Changed crank lengths, will my vector powermeters be accurate on the trainer?

power-meter
#1

Hi all,

I just went from 172.5 to 165 cranks. On my Garmin I calibrated my Garmin Vectors to 165mm crank length, but do I need to change anything while on the trainer and using TR? My trainer is a 2018 Wahoo Kickr.

Thanks!

#2

No, there is no setting for crank length related to the Kickr.

Crank length only matters for pedal and crank arm based power meters, never for trainers.

2 Likes
#3

You’ll have to spin a higher rpm in the same “gear” in slope mode. Erg mode shouldnt be an issue since resistance will adjust

1 Like
#4

Thank you coach chad! I’m actually using the vectors as my power meter on the trainer.

1 Like
#5

Thanks!

#6

Power pedals use the crank length to calculate power (it’s based partly on the cadence, the strain on the pedal spindle, and the crank length though there are probably other factors involved the precise calculation). They then transmit the calculated number to whatever device they are hooked up to, and that’s what TR uses.

So long as you set the crank length correctly through your head unit, then whatever device you attach to the Vectors will receive the correctly calculated power.

2 Likes
#7

I have a very related question. I switch my assiomas between my track bike (165mm) and my road bike (172.5mm). I have noticed there is no way to accomplish this through the app. Does anyone have any best practices to calibrate before going to/from the trainer? I have been using the assioma app to set the crank length back to 165mm but I really have no way of verifying that TR is using that value for power calculation. It would be really helpful if you could set, or at least view, crank length for pedal based PMs in the app

#8

OMGgg, Are you serious? I missed that I was supposed to account for this. This may very well contribute to some of the issues I was having in training. I travel for work and carry Vectors to throw onto spin bikes at hotels and gyms. I have never thought to change/account for crank length.

I ride 175mm on my road bike. Isn’t the “standard” length 170? Not that I’m going to go and correct anything now, but say I did my ramp on the 175, what would that mean for my workout on a 170? Would it over or under estimate my watts? I’ve always felt like it was way easier to put out numbers on my home bike. Granted that could just be because it’s my bike, but I secretly hope that if I did the test on a longer crank and moved to a shorter crank, that it would underweight my output (i.e. I’m putting out 350 in reality, but the vectors are reading as 325)

#9

I believe longer cranks on the same gearing would show a higher power output than shorter cranks at the same cadence. I don’t know by how much though, and I think standard crank length these days is 172.5mm.

#10

Having 175’s when your pedals are set to 170’s would show your power about 2.9% lower than it actually is. This is because power is calculated from the torque (force x crank length) and the 175’s are 2.9% longer than the 170’s

2 Likes
#11

By the ratio of the crank arm lengths. If you set on long and run on short, you will overstate the power, and vice-versa.

#12

The crank length is stored in the pedal, not the head unit.

1 Like
#13

Correct, but my question was mainly aimed at determining the current crank length setting stored on the pedal and adjusting if incorrect. I outlined the procedure I typically follow but was wondering if anyone had a different or improved process.

Viewing and modifying the crank length setting for pedal based power meters would be a welcome feature addition but I do not know the feasibility of this.

#14

The pedal reports power, and that’s what TR listens to.
If you set the crank length in your pedal app (or your head unit) and then sync that to the pedal, you are updating the setting inside the pedal. It will then report the correct power to TR. Your process is fine (I use my headunit to do the same on my P1s).

2 Likes
#15

Thanks

SO for us switchers, switch, connect to garmin - reset and calibrate - ride for a second - turn off garmin, ride TR.

Dear lord the data loss…