Polar 720 Power Installation Question

Discussion in 'Cycling Equipment' started by HarryS, Jan 25, 2004.

  1. HarryS

    HarryS New Member

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    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
     
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  2. beerco

    beerco New Member

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    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?
     
  3. sgb

    sgb New Member

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    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
     
  4. Squint

    Squint New Member

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    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.
     
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