HAC4 vs Powertap

Finally sorting out a fixed bike is good news but gives rise to the question of how I log rides in WKO, since natch it has not got any power meter.

However I do have a HAC4 computer, which gives a measure of estimated power based on riders speed, weight and ascent. The manual that comes with the HAC says that this is accurate to +- 20W on climbs and +-40W on the flat.

I thought I would carry out a test to check this out. So yesterday I made two rides along the same 18km course, one with my fixed bike+HAC4 and the other with my Giant winter bike+Powertap.

Both bikes had the same tyres (newly fitted Schwalbe Marathon pluses) and both+me weighed pretty much the same 85kg (which was the weight I entered into the HAC)

The weather was cold but very still, with no wind to speak of.

I chose a course that took me out and back to Kingsclere, the next village. It consists of a short climb just after the start and again just before the finish. (see below). There are no junctions or other reasons to stop on the road so I could make the rides pretty similar. I did the first on the fixed then repeated on the Giant. For the second ride I used a constant gear with the same ration as the 42x18 on my fixed. I used HR to pace, trying to keep second ride same as second, taking both rides easy. The only issue I encountered was that I accidently switched my light on during the first couple of minutes on the fixed, which messed about with the HR until I switched it off.

(Course profile above showing 2 climbs and the rolling middle section.

Once done I downloaded the Powertap file into WKO and exported into a csv file so I could use Excel to compare. The HAC file was loaded into Hactronic, then I used Snagit to save the raw data into a txt file, again to be used in Excel (pain having to do this but don't know of way to save raw data with power).

Results were quite instructive. I managed pretty well in terms of HR as the graph below shows.
(HR comparison, Powertap is red HAC is green) The funny HAC data at the start of the ride can be seen but afterwards the two graphs pretty much track each other, being slightly offset due to differences in ride speeds)

The power readings make interesting viewing:

Here the green HAC line seems to track the red Powertap one quite well through the two climbs but in the middle, rolling, section is much smoother and consistently less than the Powertap. (The smoothing is probably due to the HAC sampling every 20secs rather than 1sec, but this does not account for the difference in outputs.)

In terms of numbers a comparison of the WKO evaluation of the two rides is below:

This confirms I did pretty well in keeping HR constant and also shows how the power measures (Work, TSS, NP and Power) are all 11>25% up when measured by Powertap.

Breaking the ride into sectors explains the difference a bit more:

The climbs show much better correalation between HAC and Powertap in terms of average. (Maxes will always be a problem for the HAC given its sample rate). However the rolling mid section shows the average power is significantly different, in fact pretty much by the 40W HAC say will happen.

I used this to play around with the HAC file. After a bit of trial and error I found that adding an arbitrary 20W on each HAC4 record with a gradient of between 1 and -4% yielded a TSS and average power prettty much the same as the Powertap and the graphs between the two were more alike. Most of my rides will be similar in terms of terrain so will use this as a frig factor for purposes of upload to WKO in future.