The Descents of the 508

When you tell most people about fixed gear riding, they imagine the biggest challenge comes from climbs. This may be true for steep grades of over 8%, but you will find for most riding, far and away the largest challenge, and where the most time is lost compared to standard road bikes, is in the descents.

When climbing, you lose some of the momentum of keeping a higher cadence, and you are forced to put more strain on your muscles from the larger gear. In comparison, descents with a freehub are just that - free. Your heart rate can settle down to a minimum, your legs can completely rest, and you can stretch out and assume any position you want.

On a fixed gear, your position, legs, and cardio system must all accommodate the rapid cadence required to go at higher speeds. Training and technique development can lessen the impact of high cadence motion, but it will always be there. I was able to sustain decent speeds on the descents of the Mount Laguna Bicycle Classic, maintaining speeds over 30 fairly comfortably, but in reviewing my data, I see that the effects of the higher speeds were taking a toll on my performance. Both in raised heart rate levels (which compromise digestion and recovery) and leg fatigue (especially for climbs directly following a descent).  For a century ride, this is an acceptable tradeoff. For something as epic as the 508, I will have to be more careful.

Full analysis here.

 In brief, I marked all of the sections where I topped 26 mph, and made totals of distance and time for each. Over the whole course for 2009, I spent 133 miles more than 25% (of distance) at speeds above what I should sustain with my chosen gearing (46x16). On these sections I averaged 33.5 mph, totaling almost 4 hours of descending or very fast flats.

It is an oversimplification, but I think I can get a good estimate of the extra time my descents will take by simply multiplying by the ratio of my proposed average speed. I think it's a safe bet that I can average 26 mph on all these sections, taking into account all the lost momentum from the 45 mph+ descents, and my decreased capacity to descend as the race wears on.

• A 26 mph average  (average cadence a civil 125 rpm) would add 75 minutes.
• A 28 mph average  (average cadence 135 rpm) would add 49 minutes.
• A 30 mph average (average cadence 145 rpm) would add only 30 minutes. (but is doubtfully sustainable). 

 Seeing these numbers - I think it is worth it to limit my speeds early on (note that over 100 of those 130 miles comes in the first 220 miles of the race!). Although it is frustrating to lose time on descents, the 45 minutes gained by the significant effort increase from 125 --> 145 cadence would likely be more than made up for by the decreased leg, and cardio stress of the lower cadences.

It will be important not to overestimate the time gained by fast descents - or underestimate the toll fast descents will take on my body. All the time I spent over 26 mph during the whole 508 was less than the time it took to travel the 44 miles from Furance Creek to Ashford Mill, during the hellish windstorm in Death Valley!