Since someone threatened math…
There are three conflicting goals in pacing.
Mechanically, you want a constant speed. This is “going harder when you’re going slower”. It’s because of wind resistance. Wind resistance scales faster than your speed, so you get the best average speed by going at a constant speed, so that aren’t spending time doing “extra work” against the wind by going faster than average.
Physiologically, you want a constant power. You can see this with the Normalized Power model. Average power is what moves the bike, normalized power is what impacts your body. If you can do, say, 300 W Normalized Power for 1 hour, you’ll get the highest average power by doing exactly 300 W the entire time. Any variation, and either average power goes down or NP goes up.
Psychologically, people are often well-served by a negative split – going faster in the second half. Part of this is the difficulty of pacing. How much power you can actually put out varies from one day to the next. The cost of undershooting a little bit is that you go a little slower. The cost of overshooting is that you might blow up, which makes you a lot slower. So a conservative first half is safer.
You can run the numbers on constant speed vs. constant power. That’s exactly what Best Bike Split does. It won’t account for the benefits of a negative split because it doesn’t have that in its model.
Incidentally, the BBS recommendation for a windy flat TT are pretty close to constant power.
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You do not want to push less power into a head wind and/or up a grade. As the wind or grade increases the more variable the power (assuming you can hold the power target).
What happens to many (me included) in situations as stated in the OP is we start out at ftp with the tailwind. We feel like superman because even though 10mph tailwind isn/t much if you’re used to high winds in other sports (kiting/windsurfing for me) the speed and ease to stay on top of the gear will be way way way different compared to the return leg into the wind (it feels effortless).
However, when you turn hit the headwind you will most likely bog down a bit, lose a bit of power, watch speed drop, then overcompensate with too much power, have to recover at a much lower wattage and speed falls more. If not aware the cycle continues.
Hence my whole mentioning negative split with a tailwind on the out leg. Such a good opportunity to hold back a bit on the first leg to make sure you can hold FTP or whatever target you think you can hold on the headwind leg where you can lose more time at equal wattage.
Again, push as much or more power on the leg into the wind compared to the tailwind leg.
edit: if you are confident on the day and there is a tailwind 1st leg that you can do 305w out and 305 back then punch it. It makes no sense to do 305 out and slowly fade to 295, 290, etc…into the wind. Happens all the time is all I’m trying to drive home. If you think you can do 305 out and 305 back to begin and 310-320 as the second leg progresses, even better.
To report back - the week long crash course ended up with pretty good results, managed a 36:28. The winds reversed half way through the day so we had head out/tail back which mentally helped. I did negative split from a time perspective however my wattage was mostly steady, if any thing hammock shaped, but not drastically. I definitely looked up way too much and could be way more aero; some position work and a helmet would likely go along way. Perhaps I’ll give some more TT’s a chance this year, just wish we had more stage races. Thanks all for your help thinking about this beforehand. Log In to TrainerRoad
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I’m a little late with this post, but this may help for the future. Hope it went well!