Downhill Time Trial May 18, 2008

SRM File

These are the saved results of a downhill time trial on May 18, 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, on aerobars, 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 67/42 with 11-21 gearing, with an SRM recording data.

This effort took some creative methods to pull off. First, I had to climb the mountain to do the run. Since there is a huge difference between the 42 small chain ring and the large 67, I had to remove 8 links of chain, ride up the hill, then replace the 8 links (with 2 master links), and manually move the chain to the large ring. Not a big deal, really, but unusual.

Got up at 4 a.m., so I could get ready to drive 20 miles to the bottom of the hill, then take about an hour to climb the hill, and do the run before there was any substantial traffic. Also, the wind tends to be low first thing in the morning. For something like this, I’d prefer no wind at all, as cross winds can be a challenge with an aero bike, using aerobars, at 50+ mph. Started up the hill at 6 a.m. sharp. It’s a fairly long climb, which makes for a great warm up. With the 42x21 low gear, it required a whole lot of standing. Any hill you can hit 57 mph coming down is not going to be an “easy” climb, even though there are many harder climbs around.

Even with this enormous chain ring, still got spun out in many places (100 rpms is 47.7 mph). By the way, these speeds on this particular road were no concern at all. Just rode out in the right lane of the 4 lane road, and only 2 cars passed the whole way, of course right before I was ready to pass a Jeep. Passed the Jeep at 57 mph, while pedaling. Too focused on the road to glance over and see the look on the driver’s face, though.

Results were a time of 10 minutes 24 seconds, with average speed of 49.6 mph, and top speed of 57.5 mph.

Here is the bike: click here

Here is a graphic of the SRM file. For some unknown reason, recording stopped and the display went blank less than 8 minutes into the run. Calculated final time and average speed from a stopwatch. Was unable to record power and cadence, as the 67 tooth chain ring does not fit the SRM power meter. The results show reaching 50 mph in about 25 seconds, then hovering just below or above 50, depending upon the slope and power applied. At the steeper sections, coasting resulted in speeds well over 50 mph, and heart rate goes down. In the slower sections, a whole lot of power was necessary to maintain high 40’s, or push above 50. The top speed over 57 mph was both in a steep section, and with lots of power, as heart rate shows. For comparison, a few weeks ago I descended this hill on a “normal” road bike, no aero features or equipment, riding in the drops, regular clothing, and averaged only 31 mph, top speed of 41, and took around 16 minutes.

On the graph, note that speed is in lavender, and the scale for speed is on the left, ranging from 30 mph to 60 mph. No need to display under 30 mph, when almost all of the time is around 50 mph. The first peak speed is right at the 52.5 mph line, holding around there, then dropping into the high 40’s on the section that is less steep. Around 4 minutes, speed gets back up to around 55 mph, and heart rate is lower, meaning there was more coasting or at least backing off the power. Peak speed comes around 7:30 minutes, and heart rate goes way up there and after that, too. This is where I passed the Jeep at just short of a sprint.

Strategy for next time will be to simply coast at speeds over 50 mph, not trying to rack up high numbers, and limit pedaling and more power to the slower sections. Spinning very fast over 50 mph does not gain much speed, but makes the legs blow up really fast. Also will lower the handlebars even more, as the bike has an adjustable stem. Also, may do a run where I’ll mostly coast until the steep section near the bottom, then apply maximum power to see if 60 mph can be exceeded. If the cars would stay out of my way, I am confident I can.

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