Saturday, November 28, 2009
Thursday, November 26, 2009
Here's a quadrant analysis chart of two very different efforts. The one in green is a force rep workout (20-sec of hard low-cadence interval with spinning in between). The one in purple is the 2008 Elberton Crit. All the points above the red line represent segments of effort above my threshold power.
I'm not sure what to do with the information yet, but it's interesting to look at. The most obvious use would be to compare race efforts to training efforts to see if my training is properly race-specific.
Sunday, November 22, 2009
It's raining, I'm in a rest week, and I needed a break from house chores and work; so instead of a ride today I did a full range of Power Tap static calibration tests, as suggested by Dr. Chung.
I found that my PT is pretty accurate. It appears to read an average of about 2.3% low overall, which is within the advertised range, I think.
The errors are slightly larger than that (up to 4-5%) when I'm in my 25-tooth cog or in my 12-tooth cog (climbing or sprinting, just when I'd like to see big numbers the most!). The errors are slightly lower than that (0-2%) when I'm in my middle cogs (16-19 tooth range). It makes sense that the PT strain gauges might have more trouble measuring torque when it's applied to the very ends of the measuring cylinder (hub body).
Errors that small won't make bit of difference to riders when using their PTs for training. But I can use the information when Chung testing to get more accurate results. I'll just keep the chain on the 17 cog and scale my power numbers back by about 1.5% before doing the calcs.
And I'll run a few tests every 3 months or so to give me confidence that my PM calibration isn't drifting.
Friday, November 20, 2009
I did an FTP test today on the Cervelo - first of the season - and it hurt. I have a very aggressive position (CdA 0.217). My FTP on the TT bike was 96% of what I recently tested on my Tarmac, but my pacing was terrible (the first 5 minutes was 108% of road bike FTP - my PT head unit was in my back pocket and I couldn't see it). My threshold heartrate - last 20 min ave - was exactly the same as on my Tarmac, 162 bpm, so I know I was going just as hard (it hit 168 during my too-fast first 5 min, though).
But that was 'bout what I expected. Good pacing today might have yielded 98% of road bike FTP. I want 99% road bike FTP and CdA=0.210 by February. A new tri-spoke or deep rim front wheel with 20mm tubular running at 160 psi will probably lower my Crr to maybe 0.0045 also.
I'm going to keep my head unit out of sight so I can learn proper pacing based on perceived exertion. I won't have the PT to help me pace when I mount my Zipp disc aero daddy for the Tundra in Feb. But I think I will mount a Polar CS200 on the stem so I can use HR to prevent overcooking the start.
You'd better add an extra interval tomorrow Neo; I'm coming after you.
Wednesday, November 18, 2009
I performed a static torque test on my Power Tap at lunch today.
I took three weights:
1 - a 45 lb olympic plate (which actually weighed in at 45.5 lbs)
2 - two 45 lb olympic plates (90.5 lbs)
3 - my body weight
I ran three tests. The instructions say to use 50-lbs or more, which might explain why the first test at 45.5 lbs was off a little. In the first two tests, I hung the weights from the right pedal when it was parallel with the floor and the rear brake was locked. In the third test, I balanced myself in a door jamb such that all my weight was on the right pedal and the front brake was locked. (hanging 90 lbs from the pedal of your bike is more difficult than you might imagine).
It turns out that my PT is DOBA (dead on balls accurate) at the two higher weights. That makes me feel better about my recent testing. Now I need to do the same thing with my teammate's PT to see if I can eliminate a source of potential error with his results.
Test 1: expected 69.96 in-lbs, measured 63.0 in-lbs (thought maybe I was in trouble here)
Test 2: expected 138.39 in-lbs, measured 139 in-lbs (DOBA)
Test 3: expected 239.86 in-lbs, measured 240 in-lbs (double-DOBA)
Here are instructions for performing the Power Tap static torque test (I swiped them from a wattage Q&A posted by Dr. Coggan):
Technically, the PowerTap cannot be user-calibrated, but its accuracy can be checked using a simple test that is similar to the SRM
calibration check. First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque
mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply
the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right
crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’!
Measured torque = (weight in lbs) × (crank length in mm) × (1 in/25.4 mm) × (cog teeth/chainring teeth).
For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs × 175 mm ×
1 in/25.4 mm × 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as
(measured torque - displayed torque)/measured torque.
So if you find your strain gagues are not calibrated properly, there's nothing you can do but send it to Saris for recalibration, I guess. You can rezero the thing yourself (actually most PTs are set up to re-zero themselves while you are coasting), but you can't modify the strain gauge settings yourself.
