On this episode of How Things Work, we take a basic look at how car tyres achieve and retain their grip and what factors influence that grip.
Estimated reading time: 6 minutes, 43 seconds.
If you can’t watch the video, you can read the video transcription below:
Edited for clarity and readability
Welcome to this week’s episode of How Things Work. And in the next two segments, we’ll be looking at how tyres achieve their grip and, to add a twist to the subject, we’ll be comparing car and bike tyres and how they achieve grip in completely different ways. So first let’s have a look at car tyres. The major difference between a car and a bike tyre is that a car tyre does very little leaning. The other obvious difference is that a car with four tyres suffers from weight transfer, which loads and unloads the various tyres differently at the same time.
What affects the available grip on a car tyre? Well, aside from the type of tyre and the tread we have on it, downforce, weight, and road surfaces, there are dynamic forces that act on a tyre which influence the amount of remaining grip we have and the best way to illustrate this is with Kamm’s Circle.
So as we can see with Kamm’s Circle, we can illustrate longitudinal forces, so braking and acceleration, and then our lateral forces, which occurred due to our steering. So if the diameter of the circle represents the maximum grip levels that we have in our tyres, and let’s say that is expressed as 10, if we do braking, for example, just by itself, we might create a longitudinal force of seven out of 10. But you can still see that that’s well within our grip levels or, we might accelerate with a force equal to seven out of 10 and we still have grip levels. However, if we brake and steer at the same time, so let’s say we steer with the lateral force of seven out of 10, we brake with the lateral force of seven out of 10, the resultant force between those two vectors will put us outside the total grip levels that are available in our tyres, and that is where we lose grip.
So it’s critical that we understand how the various loads acting on our tyres when we are accelerating, braking, and steering will affect overall grip. Now that we know how the lateral and longitudinal forces use up the grip, we now need to consider that the inside wheels travel a shorter distance relative to the outside wheels when cornering and how this affects our steering grip. We know intuitively, that because the inside and outside wheels turn through different radii, one of the tyres has to slip and a lot of design work has been done to reduce the effects of the wheel slip with designs, such as the Akermann steering, which is a special geometric design of the steering linkages aligned for the front wheels to pivot at slightly different angles when turning.
But here’s the interesting part. We still need a little slip to get some grip, not too much because that’s a skid. (I’m a poet and I don’t know it.) But for this to make sense, we first need to explain that there is a difference between friction and traction. So, friction is a force that develops between your tyres and the road, and it can be static or kinetic. You can’t see it, but you can feel it and observe its impact on your driving. That said, friction can impact your entire car’s handling. Traction specifically describes the type of friction that develops between your tyres and the road. Without traction, your car can’t develop tyre grip with the road and you can rapidly lose control of your car. However, given the design and construction of our tyres, there is always a little movement between the tyre and the surface, which occurs, especially as the tread blocks settle, as they make contact with the road.
On top of that, the carcass of the tyre itself twists when we turn the steering wheel and this causes the centre of the tyre to be misaligned relative to the leading and rare edge of the tyre, which causes slippage to occur. You can see that combined with our Akermann steering angle, we have a lot of movement between the contact patches of the tyre and the surfaces of the road. But as long as the movement is not more than the traction the tyre generates, we’ll be able to maintain control of the car and not lose grip. A major challenge we face of course, are the various conditions under which we drive our cars. Between wet and dry surfaces, contaminants on the road and different road surfaces, our poor tyres must cope with all these conditions. And this is why all passenger cars have tread patterns. Now there’s many a debate that have taken place about tyre treads, but before getting into the details, remember that there are different types of tyres, whose shape and tread are dependent on the task that they are required to perform.
So you’ll notice that with a performance tyre, it’s usually a square tyre, which assists with a crisp turn into the corner. It also tries to maximise grip with little emphasis on displacing water. Whereas wet weather tyres use an even softer rubber compound than performance tyres, to create as much mechanical grip as possible. They also have more siping to displace as much water as possible, which anyone who has slipped on a wet floor wearing slops, will certainly appreciate. All weather tyres are what you typically find on most production cars. They’re designed to be a compromise between grip and performance, longevity, noise, wet weather, and safety. For increased tyre life, they’re made up with an even harder rubber compound which sacrifices outright grip and cornering performance. The tread block design is normally a compromise between quiet running and water dispersion. So the tyres should not be too noisy in normal conditions, but should work fairly well in downpours and on wet roads.
All terrain tyres are typically used on SUVs and bakkies. They are much larger tyres with stiffer sidewalls and bigger tread block patterns. The larger tread blocks mean the tyres are very noisy on normal roads, but grip loose sand and dirt very well with almost a gear interlocking type of process to create grip. As well as being noisier, the larger tread block pattern means less tyre surface in contact with a smooth road surface like tar. The rubber compound used in these tyres is normally middle of the roads, so neither soft, nor hard. And at the extreme end of the all terrain tyre classification are mud tyres and these have massive, super chunky tread blocks, and really shouldn’t ever be driven anywhere else other than on loose mud and dirt. And the tread sometimes doesn’t even come out in blocks anymore, but it looks more like paddles built into the tyre carcass. So you thought the tread was the shape of the rubber blocks around the outside of your tyre, didn’t you?
Well, it is, but it is also, as you can see, so much more. The proper choice of tread design for a specific application can mean the difference between a comfortable, quiet ride, and a tyre that leaves you feeling exhausted whenever you get out of your car. A proper tread design improves traction, improves handling, and increases durability. It also has a direct effect on the right comfort, noise level and fuel efficiency of the tyre. We also need to keep in mind the importance of correct tyre pressures to suit the conditions. Correct tyre pressures ensure the structure of the tyre and the consequential grip it provides, is retained. And pressure also influences the heating and cooling of the tyre, which also impacts on how the tyre compound provides grip.
So that’s a basic look on how car tyres achieve and retain their grip. Next week, we’ll make the comparison with bike tyres and I can’t wait, because the way bike tyres create grip is a fascinating concept to say the least. So we look forward to seeing you then.
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