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September 6th, 2007:

The Magical Coefficient of Motorcycling

There are a few things in Nature that seem like magical numbers. One is e. Another is the Golden Ratio. Another more applied version is 4 degrees C, the temperature at which water has its maximum density (thereby guaranteeing the action of thermal inversion, which prevents all of life from going extinct every few thousand years during an Ice Age).

There’s one for the world of motorcycling as well. But it’s more boring if I just tell you want it is, so here’s a bit of setup:

Think about flying down the freeway at 60 or 70 MPH. Sure, you are ATGATT (all the gear, all the time — no skin below the chin) and are helmeted, but doesn’t it seem crazy not to wear a seatbelt or something? (This sentiment dates me as being of the generation that grew up with mandatory seatbelt laws.) How the hell can you hold on to something going 70 MPH?

Well, it’s only sort of crazy. It turns out that holding on to (or rather, not falling off of) something going 70 MPH is a damn sight easier when you’re going 70 MPH along the same vector. What really matters is the acceleration involved. You at 0 MPH, bike at 70 MPH = bad news; you and bike both at 70 MPH = OK.

But, you may counter, how the heck can you get to 70 MPH? Isn’t that quite an acceleration, one that we usually encounter while strapped in with our backs against a DOT-approved seat?

Well, yes. But your acceleration is really bounded by one thing: the friction of your tires on the pavement. (You can have a stronger engine or tighter brakes, but they only are effective to the limit of your tires’ grip.) As it turns out, the natural properties of rubber make it so that you get about 1.1 g of traction on dry pavement. Torque your wheel faster, you spin out (or slow it more than that, you’l skid). Hence, as long as the bike isn’t cheating — starting or stopping using something besides the tires (like a rocket JATO or a brick wall) — you’re basically limited to applying about 1.0 g.

And guess what? Human bodies and minds happen to be really good at dealing with 1.0 g. In fact, you might say that the whole of bipedalism is just drill for how not to tumble against the pull of 1.0 g. So, from the day you first pulled yourself up as an infant, you were practicing the skill needed to hang onto the handlebars during a 1.0 g acceleration.

And that, my friends, is the magical coefficient of motorcycling: the traction of rubber on dry pavement. And that is why it’s maybe not so crazy to go 70 MPH without a seatbelt after all.

(Pace, engineers: I know that my explanation is oversimplified, but it’s roughly right.)

(And to the safety-concerned: your biggest concerns, of course, are exogenous to the question of traction: it’s when you do hit a brick wall, or when you start testing the friction of leather on pavement, that you really get hurt.)