Polar 720 Power Installation Question

[HarryS]

I have just installed the second of these unit on my race bike after having pleasing results on my training bike. However, the readout of power on the same trainer are much more erratic. That is the smaller cogs read correct whereas the large ones are very much out of line. Main difference between the bikes is the use a Compact Crabon crank, i.e. 50/34 instead of 53/39 crank. Air gap is around 5mm low/30mm high as on the other bike. Should I fiddel with moving the sensor for and aft? It currently sits smack in the middle. Cahinstay lenght is 410mm. Any thoughts? Here are the values (trainer at 165Watts):
50_23 300 Watts
50_21 173
50_19 215
50_17 220
50_15 185
50_14 165
50_13 170
50_12 171
50_11 190

34_23 250
34_21 250
34_19 230
34_17 190
34_15 164
34_14 167
34_13 165
34_12 178
34_11 196

 

[beerco]

This is a common problem on the Polar. You might have to shim it up to get good readings in the bigger gears you'll be using on your race bike.

I'm a bit surprised though, I'd expect it to work better on the 53/39 than on the 50/34.

Edit: oops, I now see that the 50/34 is your race bike (I'd have expected the opposite). You'll probably have to set it up so it's almost rubbing (or rubbing) in the 34x11 for it to work on the full range of gears in the 50.

I'd recommend swapping cranks actually - who needs a wide ratio gear set on a race bike?

 

[sgb]

hi. I have just fitted a unit and it is awful. On 5 four minute intervals at a rock steady 98 revs (450 cateye watts-not accurate but a guide) i got 5 different power averages up to 50 watts different. the chain was 20mm from the sensor. I can not hold a steady wattage even up a climb, in an even caddence, super smooth. It jumps as much as 100 watts a time and normally 50 watts. it even goes up when I ease off. It's going back to the shop and I will have to bite the bullet and get SRM with are superb. I have read a lot of negative feedback but unfortunatly, after I bought them. Plus i can only get a pedling index of 20! no matter how round i pedal and, the l/r balance is complete garbage too. any other users given up

steve UK

 

[Squint]

There's a reason why almost all pros use SRMs. They don't have time to mess around. Also, SRMs have been around for a long time so their products are very mature.