I was pretty annoyed at the Whiskey Island race when people passed me on the bumpy grass sections even though I wasn't riding near my limit. I set out today to figure out what the problem is. I took the 'cross bike to a local park (that shall go unnamed) with grass paths, some single track and two bridges.
The bumpy grass at the park was similar to Whiskey Island. The ground has been dry for several days, so it's almost concrete hard, and the surface is rippled and undulating and there is about 50 feet elevation difference from one end of the park to the other.
On the bumps if you were to ride along with locked arms and sat rigidly on the seat, your body would slam up and down. If you didn't react by relaxing your arms, it would be impossible to maintain control. Also, your body will act as a damper and absorb energy that's imparted to it by the bumps. The energy would be absorbed by heating and making minute deformations to tissue. Effectively, that acts as a brake.
The solution is pretty simple and obvious, although I didn't figure it out during the race, actually I did the exact wrong thing. The bike needs to move independently of the body with arms and legs acting as suspension. The way to do that is either get out of the saddle and push a bigger gear, or stay 90% seated while pushing a bigger gear, and pulling forward on the bars--moving the saddle forward helps with this.
During the race, I shifted down and spun while sitting, so my weight was almost completely planted on the saddle.
The difference between these techniques on my training loop today was stark. When seated and spinning, one lap took 7 minutes 14 seconds. My best time with a combination of out of the saddle pushing a big gear, and in the saddle with my legs carrying some weight was 6:05!
It will be interesting to see if there's any difference in my placing in upcoming races. As long as the ground stays dry, I should be able to move with a little more efficiency than I did in the season opener.
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