Still struggling with combo mount settings

Still struggling with combo mount settings

Postby djaustin » Wed Apr 19, 2017 6:15 pm

Running my two powerpods, the Time Trial unit is providing consistent reproducible data with an FTP of about 290 Watts (for my 67 kg). I've tested this by calibration, a 25 and a 10 TT (attached file with race from about 27 minutes).

I'm running a second powerpod, that when mounted on the bars this gives reasonably consistent values. However my road race bike can't take this mount, so I use an out front combination mount due to the profile of the handlebars bars not being round. What I note is consistently higher (+30 Watts or so) values, and I don't know what needs tweaking. I think it is wind scaling that is over-optimistic for the mount. I attach a race file on the same circuit that had a lot of drafting (of course), but I don't believe my NP was higher in the two races compared with the TT.

The TT bike has lower Crr (not much), weighs about a kilo more and has more aero wheels. The road races had drafting, but I was still going hard and the second was basically an FTP test! I'd like the two to match at about 300 watts really.

Can you advise as to what tweaking is needed please for the combination out front mount? I did calibrate with the outfront settings, but still don't get agreement.
Attachments
iBike_04_18_2017_1729_84_km.ibr
Road races on same circuit
(412.83 KiB) Downloaded 8 times
iBike_04_19_2017_1842_31_km.ibr
TT race
(164.66 KiB) Downloaded 8 times
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Re: Still struggling with combo mount settings

Postby djaustin » Thu Apr 20, 2017 5:18 pm

Just as an update, I ran some time for power calculations to predict my TT time based on reported average power of 289 Watts using bikecalculator. The predicted time was within 1% of my real time, so I am certain the TT calibration is spot on. Of course one can argue that the model uses the same algorithm as the powerpod so it ought to agree. So then I predicted the time of the winner based on their DFPM recorded 395 Watts, and this too was within 1% of their time giving model validation. Those TT numbers are for a closed circuit, 11 laps with setting for tubular tyres, 9C, 33 metres elevation and aerobars.

I'm impressed and now have a target average of 320 Watts.

If only I could get the consistency on my road race bike to help with the training.
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Re: Still struggling with combo mount settings

Postby Velocomp » Thu Apr 20, 2017 5:33 pm

It looks like your road bike ride involved a lot of drafting. That's OK, but it is impossible for me to determine if anything is wrong.

Please post a solo training file and I will see if I can help.
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Re: Still struggling with combo mount settings

Postby djaustin » Fri Apr 21, 2017 2:56 am

Commute to work on the same day attached. No drafting this time. NP is listed as 227 Watts, but this was a very gentle ride. I'll do the same 11 lap TT on my race bike to get something for comparison, as I like the closed circuit as a means of field testing - it's always windy though.

Thank you John.

PS I get a lot of interest at the TTs I enter here in the UK
Attachments
iBike_04_18_2017_0945_22_km.ibr
Commute
(139.04 KiB) Downloaded 8 times
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Re: Still struggling with combo mount settings

Postby djaustin » Fri Apr 21, 2017 8:40 am

And for further reference, here is today's 40 km commute with the PP mounted on the handlebars of my "roadified" cross bike (8 kg, with tubeless 28c high quality clinchers). The power readings here are entirely credible, consistent with my TT bike and RPE, and involved no drafting. I want the "outfront" profile to match this one really. At the moment they do not, and I am sure it is just wind scaling. I ride predominantly on the drops.

This profile was calibrated on my fixed wheel road bike (about the same weight, tyres, position), and I just moved the pod and speed sensor across (I have three bar mounts).
Attachments
iBike_04_21_2017_0842_40_km.ibr
Bar mounted commuting ride
(245.07 KiB) Downloaded 7 times
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Re: Still struggling with combo mount settings

Postby Velocomp » Fri Apr 21, 2017 5:36 pm

For your road bike, please do an "out and back" calibration. Use Isaac Device/Device Setup, and select "Best Accuracy" on the final step.

You'll find O&B instructions elsewhere on the forum.

After doing an O&B ride please post your subsequent ride file.
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Re: Still struggling with combo mount settings

Postby djaustin » Wed Apr 26, 2017 4:47 am

Today's updated calibration ride. I'm racing the bike this evening (more drafting), so I though I'd get a calibration in first. I think it is still reading a little high (30W or so), but will try some more gentle rides rather than road races to check this out. It was a little windy today, but the ride was without issue. Once this is dialled in, I'll probably buy a BLE version for the other non-out-front road bikes.

I rode the 100 km loop on a (third) different bike with same profile handle bar and speed sensor at the weekend with same power results as seen on other bikes (including my fixed wheel). The handlebar mount profile really looks dialled in. I also used the PP reading to set my target effort of 3.3 W/kg for three hours, and this is exactly what I achieved. That's exactly what I want from a power meter.

Many thanks,

Daren

EDIT: One final point. The out-front profile from this ride is on my Giant Propel SL. This is an aero bike with 60 mm deep section wheels and very high quality 25c Vittoria Corsa G+ tyres at 100PSI. Obviously Isaac doesn't know any of this, so I have set the rolling resistance to 23c tubulars, but it is conceivable that the standard CdA is too high, and hence the watts for speed algorithm is also high. I would be surprised if this accounted for such a large margin, but perhaps Isaac needs a profile for "aero road bike" after all?

Test results here
Image
Attachments
iBike_04_26_2017_0859_6_km_CalRide.ibr
Out front race bike calibration ride
(49.65 KiB) Downloaded 8 times
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Re: Still struggling with combo mount settings

Postby djaustin » Fri May 12, 2017 5:16 pm

Thought I'd post an update. I took the race bike to our closed 1 mile circuit (Hillingdon), rode 11 laps after a race, then when I got home used check calibration from this ride to tweak the profile. Hey presto, the aero and wind scaling are adjusted appropriately and all is good and the Powerpod is now providing robust readings.

I've found the same for the TT bike. I rode a 9 mile loop five times in a new TT position, then used check calibration once home to tune the profile. Raced on Tuesday with very good power figures (punctured at 9 miles though :-( )

I'm now of the opinion that these calibration rides are much more robust than the out and back rides. They take a little more time, but they give much more reliable outputs.

For my out front combo mount on my Propel, the profile has Aero 0.622 and Wind scaling 1.930, CdA is 0.322 on the drops. Friction is 10.004, this for Crr of 0.0044.

EDIT: calibration ride attached for those that are interested. You can see the 11 laps nicely
Attachments
iBike_05_09_2017_2052_16_km.ibr
(146.47 KiB) Downloaded 9 times
Last edited by djaustin on Sat May 13, 2017 4:09 pm, edited 1 time in total.
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Re: Still struggling with combo mount settings

Postby andrewkerr1991 » Fri May 12, 2017 5:50 pm

That's interesting I might try this. Did you do any out and back calibration first?

I have done an out and back but made some adjustments to profile. I guess it's best to do and out and back calibration then do some laps of same circuit and use the check calibration tweek.
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Re: Still struggling with combo mount settings

Postby djaustin » Sat May 13, 2017 4:02 pm

Yes, I've done out and back, but found them to be nowhere near as accurate as these rides. Just done a shakedown on my TT bike - 12.5 km up the road and back, and the agreement with bikecalculator.com (for TT position and tubulars) is to within 1% (Without the usual pointy helmet and skinsuit). File attached if you are interested.

Racing a 25 tomorrow on the same course I calibrated it on last week.
Attachments
iBike_05_13_2017_2026_13_km.ibr
(65.99 KiB) Downloaded 6 times
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