Different watts when going up and down, same effort


(Oddvar Grenning) #1

Hi!

I’ve had a feeling that Zwift, or my Tacx Flux, is showing more watts when I’m going uphill than downhill, with the same effort. It’s frustrating to see the watts falling when going over a hilltop and down.
So, I did a test, riding a long hill up, and then down, keeping the pulse steady. No drafting. The difference was about 40 watts (!). Here’s the data from Strava around the peak:

Going up:

Going down:

It’s hard to read the curves, but look at the data at the rulers.

Can anyone explain this?


(Paul Allen) #2

Zwft does not manipulate watts in any way. Whatever is being broadcast by your trainer is what Zwift will display and use within the game.
I would guess that your shifting and cadence would be the cause of this. BTW, you are only show an extremely small sample size, you will at least need about a minute snippet to compare the 2.


(Gerrie Delport) #3

Here is my 2c.

On the uphill your trainer has the break on so you have to peddle against it but when you go over the top the tension gets released and you don’t have anything to push against to produce the power.


(Russell Gowers [CRANKS] KISS (C)) #4

You’re not putting in the same effort. You’re increasing the cadence and the watts are decreasing which means the resistance is decreasing, as it should do - you are going downhill! PE may be the same but it will take time for your heart rate to respond to the easier workload.

If you want to continue to push the higher watts, change up a couple of gears as you go over the top, just like you would in real life.


(Oddvar Grenning) #5

Thanks for the replies, guys!

Paul, Zwift uses whatever the trainer is broadcasting, true. At the same time, Zwift is telling the trainer about the terrain. The trainer behaves very differently when going uphill and going downhill.

Zwift is trying to simulate downhill to some extent. If the hill is steep enough, you don’t have to pedal (at least on my trainer). So Zwift is in the loop.

My sample is quite short, unfortunately. About 5-6 minutes going uphill, and 1 minute going downhill. In my experience, a 40 watts drop will affect my heartrate well within a minute.

Russel, power in watts are given by the following formula:

Power = Torque x Cadence

An increase in cadence will result in an increase in watts, at the same torque, not a decrease.

What happened during my run was that I shifted to the lowest gear, which is not very low, mtb, and had to increase the cadence to keep the effort at the same level (and thus lowering the torque).

I think a good test would be to use watt-pedals. If the pedals show the same as Zwift, it’s all in my head. If not, there is something wrong somewhere.


(Daren Chandisingh [Vision]) #6

The issue here I think is that “gravity” is working with you on the downhill. It’s the same reason why it’s “easier” to put out 250W uphill than it is to do 250W on the flat I reckon.


(Oddvar Grenning) #7

Comment: I ment highest gear, of course.


(Oddvar Grenning) #8

Daren, thanks for your reply.

Do you mean that Zwift subtracts the effect of gravity from the watt number?

It might feel like it’s harder to put in the same watts on a flat than in an uphill, but the power you enter into the cranks does not care about the terrain, it’s all you. You only go faster on the flat with the same watts.


(Gerrie Delport) #9

It is like pushing your car up hill the force on your hands is a lot and it is hard work, but once you are going over the top you need to run very fast to even keep your hands on the car.

Same with your trainer, while going up hill the trainer has a break that slow you down and you need to push against it, but on the downhill it does not break so you have not much to push against to produce the power.


(Oddvar Grenning) #10

If you run fast enough, you can maintain the force. That is my point, I’m ‘running’ fast enough, but Zwift, or the trainer, says I’m not.


(Oddvar Grenning) #11

Gerrie, to elaborate over what happens over the top.

I believe this should happen: As you say, the trainer breaks less to simulate the effect of gravity. If you keep the effort up, you will accelerate. At some point the air resistance will stop the acceleration, and at this point, if you keep the effort steady, the trainer will (should) break the same amount as on the way up.


(Erik Schumacher) #12

The point is, if you ride downhill the trainer spins freely, which means the torque decrease and will not stay at the same level. Even if you shift in a lower gear you gonna hardly get the same power output like uphill.


(Oddvar Grenning) #13

Erik,

If the wheel spins freely, power ~ 0, my cadence should hit the roof, which is not the case, it stabilizes.

Cadence = torque / 0 = infinite.

As I said, the air resistance will at some point balance out my effort + the effect of gravity. The trainer should, and is, simutaling this, it’s the watt number it shows that I have a problem with.


(Russell Gowers [CRANKS] KISS (C)) #14

I’m fully aware of how to calculate wattage. I was trying to explain that, since your cadence is increasing and the wattage is decreasing, the resistance (and therefore the torque required to overcome the resistance) is decreasing also. As others have said, it simulates gravity.

You are interpreting this as “I’m working just as hard but Zwift is telling me I’m putting out less power.” I’m saying that this is incorrect. Zwift is simply rebroadcasting the information which it is getting from your turbo trainer. If the Flux says that you’re putting in a lower wattage then you are, whatever your perceived exertion.


(Gerrie Delport) #15

@O_G. Yes you will stop accelerating that is true but “if you keep the effort steady, the trainer will (should) break the same amount as on the way up.” Remember Gravity work against you when you go up and with you on the way down.


(Oddvar Grenning) #16

I didn’t mean to question your physics skills, Russell :blush:. I must have misunderstood what you meant.

I have addressed the lowering of the resistance due to gravity in earlier comments. This will enable you to accelerate, spin up the flywheel. The acceleration simulated in Zwift will come from both gravity and your power into the cranks.

The increasing air resistance, due to higher speed, will build up the trainer resistance and will eventually stop the acceleration. Balance out the effect of gravity and your power into the cranks.

Would you agree with this reasoning?

One should just as easily be able to put 200W into the cranks going down or going up.

If, as you say, the Flux is correct, the missing watts may be due to my body having less efficiency at higher cadence.

I really should get hold of some power pedals to test this. Or a bike with higher gears so I could hold the cadence steady.


(Gerrie Delport) #17

The increasing air resistance, due to higher speed, will build up the trainer resistance and will eventually stop the acceleration. Balance out the effect of gravity and your power into the cranks.

Would you agree with this reasoning? NO At some point you will stop accelerating but your wheel will be going very fast lets say 80km/h, that mean you will need the gears to actuly pedal at 80km/h that will be at 0 watt, then you need to go even faster to reach 200W (if you went up at 200w).

For every positive action there need to be negative reaction, and on a downhill it is hard to get that negative reaction due to gravity dragging you down.

You will need a 60/11 gear ratio to reach 82km/h at 120rpm.
so if you only have a 52/11 your max speed will be 71km/h and if your bike is already going faster than that then you wont be helping your bike going forward, therefore there is not negative reaction force.

image


(Oddvar Grenning) #18

Gerrie, this makes perfect sense :+1:.

Then, Zwift sends us a curve ball: Trainer Difficulty.

I did not have Trainer Difficulty set to a 100% in this test. This setting will change the grade of the hills, so your gear ratio computations do not apply.

I think I should du the test again, on a less steep slope and with trainer difficulty set to 100%, to remove some variables.

Thanks for the exchange, Gerrie and everyone, this has been a good step forward.


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(Gerrie Delport) #20

@O_G

The Trainer Difficulty does not change the the grade of the hills, what is does is it change your gear ratio, giving you bigger gears at the top or bottom depending on what your preference is.