# Help on power/watts

Discussion in 'Power Training' started by Diesel legacy, Jan 9, 2017.

1. ### Diesel legacy New Member

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Coming back from an accident, vertebra compression fracture T12 area, 6 months back.
Hit by a motor vehicle while cycling

Riding now for a while and getting caught up in this whole watts talk.

I don't have any fancy gizmos of such, just my mobile, in my back pockets top, using Strava.

Is there any information on there that could help in roughly giving an estimate.

I don't mind sharing any info needed to help.

Thank you

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The short answer to your question is that yes, you can estimate power as a function of rider, speed and resistance. For example, Strava has an algorithm that will estimate power. And, the iBike computer (now marketed by Velocomp) derives power from speed and estimated resistance.

The problem is the accuracy of the estimate. The reason there is a wide variance in accuracy is because it is extremely difficult (actually, it's impossible) to estimate resistance. The logic of deriving power from speed is that power is required to produce speed given total resistance, so if I know total resistance then I can impute the amount of power required to produce the speed. And, that's true because it's simple physics. But, here's the rub. Total resistance includes some variables that are relatively constant, such as the weight of the rider and bike, rolling resistance from the tires, frontal area resistance due to the rider's position on the bike, and the frontal area resistance of the bike. But, then you have variables that change constantly, such as grade and wind. The truth is that road gradient changes constantly and wind direction and velocity changes constantly. So, no matter what values you use, they are only accurate some of the time. So, the bottom line is you should use any estimates of power not based on an actual power meter with the knowledge that the accuracy of the estimate is subject to a large variance.

If you want to play with the assumptions and estimate power based on assumptions, here's a site where you can do that: http://www.analyticcycling.com/

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3. ### Diesel legacy New Member

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Very informative and very knowledgeable you are.

Thank you for the site adress, I'll have a look

4. ### Diesel legacy New Member

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The site is very informative, overwhelming. Thanks

I found a section that shows cadence at a given speed and gear ratio you in.

Is it possible to get watts from such a model.
I've got an entry level giant indoor trainer, magnetic one.
I put my cycling clock on so I can keep track of my training, speed, distance,time.

Out of scanning the web I don't get any graphs for the idt giant.
But every other idt on the planet I can.

Thank you

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It's virtually impossible to develop a reliable formula to translate resistance & speed to watts for any trainer, and here's why. Even at the same resistance setting and speed, there are other variables that greatly affect watts. For example, power is very sensitive to tire, tire pressure, tire wear and heat. Most trainers and tires heat up and the watts required to produce a given speed at resistance will change as the trainer heats up and as the tire heats up.

What you can do is to calibrate your trainer with a power meter. If you know someone with a PowerTap hub (or any other power meter), see if they will help you calibrate your trainer. In other words, develop a table of the relationship between resistance, speed and power. But, take note that the calibration is only valid if you use the same tire, with the same tire wear and the same tire pressure and the same trainer warmup duration. The bottom line is that there is no simple, painless way to estimate watts without a power meter.

6. ### Diesel legacy New Member

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Thank you...

Getting someone with a power meter and running my idt at certain speeds will give me a good idea where I'm heading when training.

Thank you for speedy reply and informative information.

7. ### Diesel legacy New Member

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Hi

I found this.

See the attachment.

Kind of all what i was looking for..

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Such broad assumptions about the relationship between power and speed are basically worthless. You have to ask yourself why you want to know your power. If you use a number that is at best an approximation with huge variance, then I say it's a worthless number. Do some reading at the link I gave you and you'll understand the variables that determine total resistance. It's a long list.

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