(Last month we discussed inertia and how it affects your riding.
This month, we'll look at "center of gravity.")
Center of gravity (Cg for short) is the basis of most "BMX
physics". Cg is sometimes called your "balance point".
It's the point where someone could stick a big metal stake through
your body, tie a string from that stake to the ceiling, and have
you "hang evenly" like a picture. That's probably not
likely to happen, but isn't it nice to know that it could? Another
example of Cg would be found by placing a spoon across the tip
of your finger and trying to balance the spoon. You'd probably
find that the Cg of the spoon is very close to the big round part.
I've posted a semi-complete
explanation of Cg on my web site, but here's the short version
for those of you who are too busy to visit my poor, lonely home
page. (Obviously you're already here, so click the link to
go to the complete discussion if what you see below whets your
appetite in any way---JB) Cg exists in three dimensions -
up/down, left/right, and back/forth. When you learned to ride
your bike, you learned to keep your Cg centered above it as far
as the left/right part goes, so we won't worry too much about
that.
You have a Cg, and your bike/rider combination also has a Cg.
You change your bike/rider Cg by moving your body and by moving
the bike through the four "interface points" of grips
and pedals. Let's say you want to do a manual. You do this by
leaning back until the front wheel comes up. You then shift back
and forth, keeping the front wheel up. If we could see a rider's
Cg, instead of just imagining it, we'd see that when a rider manuals
his Cg is directly above his rear axle. If the Cg moves ahead
of the rear axle, the front wheel comes down. If it moves behind
the axle, the rider falls off the back of the bike. The heavier
your bike is, the more of your weight you have to "hang back"
to balance the manual.
You know that "pulling up" or manualing a small jump
is faster than just riding it, but why is this so? It goes back
to inertia, which we talked about last month, and Cg gets to play
a part here too. Let's say you're riding up to a jump. If you
just ride the jump, the "bump" of hitting the jump with
your front wheel will slow you down. Plus, your entire weight
will have to travel up the jump and back down it, wasting even
more time. What a bummer.
If, on the other hand, you pull up and hang behind the bike, your
Cg is placed up and back from where it would normally be. When
your back wheel hits the jump, it slows down, but your Cg keeps
moving forward at the original speed. As the Cg moves forward
relative to the bike, the front wheel drops, just like leaning
forward during a manual. The front wheel touches down on the backside.
Now your Cg is where it normally should be, and the ride continues.
For a further illustration, imagine you're sitting in the front
seat of a car and you see a big pile of sand ahead. You climb
into the back seat before the car hits the pile of sand. When
it does hit, it slows down and you fly forward - into the front
seat, and you keep driving. Goofy, huh? But if you'd stayed in
the front seat, you'd have gone out the window.
To make this work, you've got to make sure your front wheel clears
the jump.
I was taking practice at a local track a few years ago and "tapped"
my front wheel on the top of a jump. When this happened, my Cg
went forward, just like it was supposed to, but the front wheel
couldn't dip because it was touching the ground already. The law
of inertia tells us that my Cg kept moving, and I flipped forward
over my handlebars like Mary Lou Retton flipping into a Wheaties
commercial.
If a particular section of a track is giving you trouble and causing
you to "wobble" or flap your handlebars back and forth,
try keeping your Cg farther back. Last year's Morristown Nationals
had a rhythm section that caused a lot of riders to put their
Cg too far forward, which caused unwanted handlebar flapping,
which led to swerving off the track and bailing. The farther back
your Cg is, the less likely you are to steer yourself into trouble.
Last month I mentioned how a wet front tire caused me to slide
out and lose a race. Had I been leaning back going into the turn,
I'd have been able to feel the slide and put my foot down to save
things, instead of going head-first into the berm. As I've been
saying for three and a half years, "Never use your front
wheel when you can use your back wheel." You'll be safer
that way.
Jumping is a Cg manipulation. As you ride up to a jump, you lower
your weight on the bike. When you start riding up the slope, you
push against the pedals, throwing your Cg up. As you do that,
you pull the bike up and into your body, bringing it higher. Without
this Cg "bounce", you're just riding the jump. Most
of us know how to "pull up", but aren't conscious of
the "pushing" motion that precedes it.
