I have been having an ongoing knee issue, so I decided this past December to go in for Bike Fit Tune ups on my road, gravel, and “trainer” bike. I decided to start with the trainer bike as it was the one I was using most frequently given the weather in the Northeast at this time of year. We made some adjustments, which unfortunately resolved the knee issue, but caused issues in adjacent soft tissue. So, I went back in for another adjustment this past week. Too soon to tell whether these are the correct adjustments, early indications are promising.
One of the changes made was to add 19mm pedal extenders to help my knees track better. This is all fine for the trainer bike, as I don’t have a power meter. I source the power from whatever smart trainer I have the bike on, currently a Kick’r Move.
My concern, my road and gravel bikes are using dual side Garmin Rallys (RK200 for the road, and XC200 for the gravel). After a discussion with Garmin support, I know that pedal extenders are not supported for two reasons (1) it will block the antenna/transmitter that is at the end of the spindle; (2) Could potentially lead to misleading power numbers because of some changes to how the strain gauge is deflected.
I am not very clear on how an extender will affect the measurement. Which leads to my next question.
I reached out to several power meter manufacturers (Wahoo Speedplay, Favero Assioma, Stages, SRAM/Quarq. All provided the same response, pedal extenders are not supported, they will import the values being reported. Leaving out Speedplay and Favero, how does a pedal extender affect the values being reported by a crank arm, spider, or spindle-based power meter?
I suspect the easy answer is that power meter companies have never tested their product with pedal extenders. They’ve likely validated (and internally certified) their products against their own direct-mount equipment and (I hope) a range of commonly used cranks.
Adding an insert of x length, of y stiffness, and installed with z torque… that is likely to changes the force across the spindle/gauges is a step too far for them to officially support.
On a similar/related topic - I’ve recently been using a set of Look carbon cranks with a tri-lobe insert that allows multiple crank lengths depending on the position of the crank insert the pedal screws into. The power numbers I was seeing on the pedals were lower than expected… only by a few watts but when switching out the cranks for a set of Shimano alloy, I was seeing better numbers from the pedals. It was only a few watts… but… More data needed to be confident it was the tri-lobe (it could have also been the cranks which had a lot more lateral flex than the Shimano cranks).
Shane, Thank you. I look forward to your results. Hopefully I can use that as a guide as what I should expect once I change power meters and add the extenders. I realize my “mileage” my vary.
The force offset shouldn’t affect a spider or spindle based power meter at all. Those are measuring the forces in a vertical plane. The pedal extenders are creating extra sideways force because they give your feet extra leverage from the crank arm.
I could maybe see it somehow affecting a crank arm based meter. However, there’s already similar sideways forces from every pedal, since they all extend to the side as a cantilevered beam. You’re effectively just lengthening the beam and the power meter doesn’t know the beam length of whatever pedals are used. Therefore, they must be measuring in a way to ignore or cancel out that force.
Try contacting 4iiii and Stages and see what they say. Worst case, you can buy a used one and try it in the trainer to validate it. If it doesn’t work out, resell for the same price.
Unfortunately, using a crank arm is not an option for two reasons. My road bike already has a SRAM Red crank. Neither Stages nor 4iii make a crankarm. Also, I want to continue getting left/right power. I’m curious to see if the pedal extenders will help resolve an imbalance I have.
Thank you for the idea. I can place the road bike on the trainer (and use virtual shifting) to get an idea what the Garmin Rally and the Kick’r Move are reporting for power over a few workouts and then do the same after I swap power meters. Then I will have an idea as to how much the pedal extenders will skew the numbers, if any.
I have the same issue and am going to try the Favero Duo Shi power spindles which insert into a Shimano Ultegra Pedal. They have a Q factor of 65mm, which is less than the 71mm with an extender on a Shimano pedal, but much more than the Garmin Rally pedal. If needed, my fitter would then add some cleat offset to widen the Q factor further. Power Meter City sells the Faveros, and I’ve read good things about them.