Interesting results with Power Meters

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Interesting results with Power Meters

Postby 1q2w3e4r » Sun Feb 07, 2010 2:23 pm

Hey guys, thought I'd post this up. Its got me scratching my chin a bit. For each result the bike is the same, shoes are the same, my position is pretty much unchanged and the two TTs have clip on bars. Only variables are wheels and tyres.

I've been out to have a crack at the Calga TT, the result was 43:30 ish at 227 watts NP (208 watts average power). 65mm Aero wheels, Bontrager TT tyres

I did a ride of it back on 28th December after being back riding for 4 weeks in 48:00 with 225 watts NP (207 watts AP) Training wheels running Bontrager hardcases. Everything else the same (bike, shoes, weight etc)

Between these two rides, in the last 2 weeks I've been out and had a hitout at the local crits, 30 minute power from the first crit where I did actually work on the front, chase down some breaks and have a hit out was 240 watts. 50mm tubulars.

I did the same crit last week, 45 minutes with a NP of 245 watts (AP 222 W), the 30 minutes following the first few warm up laps we kicked off a 3 man breakaway which we managed to stay away with till the end, I was toast after this one. 30 minute NP was 255 watts with an AP of 236 watts. 50mm tubulars

Huge differences in the power outputs over simular distance rides and was wondering what peoples thoughts were? The Calga TT today I was harder than the crit two weeks ago but felt on par with the one done 6 odd days ago, and I was struggling during the crit to hold the wheel in it at points, it was pretty much just 30 minutes of pain on the limit.

I thought maybe my PTs were out, but I followed the stomp test protocol TTL posted up in his thread and they all came back line ball with each other so it isn't a calabration issue I wouldn't think. Although I had to be creative to get 30kg hanging of the right pedal :lol:

Still, its probably the rider more than anything else, once I've been for my bike fit next month I'll be back to Alex for another training program I think.
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by BNA » Sun Feb 07, 2010 2:37 pm

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Re: Interesting results with Power Meters

Postby twizzle » Sun Feb 07, 2010 2:37 pm

Sorry - too confusing for me.

- One bike, two sets of wheels and running clip-on bars for TTs?
- You are trying to work out the power difference between which events? How about posting up date, AP/NP/TSS/IF/Kj for each and your FTP.
- Which ones are you trying to compare? Crit to crit & TT to TT? Or Crits to TTs?
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Sun Feb 07, 2010 3:23 pm

Sorry Twizzle, its a bit of a jumbled mess of a post. I didnt get a huge amount of sleep last night.

Yes thats right, 1 bike, different wheel sets (training wheels for the December TT ride, 50mm wheels for the 2 crits, 65mm wheels for todays TT at calga) and clip on bars for the TTs.

I was mainly interested / curious in the discrepancy between the crits and TT. I do find I have trouble keeping power up on the return leg at Calga though everyone else seems to manage it fine, I probably just need to HTFU!

The TT's are 5 weeks or so apart and seperated by the 2 crits. Power for Calga on December 29th was 225 NP (207 average watts), TSS was 73.7 (0.959) with a ride time of 48:00 I had FTP set at 235 watts from last time I tested it back before I had a lot of time off (8-10 weeks with very minimal riding, some with weeks with no riding at all). However I had 4 weeks under my belt as I started again in December.

Crit 1 middle of January results were 234 NP (206 watts average), TSS 95.6 (0.994) with a ride time of 58 mins

Crit 2 results first week of February were 245 NP (222 average watts), TSS 76.5 (1.044) with a ride time of 45 minutes (included 30 mins AVG watts of 236 and NP of 255 W)

TT today was 227 NP (207 watts average), ,TSS 67.6 (0.965) with a ride time of 43:35.

So there is a 4:25 ish improvement over the same course with the same power. Position is the same, I find it hard to believe its all down to wheels and tyres! Conditions were a bit different in that it rained today, but no wind in either ride.

