SRM 7900 Wireless Power Meter for Felicia

Thank you to the good folks over at SRM for their great support of my Epic Adventures.  Shaun you rock– thank you!!   After five years of riding a Power Tap I am finally free to ride any wheel I want.  My days of building race wheels with a Power Tap are over!  What’s more is I can finally use my Zipp ZEDTech 2’s that have been sitting collecting dust in my garage.

The SRM Dura-Ace 7900 wireless power meter was a breeze to install.  It literally took all of 10 minutes to install the SRM onto my 2010 Felt F2 with Di2.  I have a spare magnet and handlebar clamp for my TT bike.  I haven’t decided whether I will take two bikes to the Furnace Creek 508 this year because I don’t know if I want to have my crew members swap the crank out …but I just might.  It will take less than 5 minutes because the head unit mount for the handlebar and the magnet on the bottom bracket will already be installed. Just the crank has to be removed and installed on the other bike.

Tomorrow will be my first training ride with the SRM.  It will be a long ride (century plus) on a hot day (100F) with lots of climbing (15,000 feet).  I’ll blog about it tomorrow.

2010 Felt F2 with Di2 and SRM 7900 wireless power meter with Power Control 7

12 thoughts on “SRM 7900 Wireless Power Meter for Felicia

    • Chris,

      Thank you for chiming in. The two systems WILL report different data. The two primary reasons are:

      1. They collect the data from two different points — The Power Tap is in the hub and makes calculations on torsion put on the torque tube. The SRM collects data from the crankset much closer to where power is initially produced.

      2. They both sample and record the data differently. Here is some more detailed information

      Signal rate – the number of times torque is measured in a given period. The strain gauges in the PowerTap hub do this 60 times per second (Hertz), while the sampling rate for the SRM is 200 Hz. The Ergomo measures torque 72-144 times second, i.e., every few degrees, which is why it or a future model has the potential to provide data on variations in torque within a single revolution.

      Display (or refresh) interval – the length of time between each update of the readout (display).

      Recording interval – the length of time between each record of elapsed time, distance, speed, power, and cadence that is stored in memory for downloading.

      It can get really complicated explaining it but the key to understanding these vastly different power meters is that they collect the data from different sections of the drivetrain and then they calculate the data differently based on their sampling rates. However, it is well documented and widely accepted that the SRM is the gold standard. It is the benchmark. The convenience of any wheel for any condition makes the SRM the ideal power meter.

      So why didn’t I have an SRM before? Mainly because of it’s cost but also because I owned so many bikes the Power Tap made more sense to me.
      The SRM was dedicated to the one bike that the crankset and all the wiring was installed on. The Power Tap was just a rear wheel change. With the advent of the wireless technology and ANT + the SRM is now much easier to move from one bike to another.

      Yes the two power meters can be run simultaneously since one is a hub and the other is in the crankset.

      The Red Eyed Vireo

  1. Hey George, I saw you running this on your EVO. Is there any significant disadvantage to running this 24mm spindle crank on the oversized BB of an EVO or say a S5?

    Lastly.. why do they still use the 7800 arms? even on the new 9000 SRM!


    • Hello Neil,

      Thank you for reading my blog and taking the time to make a comment. I don’t see any disadvantage with running a 24mm spindle crank on a BB30 bike like an EVO or S5. To be perfectly honest with you BB30 is a terrible system. Bearings don’t last. I go through bearings in fewer than six months. There’s a reason why Shimano has not made BB30 cranksets. It’s not a good system, and they know it. They have done plenty of testing and have not found any advantage to having a BB 30 crankset. so to answer your question I see plenty of advantages in using a 24 mm spindle crankset on a BB 30 bike.

      Lastly… if you look at the 7900 crankset and the 9000 crankset you will see that there is no way to cut out the spider and then install the power meter. So the only way to do it is to use the 7800 crankset arms, attach the power meter and then the bolt that chain rings to the power meter. It’s the only way! Shimano is still making 7800 crankarms JUST for SRM!

      george “Red Eyed Vireo” vargas
      Furnace Creek 508 Hall of Fame
      REV Endurance Cycling
      Directeur Sportif
      REV Endurance Cycling

      • thanks for the fast/detailed explanation — it all makes sense now. Right now i have a the threaded BB and am using Rotor 3D crank arms, 24mm spindle so it drops right in the shimano BB.

        I had been debating for so long if i was going to just upgrade the spider to SRM or Quarq. I had decided to go with the SRM for reasons that they are the flagship of powermeters and if im going to dish out $$ it might as well be into something thats been working for years.

