Garmin Edge 305 or Polar CS600
- Thread starter jechilt
- Start date
Isn't the 'proof of the pudding' going to be when someone (independent) hooks a power hub and an iBike up to the same bike and compares the readings? Or is that just too simplistic?ScienceIsCool said:
The engineering behind the iBike is technically entirely feasible, but there are so many issues in implementing the theory accurately that I think some simple raw comparative figures would help to prove that the practice is actually matching the theory.
John, I must be missing something. My understanding of the iBike (and HAC 4) is that it uses a formular similar to the this one: POWER=SPEED*((0.5*1.18*(SPEED*SPEED)))/2)+(WEIGHT*9.806*(0.006*COS(ATAN(SLOPE/100))+SIN(ATAN(SLOPE/100)))). Which I use to calculate my approximate Power wattage from GPS track data. See my previous post http://www.cyclingforums.com/showthread.php?p=3226740#post3226740ScienceIsCool said:
The iBike has a wind sensor, which adds an independent variable, which I am assuming a constant (the post goes into more detail). I know the iBike (or HAC 4) is not a GPS, but it determines slope (derived from altitude), speed and wind to calculate Power without a strain gauge.
John, let's keep this intelligent conversation civil, no flaming. I do not know what the Alienator problem (read prior posts on this topic) is, but I do not care to see a repeat that poor behavior.
Thank you
digi-bike said:
How courageous you must be, referring to me in the third person. That earns tons of respect.
You cannot assume zero wind resistance for a loop. There is no basis for such an assumption. Even if you have a zero wind day, you still have to account for aero drag from your bike/rider system.....which you don't. With wind, the problem becomes even more complex, because nothing constrains the wind to blow at an angle of attack of zero degrees. And nothing constrains the wind to blow in the same direction over a given area. For example, in my area we often have strong, gusty winds that do NOT blow in the same direction. Wind directions here are a function of terrain topology, local heating, and etc.
Second, GPS units typically do not have accelerometers. iBike does, and it's accelerometer provides high resolution measurement of inclines. A GPS sensor would have to use a differential altitude measurement--between one point and the next--to arrive at an "average" incline. Meanwhile, iBike is accurately measuring the incline several times over that same interval.
At any given measurement, iBike provides road slope, air speed, road speed (via fork mounted sensor), and altitude. Calibrated into iBike are the appropriate constants: aero dynamic drag and mechanical drag. With those constants programmed in (by the user, for a given position on the bike, for a given bike/tire/tire pressure combo), a complete equation of motion for the bike can be solved. Note that is a COMPLETE solution, not an estimate. Yes it is indirect in nature, but so are most measurements in real life.
If you are uncomfortable with indirect measurements, then you had better stay far from pure science and engineering: there are many physical constants, printed on the inside covers of many physics and engineering books, for which the measurements to arrive at these constants were indirect. For instance, the last time I checked Heisenberg's little principle of uncertainty pretty much makes it impossible to measure directly the mass of an electron.....yet we have such a value that all of science abides by. And you should be doubly troubled by Schroedinger's Equation and Einstein's wonderful little expression....E^2=m^2*c^4 + p^2*c^2....because those wonderful things weren't even derived: they were just thought up, without proof. And your whole GPS world rests on their foundation!
At any rate, the equation of motion that iBike has defined is complete and sound in terms of physics.
As for your ideas of "consitency," they're incorrect. Accuracy is what all power units strive for, and all of them fall somewhat short of achieving absolute accuracy. That is ok, however, since the difference between absolute accuracy and accuracy is typically an offset that can be characterized. Your power equation is not accurate. Variations in air speed, alone, along the course of one of your rides will provide a nearly randomn variation in "readings." Also your treatment--or lack thereof--of mechanical and aero drag will cause at least an offset with some linear and squared dependence. At best, your power measuring technique, might provide a semi-decent first order guess.
+1 Yup. Well said. Cees occassionally posts a pretty good model of the equation of motion for a cyclist, including all the terms such as rolling resistance. That is the basis of the iBike measurement and it's a good one.alienator said:
John Swanson
www.bikephysics.com
John, what is the equation for iBike? You seem to know. My equation covers all the aspect tghe Bully mentioned. What are the differences with my equation or similar?ScienceIsCool said:
I have yet to see any real proof. I am not convince iBike is worth the money.
Keep it civil. Take the high ground, unlike the alienator.
Here's a good example of a standard model of a cyclist's equation of motion that, based on what I've read about iBike, is what they are using. Using a rundown test and plotting acceleration versus velocity will give you most of the constants needed. The rest can be measures in real time, such as air pressure, incline, velocity, temperature, humidity, etc, etc,digi-bike said:
http://www.analyticcycling.com/Glossary_Disc.html
John Swanson
www.bikephysics.com
I do not respect you, most people do not respect you. You do not respect yourself.alienator said:
If you knew how to read, you would see that the assumption was a simplization, and thus a limitation. You brain is so twisted, you cannot see straight. I even provided an improved equations to improve accuracy. It is limited, but I worked with what I have, just a GPS.alienator said:
First, you are an idiot. Next, an accelerometer does not measure altitude, it measures acceleration. A GPS receiver measures acceleration, along with velocity and jerk. A GPS is not as accurate, but consistency is all that matter.alienator said:
iBike does have the advantage of accuracy with slope, air speed and altitude (used to derive slope). But mechanical drag is just entered into the equation by the user, same as my equation (read the post).alienator said:
GPS recievers use Einstein's theories daily, even time theories.alienator said:
What is that "defined" iBike equation. I keep hearing about it as being complete, but that is all is ever see.alienator said:
My equation take into account mechanical and areo drag, they are user defined constants. It has limitations, but it is consistent with my bike and cheap. Read the post.alienator said:
Consistency is what truely matters. Accuracy is nice, but not neccessary for performance training.