Hence the query and why I carried out the stomp test on the PT's ,results were all pretty much identical and I carried them out one after the other. Could it just be the slight differences in ride times influencing the power between the TTs and Crits I wonder. Although I would have thought once you hit 40-45 minutes there wasn't a huge drop in sustainable power to the 1 hr mark.
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Re: Interesting results with Power Meters

Postby toolonglegs » Sun Feb 07, 2010 3:51 pm

1q2w3e4r wrote:Sorry Twizzle, its a bit of a jumbled mess of a post. I didnt get a huge amount of sleep last night.

Yes thats right, 1 bike, different wheel sets and clip on bars for the TTs.

I was mainly interested / curious in the discrepancy between the crits and TT. I do find I have trouble keeping power up on the return leg at Calga though everyone else seems to manage it fine, I probably just need to HTFU!

The TT's are 5 weeks or so apart and seperated by the 2 crits. Power for Calga on December 29th was 225 NP (207 average watts), TSS was 73.7 (0.959) with a ride time of 48:00 I had FTP set at 235 watts from last time I tested it back before I had a lot of time off (8-10 weeks with very minimal riding, some with weeks with no riding at all). However I had 4 weeks under my belt as I started again in December.

Crit 1 middle of January results were 234 NP (206 watts average), TSS 95.6 (0.994) with a ride time of 58 mins

Crit 2 results first week of February were 245 NP (222 average watts), TSS 76.5 (1.044) with a ride time of 45 minutes (included 30 mins AVG watts of 236 and NP of 255 W)

TT today was 227 NP (207 watts average), ,TSS 67.6 (0.965) with a ride time of 43:35.

So there is a 4:25 ish improvement over the same course with the same power. Position is the same, I find it hard to believe its all down to wheels and tyres! Conditions were a bit different in that it rained today, but no wind in either ride.

Hence the query and why I carried out the stomp test on the PT's ,results were all pretty much identical and I carried them out one after the other. Could it just be the slight differences in ride times influencing the power between the TTs and Crits I wonder. Although I would have thought once you hit 40-45 minutes there wasn't a huge drop in sustainable power to the 1 hr mark.



Well we all know who is the best to answer these questions...but I think he has been away at nat's this last week or so.
What I find interesting is your NP to Av in TT's.I would have thought they would have been much closer (within 5 watts)...and as you say you have trouble keeping power on the return Calga leg would suggest that that is your problem...ie:meaning you can't keep the power on on the downward sloping sections...need to work on that (just an idea,I have only done RR's at Calga but love powering the return).
Do you change your set up in anyway apart from clip on bars?.
The ride time difference is certainly huge for the TT's...I think you would have to look at your speeds in certain sections to find out where the improvements came from.
Crit 2 is suggesting you are close to your ftp...have you done testing lately?.Normally I suppose a 40minute TT would be a good test but you are obviously soft pedaling a bit in Calga so may need to find somewhere more suited for a 20mm / 16 k tt.
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Sun Feb 07, 2010 4:03 pm

toolonglegs wrote:
Well we all know who is the best to answer these questions...but I think he has been away at nat's this last week or so.
What I find interesting is your NP to Av in TT's.I would have thought they would have been much closer (within 5 watts)...and as you say you have trouble keeping power on the return Calga leg would suggest that that is your problem...ie:meaning you can't keep the power on on the downward sloping sections...need to work on that (just an idea,I have only done RR's at Calga but love powering the return).
Do you change your set up in anyway apart from clip on bars?.
The ride time difference is certainly huge for the TT's...I think you would have to look at your speeds in certain sections to find out where the improvements came from.
Crit 2 is suggesting you are close to your ftp...have you done testing lately?.Normally I suppose a 40minute TT would be a good test but you are obviously soft pedaling a bit in Calga so may need to find somewhere more suited for a 20mm / 16 k tt.


TLL I think your right, I really find it hard at Calga to pedal hard up to the wattage on the return sections I also coasted some of the descents as well. If anything it shows me I have a lot of work to do improving with the TT's to get the average power levels up closer to the NP for the same ride, its probably just practice, practice, practice. Which I suppose is a good thing, it means I have an identifiable way of improvement perhaps.

No changes in setup, its the time difference in the two TT's that has me the most confused! I'll need to break it down in WKO by km and look at the avg power, time taken, speed, cadence etc to see if it gives me a better idea of whats happening there.

