Counterintuitive

So my friend Alex has been know to post pictures of Selma Hayek on his blog in a tongue-in-cheek attempt at driving traffic. I have no idea if it works, but you’d think there’s likely more people in this world interested in looking at Selma Hayek than are interested in reading about a stand of 80,000 year-old aspen trees near a remote mountain lake in Utah.

Likewise, one would assume that more people would be interested in looking at revealing pictures of supermodels—even if the supermodels are limited to those who have dated Billy Joel—than are interested in looking at revealing pictures of me.

But if you thought that, you’d be wrong. Because last week when Elden posted a revealing picture of me showing off my post-crash road rash, traffic to this blog was more than double what it was the day I featured both Christie Brinkley and Elle Macpherson in scanty bathing suits.

Of course Elden’s daily readership is something like 30 times what mine is, but I’ve seen the Fatty effect when he’s linked to me before, and it was nothing like what it was last week. I should pull down my pants for the camera more often.

Now if you’re wondering what any of this has to do with cycling or skiing, the answer is absolutely nothing. Except that there’s something similarly counterintuitive to think about next time you inflate your bicycle tires.

Allow me to illustrate. Last week, before Elden attempted the Clark’s time trial, we had the following exchange via IM:

Elden: Tire pressure will go up to a very rare 30psi for this effort I think. And a sleeveless jersey.

Me: now you're getting serious. I'd keep the tire pressure low, though. Lower pressure = less rolling resistance. And you'll want the traction climbing the rocky parts.

Elden: Lower pressure = less rolling resistance? Are you high?

Me: Nope. I'll explain in an email. But I am correct.

Elden: I'll be interested to read that, cuz it sounds counterintuitive as hell.

After sending the email, Elden suggested I turn it into a blog post. So here’s the explanation why lower tire pressure actually decreases rolling resistance.

On a perfectly smooth surface, a harder wheel will have less rolling resistance because there will be a smaller contact patch, thereby creating less friction (think ball bearing inside the bearing shell). But on a rough surface, such as a mtn bike trail or, to a lesser degree, asphalt, a very hard wheel would not roll smoothly at all because its travel would be disrupted by the roughness of the surface (think of those old roller skates with metal wheels on asphalt).

A softer wheel (less tire pressure) is able to deflect when it hits rocks and other vertical obstacles, or even as it rolls across a rough asphalt surface, thereby smoothing and shortening the horizontal path of travel. Obviously there are limits, which is why you run higher pressure on a road bike than on a mtn bike.

A road is smoother, so the ideal tire pressure is just enough to allow deflection to smooth the road surface without being so soft as to create additional friction from a too-large contact patch or to increase the risk of pinch flats. A 25mm tire has less rolling resistance than a 23mm tire because it has higher sidewalls, which deflect more when they hit obstacles. The rolling resistance benefits of the 25mm tire, however, are offset by it being less aerodynamic and heavier.

On dirt, where aero and weight are less of a priority than rolling resistance, a fatter, softer tire is better. Most pro XC racers run 18-22 PSI because it offers lower rolling resistance and better traction. There are limits to how low you can go, however, because you run the risk of pinch flats or, with a tubeless setup, the tire blowing off the rim. The reason tubular tires are so popular for 'cross is because they allow you to run lower tire pressures, thereby reducing rolling resistance, without as much risk of pinch flats.

Another way to think about it is if you consider the path of travel of the outside diameter of the wheel rather than the outside surface of the tire, it's easy to see why softer is better. You want the wheel to travel in as close to a horizontal path as possible. To effect this, ideally you would have a soft tire that's able to deflect and absorb vertical obstacles without disrupting the horizontal path of travel of the wheel.

At first blush you think people would rather look at supermodels than look at me and that a harder tire is a faster tire. But the reality is that the opposite is true. At least in one case you now know why.