Power Meters in Turbo Trainers

SirLouen

I was thinking today: How it's possible that Turbo Trainers calculate power output?

Most turbo trainers calculate it? For example, Elite Turbo Muin, TacX Flowsmart,... ?

wongataa

They either calculate it (cheaper ones) or have a built in power meter (more expensive ones). The ones with built in power meters would work in a similar way to how hub based power meters for bikes work. Power=torque*rotational velocity

To calculate power you need to know the rotational speed of the flywheel and the mass of the flywheel.

SirLouen

Hello @wongataa
I'm pretty confused about all this thing

According to my knowledge, there are certain fundamental variables to calculate Torque (Force and Radius of the different axis of the gear set). And probably, the most important one is the Weight Force in N (Weight x Gravity Force).

But also Speed or cadence it's important to calculate the Power. Therefore, only a Crank or a Pedal, can calculate directly the Torque with a cadence sensor.

I can't understand how an expensive Turbo Trainer can calculate the Torque, because it only has very simplistic variables: Speed of the chain, cadence of the gear and cadence of the wheel at best and obviously in case of a smart trainer, the resistance level as an indirect force. So I understand it's an indirect measure of the Torque: For a certain resistance of the gear (Brake Force), at a certain cadence, the Weight Force must be X, so they have the Torque, and the Cadence, and an indirect measure of Power.

I understand that never in life, they will be as precise as a Pedal Power Meter like Garmin Vector or Powertap P1 and they are extremely contigent to the quality of the brake force friction measurement.

And the second question:

Do most turbo trainers can calculate power? For example, Elite Turbo Muin, TacX Flowsmart,... ?
I've seen for example that TacX Flowsmart can calculate up to 800W:



But it has to be extremely imprecise because the auto-braking system it's not embedded in the gear (like the Wahoo KICKR), but in the friction of the back wheel with the trainer.

Any insight about this?

NOTE: I'm asking this, because I'm thinking that maybe I should stick with a power meter pedal (like powertap p1s ~$600, of really good measurements) in my spin bike and keep manually modifying the resistance, instead of buying a Bike + Power Trainer for indoor training ($300 road bike + $300 of a poor quality turbo trainer).

frisbee

Wheel on trainers require you to set them up in a specific way and perform a spin down, they'll have a pretty good idea of resistance due to the tyre-roller interface.

Force and speed can be measured anywhere on the drivetrain, its not indirect measurement if you don't happen to measure it at the foot, if anything its more direct the closer you get to the hub as you are including drivetrain losses.

StillGoing

The Elite turbo trainers with inbuilt power meter are extremely accurate to within +/-2% and the Direto to within +/-1% as tested by D.C. Raymaker and Shane Miller. I’ve ran my Kura alongside my P2M and there’s absolutely no difference.

wongataa

SirLouen wrote:

Hello @wongataa
I'm pretty confused about all this thing

According to my knowledge, there are certain fundamental variables to calculate Torque (Force and Radius of the different axis of the gear set). And probably, the most important one is the Weight Force in N (Weight x Gravity Force).

But also Speed or cadence it's important to calculate the Power. Therefore, only a Crank or a Pedal, can calculate directly the Torque with a cadence sensor.

I can't understand how an expensive Turbo Trainer can calculate the Torque, because it only has very simplistic variables: Speed of the chain, cadence of the gear and cadence of the wheel at best and obviously in case of a smart trainer, the resistance level as an indirect force. So I understand it's an indirect measure of the Torque: For a certain resistance of the gear (Brake Force), at a certain cadence, the Weight Force must be X, so they have the Torque, and the Cadence, and an indirect measure of Power.

Forces are usually measured with strain gauges in power meters. These can be in the pedal axles, the crank arms, the crank spider, or the rear wheel hub. Rotational speed is easy to measure with accelerometers. From this data it is easy to calculate power as the manufacturers know how far away the strain gauges are from the axis of rotation (pedal power meters don't know this but you have to input your crank length for them to work properly). Higher end Elite trainers use an optical torque sensor for the power meters in those units. A quick Google will let you know how those work. For other wheel off trainers having strain gauges in the hub is the way they would go.