Ok, Grasshoppa - time to rush home and hang weights on your cranks to see what's up.
Tuesday, November 17, 2009
My last post presented my nice neat Chung results. Everything turned out exactly as expected once I measured my test hill and input the real numbers.
But now I have a problem. A teammate tested with me on the same hill at the same time. He used a power tap just like me, he weighed in and weighed out after each test segment just like me. The weather conditions were the same. The course was the same.
But when I processed his numbers, the only way I could get the virtual hill to be the right height was to use an unrealistically large Crr, 0.007. That yeilded an unrealistically low CdA for him - 0.190 m^2 (he's 6'3" and about 180 lbs - I don't think he has a CdA under 0.2).
So I looked at all the inputs. Most were identical to my tests as mentioned above. Same spreadsheet, too.
I looked at weight: he tested using two different bikes, so his weight was actually about 2 pounds different between runs, but the two runs still yeilded exactly the same Crr. That tells me the weights were right (and we used the same scale).
What about distance/speed? We both calibrated our Power Taps using a three-revolution weighted rollout, averaging three rollouts, before the test. And I checked the distance measurement of the hill versus both of our data sets and everything matches there. So I don't think the problem is distance/velocity.
That leaves the Power Tap power measurement as the only other source of error I can think of. I thought I had a culprit. To obtain a data set that would give me a Crr for him on my hill near the expected 0.005, I was required to lower all of his power numbers to 95% of their measured values. That got the hill height right, but his CdA would still be around 0.210 on his TT bike and around 0.260 on his road bike. I don't believe his Power Tap is wrong AND he has an unrealistically low CdAs. I'm stumped - but only temporarily - I'll figure it out.
My next steps - let him do runs using my Power Tap and put him on my calibrated Computrainer to see if the Power Taps read the same. Then we'll go from there.
Because there is very little air resistance on an 8% climb at 150 watts, I was able to accurately discern my Crr using the climb side of the virtual profile. Then with a little iteration, the descent side of the profile, where CdA greatly overwhelms Crr as a force against the bike at 43 mph, allowed me to determine my CdA. I found it to be very close to the CdA result I got about a year ago when I did extensive coast-down testing on a 1/2-mile 3% descent. I found it to be 0.232 then, and this week's testing shows0.230. That's a pretty good number, but I'm not a big guy (5'9" and 146 lbs) and I ride in a very aggressive position, so it's not completely unexpected.
I trimmed all my data away except for the climb and the descent for each run, and I reset each run's virtual elevation at the known elevation.
Monday, November 16, 2009
I couldn't stand the suspense, so I took a survey instrument (my trusty Topcon GPT-3000 W, shown at left) out there this morning to check it out. Guess what? The contractor built the road with 3.51' more vertical drop than it was supposed to have according to my design (I guess he wanted to save some cash on fill dirt).
The Chung Method is definitely no joke if you use it right. I don't know what Garmin Slipstream pays for tunnel time, but I think I have a comparable tool now for free.
I moved each of my aerobars inward by 3/8" yesterday, and apparently I was able to discern the difference in CdAs from that tiny change (which, by the way, would mean 11 seconds in a 40k TT). If I can see that type of change, this process is going to be fun (and fruitful).
More details to come.
Sunday, November 15, 2009
The CdAs I calculated for my road bike seemed a little lower than what I would have expected. After reading 106 miles on my Power Tap odometer yesterday for a Claxton Century course that was advertised as 104 miles, I figured the problem might be my PT odometer. I did a very accurate 3-revolution, weighted calibration of my PT wheel this morning and found the circumference to be dead on 2100 mm. I had had the PT programmed to 2098 mm. That means I'm going 2100/2098=1.00095 times as fast/far as I'd thought. Not only is that probably insignificant, it would LOWER my CdA calculations, not raise them. (A quick check revealed that my fastest CdA - when I was in the drops - was lowered by 0.001 m^2 by accounting for the PT circumference change). So I can scratch that as a significant source of error, although the fact that I picked up any change in CdA at all from a 0.095% change in speed is surprising - this method really does provide incredible data resolution).
Then I read Dr. Chung's comment on my last post: He noticed that I had used a simple approximation of acceleration in my Chung Method spreadsheet. To calculate acceleration, I used: change in velocity from T1 to T2 divided by the time interval a = (t2-t1)/dt.
That seemed correct to me. But he suggested I use a more robust approximation suggested by Adam Haile: a = (t2^2-t1^2)/(2*t2*(t2-t1))
As you can see in the highlighted columns in this screen shot of my spreadsheet, the two methods result in very similar, but differing, values of a.
