On car tires, shoes and other such items, having less of the material in contact with the surface underneath increases traction. Why is that? Isn’t friction a function of the contacting area and speed?
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On a smooth, dry surface, you are correct, a smooth tire increases traction due to maximizing contact with the road. On an uneven surface, a wet surface, or a loose surface such as dirt, tread patterns increase the surface area in contact with the road when compared to a smooth tire. Tread patters can additionally contain channeling to allow water to flow through the tread instead of sitting between the tire and the road, increasing traction in that way.
Wider tires are better in dry conditions as they have more surface area to grip, but can hydroplane when the surface conditions are wet.
Narrow tires have increased surface pressure and will be better under wet conditions (heavy rain, snow or ice).
Under ideal conditions maximizing surface area will help traction. This is why for example drag racing tires are smooth. So in that sense your intuition is correct.
But think about things that can cause that ideal to not work out. Suppose you have small balls between the two flat surfaces, that is going to result in much lower friction because the surfaces don’t get to touch! Perfectly flat shoes on perfectly flat floor would be great traction as long as the floor is perfectly clean, but add a little sand or grit and suddenly it isn’t good at all.
Similarly if the surface being walked on isn’t perfectly flat then a flat shoe won’t make good contact. By adding gaps in the shoe surface there is somewhere for debris to be pushed aside and space for the shoe material to deform to match the surface.
Car weighing 5000# with a contact patch of 50 sq inches = 100 psi of pressure on that patch.
Car weighing 5000# with a contact patch of 40 sq inches = 125 psi of pressure on that patch.
In other than ideal conditions traction is enhanced by increased “weight/pressure” on the contact patch. It is why “winter tires” are generally narrower than all seasons.
>On car tires, shoes and other such items, having less of the material in contact with the surface underneath increases traction.
This is a faulty inference. Since most roads and walkways are fairly rough, having a knobby, textured surface in the soles of shoes or tyre treads tends to actually increase the area of contact, not decrease it. The same reason why shoes and tyres are made of highly flexible substances like rubber. This is because it makes the surface of the tread or sole more compliant or squishy. This brings more surface area into contact with the road or trail, not less, because it conforms to the surface in the same way that a soft mattress conforms to the bumpy surface of your body. Moreover, dirt or gravel particles will wedge themselves into the grooves and crevices, further increasing fraction. Such a design is more resistant to skidding in dirty, sandy, or gravelly conditions.
Most importantly, the grooves act as channels for water or oils which prevent a layer of water being trapped between the soles or tyre, and the surface of the road. This reduces the tendency for a phenomenon called “hydroplaning” wherein water acts to lubricate the road surface by formingba thin liquid film between the two, which drastically reducing traction. In cars this can have disastrous consequences during a downpour, by causing tires to skid and causing the car to lose all control. Grooves and channels in the tires act to spoil this tendency by provinibg a lower pressure flow path
All things being equal, however, racing tyres are often smooth, when they are used on clean, dry, mostly gravel and dirt free racetracks. The reasons why slick tyres offer a better grip in racing conditions is complex. It’s partially related to heat retention. During drag racing the drivers usually deliberately spin their rear tyres to heat them up to improve grip. Likewise if you mount a thermal imaging camera on a F-1 car, you can see the tires heat up during braking and turns. However using slicks on a wet racetrack would be very dangerous. Because racecars typically use oversized tires precisely to increase the contact surface are with the road. But this reduces the average ground pressure and makes hydroplaning much easier.
Well….you’re talking more about the loose concept of “grip” than you are about just friction.
Friction is the coefficient of friction times force. That’s it. Contact are doesn’t matter in the slightest.
When you’re talking about real-world things designed to handle a variety of circumstances, friction isn’t the most helpful thing, because it varies a lot and isn’t the only force involved.
Off-road with appropriate tires, you’re digging into the dirt and actually pushing against it, not just dealing with the friction of it as a magical high-school class indestructible surface. So those tires act like cleats or hiking boots and dig in, and more/larger cleats can dig in to more dirt, thus applying more force before they start flinging dirt instead of moving whatever they’re supposed to move.
On-road, but with wet pavement the biggest threat to traction is that you’re not actually contacting the pavement at all, but rather the water on top of it. So instead of a smooth tire that may trap a thin layer of water between tire and pavement, you use a tire that is textured with channels to shed that water away and get down to the pavement where there’s much more friction. Same idea with shoe treads, rain boots have chunky treads with channels available for water to escape so you can make sure you’re getting to the ground rather than slipping around on the water on top of it.
On road and try, a smooth tread works just fine. In extreme circumstances, when you’re looking at stuff like racing slicks, the smooth tire may actually be heating up from the use. It both softens and conforms to the road, allowing it to push laterally against the micro-terrain of the road surface, but also in those race conditions actually becoming adhesive in which case, more surface area of adhesive means more grip.
Friction is…shaky. It can be different 2″ apart on a road depending on if there’s a drip of oil there or not. Grip is a composite of many aspects, not just friction, and a lot of them are served by digging into the surface or shedding any intervening things like water. Hence treads.
Friction has *no relationship to contact area*, or to the speed of the materials.
What matters is what two materials are in contact.
Where surface area can start to have an impact in practice is basically the ‘probability’ that puts two two materials in contact.
Wide tires allow a tire a high chance of being in contact with concrete. Which has a high amount of friction with the rubber of the tire.
But some situations, like a wet road, a wide tire may not find the concrete. In some of those cases a narrow tire may be able to push one material (water) out of the way, to make contact with the concrete underneath.
To throw a complication on top of this, wide and narrow tires are typically made of different materials as well.
Wide tires usually have softer rubber and better grip. One reason they are wide is to reduce the pressure and wear on the softer material. Note that the width in this case is not to ‘improve grip’ directly. The width does allow the use of a material with better grip though.
For tyres, in ideal conditions, a bigger contact area means more friction. That’s why racing tyres are wider and usually don’t have any grooves on their surface.
Now, on the road however, we usually don’t have ideal conditions. First of all, there can be water or snow on the road. The tyre profile helps to push water/snow away so the tyre can actually have contact with the road in first place. Otherwise the water would have nowhere to go and you’d basically “swim”. It’s similar on a rough surface – the profile helps the tyre to basically move around the rough surface, providing an overall bigger contact area rather than only on the peaks of the surface you’re on. For shoes, it works similarly. The grooves in the bottom make sure that the sole can adapt to rough ground which overall increases the contact patch.
Actually no, friction is *not* a function of the contacting area and speed. It is a function of weight (i.e. the force pushing straight down onto the surface) and the coefficient of friction between the two materials (e.g. rubber on dry asphalt has a high coefficient of friction while wood on teflon has a very low coefficient of friction).
The coefficient of friction is always higher when the two materials are not sliding on each other. When they start sliding, the “static” (not moving) coefficient of friction changes to a much smaller “kinetic” (moving) coefficient of friction.
To feel this in practice, try the following:
Standing in sock feet on a slippery floor, try to push yourself away from a wall until you start sliding. Now lift one foot, and try again. Chances are you will need just about exactly the same amount of force to start sliding on two feet as you will when you’re standing on one foot.
When you are standing on one foot, there is only half the surface area (one foot vs. two feet) to keep you from sliding, but on the other hand, when you are standing on two feet there is only half of the weight per foot.
Reducing the width of the tire (surface area) will increase the psi of the tire on the driving surface. Increasing the width will decrease the psi of tire on the driving surface. More psi on the driving surface equals greater friction. It’s important to match tire width to the weight of the vehicle as well as the use case and driving surfaces / conditions you expect to encounter.
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