Empiricism, I'm for it.

Went out with my friend John today to pre-ride the bike course that is part of the Tri for Fun in Pleasanton, CA. He can usually destroy me on flat terrain, so the plan was for me to lead one lap of the course and push my own air and see how fast I could go. Then he'd go out front and go faster, and I'd try and hang in his draft and see how much effort that would take compared to doing all the work myself. The results were pretty interesting.

Conclusions:

John is much faster than me on flat stuff (but I knew that already).
Drafting is less useful when you have a strong tail wind.
Drafting is immensely useful when you have a strong headwind.
If your Tri allows drafting you are giving up a lot by not doing it.
Pre-riding the course was a very good idea. Vineyard towards Bernal was very challenging. It is slightly uphill into the prevailing wind for a long way then has a short steep hill just long enough to run you down into granny gear.

One thing I would not have guessed after both loops without looking at the data is that my average heart rate would be only 1BPM lower when I was following versus when I was leading. But when you look at the data you can see that for part of the lap I was working harder to hang with him because he was going a lot faster than I could on my own, and the draft was weak. But once we got to where the draft was strong, my effort level dropped substantially. Plus the second lap was about 10% faster with John leading, so going faster but having a slightly lower heart rate at the same time is saying something.

Here's the course. It is mostly flat, with a quick climb where Old Vineyard meets Vineyard just below the park, with a long downhill from there to Bernal. The wind was coming from the top left of the image to the bottom right and was pretty strong. Forecast said 10-20mph.



Here it is in map form, so the point to point timings are more clear.



Here's the point to point comparison. It was hard to pull these from the logbook so there are some obvious rounding errors if you look at the data. Err on the side of John being faster.

start point to right turn at Stanley/Isabel

me 7:04 -- 20.6 mph
john 5:41 -- 24.6 mph


Stanley/Isabel to Isabel/Vineyard

me 4:55 -- 20.7 mph
john 4:46 -- 21.2 mph


Isabel/Vineyard to turnaround

me 3:13 -- 20.4 mph
john 3:04 -- 20.1 mph


turnaround to Isabel/Vineyard

me 3:23 -- 18.5 mph
john 3:13 -- 20.0 mph


Isabel/Vineyard to top of hill on Vineyard

me 7:31 -- 17.5 mph
john 6:25 -- 20.6 mph


top of hill on Vineyard to Vineyard/Bernal

me 2:37 -- 19.5 mph
john 2:40 -- 19.8 mph


Vineyard/Bernal to Bernal/Stanley

me 1:14 -- 19.3 mph
john 1:06 -- 20.7 mph


Bernal/Stanley to finish

me 2:27 -- 20.1 mph
john 2:30 -- 20.9 mph

Total

me 32:44 -- 19.4 mph
john 29.06 -- 21.4 mph




Here's the heart rate chart twice, with my lap highlighted on the first image, John's on the second.





You can clearly see from these that my heart rate was highest (for a while) while trying to stay in John's draft as he hammered it to start his loop. At the end of the loop when we had turned back into a strong headwind it was a lot easier for me and you can see my heart rate is much lower than the same part of the loop where I lead.