Q: Why is it important to learn about your center of gravity (Cg for short)?
A: Nearly everything we do in BMX is in some way influenced by our Cg. For example, it's a Cg shift that gets us out of the gate in a hurry. If you wash out your front tire in a turn but your rear tire stays put, that's an indicator of misplaced Cg. The front-wheel-down jumping style that has become popular in the past couple of years is extremely dependent on proper Cg manipulation.
Q: Can't I just figure all this stuff out without worrying about physics?
A: Absolutely. Most riders do pretty well without ever worrying about the physics behind their riding. If you want to be able to analyze your riding with any kind of efficiency, however, you need to think about Cg. A rider can also really benefit from thinking about Cg if he is having a lot of trouble with a particular section of track and wants to plan a course of attack., or if his practice time is limited (like at a National) and he wants to make the most out of his laps.
Q: Okay, fair enough. What is Cg, and how do you find it?
A: Your center of gravity, or Cg, is your "balance point". For a demonstration, take a spoon and a piece of string. Tie one end of the string to the middle of the spoon. Hold the other end of the string so that the spoon is up in the air. What does the spoon do? If your spoons are anything like mine, you'll see that the heavy end (the end that scoops your Apple Jacks) points towards the ground, while the light end (the end you hold) points up. Move the string towards the heavy end, and try it again. Eventually, you'll be able to "balance" the spoon so it stays level in mid-air. When you can do that, you will have found the Cg of the spoon.
Q: Yeah, but I don't ride a spoon---I ride a bike. How do I find the Cg of my bike?
A: The Cg of your bike doesn't matter. It's the Cg of the bike-rider combo that counts. Since most riders outweigh their bikes, where you place your body has a lot to do with your Cg. Let's say you're doing a manual on the street. As you ride down the street, getting ready to do the manual, your Cg is somewhere above and between the two wheels of your bike. When you lean back to do the manual, your Cg is behind and above the rear wheel, causing you to start falling backwards. As your front wheel comes up, you lean forward a little bit, placing your Cg approximately above the rear wheel. If your Cg moves ahead of the rear wheel, your front wheel will come down. If your Cg moves behind the rear wheel, you'll fall off the bike.
Q: But when I look at other people doing manuals, they are really hanging off the back of the bike.
A: That's true, but remember that ALL of a bike's weight is forward of the rear axle, so if my bike weighs 25 pounds, I have to hang 25 pounds of my weight behind the rear axle to keep things balanced. If I only weigh 125 pounds to begin with, that means I'm going to hang a lot of my rump back there to keep things even. Even wonder why Barry McManus is so great at manuals? Not only is he athletic, which makes it easy for him to balance, he's heavy, which means that he doesn't have to move around as much relative to the bike. It's a lot easier for him to find 25 lbs to put behind the rear axle than it is for a smaller rider.
Q: So I'm doing a manual. I'm starting to fall off the back, so I touch my brakes a little bit. Why does that keep me from falling off?
A: What you're describing is actually the key to a lot of what happens in BMX. We've already learned that the rider is the biggest part of the bike-rider Cg. A rider is connected to his bike in four places---the two pedals and the two grips. In mountain biking, the seat also counts, but we usually don't sit during a race in BMX. Whenever a rider acts to change his Cg, he does it through those four interface points. Imagine you're riding a skateboard down the street. You and the skateboard are moving at the same speed. Your feet are the "interface points" with the board. Unfortunately for you, however, you hit a curb. Your skateboard stops, of course. If your feet were welded to the board, you'd stop too, but since they aren't, you keep moving at your previous speed, which means that you are in a lot of trouble. You're going to sail through the air, at your previous speed, until something stops you, like a tree. Ow. Let's go back to that manual. You touch the brake, so the bike slows down. YOU don't slow down immediately, however; your body keeps moving forward at the "old" speed. Your body "overtakes" the bike, which moves your Cg forward, causing your front wheel to drop a little. To bring the front wheel back up, you just lean back again. Simple, huh?
Q: That's why when I see a rider doing a manual for a long time, he's moving his hips back and forth. He's moving his Cg without moving the bike.
