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Old 10-20-11, 06:54 PM   #1
jmX
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Speed vs predicted speed given watts

I did my FTP test on a flat unobstructed 20mile long course (out and back) on smooth pavement at sea level with very light winds today. I'm making progress and all is well.

As somebody who'd like to eventually do the local time trials, my question is more about a curiosity I have wondering why my speed numbers are so much lower than what formulas might predict.

Obviously there are a million variables and I understand equations and their inputs are all approximations, but the data spit out from http://www.analyticcycling.com/ForcesSpeed_Page.html is so far off from my numbers I wonder if I'm missing something. In fact, it estimates I should be able to go the speed I did at 150 watts, which is a huge delta between what I actually put down.

Even if I enter worst case scenario type numbers I can't get the speed that low in the calculator.

The FTP test results:
http://connect.garmin.com/activity/123071041
Avg power was 239w WITH zeros. I'm 6'1"@159lbs. I'm not wearing MC Hammer pants or any baggy clothing. Was in either the drops (5%) or clip-on aerobars (95%). If I ride solely in the drops I'm about 0.5mph slower.

So analytic cycling predicts 24.7mph at 239w, compared to my 20.7mph at 239w. 4mph is a huge difference at these speeds. Anybody have input?
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Old 10-20-11, 07:04 PM   #2
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I'd say you underestimated the area part of CdA

for me, at 240w with my normal road bike in the drops, I would expect 22ish under those conditions. I assume I am smaller than you.

24.7mph at 239w is something I have only seen on my TT bike with full aero gear. It's a touch on the slow side (maybe half a mph) so I tend to see those numbers on windier days or on courses with slightly more elevation gain.
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Old 10-20-11, 07:27 PM   #3
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I'd say you underestimated the area part of CdA

for me, at 240w with my normal road bike in the drops, I would expect 22ish under those conditions. I assume I am smaller than you.

24.7mph at 239w is something I have only seen on my TT bike with full aero gear. It's a touch on the slow side (maybe half a mph) so I tend to see those numbers on windier days or on courses with slightly more elevation gain.

Agreed. 24.7mph at 239w is pretty dang aero. I'm a tiny guy and get really aero, and for example, on my TT set up I'm at roughly 210 watts to 25+ mph. On a road bike I wouldn't be close to that.
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Old 10-20-11, 08:17 PM   #4
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You could try using this calculator:

http://www.noping.net/english/
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Old 10-20-11, 08:41 PM   #5
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Agreed. 24.7mph at 239w is pretty dang aero. I'm a tiny guy and get really aero, and for example, on my TT set up I'm at roughly 210 watts to 25+ mph. On a road bike I wouldn't be close to that.
pfft.
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Old 10-20-11, 09:01 PM   #6
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Anybody have input?
Yup. Either 1) your power meter is off; or 2) you're a parachute. Or maybe both. Check your PM, then check your CdA.

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Originally Posted by mollusk View Post
You could try using this calculator:

http://www.noping.net/english/
That thing is terrible.
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Old 10-20-11, 09:56 PM   #7
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You could try using this calculator:

http://www.noping.net/english/
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Originally Posted by RChung View Post
That thing is terrible.
I actually comes pretty close for me. +/- .3 mph for the Lowes Motor Speedway TT series.
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Old 10-20-11, 10:27 PM   #8
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I actually comes pretty close for me. +/- .3 mph for the Lowes Motor Speedway TT series.
Oh, I didn't mean that it was wrong in an absolute sense. I meant that it's nearly impossible to find the parameters he used to set that thing up, and it's not very flexible in that you can't really customize them. If you happen to be lucky and your CdA and Crr match his models then you're golden, but if you're unlucky the default values can be way off -- and in general, the default parameters for recumbents are optimistic and the parameters for conventional bikes is pessimistic.
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Old 10-20-11, 10:45 PM   #9
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the parameters for conventional bikes is pessimistic.
And yet even that one gave me a 22.5mph at my wattage in the drops, not including my aerobars. I don't think my powermeter is in error just based on what I see from other powermeter users in my weight bracket on hill climbs on strava, but anything is possible. I use a garmin edge set to autocalibrate with the powertap , and I wasn't aware there was any way for it to read high. I thought the failure mode for these was low readings. Is that not the case?

The only thing that comes to mind is the tall head tube on my Roubaix. The stem is at -8 degrees w/0.5" of spacers, so I can go a bit lower but that could affect my hip/torso angle and I'm generally pretty happy with my fit. Wheels are 32mm deep in front, 46mm in back. GP4000S @ 100psi.

I'm skinny, and look like every other cyclist out there so again it's why I'm a bit shocked at how far off the estimates are. I don't feel like I'm an outlier when it comes to size and position on the bike.

Last edited by jmX; 10-20-11 at 10:48 PM.
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Old 10-20-11, 10:59 PM   #10
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So analytic cycling predicts 24.7mph at 239w, compared to my 20.7mph at 239w. 4mph is a huge difference at these speeds. Anybody have input?
I just dug through some data to find a similar effort for me. A few months ago I did a constant 67 minute SST effort around a closed loop at exactly 239 watts. I covered 23.2 miles. Average speed, 20.7 mph. Now I'm shorter and lighter than you (I usually do SST work on the hoods) and we've got a hundred other variable to factor, but I was pretty interested to see how it matched your numbers.
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Old 10-21-11, 03:33 AM   #11
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And yet even that one gave me a 22.5mph at my wattage in the drops, not including my aerobars. I don't think my powermeter is in error just based on what I see from other powermeter users in my weight bracket on hill climbs on strava, but anything is possible. I use a garmin edge set to autocalibrate with the powertap , and I wasn't aware there was any way for it to read high. I thought the failure mode for these was low readings. Is that not the case?
Either 1) your power meter is off; or 2) you're a parachute; or 3) it was windier than you thought. Or maybe all three. Check your PM, then check your CdA. If your PM is reading right and the wind really was negligible then .44 m^2 for CdA while on the aerobars means you're a parachute. That's street clothes on bar tops of commuter bike territory.
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Old 10-21-11, 06:39 AM   #12
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Here's some laps I did at 230w at an outdoor velodrome, on my road race bike with normal kit and helmet, in the drops.