It turns out those differences are enough to change my CdA calculations significantly. My road bike CdAs for hoods, horns, and drops changed from 0.310, 0.245, and 0.240 to 0.338, 0.258, and 0.255, respectively. Those numbers seem more realistic, although they are still lower than I would have guessed.
My Crr changed from 0.0054 to 0.0055 (a difference that is probably not within the resolution of the method).
I think I have everything ready now to do some baseline TT bike tests. Then I'll begin to tinker with my position and see what happens.
Friday, November 13, 2009
A couple of years ago I read Robert Chung's presentation, "Estimating CdA with a Power Meter," on aerodynamic testing using nothing more than a known road profile, internet weather info, a bathroom scale, and a power meter.
I remember finding it interesting at the time; but I didn't follow up because I didn't have a power meter on my TT bike. And I didn't really care what my CdA was on my road bike.
A few weeks ago, I read about Colin Griffiths's recent Chung testing in the UK. I now have a Power Tap wheel that I can easily move to my Cervelo TT bike, so re-enter Chung testing.
Basically, Robert Chung took the equation for all the forces acting on a rider: power, air drag (CdA), rolling resistance (Crr), and gravity:
w = wrr + wPE + wKE + waero
w = Crr m v g + s m v g + a m v + CdA ρ vair 2v / 2
and he solved the equation for slope:
s = w/(m g v) – Crr – a/g – (ρ CdA v2)/(2 m g)
Then he used slope and known horizontal position at each time interval to build a virtual profile (using a spreadsheet). By adjusting values of CdA and Crr until the virtual profile matches the real world profile, you are able to solve for both.
Lucky for me, I'm an engineer and a surveyor (lucky in this example, anyway - most of the time I'd rather be a rocket scientist or the base player for the Stones). And lucky for me, a few years ago I designed and staked out a new subdivision street about a mile from my house, so I know the EXACT profile of the road. And lucky for me, the developer has barely sold a single lot in the subdivision, so there is ZERO traffic. And lucky for me, the profile is a perfect U shape with cul-de-sacs at each end so that I can turn around at both ends without touching the brakes. And lastly, the road is very well protected by tall pines, reducing any minor breezes that might interfere with my results.
So as a test run, today I Chung-tested my Tarmac, and I got perfect results. Here's the procedure:
Here's a screen shot of the spreadsheet I created to do all of this. Download it from my eSnips account if you want a copy.
It worked like a charm. I did three runs of the course in each of three different positions: 1 - on the hoods; 2 - on the "horns" (hands wrapped around the tops of my Shimano shifters and elbows sort of low; and 3 - in the drops.
I did a trial-and-error adjustment the CdAs for all three positions to get an almost perfect and consistent profile. What little profile variation I saw was likely due to a very light breeze or hitting a rock in the road. A little tweaking of the Crr (rolling resistance coefficient) got the amplitudes right.
The fact that it worked is cool enough. But now comes the fun part: using the new technique to play around with different positions and equipment on my TT bike.
I also learned something very interesting and useful that I will use while training and racing on the road bike. I've always wondered how much more aerodynamic it was to ride in the drops as compared to on the horns. On the horns is so much more comfortable and seems more powerful, too. I turns out that my CdA on the horns is a LOT lower than on the hoods (somewhat expected), and only very slightly less aero than riding in the drops (I was surprised the difference was so little).
So there will be no more training or riding in the drops for me. The almost immeasurable benefit isn't worth the more aggressive, less comfortable position. I'll just ride on the horns. I guess I'll only use the drops for standing and sprinting.
Monday, November 09, 2009
Ok - this is a work in progress. I don't have as much data as I'd like to have, but the general trend is clear. The problem with collecting additional data is that my aerobic system is improving, which makes establishing a true best-fit line a little like trying to hit a moving target. But you'll get the gist of it. The magenta line is a best fit for trainer data. The red line is a best fit for road data.
The not-so-surprising result: it's harder to ride the trainer than to ride on the road at the same power (all the power data was collected using my Power Tap). For now I'll target my training zones on the trainer using a target heartrate along with the trainer curve to get a target power range. Later I'll do a CP30 test on the trainer, set up power zones from that and see how they compare to the first method. I find it interesting that the difference in NP vs AHR is lower close to threshold than at lower efforts. I would have expected the opposite effect.
Gotta do something to keep the trainer workouts interesting, right? But I have discovered a new indoor toy for indoor rides: I downloaded the new Computrainer scenery pack. Now I can ride cyclocross trails through downtown Atlanta scenery. That's different.