A: Right. Remember that your bike is your "junior partner". It has about 25 lbs of say in your Cg. You have the rest. Since I weigh close to 250 lbs, I have about 90 percent of the influence in my bike-rider Cg. It's vital that I stay balanced on my bike, because it can't help me too much. When I ride a motorcycle, it has most of the influence, and my balance isn't as important. That's why all those old people who ride around the country on Honda GoldWings don't crash. A GoldWing weighs 850lbs, which makes it awful stable on the road. Try to manual a GoldWing sometime! It's physically impossible.
Q: How does Cg come into play on the track?
A: Let's start with your gate. You want to have a good start, which means that you want to come out of the gate quickly. The best way to do that would be to jump the gate---to start your front wheel over the gate before it drops. You can't do that, however (unless the factory you ride for has a lot of influence with the referees) so you do the next best thing. You start your Cg moving forward, so that when the gate drops you can catch your bike up with it. You start with your hips way back. Your Cg at this point is behind the rear wheel. As the lights flash, you shove your hips into your handlebars. Obviously you can't ride around the track like this, but that doesn't matter. What matters is that your Cg is in motion before the gate drops. Once the gate drops, you'll catch your bike up. If you watch a pro do a gate, you'll see that he throws himself out of the gate with his weight above his front axle. Once the gate has dropped, he'll bring his bike forward.
Q: That's why you flip over if the gate doesn't drop right. Your Cg is in motion, but the bike can't move, so just like a skateboard rider hitting the curb, you keep going and flip.
A: Exactly. It's also why a two-pedal start is important. You can get great power out of your first pedal from a one-pedal start, but you can't do that weight shift before the gate drops.
Q: So I'm out of the gate. I see the first jump. What does Cg do now?
A: Let's say you decide to manual the jump. As you come up to the jump, you lean back, placing your Cg slightly behind the rear wheel. Your body and bike are going at the same speed. Your front wheel doesn't touch the jump, but your rear wheel does. Your rear wheel starts climbing the jump. That vertical motion steals some of your bike's speed. Your body is still moving at the "old" speed, so it moves forward on the bike, which brings your front wheel down, just like you've hit the brakes during a manual on the street. As you ride down the back of the jump, your bike will "catch up" to your body.
Q: So my bike slows down, but my body doesn't. And since my bike is only about 10-20% of my Cg, it doesn't slow the two of us down very much.
A: Exactly. But what if you'd ridden that jump? As your bike slowed down, you would have had to slow down too or flip over the bars.
Q: Why don't I flip over? My body is still moving faster than the bike.
A: Right, but you use your arms and legs like shock absorbers to slow your body down. That's why your elbows bend when you ride a jump. Your body wants to keep going and flip over the bike, but by letting your elbows "take up" that extra speed, you keep it from happening.
Q: So when I manual a jump, only 10-20% of my weight has to slow down, but when I ride it, all of my weight has to.
A: Right. Let's do the math. Take a typical rider of 150lbs and a typical bike of 25lbs. The bike is about 15% of the Cg combo. The bike and rider are going over the jump at 20mph. Rider A manuals the jump. His bike slows down to 15mph. He keeps going at 20mph. When Rider A makes it over the jump, he has to bring his bike back up to speed, which means he has to give the bike some of his speed. 20mph x 85% (rider) and 15mph x 15% (bike)=19.25 mph.
Rider B, on the other hand, rides the jump. Both he and the bike are subject to the same loss of speed as Rider A's bike was, so he slows down to 15mph, assuming he doesn't wreck over the jump or lose more time in the air. He now is travelling 4mph slower than Rider A, so he will have to outspin Rider A by about 30-40 rpm for the next minute of the race simply to catch up, let alone pass Rider A. That's why you shouldn't just ride ANYTHING that you can manual safely.
Q: Okay. In your last column, you talk about using the "Compress" method if you don't feel secure about riding a jump. How's that work?
A: To ride a jump safely, especially a steep one, you have to manage your Cg well. The "Compress" method works with Cg as follows: As your front wheel rides up the jump, bend your elbows. This lets the rider continue to go just a little faster than his bike, without flipping over. Once you are on the top of the jump, extend your arms. This brings the bike back up to the rider's speed. You have some of the benefits of speedjumping in that your body never fully slows down, while avoiding the frightening aspects of lifting over a jump. There's another Cg benefit in the Compress method
Q: Which is?
A: Simple. Your Cg exists in three dimensions. We've only talked about Cg being forward of or behind the bike. But Cg also has a vertical component. The lower your Cg is on the bike, the more stable you are.