I am sure you will respond with religous insults and take the LOW road, as usual. Your mother never taught you manners.
digi-bike said:
You mean, take the high ground like you? Heck fella, you Christian types really are what you preach, ain't ya?
I seem to be readin' a lot of name callin' comin' from Jesus-blessed lips, brother. All the while, though, I see very little coming from you in terms of scientific knowledge or thought. Wow. Impressive. I can see how you made it to the lofty heights of "GPS Engineer."
FWIW, no one owns a given equation. See that's part of that whole science thing. Scientists are free to go out and use proven expressions. Sorry, it it's different in the rarefied air of GPS engineering.
I'll give you two forms.
The force equation for a bicycle is:
F=(P/V)(1+Sine(2RT))-(1/2)CdRhoAV^2-MgCrrCosine(S)-MgSin(S)
where
- P=power
V=velocity
R=crank angular velocity (rad/sec)
T=Time
Cd=aerodynamic drag coefficient
rho=air density
A=correctly calculated frontal area
M=mass of bike/rider system
g=gravitational acceleration constant
Crr=coefficient of rolling resistance
S=slope of road at time, t
In the above equation time, T, is some interval, t(f)-t(b). Note that the second and third terms are specifically dependent on time, and that in reality, the whole equation is dependent on time since the leading coefficient, P/V, is itself dependent on time, since power output will of course vary with wind direction, wind verocity, aerodrag, Crr (which varies with road surface).
If you prefer, this pdf provides an alternate formulation, which you will find, only varies in that it uses air speed specifically.
There. Hate me, but don't hate the science. Oh, BTW....the science proves you wrong.
digi-bike said:
Actually, your attitude and the implied ethic of your religion, is an insult to me, as well as a lot of people, including honorable religious people.
Your equation does not take into account any time dependencies or, if you want, spatial dependencies. Rolling resistance changes with road surface, and as everyone else realizes, air speed changes with time. Your equation, again, accounts for none of that. In fact, difference in air velocity can be so large as to blow your assumptions out of the water, especially when aerodynamic losses are the largest losses experienced by the bike/rider system. That you minimize them or "average" them out by saying the sum of air speeds over a closed course is zero....well, that plainly shows that you don't understand the highly time dependent nature of a bicycle's equation of motion. A person doesn't even need GPS to do what you do. A person can do that with a calculator and a stopwatch.
It is apparent that you will not understand why your idea won't work and is not accurate. I suspect your grasp of physics is really poor, especially since two people with physics degrees have now explained things for you, and in both cases, you failed to understand.
digi-bike said:
How little your pious self knows. Does you think your self-righteouness looks good on you?
digi-bike said:
A fatal limitation which can tell you nothing, especially if you're training.
digi-bike said:
Who said an accelerometer measures altitude? Not me. It does measure slope. What is jerk and how do you measure it? Did you have to take English when you were in school? Consistency? Accuracy? Your method has neither.
digi-bike said:
Wow. Apparently you missed the point, which was that indirect measurement is the usual practice in scientific measurements and that, in fact, some things are just thought up.
I'm willing to bet the things you don't know about such theories is staggering. Time theories? What are those, pray tell?
digi-bike said:
Your "estimations" won't even be in the ball park for training. Not even close. The fact that you don't understand the things you averaged out or "estimated" only points that out more.
digi-bike said:
Oh, dear. You brought mommy into it.......how elevated of you. Christians the world over must be proud of you. I find the most astonishing or remarkable thing about self-professed "religious" people is the amount of hypocrisy they need to exercise to get through each day.
digi-bike said:
Why do you want to know my job? Is there a reason to care about you that much or about what you think?
You love me? Oh, how wonderful. Do you want to be the top or the bottom? Iffin' you're going to be the bottom, I hope you like it bare bottom, 'cuz I like the feel of flesh against flesh. I don't want nuthin' coming between my skin and yours, sailor!
This whole thread seems to have veered far off topic, and I really have no inclination to waste my time wading through the garbage in here. But I gather that (some of) the debate is about the accuracy or lack thereof of the iBike system. On that topic, it is easy to see the flaws in their approach: Frist of all, aerodynamic drag depends a lot on the rider's position. Since iBike seems to accept only one calibration for that (and it would be impractical to have more than that anyway), its readings will also only be somewhat accurate for that one position. Moreover, its airspeed readings also cannot properly take into account the effects of drafting, in particular on the rider in front (whose drag coefficient will also be reduced when somebody rides closely behind).
Overall, iBike seems like a low-cost and easy-to-implement solution to the problem of power measurement, but you get what you pay for: Don't expect the results to be accurate to within less than maybe 10to20% or so, at best.
P.S.: I should probably add that, for the reasons stated, iBike will not only have problems with accuracy, but also with consistency, a potentially even more serious flaw.
Overall, iBike seems like a low-cost and easy-to-implement solution to the problem of power measurement, but you get what you pay for: Don't expect the results to be accurate to within less than maybe 10to20% or so, at best.
P.S.: I should probably add that, for the reasons stated, iBike will not only have problems with accuracy, but also with consistency, a potentially even more serious flaw.
