Thursday, January 22, 2009

What the F is a CP30?


About a month ago, my teammate Rocket Pants and I did a bike time trial. I use a PowerTap on my race bike, so this was a great opportunity for me to calculate my CP30 as described in Training and Racing with a Power Meter by Hunter Allen and Andrew Coggan. CP30 is your "30 minute Critical Power", or in english - how hard you can push your pedals for 30 minutes. Fair warning - triathlon is full of confusing jargon, and analyzing power data magnifies that problem. If your eyes gloss over at the thought of dealing with lactate threshold, anaerobic threshold, VO2Max, etc. then you may want to skip this one. If you're a data geek like me, however - you're in for a treat!
I'll work backwards from the workout, since that way I can avoid dealing with all the theory. What did RP and I actually do? We met at Fiesta Island in Mission Bay, and did a 12 mile time trial. Fiesta Island is San Diego's gift to runners, cyclists and dog owners. It's basically an undeveloped island with a 4 mile one way road around it's perimeter. In actuality it's not "land" so much as "landfill", but that's unimportant for now. The point is - it's a practically closed course bike track - the perfect place to do a time trial. So we did a bit of a warm-up, and then did our test set - 3 loops (12 miles) as hard as we could go. Simple enough.
Next, I took my powertap home and downloaded the workout data to my computer using the TrainingPeaks WKO+ software. I would highly recommend WKO+ over the software that comes with the PowerTap - especially for triathletes since WKO+ can handle run and swim workouts. The raw data looks something like this:
You'll notice I started with a warmup (grey area), going about 15MPH for a few loops. Then, around the 39 minute mark, I began my test (black area). Unfortunately my heart rate monitor went flakey on me (red line), and I don't have any HR data for the test period. You can see some summary data about my test on the left side of the image - here's the important parts of what it says:
Duration: 33:47 (33:50)
Distance: 12.373 mi
Min Max Avg
Power: 34 682 242 watts
Cadence: 66 131 96 rpm
Speed: 14.7 25.8 22.0 mph

So, I did 3 loops of the island and averaged 22.0 MPH. Without a power meter, that's pretty much the extent of what you would know from having done this test - and while that's valuable information to have, the extra info is pretty damn cool.
First, the powertap records my cadence - it says I averaged 96 RPM for the 34 minute test. That's about right for me. I follow the conventional wisdom of the last few years that 90 RPM is the sweet spot for most cyclists - and when I race I end up being closer to 100. Of course there are those who argue that you should have a low cadence (Chrissie Wellington claimed hers was only 70 during the 2008 Ironman World Championships) - I really don't know what the "correct" approach to cadence is. But I do know exactly what my cadence was - which at the very least tells me if I'm doing what I intended to, and in this case I was. The PT also tells me that I averaged 242 watts during the test. Watts is the unit of measure used for power, the higher the better. 242 is a decent number, however a pro cyclist will average 350 or even 400 watts on a stage of one of the grand tours - so I'm not turning pro any time soon. You're absolute wattage isn't really that important, what you're trying to do is improve that number over time - so what I really want is for that number to go up the next time I do this test. I can also look at my power output for each lap, it was: 245, 241, 241. So I did go out a little hard on lap one - but overall I was very consistent with my pacing, which is an area I am focusing on because I suck at it.
OK, I did 242 watts over 12 miles and it took me 33:47 to do it. Now what? Well, CP30 is the wattage you can hold for 30 minutes - and that's pretty much what I just did (I went 3 minutes long, but that's no big deal). So my CP30 is equal to 242 watts, easy! But unfortunately CP30 isn't really the number we want to know. It turns out 30 minutes isn't a long enough test - you can push yourself too far anaerobic for that short a time, and so your power output will be higher than it would be for a long race. The number we really want is the CP60 - the 60 minute Critical Power threshold, also known as Functional Threshold Power (FTP). How do I calculate FTP using the CP30? Oddly, you don't. What you actually need is the CP20. I know at this point you think I'm just fucking with you, but believe me there is a method to the madness. If I wanted to find CP20, why not just do a 20 minute time trial instead of a 34 minute one? And why not just do a 60 minute TT and measure FTP directly?! Because if you just do 20 minutes on fresh legs, you're numbers are skewed to the high side - because you are well rested. You need to get a bit of a warm up, as well as a bit of hard effort behind you before you start taking data. Doing a 60 minute time trial would be great - but it really trashes you, and it takes a few days to recover. Luckily WKO+ will tell me what 20 minute period had my highest power output - it is minutes 51 through 71, consisting of most of laps 2 and 3 of my test, and my power output was 243 watts. So my CP20 and CP30 are nearly identical, at least I'm consistent. The rule of thumb is that your FTP is going to be 5% lower than your CP20: FTP = CP20 * 0.95 = 243 * 0.95 = 230 watts.
I know this is a long way to go to come up with a seemingly arbitrary number, but it's actually very important. Now that I know my FTP, I know exactly how many watts I should be able to put out in a 60 minute race - like the 26 mile Bulldog race I'm doing in February for example. Having that number is an invaluable pacing tool - it can help me prevent myself from going out too hard at the start, and dying at the end - which is my natural tendency. But most importantly, that's the metric I will work to improve over the course of the season. Doing these tests regularly will help me gauge whether I'm making progress as a cyclist, and how effective my training has been. Plus, there are lots of pretty graphs!

You'll notice my "Mean Maximal Power Curve" on there - I'll talk about that next time. Below is all the gory details from the test.

