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  1. #1
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    Confusing race power numbers

    Hi guys,

    I'd like to share with you some power data I have collected and get your input to help me understand it.

    Background:
    This weekend was the state championship here in Minnesota, and I raced both days on the same course in the same temperature, (although it was a little slicker by Sunday after with a weekend of races).

    Saturday morning was cat4 and I did 3 laps, Sunday afternoon was 35-44 and I did 6 laps.

    Both days I used the same bike with the same tires (Michelin mud2s with latex tubes) although on Saturday I used 36 psi front, 38 psi back, and Sunday I risked 33 psi front, and 35 psi back due to the slick conditions.

    My normalized power data and lap times for both races are below along with an average heart rate for the race (lap 0 was a little lap around a baseball field to stretch out the field). The power data was recorded on a 2 month old powertap wheel.

    Saturday
    Avg Heart Rate 175
    Lap 0: 1:32, NP NA
    Lap 1: 9:44, NP 246
    Lap 2: 9:38, NP 247
    Lap 3: 9:45, NP 236

    Sunday
    Avg Heart Rate 166
    Lap 0: 1:27, NP NA
    Lap 1: 9:19, NP 232
    Lap 2: 9:38, NP 215
    Lap 3: 9:47, NP 201
    Lap 4: 9:47, NP 207
    Lap 5: 9:44, NP 209
    Lap 6: 9:37, NP 215

    After the race I was shocked to see my lap times were consistent between the 30 minute and the hour race. Weirder still my power was much lower on Sunday. I'd blame a dodgy power meter but my heart rate is also much lower on Sunday in a manner that is very consistent with the lower power output.

    So I guess the question is what is going on?

    Is this the impact of the lower tires pressure?
    Is it just me learning the course? (I did 3 practice laps prior to Saturday's race and 2 prior to Sunday's)
    Could it be the impact of racing two days in a row? (although I did a short intense training session on Friday to prep for the weekend)

    I'd love some opinions on what is going on here.

    Thanks,
    Matthew

  2. #2
    J-K
    J-K is offline
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    If you calibrated before each race, temperature differences should not have had any influence.

    Many things influence power numbers:

    - Handling skills. Yup, by sunday, you probably learned every line and alternative line on the course. Maybe you just had a much better bike handling day! Maybe because of the slickness, your speed into corners was a bit less, so less mistakes were made.
    - Drafting. Were you able to draft more on sunday?
    - WHERE you apply the power. Maybe the slickness dampened the possibilty to accelerate hard a bit, reducing fatigue. Sometimes steady power is the way to really fast lap times, instead of a sprint-recover pattern.
    - A combination of all of the above and more.

    What was your average power? If the percentual difference between average power saturday vs. sunday is less than NP, chances are power spikes have a lot to do with this.

    But ... do not over analyse. With some experience and data about other weekends, you'll be able to zoom in on what influences power numbers a lot more.
    Last edited by J-K; 12-06-2012 at 01:57 AM.

  3. #3
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    I look at that and see lower power required to get the same lap times. Not necessarily a bad thing.

    Check your variability index to compare if the two rides are similar in your application of power on the race course or if one was "choppier" than the other.
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  4. #4
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    I'm thinking the course also smoothed out between days.

  5. #5
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    As much as it can hurt, in a cross race, I've always placed better the second day of racing. I don't do it often, its expensive. But when I do, I typically place 3-5 spots better in the same or even larger fields.

  6. #6
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    Why the difference in lap numbers? Did you get pulled off after being lapped or is it a timed race?

    If its the latter, perhaps you were mentally and physically prepped for a sprint race on the first day, then adapted for the long haul on the second.

  7. #7
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    Thanks for the input guys. I'll review and post the average power to see how that compares to the normalized power tonight, that's a good idea. Ditto with the variability index. In general I do feel like I was smoother Sunday both due to the slickness of the course and my desire to save energy for the longer race.

    As to why there were less laps: at the MN state championships this year, Saturday was for categories and Sunday was for age groups. So Saturday was my last race as a Cat 4, it was the typical 30 minutes race. I had really trained and focused on peaking for this race and I was really happy with the result. 12 of 81, my best race of the year. I had started this year at the back of the pack so I've been working my forward and this was a great result for me. This CX season was mt first bike racing season (I'm not a kid just a late starter). As I mentioned in another thread, I had planned to try to get on the podium in cat 4 next year but now that's not an option. Cat 3 here I come!

    Sunday was the 35-44 age group so it was Cats 1-4 and a full 60 minute race. I finished about 3/4 of the way back but crossresults thinks I did a better job than on Saturday (US cycling's ranking system doesn't agree). I got lapped right before the bell for the final lap so I did 6 laps while my betters did 7.

    I've got photos of me from the long run up on both days and on Saturday I look fierce, while in the Sunday photo I'm smiling my face off. Sunday was a free race for me, my season was over and I had nothing to prove against the elites in my first non 30 minute race. It was a great weekend of racing on a great course.

    Matthew

  8. #8
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    > Is this the impact of the lower tires pressure?

    In part, sure.

    Everything else being equal I would expect lower rolling resistance with lower tire psi, especially when you're running pressures a bit higher than you need to. Tire pressure is more than traction for cornering: a softer tire conforms more easily to a bumpy course, meaning less energy wasted deforming the tire, less rolling resistance.

