Does crank length effect power accuracy?

Hi All,

I am a user of the original Vector (1) pedals. I have about 20k kms on them (since 2015??) and other than a few teething problems initially, they have been great.

Recently I changed my crank length on my bike from a set of Specialized FACT carbon cranks in 172.5 to a slightly later model FACT carbon cranks in 167.5. When I refitted the vectors I made sure to put more than the minimum required torque as this has caused me issues in the past which mostly were visible in inconsistent L:R ratio. After the installation, I am finding the L:R ratio is consistent and similar to before the crank change indicating that the pedals should be torqued enough.

I use the vectors with my Edge 520. I adjusted the crank length in the settings on that and did the necessary angle setting and calibration . I have been riding for a few weeks since this change and calibrate before each ride (and confirm the 167.5 setting) and also did an angle reset in this time to be sure. I am noticing that my power output levels have gone up since the change, but my performance seems similar. For example I do a maximum effort 10 minute climb every week at about 300W. I seem to have picked up about 10W since the change, but my times are roughly the same (within the limits of condition effects so +/-15s). Previously (old cranks) I did find my power readings did seem lower than others (with various PMs) of similar weight to me (by more than 10W), so perhaps it is closer to others now, but either way, it does seem different.

Now it is possible that the shorter crank themselves could have increased my power outputs whilst not increasing my speed? Perhaps I am putting out more power and just going no faster due to some aero/physioogical effect). I am just posting here to see if anyone is aware of any attribute of my Vectors (power pedals in general) than can cause power to read differently at different crank lengths. Would be great to be able to compare power now with the years of power data I have from before.

Thanks!

0 lustyd over 5 years ago

Shorter cranks will change torque but not power for a given speed in a given gear. You didn't mention, but are the chainrings the same size? It's possible that it's a firmware bug and Garmin have not tested with that size of crank - try setting to 172.5 and see what happens. Aero would be better with shorter cranks and certainly wouldn't affect climbing without a big pile of EPO!

You could try some calibration tests with weights too, GPLama would be useful for getting to the bottom of it as he often works through this kind of stuff.

0 vosadrian over 5 years ago in reply to lustyd

The chainrings are the same (52/36) before and after. So the only thing changed is the crank arms. I have not measured the crank length but they are clearly marked as the lengths reported. I assume they don't get that wrong? It seems like a bit of mathematical bug to me. Literally the same pedals, so a calibration issue would have been there before and after. Not sure if I should go to the trouble of testing with weights. Not a big issue for me. Just something I was wondering if anyone else had noticed similar. My numbers are better now, so more bragging rights!! I have PR'd by around 5W up the climb!! Pity I never went any quicker!

0 t_smit over 5 years ago

In the Vector pedals, the crank length setting is used as a multiplier to calculate torque from the measured force. So with your shorter cranks (and since you entered the shorter crank length) you are getting about 3% less torque for a given pedal force application, compared to your 172.5 cranks. If you're measuring more power then it's either that you are actually working harder or there is a minor calibration difference depending on how the spindles are installed in the cranks. The latter effect does occur and it's more significant in Vector 1/2 than Vector 3, but it should be within the +/- 1.5 percent range.

On the other hand: at 300W, 10W is about 3%. On 10 minutes, 15 seconds is a little over 1%, so if you went 15 seconds faster over 10 minutes, and recorded 10 watts higher, that would be within expectations.

0 vosadrian over 5 years ago in reply to t_smit

Thanks for the response. It seems this is not an issue that is common which is what I wanted to find out. I am probably reading accurately. If anything I may have been reading under before, as I believe I have applied well over the minimum torque this time with installation and the last installation was done years ago. Previously I found if I used a torque wrench with recommended values I got inconsistent results. These days I use a 40cm bar and fair amount of force and can be sure I am well above minimum torque and when I started doing that I found the Vectors we much more consistent. These days I always hit between 48:52 and 50:50 L:R with 90% of the time being 49:51. The 49 is my problem hip. This was similar before fitting shorter cranks. When I first got the vectors and under torqued (well set them up using a preset torque wrench that may have been under) them I was seeing 60:40 regularly and getting very low power outputs compared to others.

I do find the shorter cranks are helping with my power and endurance (I have hip issues and put them on for that reason). I can believe I put out more power. I feel better when I arrive at the top of the climb than I used to with the longer cranks and I am quicker in the latter half of the ride. I have actually gone a little quicker since fitting them, but we are talking in the 10s range. Could easily be explained by atmospheric conditions also.

0 Luko79 over 5 years ago in reply to vosadrian

what kind of climb is it? (strava link maybe) how steep? If not 10% or above, the different wind direction, a little different body weight (and setup weight, i mean different liquid level in bottle between 2 days, pump, innertube/s/, etc), different riding position could easily cause 10s (or even much more seconds)

0 vosadrian over 5 years ago in reply to Luko79

Sorry, I must have forgotten to respond. This is the climb: https://www.strava.com/segments/612442 6% for just under 3km. Certainly could be effected by wind and other things, but typically I am pretty consistent with time vs power, but I have just noted a change in that since the shorter cranks. It feels like I am reading about 10W higher now. I am actually lighter than I have ever been while cycling so there is no chance I was lighter when reading lower power.... but early spring here so this may even out with heavier kit (carrying lights and toe warmers, long sleeve jersey etc.)

My top 3 times are drafting to varying degree (shared on front vs staying at the back so not entirely consistent) with old crankset:

* 8:50 - 300W
* 9:12 - 292W
* 9:18 - 302W

Now for my old crankset solo top 3:

* 9:36 - 296W
* 9:43 - 287W
* 10:02 - 283W

Now for my new crankset solo top 3:

* 9:34 - 305W
* 9:39 - 300W
* 9:55 - 293W

As you can see the top 3 for old and new crankset have similar times but the new crankset is about 10W higher. I am pretty sure that the bike configuration was identical except crankset, but I do have some light climbing wheels that may have been used on the times I was drafting. Pretty sure all the solo efforts are my 50mm deep wheels. I am trying the climbing wheels tomorrow to see what difference that makes. They are about 500g lighter. Just hop I can put out my numbers for the test!