Power = work/time
 

Discuss.

whats there discuss. this is obvious.

When I first read that, it seemed more than just oversimplified, but rather entirely wrong unless I'm missing something.

How do powermeters measure power? Dont they measure torque and use that? Or do they know what gear your in?

Pt has a torque tube in the hub. as well as the Ergemo I believe has one in the BB.

measure by strain

my PT had a-torque tube, ergemo i'm not familiar with

PT measures torque and wheel speed to give you force and distance per time. From that you can calculate work and power.

I agree - he probably shouldn't have taken the risk. :D

Since work = force x distance, I can see where Friel equates distance with gear size. But reading that text you could infer that he sees it as a linear relationship. I think he's missing (or choosing to overlook for simplification) the relationship that force must increase with increasing proportions of distance over time (i.e. velocity).

It's a good book, but there are a few areas like that where it could have benefited by better editing.

I don't know what you guys are *****ing about, it's well explained and makes perfect sense.

This is a better way of looking at it:
I can push a 53x13 on a flat surface at 90 rpm with no headwind.

Does my power change if my speed, gearing and cadence stay the same but now I'm going downhill? Or uphill? Or into a headwind? Or into a tailwind?

Obviously yes. There is more to power than gearing and cadence.

Power is also going to depend on the weight of the rider (ie. when going uphill). We did power tests in physics, and this simplified explanation of power would only apply in a setting with zero friction from both rolling resistance and wind.

power = work/time.

change some (fundamental physics)stuff around and you get

Power= torque*angular velocity.

Angular velocity varies directly with cadence, so
Power = torque*cadence to a constant.

This applies both at the cranks and at the hub on the wheel. The gearing system can trade out torque for cadence, but power will be the same under the came conditions (speed, incline, etc) regardless of cadence.

So what you're saying is watt/kg is important.

...

on a hill, watts/kg are going to determine your steady state speed.

On a flat, watts/frontal area are what matters

For sprinting, it's some combo of the 2, but it's mostly positioning.

This is true even with air resistance. Generally it's safe to ignore rolling resistance in this type of discussion, because it's minimal (i would say negligible) compared to air resistance at high speeds, and it's again minimal compared to the force of gravity at low speeds.

Generally a heavier rider will have a higher overall power output, but a lower power/weight ratio.

This is itself a gross oversimplification, and outside of the professional peloton, it's mostly wrong.

~13% of total power at 25 mph for a 75 kg bike + rider on level ground
~7% at 250 W for the same rider up a 5% grade. (per analyticcycling.com)

yes, rolling resistance is not negligible, as you indicate. it takes significantly more power for me to average 25mph on a still day than it does to average 15mph into a 10mph headwind.

I'm not sure what there is to discuss.

FWIW, this is how I do trainer and roller interval sessions. I don't use a heart rate monitor or powermeter or anything, I just pick a cadence and gearing that has me feeling the right amount of pain, then use the speedometer to maintain wheel speed, and therefore, my power output, constant over the interval. If my speed starts drooping, then I know that my power output is drooping, and I need to step it up.

Your power does not vary with the gear if your speed remains constant.

What varies with gear is the force and the distance equation as jayhawken said. The higher the gear, the more you need more force over less distance, with distance being the circumference of your pedal stroke.

Of course it does. Drive train losses are proportional to the tension in the chain and diameter of the sprockets. Now, how much of a change that makes is another story.

Next to nothing unless it's so tight that your derailleur breaks off. All you need is a fixed gear and moving the axle/back forth to see the effects yourself.

You mean like rolling resistance? Put a number to it.

Well, as a guy who drafts trucks at 40mph a couple times/week, I'll tell you that rolling resistance can be quite substantial. With pretty much no air hitting me, I can't hold 40mph on the flats for longer than maybe five minutes (368W 5-minute power).

This is true only for measured power, if it's measured at the hub. Your actual power at the cleat doesn't change with gearing.

Look at the original statement. Speed remains constant; therefore, power to the road is constant. Since drive train losses change (slightly) with gearing, the only way for speed to stay constant is for power from the rider to change. This has nothing to with how power is measured or even that it be measured at all.

I'm too tired for this.