Time Trial:
Duration: 33:47 (33:50)
Work: 491 kJ
TSS: 67.6 (intensity factor 1.095)
Norm Power: 244
VI: 1.01
Pw:HR: n/a
Pa:HR: n/a
Distance: 12.373 mi
Min Max Avg
Power: 34 682 242 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 66 131 96 rpm
Speed: 14.7 25.8 22.0 mph
Pace 2:20 4:05 2:44 min/mi
Hub Torque: 10 234 73 lb-in
Crank Torque: 42 670 214 lb-in


Lap 1:
Duration: 11:17 (11:19)
Work: 166 kJ
TSS: 22.7 (intensity factor 1.099)
Norm Power: 245
VI: 1
Pw:HR: n/a
Pa:HR: n/a
Distance: 4.106 mi
Min Max Avg
Power: 145 682 245 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 68 122 96 rpm
Speed: 15.2 25.8 21.9 mph
Pace 2:20 3:57 2:45 min/mi
Hub Torque: 41 234 75 lb-in
Crank Torque: 129 670 218 lb-in

Lap 2:
Duration: 11:15 (11:16)
Work: 162 kJ
TSS: 22.6 (intensity factor 1.098)
Norm Power: 245
VI: 1.02
Pw:HR: n/a
Pa:HR: n/a
Distance: 4.102 mi
Min Max Avg
Power: 161 403 241 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 68 131 94 rpm
Speed: 19.1 25.4 21.9 mph
Pace 2:22 3:08 2:45 min/mi
Hub Torque: 43 129 73 lb-in
Crank Torque: 105 396 216 lb-in

Lap 3:
Duration: 11:09
Work: 161 kJ
TSS: 22.3 (intensity factor 1.096)
Norm Power: 244
VI: 1.02
Pw:HR: n/a
Pa:HR: n/a
Distance: 4.13 mi
Min Max Avg
Power: 64 526 241 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 66 128 98 rpm
Speed: 19.4 25.2 22.3 mph
Pace 2:23 3:05 2:41 min/mi
Hub Torque: 19 154 72 lb-in
Crank Torque: 58 523 209 lb-in

Peak 10s (501 watts):
Duration: 0:10
Work: 5 kJ
TSS: n/a
Norm Power: n/a
VI: n/a
Pw:HR: n/a
Pa:HR: n/a
Distance: 295 ft
Min Max Avg
Power: 328 682 501 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 68 99 87 rpm
Speed: 15.2 23.4 19.7 mph
Pace 2:34 3:57 3:03 min/mi
Hub Torque: 125 234 170 lb-in
Crank Torque: 369 670 490 lb-in

Peak 30s (347 watts):
Duration: 0:30
Work: 10 kJ
TSS: 2 (intensity factor 1.557)
Norm Power: n/a
VI: n/a
Pw:HR: n/a
Pa:HR: n/a
Distance: 0.194 mi
Min Max Avg
Power: 196 682 347 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 68 103 96 rpm
Speed: 15.2 25.2 23.0 mph
Pace 2:23 3:57 2:36 min/mi
Hub Torque: 52 234 105 lb-in
Crank Torque: 162 670 314 lb-in

Peak 5min (259 watts):
Duration: 5:01 (5:02)
Work: 78 kJ
TSS: 11.1 (intensity factor 1.153)
Norm Power: 257
VI: 0.99
Pw:HR: n/a
Pa:HR: n/a
Distance: 1.816 mi
Min Max Avg
Power: 161 343 259 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 71 106 94 rpm
Speed: 19.7 23.9 21.7 mph
Pace 2:30 3:03 2:46 min/mi
Hub Torque: 48 111 79 lb-in
Crank Torque: 155 316 232 lb-in

Peak 10min (248 watts):
Duration: 10:01 (10:02)
Work: 149 kJ
TSS: 21 (intensity factor 1.121)
Norm Power: 250
VI: 1.01
Pw:HR: n/a
Pa:HR: n/a
Distance: 3.678 mi
Min Max Avg
Power: 161 400 248 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 71 128 95 rpm
Speed: 19.1 25.2 22.0 mph
Pace 2:23 3:08 2:43 min/mi
Hub Torque: 48 126 75 lb-in
Crank Torque: 123 364 220 lb-in

Peak 20min (243 watts):
Duration: 20:01 (20:02)
Work: 291 kJ
TSS: 40.4 (intensity factor 1.101)
Norm Power: 246
VI: 1.01
Pw:HR: n/a
Pa:HR: n/a
Distance: 7.368 mi
Min Max Avg
Power: 64 526 243 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 66 131 96 rpm
Speed: 19.1 25.4 22.1 mph
Pace 2:22 3:08 2:43 min/mi
Hub Torque: 19 154 73 lb-in
Crank Torque: 58 523 215 lb-in

Peak 30min (243 watts):
Duration: 30:01 (30:04)
Work: 437 kJ
TSS: 60 (intensity factor 1.095)
Norm Power: 244
VI: 1.01
Pw:HR: n/a
Pa:HR: n/a
Distance: 11.056 mi
Min Max Avg
Power: 64 682 243 watts
Heart Rate: n/a n/a n/a bpm
Cadence: 68 131 96 rpm
Speed: 15.2 25.8 22.1 mph
Pace 2:20 3:57 2:43 min/mi
Hub Torque: 19 234 73 lb-in
Crank Torque: 58 670 215 lb-in

1 comment:

derekv said...

THANK YOU,
phew, my eye balls crossed when I saw CP30,60, 180 etc... You did a great job of explaining it. Or at least I got the gist of it.

Keep up the good work and good luck as you race and push yourself

Derek V.
Let you bike take you somewhere you've never been and show you something you've never seen.