Indoor to outdoor calibration. Or, when do I get off and push?

So I did Mount Goode today. All good:
I have TrainerRoad controling the Kickr, which is in Erg mode. I also have Zwift ‘watching’ me and simulating what my power output would look like in the ‘real world’ of Watopia. It’s interesting to compare the two datasets.

In TrainerRoad, I get

which is all fine and good. I assume the speed is calculated by knowing the wheel size and the power I’m putting out at the specified cadence. I’m doing 0 mph, of course, so the speed is a calculated parameter. The Zwift data is identical in all measured data, of course:

Although I think the smoothing algorithms must be different, but the data is basically the same. Now Zwift of course calculates the speed quite differently, as it accounts for the gradient of the road, and at the same power I could be doing 1 mph puffing up a hill or 40 mph coming down it. So it must assume that the motor (me) is putting out a fixed power (which I am) and then using some infinitely-variable gearbox to match the known power and cadence to road speed. Fair enough.

Now my perceived effort is the same, of course: I’m putting out X watts on a stationary bike. In real life, I perceive hills as much harder than flats, and flats as harder than descents. I tend to pedal harder uphill to maintain speed, and I’m using TrainerRoad to train me out of that: I should be maintaining a fixed power, and so letting my road speed vary though use of the gears.

In real life, my riding becomes very wobbly below 3mph, and when riding uphill in Zwift runners were going past me. Imagine a steep hill in real life, and you’re granny-gearing it at a constant power. At what point do you just give up and push?

Also, in the simulation I’m maintaining a cadence of 100 rpm while climbing a hill. I can’t see me with a sufficiently low gear to be spinning at 100rpm going uphill at 1mph.

Basically, the question is: how slow can you go without falling off?

Hey Psychopasta!

The minimum speed that a rider can sustain without falling off will vary depending on gearing and the rider’s sense of balance. In some cases, if the hill is sufficiently steep, you will need to exceed your target power in order to stay upright.

While ideally you would be able to stick to your target power, it is far better to exceed your target power than to push your bike up the hill. If you exceed your target, you are still getting a training benefit from that portion of the ride, you just may need a few extra minutes to recover after the hill ends On the other hand, if you choose to walk up the hill, you will not get any training benefit :wink:


Thanks! So I should plan to stay in a lowish zone like Sweetspot for most of a ride, but exceed that power if my speed falls below say 3mph on a steep hill? Makes sense

The speed in TR will be determined by your gear selection and the cadence you maintain. Power isn’t relevant except that it will alter the trainer’s resistance which will have an effect on the cadence you can maintain. The speed will come from your speed sensor not TR.

As for Zwift’s speed it is calculation based on all the info that TR uses then massaged with the riders entered weight, trainer difficulty setting etc …

If I could ride anywhere near as fast in real life as I can in Zwift I would still be a Zwift subscriber. :muscle::muscle::muscle:

With regards to real hills you need to use some online calculators that will help you find out what power is required to ride a certain length of climb at a certain grade at a certain power output. You’ll need to keep increasing the set power until you are fast enough that you don’t fall off or the power required is too high for too long and you have to walk.

And from experience, walking steep hills in cleats is a sure way to ruin your calves. :frowning:

I’ve never found my slowest possible riding speed as the legs always gave out first. Riding outside with power has helped me judge my efforts and I no longer go too fast too early.

I had a lot of trouble with cramping last year and had to walk at least one big steep hill because otherwise I would have never made it to the top. For me, once I get past a certain gradient, walking is almost as fast. I wear mountain bike shoes, but I have found that to be true even with road shoes. If I’m not feeling that great, walking can be a break.

One thing I have noticed is a lot of people really bury themselves on hills. If you have to keep up with better climbers, there is nothing you can do. However, that is not a good strategy on a long ride. I ride with someone that attacks every hill in a sprint, he has noted that I often ride stronger than him at the end of a long ride.

In all honesty, I think I’m just pushing the analogy too far here. With the trainer in Erg mode, I just sit at 100rpm producing the requested watts. That’s not a realistic representation of what happens outside, or even inside if you take Erg mode off :slight_smile:

I should really pull out my first-year Classical Mechanics notes, but I think that as the cadence drops you need to produce more torque to maintain the desired power. I think you grind to a halt when you can’t produce the torque needed to turn the pedals against the gradient.

In true erg mode, you also grind to a halt if you can’t keep up the power. I think over time the smart trainer designers have realized that’s not a good way to go. My kickr seems to back off on resistance if I’m more than 10-15 watts low. When I first got my kickr, that iteration of the firmware would just keep clamping down until I stopped.

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Thanks @eekeller, and all others.

I’m just taking my Zwift thought-experiment a step further here. Presumably, if you are producing some watts X with sufficiently low gearing that you can crawl up the hill at say 1 mph, you would be better off getting off the bike any just pushing in any case? So forget about the practicality of going so slow you fall off…at some point the bicycle represents a mechanical disadvantage, and you will go faster with less energy by walking and using the leverage of your legs and whole body height.

It’s very marginal, but a bike interferes with walking. So as long as you can stay upright, you are better off on the bike. I don’t think most of us can stay on a bike long at 1 mph on a steep uphill.

So is the next must have tr accessory an incline treadmill to put next to the kickr so we can train walking the steep parts? :wink: do I have to simulate pushing the bike while I hike uphill ?

It’s the new Wahoo WALKR!

I have a direct-drive Kickr and the way it works is that the Kickr ANT+/Bluetooth streams both power and “virtual speed.” By default the Kickr will output virtual speed based on the speed of the flywheel. Lame. Probably the similar/same as wheel-on Kickr Snap, not certain as I don’t have one.

If you have an iPhone, then its possible to tell Kickr to output virtual speed based on power by toggling “Erg mode speed simulation” as described here:

Zwift has its own physics engine to estimate virtual speed based on power, grade, and perhaps a few more variables. Zwift ignores the virtual speed streamed by Kickr, and does its own calculations.

On a mountain bike or fat bike, about 2.5 or 3mph is about as slow as I go before getting off and walking. For several reasons - cadence is very low at that point and as a result, torque is high; it’s tough to balance. So it’s just easier to walk.

I’ve not had to get off my road bike - I guess that’s due to me not having ever ridden anything steep enough.

Gearing also has a big impact on this.

May main takeaway is that you’ll know it when you experience it - there’s times when it’s just easier to walk.

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