I haven't done any testing as I've had tendonitis which resulted in me taking time off the bike for most of late last year, I'm off to see Steve Hogg in early March and the plan is to then get back into riding with some goals and get a training plan off Alex.

I followed the protocol laid out by yourself in your thread for a stomp test on the PM's so I'm pretty sure they are accurate as they came back pretty much identical
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Re: Interesting results with Power Meters

Postby toolonglegs » Sun Feb 07, 2010 4:19 pm

How tall are you?...what crank lengths and gearing do you run?.Are you in the biggest gear on the return leg?.
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Re: Interesting results with Power Meters

Postby Alex Simmons/RST » Sun Feb 07, 2010 5:22 pm

10 sec / km difference in TT times for same power is a lot although not out of the realms of possibility for significantly different set ups and different conditions. However if your set up was not overly different and wind and air density were the same, then 10s/km for same power is unlikely.

Hence, I would suspect the data from one of the TTs, either that or you didn't ride exactly the same course (didn't miss the turn point?). Are you certain that the PT was torque zeroed before each race? Or are you certain wind was the same?

I have ridden the same course using road bike and TT bike and got 4 sec/km difference in similar conditions with same power.

Average power on the return leg on the Calga course is not untypically lower than outward leg due to the course profile.

The ratio of NP to AP from a well paced effort on that course is likely to be <1.05, and probably closer to ~ 1.03.
Your ratios are ~1.09 which suggests you are either going too hard on some sections, not hard enough on others, or a combination of both.

See here for some thoughts on issues that can cause differences in times:
http://alex-cycle.blogspot.com/2009/04/new-skool.html
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Sun Feb 07, 2010 5:47 pm

Thanks guys, I appreciate all the input.

toolonglegs wrote:How tall are you?...what crank lengths and gearing do you run?.Are you in the biggest gear on the return leg?.


178 cm tall, my weight is unchanged and I run 172.5 cranks with A 12-27. I was on the big ring for the entire return leg excluding the largish hill about 5 minutes from the finish.

Alex Simmons/RST wrote:10 sec / km difference in TT times for same power is a lot although not out of the realms of possibility for significantly different set ups and different conditions. However if your set up was not overly different and wind and air density were the same, then 10s/km for same power is unlikely.

Hence, I would suspect the data from one of the TTs, either that or you didn't ride exactly the same course (didn't miss the turn point?). Are you certain that the PT was torque zeroed before each race? Or are you certain wind was the same?

I have ridden the same course using road bike and TT bike and got 4 sec/km difference in similar conditions with same power.

Average power on the return leg on the Calga course is not untypically lower than outward leg due to the course profile.

The ratio of NP to AP from a well paced effort on that course is likely to be <1.05, and probably closer to ~ 1.03.
Your ratios are ~1.09 which suggests you are either going too hard on some sections, not hard enough on others, or a combination of both.

See here for some thoughts on issues that can cause differences in times:
http://alex-cycle.blogspot.com/2009/04/new-skool.html



Alex you've made me think of something, I checked the ride distance, todays ride was 500m shorter (Calga's 25km TT is 24.5 on my PT) I turned just past the service station in December (at 12.5 km), but before it today on the Calga course. This probably accounts for what roughly 1 1/2 to 2 minutes of the difference in times? The distance discrepancy would impact on the sec/per kilometer over such a shortish course. The avg speed was 33.7 km/h today The rest then would be down to wheels, tyres and conditions (aero wheels and faster tyres would make a huge chunk of the remaining 2 odd minutes up), the wind was the same (negligible) however the barometer would have been different (rained today, sunny in December) so probably beneficial today.

The rest I believe your right is purely down to my ability to ride the course properly and understand it. I'm probably pushing too hard in some sections and soft pedalling in others.

I zeroed the PTs before each ride, hence why I decided to check the calibration today. I obviously need more practice. Still the difference between todays effort and last weeks crit is interesting.

I'm going to have a good read through your paper again and have a think, I reckon I'll be back out there again in March or April to get some more data.
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Mon Feb 08, 2010 10:29 am

Hey everyone, I have been doing some more digging with the powertap and I didn't want to start a new thread on it so I thought I would just use this one. I hope this all makes sense, I have 3 PTs. Two I bought new, (Mavic and Tubular) the third a second hand race clincher off ebay (popped up right after I got the tubulars otherwise I wouldn't have bought them).

