Login required to started new threads

Login required to post replies

Power Numbers Off on Garmin Vector vs Computrainer
Quote | Reply
Hi there,

Looking for some advice from the gods of slowtwitch.

I have some garmin vector (forgot which version but they are from 2015 and only one side measures the watts) on my p2 and I actually never managed to rely on them... Because I train on computrainer all the time and I cant seem to get a consistent % difference between the power numbers on computrainer vs the pedals. I read power number of my garmin 920

Granted I should be calibrating at every ride and the pedals have to be tightened to a torque of 25-30 lb-ft (I havent exactly verified the torque numbers on my bike).

Anyone successfully training on Computrainer and races on vector? How?

Cheers
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
tamiii wrote:
Hi there,

Looking for some advice from the gods of slowtwitch.

I have some garmin vector (forgot which version but they are from 2015 and only one side measures the watts) on my p2 and I actually never managed to rely on them... Because I train on computrainer all the time and I cant seem to get a consistent % difference between the power numbers on computrainer vs the pedals. I read power number of my garmin 920

Granted I should be calibrating at every ride and the pedals have to be tightened to a torque of 25-30 lb-ft (I havent exactly verified the torque numbers on my bike).

Anyone successfully training on Computrainer and races on vector? How?

Cheers

I had first generation Vectors (silver pods) and they were upgraded to 2nd gen (black pods). Both worked the same way.

I would torque them religiously, I would zero them on every ride and I would get good tracking between my computrainer and the pedals.
The only time they were off is on the first ride after being removed/-re-installed. This was very consistent. On the first ride after an install they would read high.

You are not helping your odds by using single side, not torquing and not calibrating. Did you at least set angles ?
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
My Vector 2S were really off v my Kickr, recently I got a Stages V3 and the Kickr tracks really closely to this.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [Gilliga] [ In reply to ]
Quote | Reply
marcag wrote:
You are not helping your odds by using single side, not torquing and not calibrating. Did you at least set angles ?

I know and thats why Im asking folks like you who seem to know what they doing :)
what do you mean by setting angles?


Gilliga wrote:
My Vector 2S were really off v my Kickr, recently I got a Stages V3 and the Kickr tracks really closely to this.

did you calibrate/torque/angle properly with your vectors?
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
tamiii wrote:
what do you mean by setting angles?


Also, with single-side PM, it will be off by 2x the percent of your power imbalance. You don’t know if you have a power imbalance, but if you do, that will be error additative to the V2’s 2% stated accuracy.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [exxxviii] [ In reply to ]
Quote | Reply
 
Sure did, I spent a lot of time and effort making sure all of that was calibrated. I found the variable numbers from Vector to Kickr was constant about 10% however, the vectors really were not accurate once above 350 watts or below 100w, so for me as a triathlete that did not matter, and I knew the difference from training on kicr to my vector numbers. However, now on Stages, I like that I dont need to worry about setting angles, I just put them on, used the app to do a zero reset and they work, and they track consistently close to Kickr across all watts, so now my training and race numbers match and I dont have to think about it.

My Vectors were original with a 2 upgrade kit, so maybe the Vector 3 is better. The original stages had issues too (my mates had them and were not happy) but now I have V3 and found them good and so have others. They were on sale $400 AUD recently on the 105 cranks so that got me in, I went 6 months no PM as sold the Vectors and just went back to HR for a while.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [Gilliga] [ In reply to ]
Quote | Reply
Gilliga wrote:

Sure did, I spent a lot of time and effort making sure all of that was calibrated. I found the variable numbers from Vector to Kickr was constant about 10% however, the vectors really were not accurate once above 350 watts or below 100w, so for me as a triathlete that did not matter, and I knew the difference from training on kicr to my vector numbers. However, now on Stages, I like that I dont need to worry about setting angles, I just put them on, used the app to do a zero reset and they work, and they track consistently close to Kickr across all watts, so now my training and race numbers match and I dont have to think about it.

