Jughed

Brand new zwift trainer, 14-16% grades up the radio tower climb, keeping the watts low/under 200 - and the trainer resistance seems to surge during the pedal stroke…

Works fine the rest of the time…

Setting issue, or just the nature of the machine at these extremes?

PeteHski

That’s pretty typical trainer behaviour. Sub 200W at 16% would be very lumpy in real life too. In fact you would probably stall with normal road gearing. The radio tower climb is a tough test and requires 300W+ to keep moving smoothly at a half reasonable cadence. This is assuming you have your Zwift trainer difficulty set to 100%. If it’s at the default 50% then maybe your trainer is struggling.

rm -rf

On my Kickr, with very low cadences, the resistance can rapidly become huge. (If you stall out, stop pedaling for 1-2 seconds, it'll reset and give you reasonable resistance again.)
In the Windows Zwift app:

I'm older and not a strong rider.

Menu --> Settings --> Trainer Difficulty
Think of it as changing to a smaller crankset, which lowers all the gearing.

On hilly courses, I set it about 1/3 from the left. (There's no number scale, you just need to estimate where you want it.) I'm spinning up very steep hills at a fast cadence. Of course, my mph can still be pretty low, due to the lower wattage I'm putting out, but that's much easier on my legs than grinding at low cadences.

Today, on the new, quite flat coastal roads, I set it halfway and stayed in the big ring.

Lowering the Trainer Resistance doesn't reduce the wattage to climb a hill at a specific mph. It just allows a must faster cadence to ride that speed.
It can even be changed during a ride by jumping back to the menu. But you'll likely be dropped by a group you were riding with.

rm -rf

Kickr pedaling stroke
When the trainer was new, I immediately noticed that it's much harder to pedal at the same watts that would be very comfortable outdoors. (My road bike has a crank power meter, and the numbers matched quite well with the Kickr's reported watts.)

I think it's something to do with the difference between the continuous trainer resistance around the whole pedal stroke. Outdoors, there can be a "micro coasting" within a pedal stroke if I let off the pressure a bit. And on Zwift, I can't really let off pedaling at all, or my speed drops considerably. Outdoors is more a mix of easy, moderate, and very hard efforts.

It became much less noticeable after a while. I think my pedal stroke adapted, and I'm "rounder" and "smoother" through the whole revolution.
Back outdoors after last winter, I could see this helped on outdoor climbs too.

PeteHski

I think what you are noticing is a difference in crank inertial load, which varies dramatically with gear ratio and therefore slope (as you use a lower gear for the same power as slope increases). I expect your trainer generates lower crank inertial loads than your road bike due to its limited flywheel inertia.

When climbing a very steep slope in a low gear at very low cadence, crank inertial load is minimal and that’s when pedalling feels most “lumpy” as there is very little inertia to help the cranks over the top of each stroke. Your trainer probably exaggerates this effect with very little flywheel inertia at low speed.

Reducing the trainer difficulty simply reduces the effective slope resistance and therefore increases crank inertial load as the flywheel speed increases with higher cadence or a higher gear. Your avatar speed is still governed by the indicated road slope and power, but the pedal stroke is smoother.

Jughed

I just now found the trainer setting function - it was at default/50%. Explains why I could stay at my 2w/kg pace up those grades and maintain 70rmp - at 2 mph...

I will try it at 100% someday - when I'm not doing base training.

Jughed

Yep, I instantly noticed the difference in pedal stroke. I try and work on pedaling circles in on the fluid trainer - the smart trainer seems to force you into it.

I did a 40 mile zone 2 group ride the other day, and like you said - the power is on all the time, constant. No coasting, no micro coasting - my watts stayed bang on for the entire ride. And I was feeling it the next day.

PeteHski

Yes, so you were effectively climbing a 7-8% slope in terms of the trainer resistance. At 2W/kg that is still probably pushing the limits of your trainer in providing a smooth resistance. I find that most low and mid-range smart trainers start to feel rough when your cadence drops below about 60 rpm with a high resistance load. That’s where the higher end trainers can feel significantly better.

Looking at the spec of the Zwift Hub it can simulate up to 16% slopes, but that will be at a specific rider weight and cadence. Probably something like 75 kg @ 80 rpm. That spec is roughly similar to my Elite Direto X, which can sim up to 18% and it does struggle at low cadence on those slopes. So I doubt that your trainer is faulty.

PeteHski

Anecdotally, I’ve heard it said many times that an hour on the trainer is equivalent to at least 90 mins on the road in terms of fatigue on a flat ride. My average power on the trainer is always higher on rolling terrain for the same perceived effort. While it is pretty much equal on a long climb, where you don’t get to coast on your road bike either.

rm -rf

Zone 2 training
I did a custom Zone 2 workout. Originally, it was a warmup, then 40 minutes of Zone 2. That didn't work: I can't stay seated and pedaling continuously for that long. Standing briefly didn't work well with the Zone 2 erg mode.
Now the workout is 8 minutes Zone 2, 30 seconds of free ride, and repeat, up to 1 hour 15 minutes. The short free ride lets me stand up and ride in a lower gear, or stretch, etc. I can adjust the workout watts with the + and - buttons. 95%, 110%, etc.

This seems way too easy at the start, but my legs feel it by the end of the session.
I can ride on any terrain doing this, even steep hill climbs (which can be at very very low road speeds).

In the Zwift Windows app:
Workout anytime --> scroll to the right -->Workouts
left sidebar -->Custom
Create a workout. Drag and drop components onto the timeline. stretch them to different durations if needed. Unfortunately, the editor doesn't allow copy and paste to repeat workout blocks. I have to drag each one separately. Grrr. (I see there is an external website zwiftworkout.com for better editing that I haven't tried.)
Save it, giving it a name. Now it shows up in your Custom page.

PeteHski

That’s because ERG mode is completely independent of terrain slope. All that changes with slope is your avatar speed. Trainer resistance is unaffected.

Jughed

OK - new issue.

Trainer gain is set to 100.

My just riding along conversation pace, 1.8-2.2 w/kg, even up to 2.5 w/kg - no issues on the flats. Road and trainer are the same. I can ride all day at this pace on the bike or online.

Climbing on the trainer, grades of 8-10% - the trainer is telling me I'm in the same range, but my butt power meter and my lungs are not - they are telling me something much different.

I've got a decent feel for what zone I am riding in - I was in high Z3, low Z4 - but the trainer was telling me I was in mid Z2. I know I was pushing 250-270W - trainer had me at 180-200.

Bike calculator website typically agrees with my road climbing conditions, my power meter, and my butt meter - and it says I should have been in the 250-270w range.

In short, the trainer seems correct on the flats, but way off on climbs. Is there a fix?

spelger

have you ever used the trainer in ERG mode? i would be curious to try that. and if you have an independent PM like you imply i'd get that running too. always good to compare.

but sorry, i have no fix ideas, only troubleshooting. i'm currently fighting my own trainer problems too.

PeteHski

I would speak to Zwift support. That is not normal for a trainer.

Jughed

I'm going to pull the PM off my other bike and see if the numbers match.

I had to abort the first attempt at a ride last night because I was humping it just to get 12 mph. Tried a spindown test but it sat at stage 3 for 5 min or so, and I aborted that as well. I simply started the next ride everything seemed fine until the climb.

I will try to complete a spindown test and then compare to my PM...

I was expecting to do a Z2 ride in London (triple loop), thought it would be all flat, because London was flat the last time I was there!!.... nope. 2k'+/- of climbing, much of it at 8-11%. I need to read the ride descriptions a little better...

PeteHski

Yeah they moved the Surrey Hills right into the city!