Tuesday, August 14, 2007

New TT position

I have worked very hard over the last 5 weeks to improve my time trial position to lower my CdA (drag coefficient x frontal area). On July 4th I performed aero testing showing my CdA to be 0.297 with my skinsuit, TT helmet, shoe covers, and the body position that I've been using for the last 6 years. That body position was set in a bike fit when I purchased the bike at All3Sports about 6 years ago.

I have made three significant changes gradually over the past 5 weeks, one at a time between rides. The first was a lowering of my bars by moving spacers from beneath the stem to above the stem. I've only lowered the bars about 2 cm so far and could probably go at least one more cm over the next month or so, but I only drop 1/2 cm at a time for acclimation purposes.

The second change has been a narrowing of my elbows by bringing the elbow pads on my bars closer together by about 2 cm.

The third change (the one that's made the most difference) has been seat position. I knew that my triathlon seat position was set a lot more forward than my road bike position. Triathletes need to save their hamstrings for the run, and the forward position also opens the waist angle to allow better breathing. I found that my road bike saddle was about 12 cm further back from the bottom bracket than my tri bike. That's a lot more than I expected. I flipped the seat mount around backwards on my Cervelo and also pushed the saddle back on the rails to move my saddle back about 8-10 cm from it's original position.

The original reason I did that was to get a similar saddle-to-pedal relationship to my road bike so that I'd use those muscles similarly and not have to train additional muscles for the TT bike. That's still a good reason, but I've discovered an even bigger benefit: by shifting my hips back 8-10cm, the angle of my upper arms relative to horizontal has moved from about 90 degrees (which maximizes my shoulder height above the bars) to an angle that might be about 65 or 70 degrees to the horizontal. That has had the effect of lowering my shoulders even further relative to my hips.

It does reduce the angle of my torso to my legs, but so far it seems manageable - I'll know more after riding that way for a while and doing some power testing on the computrainer.

The end result is a reduction of my CdA from 0.297 on July 4 to 0.270 on August 13th). At first those are just numbers on the page, but when I calculated the effect of that improvement on TT times for various distances, I was quite surprised.

IF I can hold a similar power in the new TT position as I held in the old one (a big if), I can expect to save about 45 seconds on a 10-mile TT like Perry-Roubaix (and most GA Cup races) and I can expect to save over 12 minutes on the Rock-n-Rollman half-iron 56-mile course. It seems almost ridiculous, but that's what the math shows. No wonder Levi gets himself into that crazy position for TTs - there are huge benefits to being so aero.

In the best-case scenario, I can get the aero improvement and generate more power due to the more road-bike similar position. I'm going to stop moving my position now and work for a few months on getting powerful and comfortable with the new setup.

My next task is to address rolling resistance. Based on published data, I can save chunks of time by ditching the tubular Tufos on aero wheels I'm now using (maybe Crr=.006) and replacing them with good aero clincher tires (maybe Zipp 404s) using latex tubes (maybe Crr = .0045). So next I'm onto the flat road coast-down tests for Crr calculation.