My Vectors were original with a 2 upgrade kit, so maybe the Vector 3 is better. The original stages had issues too (my mates had them and were not happy) but now I have V3 and found them good and so have others. They were on sale $400 AUD recently on the 105 cranks so that got me in, I went 6 months no PM as sold the Vectors and just went back to HR for a while.


I used the DC Rainmaker Analyzer to compare twice two different rides between the vectors and my kickr snap. The vectors are adding an extra 10% compared to the power output of the snap.

Below in blue is the vectors, in purple the kickr snap. There's a constant ~10% difference from the snap vs the vector.

PS: everytime I paste my screenshot into the text editor, it shows up fine and then I hit submit and the screenshot disappears. Even if I upload it as an attachment it doesnt stay. I'm really confused. Here's the screenshot => https://ibb.co/XV4rG0h
Last edited by: tamiii: Nov 24, 20 11:07
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
Drivetrain loss? Are the figures on the Garmin pedals high than on your computrainer (assuming the computrainer measures the power at the rear hub)?
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
Since the Vectors are measuring one foot at the pedal and doubling it while the Computrainer is measuring the total power at the hub (correct?), they are not measuring the same thing, and it's no surprise you don't get consistent figures.
While drivetrain losses might be expected to stay fairly consistent, the fact you're using single pedal measurement is a definite issue. Most riders have a significant imbalance under some or all conditions. I don;t think I'm particularly asymetrical and I regularly get figures of 47-48% Left / 52-53% Right as n average for a typical mixed intensity session. My imbalance is greater when taking it easy, especially during a recovery, but generally levels out approaching threshold unless I'm really fatigued, which also has an effect on balance. Oh yeah, and position and cadence also plays a part (I think I'm more even on extensions on Tri bike than sitting up on the Tri bike and I suspect I'm less balanced on the road bike with longer cranks and upright position but haven't been able to measure that).
So if you are like me, and probably most other riders, in that the contribution from each leg varies depending on intensity, fatigue, cadence, position, etc, then you can't expect a single sided "power meter" to be consistent with a total power measurement.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [ In reply to ]
Quote | Reply
My Computrainer needs a 20 to 30 min warm up before it reads anywhere close to any of my power meters. If I'm riding zwift, I need to do a coast down after the warm up, that means I need to do the coast down, get out of and then get back into zwift. It's a pain in the xss.

So, instead I just use my PM as the power source and let the Computrainer adjust the resistance. (I'm also not using a single sided PM and have confidence in the measurement).
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [jaretj] [ In reply to ]
Quote | Reply
jaretj wrote:
My Computrainer needs a 20 to 30 min warm up before it reads anywhere close to any of my power meters. If I'm riding zwift, I need to do a coast down after the warm up, that means I need to do the coast down, get out of and then get back into zwift. It's a pain in the xss.

So, instead I just use my PM as the power source and let the Computrainer adjust the resistance. (I'm also not using a single sided PM and have confidence in the measurement).
Ah, so one more source of significant error then!
I've no experience with the Computrainer or Vectors, just pointing out the inherent problem with single sided measurement which will just be exacerbated by any other issues like drive train losses, trainer inaccuracies, etc.

I use a Tacx 2T with Zwift. Should be among the best for power accuracy, but I still use my power meter as the source of power for Zwift and my Garmin records. I'd rather use the same instrument indoors and out since it's possible.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [Ai_1] [ In reply to ]
Quote | Reply
I have a love/hate relationship with my Computrainer, it takes so long to warm up and it's different depending which tire I use. Sometimes I use a trainer tire, sometimes a Gatorskin, it's been so long since I've used it's measurement.

I remember the power numbers being very close after a short 10 min warm up and then watched the Computrainer power go down in relation to my PM over the next 20 min. When I saw it was steady I'd do another coast down. After that it'd be really steady, except when I got off the bike for a min for a bathroom break or to get a drink then it'd be high again.

There are times I wish it would die but then I really don't want to spend any money on a new one. I've had it for seven years and I think it was 10 years old when I bought it. It's a used lab model from Carmichael training systems. I'm dreading the day I have to replace it but that'll open some options for connectivity. By then it'll probably be 2040 and I'll be in my 70's. We'll have infinite internal gearing that shifts with telepathy but only upgrades with Windows Vista.