I have done the following tests, a stomp test and torque information check from the powertap computer test mode.

The stomp test I used was as outlined by toolonglegs post here
viewtopic.php?f=43&t=24047&start=0&hilit=powertap

I used a 30 kilogram weight, 172.mm cranks and the 39x21 gears resulting in the following. 66.13 pounds x 6.791339 x 0.435897 = 195.76 as a "baseline" under the protocol.

Here were the results for both the stomp tests and PowerTap head unit "torque" information found in the test mode.

Mavic wheel: Has 4071 km on it. Stomp test results 207 1st test, 209 2nd test. PowerTap head unit torque is shown as 526.

Tubular: Has 421 km on it. Stomp test results 211 both tests. PowerTap head unit torque is shown as 520

Ebay Race clincher: Has 4340 km on it. Stomp test results 209 1st test 211 2nd test. PowerTap head unit torque is shown as 514

The stomp tests are all pretty much line ball so I'm relatively happy with those even though they fall wide of the 196 baseline as done by the calculation, as each unit provided similar results.

From reading the PowerTap booklet regarding torque information, it says the default value is 512 +/- 12 as the zero point.

Two are within spec although one falls slightly outside it. The Mavic that falls outside doesn't have a huge amount of km on it and is still within warranty as I purchased it locally in late March early April last year. Does anyone think it is worthwhile having it send back and looked at under warranty? It's only marginally out of spec, but the main thing I'm after (as everyone is I'm sure) is consistency in the data I'm getting, otherwise there is little to no point in looking at it.

Also how frequently is everyone checking their units like this? To be honest this is the first time I've done it, I have always just zero torqued before each ride. Hence the posts about it, sorry for those more knowledgeable about it than myself. I'm thinking its probably prudent to keep on top of this and at track the calibration accuracy over time. Is one method (stomp test vs computer torque information) superior to the other? Or should I just continue to do both?
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Re: Interesting results with Power Meters

Postby twizzle » Mon Feb 08, 2010 11:02 am

As mentioned in the training thread, I just looked at mine for the first time, offset is 524 and it's got 4464km on it.
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Mon Feb 08, 2010 11:10 am

twizzle wrote:As mentioned in the training thread, I just looked at mine for the first time, offset is 524 and it's got 4464km on it.


So right within spec but on the outer limit with similar km to my mavic which is slightly high
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Re: Interesting results with Power Meters

Postby twizzle » Mon Feb 08, 2010 11:21 am

Better than TLL's. :twisted:
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Re: Interesting results with Power Meters

Postby toolonglegs » Mon Feb 08, 2010 3:35 pm

twizzle wrote:Better than TLL's. :twisted:



That wouldn't be hard :lol:
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Re: Interesting results with Power Meters

Postby 1q2w3e4r » Mon Feb 08, 2010 3:52 pm

I can't believe the continual drama's you've had with yours TLL. Won't they just swap the whole thing out for a completely new unit as they have pretty much done everything else from reading your thread/posts on it. Maybe its just too much power :twisted:
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Re: Interesting results with Power Meters

Postby nimm » Mon Feb 08, 2010 7:54 pm

yeah definitely TTL.

I just checked my offset and it's 511, so smack bang in the middle of the range. On the other hand the max power I've put through it is just a tad over 1000W and my usual average power is a lower than yours... Roughly 2000km on it now I think.
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Re: Interesting results with Power Meters

Postby toolonglegs » Mon Feb 08, 2010 8:09 pm

Maybe my numbers have been off since day one 8) ...that would be nice if they were always reading low :lol: .
I may not be to far off the mark saying that because I came round a rider in sprint yesterday who said his peak in the sprint was 1300w at about 75 kilos...on a reliable SRM :D ...I went past him like he was standing still...my max 5 sec was only 1250.
One thing I improved hugely over the winter is my acceleration...very happy about that!.
OK I am dreaming but it would be a pleasant surprise...anyway I will find out what they are doing for me his afternoon.
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