip – Video Transcript
Hey everybody Matt Covert here again from howtobecomearacecardriver.com back again with another free video – this time about percentage of slip. And I saw this in a book I was reading. Carroll Smith’s Tune To Win. Boy there’s a whole lot of information. There’s probably more information about racecar engineering and tuning in the first thirty pages of that book than anywhere. So if you’re looking for racecar tuning, that is the book to go to.
And I know I’m always raving about books. I’m not an affiliate for any book. But that one is so valuable I just can’t help but rave about that one. So let’s go head and talk about percentage of slip.
Percentage of slip is very similar to slip angle. OK? And there’s a video about that, which I will link to somewhere on the screen here. As you know, slip angle is the difference between where a wheel is pointed and where the wheel is actually going. And the difference between them is just the slip angle. And it’s a great way to kind of keep track of the levels of adhesion that your tires have while being driven in various ways, etc.
OK. When we’re talking about slip angle we’re talking about forces that act laterally. Like this – I’m just going to draw a tire to kind of delineate that – going this way. Percentage of slip, also sometimes known as a slip ratio I’ve seen somewhere, deals with the forces fore and aft under accelerating and braking. OK? So let’s get into what this is.
So I’m just going to go ahead and make the curve here first before we start talking. Something like that. OK? So here we have a graph that shows you a relationship between the amount of grip a tire has and the amount of slip that it is experiencing. OK? So imagine real quick that you are driving a really awesome muscle car and you rev the motor and dump the clutch. OK? The wheels are going to spin really really fast but they don’t have a lot of ability to accelerate under that condition. And that would put you somewhere in this range, OK?
You’ve exceeded that tire’s ability to grip because it is slipping around in a circle so fast that it can’t really grab on to anything, alright? And since it’s called percentage of slip, we can just go ahead and call that maybe a hundred percent. Something like that, right up in there. It will probably – the tire will always produce some amount of forward grip but it’s not very much so just for the sake of percentages we’ll just call it a hundred. Just for the theory, alright?
And there’s never really a zero because any time a tire is move at a low speed, you’re moving. So zero isn’t really zero. So we’ll just say ten, just for the sake of theory. OK?
It’s important to understand that, like slip angle, when there is a little bit of slip on a lateral axis, a tire produces the maximum amount of acceleration or braking when it’s sliding across the surface – the driving – or sliding across the driving surface this way. Instead of this way like we talked about in slip. So you know when you’re driving across sand at a low speed and you can kind of hear it slipping under the tire. That’s the kind of slip I’m talking about. It’s not very much. Just enough to get the tire turning a little faster than the ground is moving past you. And that typically produces the optimal amount of percentage of slip.
And I don’t know what that is, it’s different for every tire. It’s probably not very much. It’s probably – yeah we probably shouldn’t have put ten here. I’m just going to go ahead and put two. It’s probably in the ten percent range if I had to hazard a guess. OK? I’m no expert, but I get all the basics and I love to pass them on to you OK?
So that’s that. That takes care of that. If you noticed, I have a cool new little thing. It’s somewhere down under this box somewhere in the corner. It’s a little button that’ll always be there in all the videos. And that just lets you link to the YouTube channel really quickly and subscribe, quick as you like. And then I don’t have to put those little green boxes on the screen quite as much, OK? And it’ll either be right there or just below the video, depending on where you watch it.
So percentage of slip – that takes care of that. I’m going to move on to something else and I’ll be back soon with another video from howtobecomearacecardriver.com.