It's not much of a scrape, about seven inches long and three wide, just disgusting enough that I won't show my treasured readers the photo I took shortly after the crash. I've certainly had worse injuries over the years, although I notice that the older I get, the more likely I am to end up with a scar from this sort of thing. Nope, this little scrape on my knee isn't interesting for what it is, but rather for how it got there.
COBRA BMX, in Columbus, Ohio, has been famous since its initial season in 1988 for having possibly the worst sand, I mean, dirt, to ever make up a BMX track. The Florida sand of which Colin Stiles and his ilk so vociferously complain has nothing on this rock-laden, razor-sharp, Teflon-slippery junk. Several applications of expensive clay have done nothing - the rain washes it off in less than a month, and COBRA's riders are once again left with this curious cross between sandpaper and T-FAL, guaranteed to torture everyone who rides it, and to rip their skin off, sooner or later.
I hadn't ridden the track in quite some time, so it was no surprise that I made a classic COBRA rookie mistake on my second lap. I jumped a mild tabletop in the first straight, said tabletop being right before the first turn, and decided to "pre-lean" in the air. "Pre-leaning", for those of you who aren't familiar with the term, is simply to lay the bike over in anticipation of turning shortly after one's landing. The closer one will land to the turn, and the steeper said turn is, the more one leans. In this case, I realized that I would overjump the tabletop by a foot or two, and I thusly leaned the bike hard - nearly hard enough to qualify as a stylish jump, hee hee.
When my heeled-over rear wheel hit the slippery COBRA sand, it gripped for the briefest picosecond - then let go completely. My initial sensation was that the bike had collapsed under me. My next-to-intial sensation was that old familiar feeling of having the itty-bitty rocks work themselves into my elbow and knee caps. Since I am of the old school in such matters, I immediately jumped up and proclaimed that I was okay, but in truth it really hurt, and I would have preferred to do what today's young riders do and just lie on the ground, whimpering, until five or six people showed up to help me stand.
If you understand how and why I wrecked, you are several conceptual steps past my fellow riders that day at COBRA, who understood neither my decision to lean nor why it led to the wreck. (In the interest of complete disclosure, I have to mention that my BMX mentor, Rich Hetzel, was present and did understand all of the above.) Most BMX racers, I think, understand very little about how bicycles turn. That's okay, because the complete "physics of turning" aren't understood by anyone, and there remains some fierce debate even today over issues like, "Why is a bicycle easier to balance in motion?" and "How much can bicycles be countersteered?"
Lack of total understanding need not be a barrier to our learning a little about how cornering works, however. We'll start with a simple experiment which you can perform at home, and which you have probably done in the past without understanding why it works. Start by riding your bike in a moderately tight cirle, say, twenty feet in diameter. Once you have some momentum going, see if you can lean your bike over more without affecting your turning radius. Hmm... you say that you can? Well, see just how far you can lean it over. You say that you can lean it quite a bit? How far? Nearly far enough to scrape a pedal? Yup, that's about right.
Having done that, now see if "hanging off" the bike changes the turning radius, even if you don't lean it over any more. It does? What a surprise! From this experience, we can create a "rule" of sorts, that rule being, "For some reason, the amount of lean has more to do with how hard a bike turns than the amount of steering does."
We have one more experiment, and this one will be easy. Ride down the street no-handed. Observe how the steering moves with your weight, but also observe how said steering isn't usually a smooth motion, but instead consists of wobbles in both directions, even when you are turning in one direction. From this experience, we can create a second Rule, that rule being, "For some reason, a bike doesn't always steer in the way it is turning, and once it is in the turn, it doesn't always keep a steady amount of steering."
Notice that I have, for some reason, begun both rules with the phrase "For some reason". There's a reason for all this "reason" business, and if your head isn't tied in a knot yet, it may be in a moment. You see, as I stated above, nobody really knows, at least not in the sense of having scientifically proven anything, exactly how bicycles and motorcycles turn. To my knowledge, the robot has yet to be built that can really ride a bicycle - most of the bicycle-riding robots out there are only successful in very strictly defined situations and at very low speeds.
We do know how steering works... sometimes. When you turn your bars, the front wheel turns. The tire, which is now pointed somewhere different from where its momentum is taking it, builds up a force against the ground. This force is transmitted through the sidewalls and rims into the bike itself, and expresses itself as a "shoving" motion. The front of the bike is therefore "shoved" in one direction or another by the force of the sideways-sliding tire rubbing against the ground.
I see you out there shaking your head, so I'll explain it differently. Imagine that you and a friend are in football practice, and that you have been commanded to run into the tackling dummies. When you hit the dummy, your body is turned a little bit, with your left shoulder forward, so you slide off the dummy to the left. Your friend has his right shoulder forward, so he slides off the dummy to the right. The reason for this is that the angle of your body will force you to move across the dummy when you hit. The same is true with your front wheel. In every instance of turning, the front tire is "sliding" for the tiniest fraction of a second before anything happens. If the track is dusty, of course, your wheel will slide for lot longer than the aforementioned tiniest fraction of a second. This is, by the way, why your front tire eventually wears out - the tiny slides, repeated thousands of times over the course of many rides.
Now I need you to really put on your thinking cap. If the "tiny slides" are what make a bike turn, what can we do to make the bike turn faster, or turn more? Well, we could put more weight on the tire. More specifically, we could put more weight on the tire, on the side we'd like to turn towards. Either technique will work, but the second works better, and it works more predictably. The bottom line, however, is simply this: Effective turning depends on controlling your center of gravity.
What's the point of all this, when today's tracks consist of nothing but trail-jumps and rhythm sections connected by the gentlest of steeply-banked turns? Well, there's the satisfaction of a "turn well done", but there's more to it than that. Even on today's competition-averse tracks, there are occasionally times when you want to turn more sharply, or with more precision, than the rest of the track sheep, if only so that you can cut across an entire line of riders and ruin their run-up to the next set of step-up triples. When doing so, you will run the risk of two distinctly different hazards: the washout, and the tuck-under.
A washout, of course, is when your front wheel slides too much and causes you to miss your turning opportunity, wreck, or both. A tuck-under is the opposite: the front wheel digs too sharply and quickly, causing you to be launched headfirst over your scoot. This happens to the best of us, and it happens more frequently when we clip in... for reasons I'm about to make clear. Both situations stem from incorrect positioning on the bike, and they can both be prevented with proper cornering technique.
This is enough of a topic that I'd like to devote a second month to
it, and include a couple of photos that I'll be taking at the upcoming
Jeff Dein clinic in Dayton, Ohio. But before I go, here's the answer to the
"clip" conundrum. Tuck-unders happen more frequently to clipped-in
riders because the use of SPD masks bad front-to-rear positioning
on the bike. Clipped-in riders tend to lean farther over their
front wheels in turns because doing so doesn't make their feet slip
off the pedals. Leaning too far forward will always make a
traditionally-mounted rider leave his pedals, because the "pedal weld"
depends on having enough weight over the axle. For that reason, most
of us non-Clippys never lean too far forward in a turn. Clippys,
however, are mechanically attached, and don't feel the "warning slip."
Something to think about until next time!