Q: Why?
A: What's more stable: walking on stilts, or creeping along with your knees bent?
Q: That's obvious, but what,s behind it?
A: The more distance you have between your Cg and your contact points, the more the lever principle comes into play. Take a pen and put one end on the floor. Put your index finger on the other end. Move your index finger left and right. You can move your finger about three inches either way without really changing the angle at which the pen contacts the ground. If your Cg is high above your bike, you can't have much influence on it without really swinging around. That's why the "Dead Sailor" jump exists. When you get your arms and legs stretched out, it's hard to control the bike in the air.
Q: Relate this to riding a jump.
A: If you keep your weight low, you will not wobble the bike as much going up a jump. This will reduce the likelihood of a crash.
Q: Simple enough.
A: As a matter of fact, you should always try to keep your Cg as low as possible. After all, what handles better---a Porsche, with its low Cg, or a jacked-up F-350 4x4? Which is more likely to "bail", or roll over?
Q: Point taken. Last but not least, what about jumping?
A: That's where Cg is really important. I'm sick of people talking about what their bike would or wouldn't do on a jump. Listen---you control a lot more of what happens on a jump than the bike does.
Q: How does jumping work?
A: Okay. Let's imagine that we put a dead guy on a bike and pushed him off a jump. Let's say this dead guy can balance the bike, by the way. This bike-rider combo would leave the ground and travel in a simple arc. To see a simple arc, make a "ramp" by putting a one end of a board on a brick. Run a Hot Wheels car off it. That's a simple arc.
Q: Most people don't jump like that.
A: Right. Most people do three things coming up to a jump. First, they pull the front end up, bringing the 20% weight of the bike up to the 80% of the rider. Next, they push against the ground with their legs, working through their pedals. This takes some of the horizontal motion component of the simple arc and changes it to a vertical one. Last, they bend their legs and bring their bike up into a tuck.
Q: What's this motion component business?
A: Okay. Motion isn't free. If I'm doing 20 mph and I go up a ramp, I'm going to slow down my horizontal motion and increase my vertical motion. A ramp is an angle, and it does work in the science-class sense of transferring horizontal motion into vertical motion. A forty-five degree ramp, in theory, takes half of your speed and changes it into height over a jump. You get what you pay for. That's why if you "pop up" over a jump you lose speed. You're trading speed for height.
Q: I'm still not all that sure about this.
A: Imagine you,re riding a half-pipe. When you hit the transition of the ramp, your horizontal motion is turned into vertical motion. You fly up into the air until gravity stops you, then you fall down back to the ramp. When you ride the transition into the flat part again, you get horizontal speed from the vertical speed you had returning to the ramp.
Q: Now I see. You're going the same speed, just up and down instead of forward.
A: Right. A BMX jump does the same thing, only it isn't straight up, so it doesn't take all your speed---just some of it.
Q: Back to the jumper.
A: Okay. He pulls his front end up, springs off the ground, and pulls the bike up in a tuck. Once he leaves the ground, the course of his Cg is set by physics.
Q: But I can change what my bike does in midair, and I can lean forward and back
A: Sort of. Your bike, remember, doesn't weigh as much as you. You can move the bike in midair. You can wave your arms. You can dump the bike. You just can't change the basic arc of your Cg. You also can't change any twisting or turning motion your Cg has when you leave the ground.
Q: A Cg can twist?
A: Sure. That's how you do a 360. But since you can't change the motion of the Cg in the air, you can't, say, do a clockwise 360 and then do a counterclockwise 360 in the same jump.
Q: True. But you can do a backflip and a twist.
A: Those aren't the same motions. They don't contradict each other.
Q: Okay, I think I see. How do I take Cg into account when I'm racing?
A: Look at each jump before the race. Decide whether you will Lift, Compress, or Hop the jump. For more details on that, read my November column. As you look at each jump, consider your momentum. Decide where your body and bike will move. Think of strategies to conserve speed. If you bail, ask yourself why it happened using Cg as a guide. If you fell off the back of your bike on a jump, consider whether or not you could have shifted your weight forward on the jump's transition, and so on. Ride smart.
Q: Jim, thanks for the help.
A: Thanks for asking and reading. I know this isn't easy to understand.
Q: See you at the Player's Ball.
A: Hasta la burrito.