    I don't know how low a psi this is true for, but it certainly seems true in the mid-20s psi. I run clinchers, too, so I can't speak for less than 22 psi or so. (I'm 180lbs, I race between 22 and 28 psi.)

    I would also imagine that the softer tire softens the ride and lets you put down more power while bouncing around less.

  9. #9
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    in cross, it's not the mean that gets you, it's the standard deviation. If I had to speculate, your numbers varied so much that comparing means might not be so useful, as there is so much coasting and oddball "artifact" numbers.

    I have used a powermeter in a cross race and found the numbers not very illuminating. It's a race for placement and perhaps your own analysis of how well you rode. The numbers don't tell you so much.

  10. #10
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    Lower psi is faster off road

    This isn't the study I was looking for, but it comes to the same conclusion:

    "Anyone who wants to ride really fast off-road needs to decrease tyre pressure. The rougher the ground, the more pronounced the effect."

    Mountain Bike Tyre Rolling Resitance

  11. #11
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    Your numbers are similar to what I see week end and week out. Several things you will find with power.
    1 your highest wattage laps and not always your fastest. being smooth is important
    2. races early in the day will be slower than races late in the day when the course get beat in.
    3. learning the course and getting it dailed in can improve lap times.
    4, each course will have a different power profile, some course your np will be 200w other it will be 280w
    5. race pace heart rates are lower when you are tired.

  12. #12
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    Quote Originally Posted by JohnStonebarger View Post
    > Is this the impact of the lower tires pressure?

    In part, sure.

    Everything else being equal I would expect lower rolling resistance with lower tire psi, especially when you're running pressures a bit higher than you need to. Tire pressure is more than traction for cornering: a softer tire conforms more easily to a bumpy course, meaning less energy wasted deforming the tire, less rolling resistance.

    I don't know how low a psi this is true for, but it certainly seems true in the mid-20s psi. I run clinchers, too, so I can't speak for less than 22 psi or so. (I'm 180lbs, I race between 22 and 28 psi.)

    I would also imagine that the softer tire softens the ride and lets you put down more power while bouncing around less.
    Wow you run mid-20's on clinchers. I didn't know know anyone our weight (I'm 170lbs) was running less than 30 on clinchers. Can I ask if you ever pinch flat?

    Thanks,
    Matthew

  13. #13
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    Depends on the course, but I will run anywhere from 22 psi up in front, 25 in back. More typical is probably 24-26 in front and 26-28 in back. I've experimented with going lower, and can easily fold tires in turns much below that, but I haven't felt any faster.

    I've flatted three times in two years. Twice on very rough mtb trails where I obviously needed a bit more air, once in a race. The flat in the race was a slow leak in the back after a pile up of several riders. I'm still not sure if that was truly a pinch flat or not.

    I run 23mm rims, which may help just a touch. More importantly, I always run latex tubes, which are much less likely to flat even we you do bottom out. After 36 races with this set up over the last two years, I'm very happy running clinchers.

  14. #14
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    I also use a power meter during cross season and have noticed that on muddy days, power seems to be less.

    There are a few potential reasons for this. First, with muddy conditions I seem to pedal less, I prepare for corners sooner and wait until I'm stable to start pedaling again.

    Slick conditions also tend to cause loss of traction which equates to wheel spin that robs some of your power.

    As far as your lap times are concerned, speed is not always determined by power in cyclocross. Technical handling and less brake application are also a significant chunk of that equation. You could have very well applied your brakes less and actually cornered more quickly and held your speed better on Sunday. Think of NEGATIVE power each time you apply your brakes.

    I think overall with the power/HR relationship your power meter is probably giving you valid numbers. You should definitely try to recall some of your thought processes and bike handling on day 2 and try to apply it again if possible.

    Less power same lap time = good.

  15. #15
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    Quote Originally Posted by trauma-md View Post
    I also use a power meter during cross season and have noticed that on muddy days, power seems to be less.

    There are a few potential reasons for this. First, with muddy conditions I seem to pedal less, I prepare for corners sooner and wait until I'm stable to start pedaling again.

    Slick conditions also tend to cause loss of traction which equates to wheel spin that robs some of your power.

    As far as your lap times are concerned, speed is not always determined by power in cyclocross. Technical handling and less brake application are also a significant chunk of that equation. You could have very well applied your brakes less and actually cornered more quickly and held your speed better on Sunday. Think of NEGATIVE power each time you apply your brakes.

    I think overall with the power/HR relationship your power meter is probably giving you valid numbers. You should definitely try to recall some of your thought processes and bike handling on day 2 and try to apply it again if possible.

    Less power same lap time = good.
    I think you've hit the nail on the head here. Its a shame it was the last race of the season, so I don't get to build on it in real race conditions for 9 months. As suggested above, I looked at the average power for each lap and VI, but there was no extra information there.

    Thanks for all the info guys,
    Matthew

  16. #16
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    Quote Originally Posted by trauma-md View Post
    You could have very well applied your brakes less and actually cornered more quickly and held your speed better on Sunday. Think of NEGATIVE power each time you apply your brakes.
    Winner, winner, chicken dinner. Less slowing means less accelerating. Fewer hard accelerations mean a lower power average with faster lap times.

  17. #17
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    i notice the power meter tends to get "fooled" a little when you reef on the pedals hard for a couple strokes, which we often do in cross, especially if there is traction. I have to expect the real high numbers this produces may tend to pull the means up.

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