The reason that I’ writing this is because I’ve found a huge power value between these two equipments (my math tells-me that it is ~15%… I’m yet to figure if this is normal or not. I don’t think I have a 7,5% leg inbalance).
I’ve used scale on 4iiii precision to diminish this discrepancy, but…
What I would like to know is:
a) is it better to use smart-trainer power source for indoors training (with ERG mode) or
b) should I user 4iiii as power source?
While doing a structured training session on Zwfit (with ERG) I’ve found that if I use 4iiii as a power source it seemed almost impossible to achieve power target… I’ve not tried on TR yet, but I assume that it will be the same.
You should be able to achieve target power regardless of the power source. Do you mean that it fluctuates around the target with one power meter while with the other is is more stable? This is probably because of smooting, as the trainer might be transmitting a rolling average of the power, which will look more like a straight line on target.
It doesn’t really matter what you use as long as you have estimated FTP using the same source.
A consideration to use the 4iiii is that when you ride outside you can then better use the inside power numbers as a reference.
That trainer is a wheel-on smart trainer. That means to have accurate power numbers you need to:
Have consistent rear tire pressure across sessions (within a couple of PSI)
Consistent environment (room temperature mainly. But also moving the trainer can throw off calibration)
Wait for the trainer to warm up for first ~15mins of riding before trusting the numbers
Do a spin down calibration regularly (every couple of weeks or whenever anything changes with your setup. And it should be done after ~15 minute warmup)
If any of those are off with your routine, the power numbers reported by the trainer might be significantly off. Wheel-on trainers are also not as accurate in general, so ~5 percent difference to “real” power is not unexpected even when you do things exactly right (check the trainer specs)
The left-only power may only report your left-only power, but majority of users don’t have that significant issues with L/R balance varying significantly.
I would just use and trust the left-only power meter as your power source and ignore the power reported by the trainer. You can still use the trainer’s erg mode with Trainerroad if you enable the Power Match feature. This way you get consistent and comparable power numbers for indoor and outdoor rides as well.