It was not a completely calm day, but the wind wasn't a major factor. The average speed came out to 22.9mph but I would expect that was a bit high due to wheel circumference issues. Bet on 22.5

I do have a rather small frontal area though. Some folks have big shoulders that are just impossible to hide. I am not one of them. I also have a pretty aggressive position on the road bike.



the faster part (where my speed is sitting above the average line) is around 250w and 23.5mph indicated (actual maybe 23.2)

It would not surprise me at all if a larger rider would be at 21mph for the same power for the overall 12 minutes, since I have large teammates who show similar numbers. It also would not surprise me if a rider with a higher handlebar position would have similar numbers to the larger rider.

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Old 10-21-11, 09:56 AM   #13
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OK, now I'm worried my PT is reading high! It zeroes at zero, so I don't know what else I can do. Live with it until the pedals come out, I guess, as the precision seems good even if the accuracy is lacking.
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Old 10-21-11, 10:06 AM   #14
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What's with the periodic drops in speed, power and cadence. It looks almost like you had stoplights?
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Old 10-21-11, 10:37 AM   #15
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So analytic cycling predicts 24.7mph at 239w, compared to my 20.7mph at 239w. 4mph is a huge difference at these speeds. Anybody have input?
One suggestion would be to find a better course where you can put down steady power. I don't know how big an impact it has but you have a fair number of periods where you're coasting for 10 seconds at a time. I see dozens of power spikes between 400-500W. If nothing else, eliminating those spikes should boost your AP.

Also the sections where you're having to slow down are costing you at least a minute.
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Old 10-21-11, 10:40 AM   #16
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It's a bike path. I'm guessing the coasting times are underpasses and bridges.
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Old 10-21-11, 11:43 AM   #17
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That bike path is not flat. The file shows 561 feet of elevation gain.
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Old 10-21-11, 11:59 AM   #18
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Indeed it's a bike path (more like a bike highway) through a very urban Southern California. It snakes under probably a dozen roads along the 10 miles and I was coasting the decline then going 400-500w up the short incline on the other side. Each one is probably <20 seconds, and I doubt my speed on those ever got under 18mph. I do not believe these affected average speed much.

Additionally there are 2 slow bridges that I had to cross at probably 13-17mph, each one lasting 30-60 seconds and on an out and back that makes a total of 4 times, plus one time I had to slow to 2-3mph to do a tight u-turn on the course. I can see that taking 0.2-0.3mph off the average.

RChung, as for it being windier than I thought, my out speed and my back speed were nearly the same so there wasn't much head/tail wind. WKO+ reports out speed was 21.2 @ 240w, back speed was 20.2@239w. Maybe there were light crosswinds.

Grumpy, thanks for the input. Maybe I'm just way too upright. I've been thinking about getting rid of the spacers so maybe I'll work those out over the winter. Had hand pain from extra weight I was putting on the hoods last time I tried though. It is probably time to evolve my fit a little as I was professionally fitted when I was brand new to the sport a year ago.

FatBoy, pretty interesting how our numbers matched up. I have to ask, just how fat are you?

It appears I'm many years off from a good 40k TT, at least on this bike.

Last edited by jmX; 10-21-11 at 02:51 PM.
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Old 10-21-11, 12:59 PM   #19
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You'd be shocked at how a few position adjustments can move your wattage at a given speed. Tires, wheels, helmet...all things things also factor in.
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Old 10-21-11, 02:34 PM   #20
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Got a picture of your position?
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Old 10-21-11, 04:01 PM   #21
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That thing is terrible.
I have found it to be pretty accurate for me using "road bike tops and drops" whenever I have real power data to compare to it.

Of course if you have real data from a wind tunnel or carefully controlled coast down tests you can do better.
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Old 10-21-11, 04:36 PM   #22
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Got a picture of your position?
Here's my imitation of a parachute, aka, my current position on the bike.

Aerobar animation:

URL: http://jmx.ls1howto.com/pics/2011/20...robar_anim.gif

Aero fit:


Dropbar position:

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Old 10-21-11, 04:51 PM   #23
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Holy Head Tube! How far are your local TT's? Are you planning on doing them on that road bike?
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Old 10-21-11, 05:13 PM   #24
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Holy Head Tube! How far are your local TT's? Are you planning on doing them on that road bike?
He's in my neck 'o the woods. 20k and under for the most part.

It's a truly horrible aero position... I'm surprised you're slower in the drops (same wattage same course?).

There's a lot going on there with your fit.

Last edited by Racer Ex; 10-21-11 at 05:20 PM.
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Old 10-21-11, 05:31 PM   #25
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I can appreciate the feedback, but would wish for a bit of constructive criticism if possible. The primary use of the bike is endurance and climbing, I never bought it under the pretense of time trialing. Doing my FTP test on the aerobars yesterday was me A) doing an FTP test, and B) checking if it's feasible to attempt a TT on this bike. From the responses here I'm gathering that's a no, but I'd still like some details and feedback on why.

The local TT is a semi hilly 11mi. Aside from TT's, I do solo organized timed centuries about 3 times a year and would think that going down this path will yield quite a bit of time on those as well.

The non-aerobar fit you see is the fit from a well known local fitter. Me slapping aerobars on it is just that and nothing more.
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