SRM Power/Watt Meter Cannondale SI/SL Compact
The SRM Cannondale SI Compact is one of the most accurate models offered by SRM. Though SRM won’t advertise it as such, it is supposedly accurate to 1%. And for 2012, the battery life is now a whopping 1900 hours.
SRM does not supply the chainrings or crank arms or spindle. You’ll need the Cannondale QC850 spindle, and FSA N10 chainrings. Though SRM replaced the 34 ring with a different variant while installing the spider; as I write this I am inquiring as the the correct model number for the 34 ring.
Update Feb 2014: crank pulled, weight as follows:
678 grams with Praxis rings, 172.5mm crank arms, spindle, lock rings.
Add +74 grams for the PressFit 30 BB adapter
= 752 grams installed.
I installed my unit on my Moots Vamoots RSL road racing bike, which has a PressFit 30 bottom bracket.
The SRM Cannondale SI Compact was my unit of choice for saving the ~4 ounces over the DuraAce model and avoiding the need for PressFit 30 adapters (+1.5 ounces), for a total savings of about ~4 ounces— quite a bit in the context of a sub-15 pound bike.
As this was written I did not have a separate weight for the crankset, but installed on the bike complete it saves ~4 ounces over the DuraAce setup, which includes 48 grams saved because a PressFit 30 bottom bracket adapter is not needed— total savings of about 1/4 pound. Not nearly as much as some of the Cannondale weight specs would have you think. Still that’s not nothing.
Power measurement is transmitted to the Power Control 7 wirelessly from the strain gauges in the SRM Power Meter crankset. Other head units can be used also, such as the Garmin Edge 500 or 800 models, but I had nothing but trouble with Garmin, and not just with power.
The SRM with the PC7 is a highly reliable unit. A head unit can be bad (this happened once) but a year of use proved to me that the PC7 is the Real Deal.
See the discussion of how SRM measures power.
This picture is of the Cannondale Hollowgram SL with N10 rings BEFORE the SRM spider was installed. Rings eventually became Praxis rings.