Downhill Time Trial April 12, 2008

SRM File

These are the saved results of a downhill time trial on April 12, 2008. The course was an 8.6 mile section of a mountain descent on Highway 168 (near Fresno, CA), starting at the intersection of 168 and Alder Springs Road, heading west and descending approximately 2700 feet (start right at intersection and finish right at the “highway information” sign, about 200 yards before the stop sign, both marked with orange paint). The road is very wide 4 lane highway, with good shoulder, smooth, clean pavement, and wide turns requiring no braking. An aero position was held the entire way. Grades vary from approximately -9% to -3% on the descent, and the last half mile is close to flat. This run was done on a Cervelo P2C, with Zipp 999 clincher wheels (disc/deep front), Zipp clinchers, Michelin latex tubes, aerobars, skinsuit, Gyro time trial helmet, and 53/38 with 11-21 gearing, with an SRM recording data.

Results were a time of 11 minutes 25 seconds, with average speed of 45.5 mph, and top speed of 52.5 mph. Speeds and cadences were too high to maintain power for any length of time, so brief power surges at close to 130 crank rpms and 700 watts was all that was manageable. Heart rates stayed under 130 rpms, but rose to above threshold on the flat section at the end. Maintaining only a tuck was not faster than pedaling at speeds under 45 mph. Despite the higher than normal speeds, on this section of road it never felt unsafe in any way.

Clearly, higher gearing would be beneficial. A 53x11 gear ratio is 49.6 mph at 130 rpms. A more sustainable 110 rpms would be about 41.9 mph, under the average speed for this run. Taller gearing that would allow a cadence of around 105 rpms at 50 mph would likely give best results. Will try higher gearing and see how much more speed that gains.

Interestingly, as an afterthought, I ran some numbers at Analytic Cycling.com. Coasting on this course, assuming an average -6% grade, among other assumptions, results in a predicted average speed of 44.5 mph, pretty close to the actual result, which included some pedaling for brief periods. Then, running the same numbers, but with 300 watts of power added, only raised the average speed to 49.9 mph. So, at these speeds, 300 watts only gains 5 mph. That shows what a huge effect wind resistance has at higher speeds.

Added a larger chain ring to the bike: click here

May 18 time trial with 67 tooth chain ring

In the profile, below, it looks like there are little uphill portions along the route, but there aren’t. I understand the Topo software goes by the lay of the natural land, not the road. It actually has no uphill or flat portion on the descent until the very bottom.

3D view of hill