Power source - Trainer or Crank Power Meter

Hey All,

I’m starting a new training plan and i’m wondering what power source to use for my training.

Currently, my winter bike is on the trainer and i’ve been using the PM on it for the data source. However, in a few weeks i expect to take that bike off and put my road bike on the trainer to use during the winter months.

I know that the guidance is that whenever you have a new power source you should redo the ramp test. However, i’m wondering if it would be better to just take the power from the trainer (Tacx Neo 2) so that i don’t have to redo the ramp test.

One thing I did notice was that when I dual recorded my recent workouts (tacx neo2 + TR vs. garmin edge + PM) there was a difference in power being recorded. The PM on the bike was reporting a 10-20watt higher power than what the trainer was reporting. Should i be concerned by the difference by the power numbers between the different sources? I did calibrate the bike PM but i don’t think you can calibrate the Neo2.

What do you guys recommend? Should I just use the trainer for power? or should i just suck it up and redo the ramp test when i switch bikes on the trainer?

Thanks in advance for your thoughts!

If you think it is important to know what power you need to put out next year outside while racing/riding on your summer bike then train with its power meter over the winter.


Just do it now and gain the consistency of a single source.

It seems that you are thinking that it’s a big deal to take a bike off a trainer and ride it outside, then put it back?

1 Like

it is a bit of a hassle. the road bike has rim brakes and a shimano groupset, whereas the winter bike is a disc brake bike and uses the XDR driver.

1 Like

It is better to estimate your FTP on the trainer if you are going to train there. The trainer measure the force at the cassette area where other power meters usually on the pedal or crank are affected by the friction of the chain and Bottom bracket bearings, so the wattage at the cassette will always be lower than the wattage at your feet/pedal… On mine I calculated between 8-12 watts difference depending of the number.

I agree. You would expect to lose around 4% of your power between the pedals/crank and rear hub i.e. drivetrain efficiency is typically around 96%. So you could expect to lose about 8W @ 200W. This is what I typcially see between my crank PM and trainer power.

But 10-20W sounds a bit on the high side, especially if you see that sort of loss at relatively low power. I don’t know what PM is being used here, but I would tend to trust the Neo 2 readings. That’s got to be better than trying to work out the difference between 2 different bike PMs.

Train with whichever PM you will do your A race on, if that event involves hitting power targets consistently.

Examples of races in which the number on your PM is important: any triathlon, grand fondos, hilly road races, XCO, XCM, gravel, enduro.

Examples of races in which the number on your PM is not important: crits, CX, DH, BMX, tracklocross (I just threw that one in because I like the name).

Generally speaking it’s better to have a single power source to train and race on.

You are better off being in a situation where your training zones match up with your outdoor zones, rather than knowing that they are different, if you use the power meter at all for pacing, eg breakaways, hill climbs etc.

If you only ever ride outdoors by feel, and passively collect power data, then go ahead and use the neo as the power source, but if you use the power meter outside, just use the same device for both to ensure you are comparing apples to apples.

Heck, even passively collecting data and using it to look at rides vs workouts, power prs etc. I’d still rather have a single source as well.

I found that the best set up for me was to use a pair of assioma duos as my main/single source of power. I use them to power match against my Tacx neo 1 indoors and then I’ll take them off to put on my road bike outside. I found that at high fly wheel speeds the Tacx neo would give fairly high power readings - perhaps 10% higher than my duos. At low fly wheel speeds I’d see the Tacx was around 1% lower than the duos (spot on in terms of accuracy).

I also have a gravel bike with a quarq dzero on. After plenty of testing I adjusted the slope on the dzero so it was a better match to the duos. I’m not sure which is more accurate but I’m happy to go with consistentcy.

My advice would be to consider pedals as your power source. The they take a matter of seconds to swap over and would give you consistency between indoor and outdoor rides

1 Like

As long as you keep it consistent and train using the source you tested with you can use whichever works best for you, so I’d consider your own priorities above anything else- whether that be the convenience of using the trainer or the specificity of a source you’ll also use outdoors.

I imagine you could also dual record your FTP tests and get a value for both, so you’d only have to swap the bike over for testing versus for every session.

1 Like

Use whatever you will use outside. The trainer might give different values. And you really want the same values indoor and outdoor.

1 Like

The problem here is that he has two bikes with two different power meters. So if he uses the two outdoor bike PMs on his trainer, they will both give different readings. Ideally you would want the same power readings indoor and outdoor at all times, but that’s not going to ever happen in this scenario.

What I do is use my trainer power indoors, but dual record the bike PM and learn how it differs so I can set corresponding power targets for outdoors. As it happens they are close enough not to require much thought. My bike PM read about 3-5% higher across the board.

1 Like

So then you can adjust the slope accordingly (if your PM allows it).

How do you dual record? I have my TR set to use my Stages PM but I could very well just use the Kickr PM. I was under the impression that the crank power meter was more precise…

Both should be sufficiently precise for training purposes, in that they’re consistent with themselves day-to-day. Accuracy is a different issue, and one that’s both hard to quantify outside of a lab setting and probably less relevant for training purposes.
I think you can pair the trainer to TR and the meter to a bike computer or something similar- as long as you have two different devices/programs recording you should be good. :slight_smile:

They should be of similar accuracy. Probably both within +/- 1.5%. But the Kickr is measuring at the hub, so will be about 4% lower than at the crank due to drivetrain losses. Is your Stages PM single or double-sided? Single-sided will be less accurate.

I record my bike crank PM on my head unit, so I can compare with my trainer power. Sure enough it’s about 3-5% higher across the range.

Thanks Peteski. My Stages is single sided. I cannot afford double sided power meters since I now want them on all my bikes!

I’m told the Kickr power model factors in some drivetrain loss.


Do you execute TR Workouts outside ?
So I would Use The PM on my bike.
If Not I would User the Trainer pm. So IT would be easier to swap bikes.