        I wish i could say that I woudl only own 1 bike, but I am really thinking of getting the S5 or R5 in the future. So i was trying to future-proof myself but getting the 3d+ which is the oversized BB crankset from Rotor. However you can already picture that I would have to get rid of my current crankset, change the BB on my threaded BB frame, and go through the headache of all that was far more than I wanted to deal with.

        Now that I understand that the 24mm will work fine in the Cervelos I just have to figure if i plan on moving to the Dura Ace Crankset or upgrade the spider on my current crank and be done.

        Regarding the 7800 arm, obviously if SRM has it on its spider, its pretty good. Im assuming there is no significant stiffness/performance loss compared to the 9000 arm?

        Thanks for clearing the air! best of luck to you this season and I look forward to more of your blogs!

        best regards

        • Neil,

          Get the SRM! Think about this the only complaint people have about the SRM is that you shift your crankset back to SRM to get them to replace the battery. But I’m here to tell you that that’s not really an issue and should not stop you from buying one. See if you can follow my math.

          Battery life: 1900 hrs
          average speed: 17 mph
          total mileage: 32,300 miles
          average miles per week: 200 miles
          number of weeks: 161.5 weeks
          number of years: 3.1 years

          So if you average 200 miles a week at a speed of 17 miles an hour it will take you 3.1 years to accumulate 32,300 miles or 1900 hrs. So basically, you don’t have to change the battery, but once every three years. And that’s if you average 200 miles a week 52 weeks a year for three straight years of doing 10,200 miles per year.

          I have used my Dura-Ace SRM on multiple BB 30 bikes. If you saw my last post. It’s currently on a Specialized Venge. I’m using adapters.

          SRM is a German company. They know what they’re doing!


          • thanks for the breakdown! thats awesome, the life of the battery and cost to replace it overtime is negligible when you look at it that way!

            Do you know if the 7900 DA SRM will work with the 9000 drive train.. Im sure i can find a few for ALOT cheaper than the new 9000. Im starting to lean towards the Dura Ace set up…sometimes you just gotta go with what is known to be reliable and proven!

            thanks again for all your time!

            • Neil,

              Of course! I put the breakdown so that other people can see it and realize that just because the Quarq allows the user to replace his battery in the field – that “benefit” does not outweigh German precision. Furthermore, when you send your SRM to have the battery replaced they will service the unit, re-calibrate on precise testing equipment (not at home with a weight and numerous human errors) and ship it back to you. The last time I sent my SRM in for battery replacement the unit was back in less than a week!!!

              Yes the cranksets are compatible. As a matter of fact for the 9000 Shimano has done away with all the intricate ramping the 7800 series chainrings had.

              Neil …ride Shimano! Campy is over rated and for show-offs that don’t really ride and SRAM is for weight weenies! You want something that last ride Shimano and this is up to you but BB30 is also worthless in my opinion. The bearings don’t last and they are constantly creaking!

              • Got it! Love your enthusiasm. Now that I know that I can be confident in that purchase i can ride without worry! I hear you in the comparison. Shimano for me!
                Ill keep ya posted and if this becomes more consistent for me I may have to hire you for some training!
                Have a great weekend and good luck to you this year in your events and ventures!

                • Neil,

                  When I was a financial planner my mentor told me “people pay you for your opinion. right or wrong they want to know that you have conviction when saying it and that you have their best interests at heart” I have never forgotten that. In retail sales I have rubbed people the wrong way sometimes because I am frank and honest but I always lead with my BEST recommendation first and then give someone the option for a second best.

                  I am available for coaching. Additionally, my REV Endurance Cycling team might be what you need to get and stay motivated. Have you seen my blog posts about my new team?


                  • i have “liked” your page on facebook, if i havent read any recent blogs it maybe becuase i had skimmed through my status feed on facebook.

                    i always respected your knowledge, i do hear about the rubbing the people the wrong way, but good friend Carlo defends you often, and i said. no need to..i get it, jsut like anyone that wants to be sucessful the best coaches/people are the hard nosed people, say it like it is.. which is also why i know your not just trying to ‘sell me something’ uve got the hours and accolades to back it up. i dont know if u watch much basketball, but you would be the phil jackson of cycling..and if there was anyone i would listen to and invest my time/money with it would be someone like you.

                    i’ll definitely consider it, but i know that i would want to give back to you what you give to me, im just now getting my cycling mojo back after a rough year but im taking the steps by having the right equipment and once the rest of the dust settles ill be putting my head down and driving forward.

                    we will keep in touch!


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