Dumb to Smart Trainer. Or, ERG does indeed equal ARGH!

After almost eight years of Suffering, I replaced my trusty Kinetic Road Machine with a Saris H3 last week. In December 2019 I’d purchased Assioma Duo pedals, so had been Suffering using the power figures from them rather than virtual power.

My first quick test ride on the H3 was Ignitor and that seemed good. The next morning I fired up Joyride and… it kicked my ass.

Here’s when I hit Joyride in August (on the Road Machine with PM pedals)

Joyride last Thursday morning (it wasn’t joyful at all) - witness the spiral of death… three times!

I’d read smart trainers were best in the inner ring, so I used that and the middle of the cassette for a good chain line. I rode GOAT on Monday and my RPM was well under target (40 to 50rpm). I thought GOAT should be harder as even in big/little I couldn’t generate enough resistance for it and would end up over target RPM to hit the power targets (especially the blocks over FTP).

On another thread I came across this blog post. My rider profile is a TTer and I do spend a lot of time in the big ring on the road (I rode 70km (600m climbing) on Sunday morning and didn’t use the granny ring once). So when I lined up for Half Hour of Power this morning, I went back to the big ring.

Now normally I’d expect to hold 100% for HHoP (in HR Z4 or just creep into Z5) but ten minutes in this morning I had to drop to 95% otherwise I would have cracked.

HHoP in October (Dumb / PM pedals again):

HHoP this morning on this Saris H3:

It’s like The Minions have sneaked into my 4DP profile and stuck another 10% on my power targets - everything feels soooo much harder. Was I somehow cheating on the dumb trainer? Or what am I missing in the glorious world of ERG?

Power measured at the input pedals/cranks can be 5-10% higher than power measured at the wheel due to drivetrain losses etc. If you are switching to doing workouts in ERG model you need to redo FF or Half Monty to recalibrate. :laughing:


It sounds like the trainer isn’t applying ERG mode correctly. It should be able to apply enough resistance at low cadences without you having to change gear or up cadence.

Firstly, if ERG is working properly then there should be no difference in your power output whichever chainring you’re in, being in the small ring and a larger rear is then beneficial as it leaves the flywheel moving slower, which makes it easier for the trainer to modulate.

Second, the RPM difference in those two Joyride sessions will make a huge difference, combined with the fact that the dumb trainer won’t have been holding you to the power target strictly, so you could cope with slight changes with your gears, while the ERG mode is demanding you pump the correct power even if you’re not getting your cadence up. Basically, in ERG mode you need to be ahead of the game with cadence a little or you can easily hit that spiral of death.

Third, have you done a new FF on the smart trainer? If not, you really should, because what you can produce on the two systems (as recorded) might be different.

You should definitely expect ERG controlled targets to be harder than level mode for the same effort, because there is no forgiveness.

The power difference is likely due to where the power is being recorded. Previously it was at the pedals, so very close to the foot and very little if any power is lost. The smart trainer records at the Hub so energy (Power) is lost through the cranks, spider, chainrings, chain and cassette before it is measured. This can reduce power very significantly.
As for ERG, it should change the resistance to match when you reduce cadence or increase it (increase or decrease resistance accordingly), but I find this can take a short amount of time to knock it up or down a few gears for a specific RPM effort