At full speed

I left off the last post with the following question.

Q: How fast should I be going?

The smart ass response is:

A: It's a race! FULL SPEED!

However when it comes to cornering, riding at full speed is not a strategy for a fast race. It is a recipe for disaster.

At any given point in time during a race, the amount of available traction is fixed and in limited supply. There are only "two" things that influence the amount of available traction: A) the coefficient of friction between the tire and the ground and B) the normal force (i.e., the weight of the bike + rider) pressing the tires into the ground.

I put "two" in quotes because the coefficient of friction is a result of the interaction between tire and ground, both of which can change. Tires can vary in the material (e.g., soft vs hard rubber), tread patterns, width, pressure, etc, and the ground can vary in all the ways that ground varies. It can be asphalt, concrete, gravel, dirt, dirt with live grass, dirt with dead grass, sand, wood, artificial grass, asphalt or concrete covered with any or all of the above, and all of those surfaces can vary in the extent to which they are wet and or frozen.

But at any given point during a race (i.e., a picture taken of a cyclocross rider) the tire and the ground are constant and the rider's weight is fixed in a single position. At that moment, the amount of available traction for accelerating, cornering, and braking is fixed. If that single snapshot becomes a video that can be played forward and backwards a few frames, the tires will not change during those few frames, and the ground will not likely change (very much) during those few frames. Although available traction may change greatly over the course of the race we can think about it as being fixed in the moment.

In auto-racing they have something called the Friction Circle to help communicate the limited nature of traction. There are also some youtube videos (video 1, video 2) that further explain the general concept. If you have a few minutes it might be useful to watch these videos and pretend they are talking about bicycles instead of cars. I very much like the idea expressed in the second video that one of the differences between more and less skilled racers is the ability to stay on the outside edge of the friction circle and maximize the use of available traction for accelerating, braking, and cornering.

The same general concept applies to cyclocross. Friction circle theory suggests that in order to maximize the use of available traction, there has to be some segmentation of the three uses of traction. The general approach is depicted below.

From My Racing Thoughts

This implies waiting until the last possible moment, then going "deep, deep, deep into the brake pedal" to decelerate to the appropriate entry speed. Using all available traction means braking almost to the point of locking up the wheels and skidding. At the entry of the turn, the force of breaking is reduced, but the force of cornering increases. I was at practice on Tuesday morning and was trying to be very mindful of being on the edge of the friction circle. (How do I know I was successful? Well, on two different occasions while turning I lost traction and wiped out. Skidding means being at and then beyond the edge of the friction circle. The second incident was much to the entertainment of all). There were two things I noticed. First, the edge of the friction circle is jagged. I could feel my front wheels sliding and skidding a little bit, and then regaining grip, as I made microscopic adjustments to the handlebar steering into any slippage. Then on certain grassy corners I could actually hear the sound of the grass tearing and snapping one blade at a time as they strained to hold the force of my turn. So just as in driving a car there is both tactile and auditory feedback.

The turn on an ideal line starts to get gentler after the apex. This frees up available traction for accelerating. This available traction may or may not be used for accelerating. There is an element of personal choice in whether or not the greater force is applied to the pedals to take full advantage of that additional traction. It is easy, especially when tired, to coast through the second half of a turn and wait until the straight-away. Each time represents a missed opportunity to close a gap in front and open a gap behind. The more turns the more opportunities to gain time through more aggressive cornering.

Another similarity that bicycles (these folk notwithstanding) share with automobiles is an ability to select different gear ratios to optimize the transfer of power from "engine" to road. Being in a lower gear makes it easy to accelerate, and being in a higher gear makes it easier to maintain momentum. Thus being able to accelerate out of the apex of a corner means being in the appropriate gear going into the corner. For a number of reasons it is difficult to shift while cornering, and I find it to be much easier to shift before or during braking at the entry of the corner than trying to shift during the corner. I also feel like it's easier to upshift while pedaling hard than it is to downshift while pedaling hard, and therefore I try to err on the side of being in too low of a gear so I can spin fast and then upshift instead of being in to high of a gear and try to apply a lot of force to the pedal while downshifting. I am always curious to know what others think about shifting before/during/after a turn.

Having introduced the general case, maybe it's time to talk about some specifics. One of the things that makes cyclocross challenging is that as previously noted the amount of available traction changes from moment to moment. Take as an over-simplified example, the nasty turn I diagrammed from my last race.

From My Racing Thoughts

I've taken away the "ideal line" because for this turn it has little bearing on where I ended up trying to ride. The purple circles of different sizes are meant to reflect that the loose and dusty dirty has less available traction than did the surrounding grass. The "best-line" that I tried is shown in blue. Others may have had different approaches, but I was trying to turn early before hitting the dirt, carry on straight through the loose dirt until I hit the grass on the far side (bigger traction circle) where I could make a sharper turn, and avoid the loose dirt on the outside. One time I brought in too much speed and shot it wide. A different lap I slowed down enough that I was able to make a turn in the dirt. A few times I may have actually followed something resembling where I thought I wanted to be. Being able to figure out where to go, and being able to go there consistently lap after oxygen depleting lap? Two separate things.