The odd lesson we learn from this is that the more we "crouch
and spring" before the jump, the higher we'll go. If you're
having trouble clearing something, and more speed isn't possible,
try a stronger crouch.
Understanding "crouch and spring" helps us figure out
how a "dead sailor" happens. Remember that you outweigh
your bike, and because of that you are the major part of the bike/rider
Cg. When you leave a jump and catch air, the bike is going along
with you, not vice versa. Most of us "tuck" our bikes
in midair and then extend our arms and legs rig before hitting
the ground, for a "shock absorber" effect. The bike
moves up and down in relation to the body, not the other way around.
A "dead sailor" occurs when you don't tuck properly
off the lip of the jump. This means your bike is too far away
from you to manipulate properly, and it sways out from under you.
Think of someone on a very tall unicycle, trying to keep the wheel
under the seat, and you'll understand why it's tough to keep a
bike under you when your legs are extended. This increases the
likelihood of landing partially leaned-over, which usually leads
to a wreck.
Long ago, back when dinosaurs ruled the earth, my mom bough me
a new Construction Yellow Redline 600c. It was my first "real"
race bike, so naturally I took it right up to our local jump and
hit it as fast as I could, which is a lot faster than I'd been
able to do on my old homemade racer. My extra speed made me jump
higher than I was used to, and I didn't "tuck" properly.
I was so far above my bike that I nearly lost it altogether, but
I settled for toppling over in mid-air and landing on my face
and elbow. Were I to do that today, I'd probably check myself
into a hospital, but back then I used a Vise-Grip to pull the
rocks out of my arm and rode the jump until the sun went down.
I was either very stupid back then or far smarter than I am now
- I'm not sure which.
If the "dead sailor" is an example of uncontrolled Cg,
then a backflip like the one Jose Yanez invented and Todd Lyons
popularized is an example of perfectly controlled Cg. As Todd
leaves the jump, he begins rotating around his Cg, the same way
a top spins around its balance point. If we could "see"
his Cg, we'd notice that its motion is the same as on any other
jump - an up-and-down curve created by gravitational pull. It's
the rotation around the Cg that makes a backflip.
Jody Donnelly, another hero of mine and a prime example of an
"ordinary guy" with extraordinary talent, used to wreck
very spectacularly on backflips by "over-rotating",
just like Cru Jones. Jody would have gladly quit doing this, and
eventually did, but you can't just decide to stop "over-rotating."
Once you start spinning around your Cg, you wait, and can't stop
until you hit the ground. Why? Inertia, of course. To prevent
"over rotation", you have to carefully match your rate
of rotation to the size of the jump. That's why backflips on dirt
are so special - every jump is different, and it takes an unusual
kind of talent to estimate the necessary spin rate just by looking
at a jump. Quarterpipes, on the other hand, are much closer to
being identical.
We'll finish by noting an example of over-rotation on my part.
In the semi of the Ohio State Race Championships many years ago,
I believed it would be important to totally clear the first tabletop.
Since we all landed on our back wheels in those days, I planned
to pull up hard, lean back, and touch my rear wheel down on the
jumps backside. Doing this would let me keep some speed and make
the main.
When I hit the jump, I pulled up hard - way to hard. Before I
knew it, I was upside-down on the air. Not knowing what to do,
I settled for letting go and falling eight feet onto my back.
My bike followed soon after and snapped my favorite Kashimax Aero
seat, along with my hopes of making the main. Although I didn't
know it at the time, that was the best year I would ever have
on the bike. In 1988 I broke my neck and changed from the snot-nosed,
hard-charging "winner" I used to be to the low and slow
Jim Boswell you know today.
If I could go back in time, I would worry less and enjoy my good
years more. But physics won't let me go back, any more than it
will let you save a bad "dead sailor" ten feet off the
jump. So enjoy your riding today. Physics doesn't guarantee us
a tomorrow.