Does a higher power number always equate to faster speed? For example, is 310 watts in Gear A faster than 300 watts in any other gear? I’m new to riding with a power meter, so any insight would be appreciated.
Yes, assuming everything else is constant. More watts is more speed.
Speed = power (watts) * cadence * gearing
You can check out the power / cadence calculator here to play with variables.
Some aerodynamic drag effect in extreme cadence examples, ie. 50 cadence v 120 cadence, in certain yaw angles and wind speeds might mean that it’s possible 300 watts is faster than say 303 watts, in the real world. Or maybe even greater. Not something that’s been tunnel tested much but we do have this:
University of Melbourne study:
Both the drag coefficient and the turbulent intensity obtain their peak values when the crank is in the near vertical position. This is to be expected as the blockage due to the leg is highest when the crank is vertical.
Assuming aerodynamics and weight stay same…then yes. More watts = more speed
No…but to most intents and purposes, yes.
Drive chain efficiency is the issue, a straight chain from a middle sprocket to ring is more efficient in transmitting power than an outermost or innermost sprocket (highest or lowest in your cassette). But this is marginal gains territory - I regard it as irrelevant to my cycling.
Yeah, so don’t worry about it. More watts = more speed.
If you like to worry about stuff (as I do) worry about aerodynamics. There’s measurable gains for normal situations there.
I understand this isn’t intended to be a scientific equation but it is anyway incorrect and misleading. Speed is not directly proportional to power. A doubling of power will not result in a doubling of speed (as your equation would suggest). Aerodynamic drag increases exponentially with speed such that successive increments of increased speed are increasingly costly in terms of power.
If your position changes in order to gain the extra watts, than it can be faster to have fewer watts. Commonly, people put out less power in a TT position, but go faster because the aero gains are worth it. Similar observations have been made about sprint form.
True, which is why I said “everything else is constant”. You’re right, this is not a scientific equation. I ignored aerodynamic drag, rolling resistance, wind speed, air pressure and many other things. I was offering an answer to the question posed by the OP to understand the relationship between speed, power and gearing. Other than being simplified, can you explain how it’s incorrect and misleading?
It’s misleading primarily because it’s not immediately obvious that the context which you purport to be referring to by “everything else is constant” is not an ordinary one but in fact a super extraordinary one which is only theoretical and not in the least realistic. Specifically, it would appear that the only context in which drag remained constant would be a vacuum. People don’t normally cycle in vacuums.
Secondly, I’m also not sure what exactly that equation is supposed to express aside from that misleading notion. How precisely does speed = power * cadence * gearing ? I’m not even sure what this is supposed to mean; what units are we using to quantify gearing (and speed) here for example? I actually doubt there is any relationship of this sort - I doubt that speed is equivalent to power * cadence * gearing -whatever units you choose.
Ok, let me try to be clearer.
Disclaimer: I’m not attempting to write a scientific white paper that factors all variables found in the universe that effect bicycle performance. I am simply trying to explain the original question posed by @bmcd70 about the relationship between watts, gear ratios and the resulting speed.
There are certain variables which, for simplicity, should be removed in order to explain the underlying principle. I’m referring specifically to rider metrics (Cda and weight), bike variable (drivetrain efficiency, rolling resistance, aerodynamics) as well as environmental conditions like air pressure, gradient and wind. The reason for doing this is to isolate the relationship between a riders effort (power measured in watts) and the resulting forward motion (speed measured in miles per hour) of the bike.
Bike speed is determined by the given gear ratio (referred to by the ratio of chainring teeth to sprocket teeth; for example a 50 chainring into a 20 sprocket will result in a gear ratio of 2.5 : 1) multiplied by the crank cadence (measured in revolutions per minute). Legs turn the cranks, which rotate the chainring, which then rotates the sprocket which then turns the rear wheel based on the gear ratio. In the example here, the rear wheel will turn 2.5 times faster than the crank. The bike speed will be determined by the circumference of the wheel.
Of course, it’s not free effort to turn the cranks. The gear ratio (and weight of the rider, rolling resistance, gradient, air pressure, etc) present resistance that must be overcome by power (watts) from the rider in order to turn the cranks. Assuming constant all the other variables identified above, a given constant wattage for a given gear ratio will result in cadence for the cranks and therefore speed of the bicycle.
If you increase watts, your cadence will increase and the bike will go faster. If you keep watts constant and change gear ratio, your cadence will proportionally adapt and your speed will be constant.
I like it…cheeky
It’s more accurate to say:
speed = cadence * gearing cadence = ‘net power’ / torque
Where net power is the power the rider is putting out minus all the various losses (aero, drivetrain loss, grade, etc.). Ultimately how fast you go is how quick you can turn the crank. Eventually you can’t spin fast enough or can’t generate enough torque to increase the cadence any further.
I understand you’re not trying to write a scientific paper.
Nevertheless I still do not believe it is in anyway helpful or informative to use made-up, incorrect pseudo scientific formulas.
Fair enough. I look forward to your alternative answer to the question posed in this thread.
This set of comments is completely unnecessary and borderline violates the rules of the Forum, that in simple terms say it is OK to attack and idea, but NOT ok to attack the person [which is how I interpret this set of comments].
As a mechanical engineer that excelled in fluid dynamics, I can appreciate your earlier comments challenging @julianoliver comments (IMO perfectly ok thing to do), as well as appreciate other technical responses in this thread that may challenge his comment. However, Julian is a valuable contributor to this Forum and should feel comfortable being able to continue to freely post his thoughtful responses and be challenged where appropriate in appropriate manner. And, FWIW, I also welcome YOUR continued participation, provided it is done along Forum guidelines. Thank you.
No - your interpretation of my comment is way off the mark. It was a concise and objective summation of what was wrong with his idea. The formula was “made up” rather than for instance an uncontroversial example of settled science. It was incorrect - speed does not equal power * cadence * gearing. And it was pseudoscience insofar as it presented itself as actual scientific information but was anything but - misleading and you can’t even do the bare minimum of plugging in values for power, cadence etc and get a reasonable solution.
I made a point of noting that his equation wasn’t really intended to be scientific when I originally critiqued it - as a means of softening the blow somewhat. Tactful I think. Despite this Julian’s “disclaimer” would appear to be suggesting that my critique was somehow unfairly and inappropriately stringent.
I was replying to this and in no uncertain terms, yet I believe what I said was both accurate and warranted. Again I will restate - I was talking about an idea: there was no ad hominem here. Please explain how this was an attack on his character or what exactly it is you disagree with. Do you believe I said something that was false or do you just think my words were insufficiently sugar-coated?
Well, to be honest I do feel like your comments are aggressive (calling someone’s ideas “made-up” and “pseudo science” isn’t nice and at best clumsy), directed specifically toward me (despite the fact that others have offered similar ideas in this thread) and without contributing anything to improve on the ideas that you clearly disapprove of. Misguided or not, my goal in this and all other threads is to help other members of the TR community. I’m more than happy for someone to challenge my ideas, but the minimum I expect is for them to offer their counter-idea in return. I’m not entirely clear what you were trying to achieve here, but it does feel like it is simply to tear me down.
Please go re-read your own post beginning with the quote above. As indicative of the second quote from your same post (immediately above), you had already made it very clear to all TR users on this thread your opposition to Julian’s idea. And it is/was accepted by all as valid criticism of the Julian’s IDEA.
However, the post I responded to provides no new information of value regarding criticism of the idea and, as such, I interpreted as an inappropriate aggressiveness toward the contributor.