Another missing watts query

[Todj]

I'm using my Gen III iPro mainly on my time trial bike in races (it's a stem mount and the RWS out on the right hand brake lever).

On the flats and upslopes I'm getting sensible figures, but on down slopes I am beating my brains out and getting very low figures.

Attached are three files showing the active Profile, a view of the original download data, and the original download race ride file itself.

You'll see that the slopes are in the the +/- 2% range. From the heart rate it can be seen that I was still working very hard on the two down slopes (I was catching riders in front so that's always a good incentive ). But on those two down slopes - miles 2 to 3 and 6.5 thru - the watts are way, way down. Both road surfaces at those points are a little choppy, but nothing OTT.

I've done several calibration rides and they've all come out with very consisient figures. I'm happy with a Crr of 0.055 for the UK roads I'm racing on, and a Cda of 0.270 as I'm partially disabled and can't get quite as low at the front as I'd like.

Tilt Calibration is good.

So......do I tweak the Profile's Riding Tilt to a different number than -0.9% (eg -0.5% or -1.5%)?? Or am I missing something else?

Thanks in advance.

[racerfern]

I think it looks pretty accurate. Change your smoothing from 5sec to 5min and you'll see fairly consistent wattage. Sure, there's a drop on the downhill, but it's really really difficult to maintain the same wattage effort downhill. You made a massive push from 19:35 to 21:35 and then leveled off after that.

Also try changing the smoothing to 10 or 20 min. and you'll see a steady decrease in the power output as you might have started out a tad too strong.

My 2¢

[Todj]

Thanks Fernando. That 2c is most useful.

I am wondering if this might be a 'vibration' issue. Looking closely at a couple of sections where the power drops to zero or thereabouts eg mile 6.7 the slope shows at -5%, whereas on the way outwards, the same spot in the road at mile 5.8 it shows as just +3%. The surface is a bit choppy on that downslope section and, taken at not far off 30mph there is a high frequency vibe. Could this vibration be upsetting the device for a while and then it is re-adjusting?

I'm concerned that if I'm trying to ride to a power level then I'm able to do that fairly reasonably upslope and on the flats, but downslope the readings are way out?

[racerfern]

I recently tried a custom mount that had some vibration issues. The problem was that it wasn't bad enough to show as vibration in the iBike software (and neither does yours). I was riding along the top of a dam which was paved and absolutely level. However the iBike kept on seeing +3% then -3%. It wasn't the iBike fault, it was doing the best it could considering the existing conditions. So to directly answer your question, this could be a vibration issue. I solved my problem by changing the configuration of the custom mount, you might have to think about the physical setup you have.

Over the course of the ride the wattage works out just fine, but if you're "staring" at the meter trying to maintain a specific effort then this could be a challenge. Can you post a picture of your setup? If it's a typical handlebar or stem mount then it could be the material under the mount or it could be something else that's allowing "wiggle".

[Todj]

Thanks again Fernando.

Photos attached. It's a standard low-angle stem mount. It has the green stickies underneath (cut to shape and hence they don't show up against the white stem). Mount is good and solid.

The RWS is taped around just to make it look less like a drinking tube.

The elastic band is my attachment to hold the RWS into the iBike. It applies just a touch of 'pull' to keep the RWS snug. (Also available in black and red ).

In particular I look to use the iBike in time trials in order to improve pacing my effort, and so I do need something that gives me a good readout in realtime.

[geoffsuperturbo]

"In particular I look to use the iBike in time trials in order to improve pacing my effort, and so I do need something that gives me a good readout in realtime"

we all want real time accuracy richard, but lets face it, we are not going to get it with ibike
. the only time it gets accurate (possibly)is an hour after the ride when the software has worked on the file to correct several "issues". not a lot of use in a 20 minute TT.
may aswell stick to pulse or even feel or push the boat out and go the powertap route like everyone else seems to. i dont have any confidence in ibike for real time pacing power. it might be close on one ride, then next day it isn't. it isn't consistent and in real time not much use at all, as a bump in the road causes a 0 reading (as do potholes) and rough roads have the same effect.

the only use it has is making nice route profile pictures and collecting some interesting data, but as was said previously, staring at it isn't going to give reliable feedback, which is basically the idea of racing and indeed training to a power zone.

stick to racing with pulse or RPE which doesnt jump around and also gives close to real time feedback of your work rate. collect the power data afterwards by all means, but dont expect consistency from one ride to the next. i managed for years training and racing with heart rate and i'm going back to it as ibike just does my head in,, too many variables to get real time accuracy, it's quite a fun toy for analysis but thats all i can see in it

[racerfern]

@Todj

Nice bike! You've got a spot of dust on the stem. Your setup seems on the money so I'll drill further into the file when I get home. One thing to consider is to either ride with a USER screen for an interval. This will give you current watts and average watts at the top window for the entire ride. It will help you pace better and keep you from chasing a number.

@geoffsuperturbo

Your opinion is welcome of course but off topic. Todj has a valid question and a good solid setup with respectable results.

[racerfern]

OK, let's drill into this a bit more:

1) At 6.3 miles you coasted for about 20 seconds. That accounts for your first zero power spike. Note, I say spike as in downward toward zero.
2) At 6.7 and 7.1 miles the iBike saw a bigger drop in slope than there actually was. The difference is minimal until you drill down to the specific instance.
3) Note that the slightest nuances in slope have a huge effect on your effort vs. your perceived effort (look at the attached picture) and you see all the variations in the hill
All in all, I think your effort could be improved by concentrating less on the actual number. Remember that any PM is going to have occasional variations for any one of a number of reasons.

Bottom line? I think you've got a good profile, though I think you could raise your wind scaling to about .669. Also, you would benefit by using USER INTERVALS as I mentioned earlier. You enter USER INTERVAL by pressing the right arrow until you see FIT TRAIN. Then continue to press the right arrow over and over until you see User (read the manual for the other options).

You can enter this area as you're riding. Assuming you're warming up you would press and hold the right arrow and USER would come up if that's the last one you used. You would do this at the start of the warmup, then at the timed section you would press the center button. That would mark a lap and zero the timer. Now you could watch current watts and more importantly AVG watts in the top window.

[Todj]

Thanks Fernando, I will try the adjustments you have suggested.

There are three or four places where I have to coast as the TT route negotiates several roundabouts. The one at 6.2 miles is very small and tight, usually with traffic.

[racerfern]

Using the PM during TTs works and seeing pros using this tool confirms it. Pros tend to train with power meters but not race with them.

Interesting that they set a first half goal, then planned to ramp it up for the second half.

http://tinyurl.com/3ja3v7z