Mark your google calendars...
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [Ai_1] [ In reply to ]
Quote | Reply
Quote:
Since the Vectors are measuring one foot at the pedal and doubling it while the Computrainer is measuring the total power at the hub (correct?), they are not measuring the same thing, and it's no surprise you don't get consistent figures.

It's measuring power outside of the drivetrain which is probably why every wheel-on smart trainer isn't as accurate, and that gives you more variables that can affect the accuracy/calibration so consistency is going to be an issue too. I've ridden with Powertaps since 2007 and nearly every time I've ridden a Computrainer it has read high. It's usually been around 10 watts, but sometimes 20+, so I haven't found it to be comparable to power at the hub. Like someone mentioned, a good warm up helps, but even then I always found the calibration to be inconsistent.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [ In reply to ]
Quote | Reply
Still using Gen1 Vectors. They're generally within a few watts (NP) of my Kickr. I calibrate them every ride. Takes less than a minute.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
Favero, of Assioma/BePro fame, have an interesting piece of documentation on how to validate the numbers their power meters output


Last edited by: lemos: Nov 25, 20 8:02
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [Supersquid] [ In reply to ]
Quote | Reply
Supersquid wrote:
Quote:
Since the Vectors are measuring one foot at the pedal and doubling it while the Computrainer is measuring the total power at the hub (correct?), they are not measuring the same thing, and it's no surprise you don't get consistent figures.


It's measuring power outside of the drivetrain which is probably why every wheel-on smart trainer isn't as accurate, and that gives you more variables that can affect the accuracy/calibration so consistency is going to be an issue too. I've ridden with Powertaps since 2007 and nearly every time I've ridden a Computrainer it has read high. It's usually been around 10 watts, but sometimes 20+, so I haven't found it to be comparable to power at the hub. Like someone mentioned, a good warm up helps, but even then I always found the calibration to be inconsistent.
As I mentioned previously, I've heard of the Computrainer, but I'm not familiar with it. Until I read jaretj's last response I actually thought the Computrainer was a direct drive setup (I know I could have checked, but I'm lazy). So, yes I agree, that's a lot more variables to be added.
I used to use a virtual power and a Kurt Kinetic Road machine for Zwift. Being aware that setup was critical to consistency of readings I always used the same rear rim/tube/tyre combination, pumped to the same pressure and carefully applied 3 turns of the roller clamping knob beyond first contact. That gave very repeatable results (as checked against my power meter when I eventually got one. I used the PM with the KK for about 18 months before getting the smart trainer and used the first few rides to figure out whether I needed to re-evaluate all my data now I had a power meter - but thankfully that wasn't necessary. The virtual figures were extremely close and consistent with my particular setup during these tests).
So, while I much prefer direct drive, wheel on CAN be pretty consistent IF your setup is very consistent, but it adds extra variables regardless.
Quote Reply
Re: Power Numbers Off on Garmin Vector vs Computrainer [tamiii] [ In reply to ]
Quote | Reply
Part the challenge you are going to have in getting the systems to match is that both correct for temperature changes different but neither one is very good at it.

The computrainer won't automatically adjust for temperature changes so you have to calibrate it after its warmed up and then it will only stay accuracte within a limited temperature range. Assuming you have Vector 2 pedals they do adjust for temperature but in a bit of a half-ass manner. Garmin actually changed the way this was done when launching the Vector 3 pedals and it significantly improved the system. With the Vector 2 pedals you are likely to see re-calibration jumps rather than a smooth continual transitions which will appear as random noise in comparison to the computrainer. This is a big reason why you should calibrate the Vector 2 pedals every ride so you are starting with the correct temperature correction and don't get a sudden large adjustment part way through the ride.

I don't have a computer trainer but did years of training with a wheel on trainer which transmitted 'virtual power' and vectors. I just ignored the trainer numbers and stuck with the numbers coming off the PM. This yielded the most consistent results.
Quote Reply