Why do narrower tires have less traction?

Because the contact patch with the road is smaller.

Because the contact patch with the road is smaller.

Friction isn't dependent on surface area. I think the answer, if there is one, has to be more complicated than this.

Friction isn't dependent on surface area. I think the answer, if there is one, has to be more complicated than this.

I'm trying to figure out the explicit physics of it, but I'm getting stuck (that could be a friction pun, couldn't it?). Needless to say, the same tire gives less rolling resistance when it's full inflated (hard) than half-flat (soft). I think the softness of the tire adds to the traction and I also though that the larger the contact patch, the more traction. E.g. this is why race cars are fitted with wide tires without tread patterns (racing slicks). I think the lack of tread not only increases the area of the contact patch but also prevents the tire from cooling, which makes it even softer for greater traction. If you could drive on narrow steel wheels without any rubber, I would think the rolling resistance would become similar to a train on rails. Hard, narrow tires, such as those of Model T Fords and bicycles have less rolling resistance than wider tires.

The general contact patch issue is pretty familiar -- narrow tires for fuel efficiency, fat tires for performance. I've always just assumed this was a friction thing, but I wouldn't be surprised to hear it's more complex than that.

The general contact patch issue is pretty familiar -- narrow tires for fuel efficiency, fat tires for performance. I've always just assumed this was a friction thing, but I wouldn't be surprised to hear it's more complex than that.

How much more complex do you want it to be? Probably there's something like the energy it takes to generate a continuous wave in the tire tread as the pressure from the road pushes against the angular momentum from the rotating wheel. You're looking for something like that?

How much more complex do you want it to be? Probably there's something like the energy it takes to generate a continuous wave in the tire tread as the pressure from the road pushes against the angular momentum from the rotating wheel. You're looking for something like that?

Yeah, softer rubber will give a larger contact patch but more rolling resistance, and will improve the ride smoothness at the cost of some handling characteristics. Tradeoffs all the way.

I wonder why these high-speed rail projects are dependent on public funding. Why can't a private company just sell bonds to raise the money for the project?

I'm pretty sure contact area does matter for friction, probably because the surfaces are not smooth and perhaps also because rubber is flexible. More contact area is probably going to increase rolling friction too. And trains are always going to beat cars at rolling friction, because steel wheels aren't all squishy and frictiony. There's also the issue of air resistance, but in the future automatically controlled cars (auto-automobiles?) could form "land trains" by driving bumper to bumper to minimize drag.

I'm pretty sure contact area does matter for friction, probably because the surfaces are not smooth and perhaps also because rubber is flexible. More contact area is probably going to increase rolling friction too. And trains are always going to beat cars at rolling friction, because steel wheels aren't all squishy and frictiony. There's also the issue of air resistance, but in the future automatically controlled cars (auto-automobiles?) could form "land trains" by driving bumper to bumper to minimize drag.

If they could do that, why couldn't they also have bi-sectional tires with a retractable steel belt in between that slightly separates the tire from the road and reduces rolling friction to the amount of a train? In fact, cars could actually engage separate retractable wheels that could be slotted into rails embedded in the road for the majority of long-distance travel.

http://en.wikipedia.org/wiki/Tweel

http://en.wikipedia.org/wiki/Tweel

Interesting development, but I don't see how it approaches the narrowness, rigidness, and frictionlessness that give train wheel/rail contact its efficiency.

Rigidness in a road tire can be a disadvantage, since you don't grip an uneven road surface and you transmit every bump and jolt into the suspension. The Tweel will let you play with the tire characteristics much more than a rubber tire would, though.

That's cool, I'd not heard of Tweels before. Two cool automotive techs to ponder from a single thread -- bonus!