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I have an old Cervelo One from 2000 with Shimano 105 drivetrain including a FC-5501 crankset. I'm looking to add power metering to the bike and there is a Ultegra FC-R8000 available with a 4iiii power meter. Crank lengths are the same at 172.5mm. Concerns: 1) My 105 set up is a 9 speed in the back, and the Ultegra is for 11 speed. Does this matter? 2) both are dual front chainrings, but mine are 52/39 and the Ultegra are 52/36. Any issues there? 3) do I need to worry about bottom bracket compatibility?

Thanks for any advice!

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The two cranksets use different bottom brackets, so you will need a replacement BB. Something like the SM-BBR-60 has an MSRP of only $32. It does need different tools to install than the old Octalink BBs, so if you are installing this yourself you will need to account for that.

The front chainring difference also presents a slight issue. The older Shimano groups had front derailleurs that weren't designed to shift a 16-tooth gap. Also, the R8000 cranks have the chainrings very slightly further apart than all the previous cranks (the gap between them is about 0.4mm bigger). The older FDs may not shift properly as they may not be able to move far enough inward (it does depend on your bike). So you would be better off getting a new front derailleur, although it can be a 105 version. If your current rear derailleur is the short cage version (which it probably is), then it may not be able to take up the chain slack with the smaller front ring. Also, your 9s chain may not work as well with the R8000 chainrings, but I think this may be less of a problem.

Last, if that 4iiii is a dual sided meter, don't bother buying it. Drive-side meters mounted on the R8000 and contemporary cranks are inherently inaccurate because the spider is asymmetric.

Overall, it would be simpler to get a pair of power meter pedals. Versions are available for Look Keo, Shimano SPD-SL, and Speedplay cleats. The cheapest of these tend to be the Look Keo-based ones, particularly Favero.

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    Ad powermeter pedals: Assioma have SPD-SL pedals on offer, or at least ones compatible with these pedal bodies.
    – DoNuT
    Aug 24, 2023 at 16:32
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    and actually Garmin have an Spd and SPD-SL version. Let me correct that. The Assioma Spd-SL version has much wider q-factor than normal, though.
    – Weiwen Ng
    Aug 24, 2023 at 16:55
  • Didn't know that, it is +10 mm per side which obviously is the toll of the sensor body interfering with the Shimano design, should have known and said Favero since I actually run these in a Keo version. :)
    – DoNuT
    Aug 24, 2023 at 17:16
  • Drive-side meters mounted on the R8000 and contemporary cranks are inherently inaccurate because the spider is asymmetric A full reading of this relatively-recent DC Rainmaker review - Shimano R9200P Power Meter In-Depth Review: Astonishingly Inaccurate - seems to blame Shimano's electronics. That's supported by the lack of R/L inaccuracy in this DC Rainmaker review of 4iiii's power meter on a similar crankset Aug 24, 2023 at 18:26
  • @AndrewHenle Different issues. any PM on the 8000 or similar series drive side will be inaccurate. The 8100 and similar cranks are symmetrical. Here, Shimano’s meter is inaccurate, presumably due to its electronics (or maybe the strain gauge placement), but two 3rd party meters are accurate.
    – Weiwen Ng
    Aug 24, 2023 at 18:27

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