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Polar S710 + Power Measurement

Here are pictures and specifications of the toy I got back in 2002; the Polar S710 bike computer with the power measurement option. Also contained below is a comparison between the Polar unit and my Computrainer.

On a historical note, I don't use the Polar system anymore. In 2003, I got a SRM Professional crank with a 110mm bcd when I got my Litespeed Ghisallo. The SRM system is much less finicky. However, it's not perfect either. I really dislike the PC V controller for a number of reasons.


  1. Pictures
  2. Specifications
  3. Computrainer Comparison: Part 1
  4. Computrainer Comparison: Part 2


First, the watch (computer) goes on top of a wired mount as shown below.

I wanted the computer to be centered just ahead of the bars, hence the unorthodox setup.

To do this I used a 3/4" pvc pipe, notched for the watch strap to pass through, as an intermediary. The wired mount goes onto the pipe and the whole thing is securely strapped to the handlebars using 4 zip ties.

(In an earlier version, I used an oversize (offset) ErgoBrain mount from Campagnolo as a base. The Polar mount was velcroed to that base. The 4-zip tie solution is much more solid and secure.)

The mount itself contains a battery, two pins on top for electrical connection with the underside of the watch and the plug you see (on the left) goes to the main sensor unit, which is shown below.

The main sensor unit houses two sensors: a cadence and a chain tension sensor. An LED next to the Polar logo flashes red and green(initially) to confirm that a signal is being received by the speed and cadence sensor, respectively. Yes, the cadence magnet is mounted on the right crank. The speed sensor is mounted on the left chainstay (partially hidden here). A third wire (seen here) going to the chain speed sensor mounted on the lower derailleur pulley.

Compare the picture above with the one immediately below. After just a couple of hundred miles, the main sensor is already all scuffed up. The Polar logo is half-worn off. I'd advise using sacrificial electrical tape.

Here is the chain speed sensor.

This wire wraps around the rear derailleur cable housing all the way to the lower pulley. The original pulley bolt must be replaced by one dedicated to the sensor. (Polar supplies three different replacement pulley bolts for Shimano RD/MTN and Campagnolo.) For a Dura-Ace derailleur, I don't see any way of zip-tying the wire to the body.

Finally, another wire runs from the main sensor unit to the speed sensor shown below. The addendum to the manual advises mounting the speed sensor as far away from the main sensor unit as possible. As you can see, the wire is long enough to run around the seat tube over to the other chainstay almost to the rear hub.

All the sensors mount on the back of the bike. There is just a single wire going up to the mounting block shown earlier.


Part Weight Comments
Polar S710 watch 53.6 (g) Includes strap
Original Watch Mount 21.2 (g) (Not used with the power option)
Watch Mount 69.8 (g) (Comes with the power option)
Sensor Pack 110.0 (g) Cadence, speed, pulley and chain tension sensors and wiring
Derailleur pulley bolt + nut/washer 5.0 + 2.9 (g) Replaces original pulley bolt 1.9 (g)
Cadence sensor 2.4 (g) Mounts on right crank
Original Speed Sensor 19.6 (g) (Not used with the power option)
Screw magnet 10.0 (g) (Not used)
Total: 243.7 (g)  
Parts replaced: 67.3 (g) (Supergo HR1000 computer, pulley bolt, chainstay protector)
Net weight penalty: 176.4 (g)  

The total weight of the system is around 243.7g. To be fair, the net weight penalty for this system is about 175g after deducting for a typical HR computer. Note, however, that the weight of tie-wraps and tape has not been factored into this calculation. Add another 10-12g for 6 to 8 tie-wraps depending on your installation.

Computrainer Comparisons: Part 1

After a few rides, I began having doubts about some of the power readings put out by the Polar unit. Depending on the gear, usually the big ring with one of the larger sprockets on the back, the power reported seemed low compared with some of the other gears.

The distance or "air gap" between the chain and the main sensor unit was an item of concern. Because I have funky gearing (50T/30T on the front and a 11-23 cassette on the rear), there is a large variation in the gap. The following picture illustrates the approximately 8mm air gap when in the small-small (30-11) combination:

A close-up:

Here are the corresponding pictures when the drivetrain is in the big-big (50-23) combination:

The close-up:

The approximately 45mm gap is substantially larger than the 30mm maximum recommended in the addendum to the Polar manual for the Power Unit. But the small-small combo means that the main sensor unit cannot be raised 15mm without dragging the chain on the unit. Perhaps the Polar unit only works optimally if one employs a narrow gearing range. I did not consider this potential downside when I purchased the unit.

But before I start tinkering with spacers or consider other more drastic remedies, let me first put the putative air-gap problem aside for the moment, and first verify that there is indeed a problem to begin with.

I decided to test the Polar unit on a recovery ride on my Computrainer. The idea was to simply try every gear (all 18 combinations) with a constant resistance. The Computrainer was run in ergo mode with wattage set to 200W. I warmed up for ten minutes and then did the coast-down calibration to account for tire/roller pressure (value: 1.73). Immediately after the wattage test was completed, I did the coast-down test again to verify that the Computrainer calibration had not drifted significantly during the test. Here are the results of the test in table form:

Big chainring:

50-11 235 8.8 77
50-12 225 14.6 78
50-13 223 11.7 81
50-14 235 19.9 82
50-15 226 16.7 83
50-17 210 11.4 83
50-19 205 18.4 85
50-21 174 11.9 87
50-23 166 11.7 87

Small chainring:

30-11 240 12.1 82
30-12 215 12.5 84
30-13 207 14.1 87
30-14 193 13.3 88
30-15 196 12.7 87
30-17 181 10.2 90
30-19 158 10.4 91
30-21 150 9.6 89
30-23 137 5.7 88


Cog 111213141517192123
X-axis 123456789

Some notes:

Computrainer Comparisons: Part 2

Jean-Joseph Cote, one of the developers of the power meter, suggested I raise the power sensor unit as far as possible, even at the cost of some rubbing in the small chainring and smaller cogs. For reasons mentioned in the previous section, it seems unlikely that this will make a big difference, but I'm willing to try anything. So I dutifully raised the sensor unit with the aid of a stack of rubber spacers.

The front of the sensor unit barely clears the front derailleur. The rear will rub in the 11 and 12 cog when used with the small chainring. A small price to pay if there is improved consistency to be found.

I re-ran the 200W Computrainer test (tire/roller pressure value 1.72) with the big chainring only. Here are the results:

Cog 232119171514131211
X-axis 123456789

(Here, I've also shown the average cadence and speed for each cog in the cassette.)

The results are quite different from those seen in the previous section! Apart from the outlier 50 x 23 combination, the other 8 cogs seem to be in reasonable agreement. The wattage reported seems independent of speed.

However, the average wattage for the 8 cogs at 240W is 20% higher than the 200W nominally set by the Computrainer. Is my Computrainer under-reading? Or am I really losing about 20% of the power to the drivetrain and tire interface? I suspect the truth is somewhere inbetween.

But at least the unit reports reasonably consistent wattages for 8 out of 9 cogs, and I can live with the one outlier. The take-home lesson here is that the Polar unit has to be set up carefully with respect to the air-gap to get good results.

Sandiway Fong
March 18th 2002 (Revised April 7th and April 28th 2002)
Addendum (January 2005)

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