Quote:
Originally Posted by stugray
In this case (simple inertia dyno) the only things that are measured are Drum RPM, and time.
Using those two numbers (drum angular velocity) and elapsed time (∆t), you can derive the power.
Power is the rate of doing work.
The work done is the change in rotational energy of the drum.
If you start from a stop, w = 0 so rotational kinetic energy is zero
The power is the final kinetic energy of the drum (in joules) divided by the time it took to spin the drum to the final speed.
So rotational kinetic energy is E = 1/2*I*w^2 ( where E is in joules, I is drum rotational Inertia in kg-m^2, and w is angular velocity in radians/second).
So total energy is: (Efinal - Estart)/time-to-reach-Efinal (in seconds).
This will give an average "Joules per second" which is the definition of Watts.
Note: this will only give you average power over the whole "pull" and will not provide the nice dyno charts we typically see which represent instantaneous power across the RPM range.
This precisely how I calclulated an average hp draw of spinning just the driveshaft.
for the driveshaft example:
Rotational inertia of the 30 lb driveshaft (@3 inch diameter) is : 0.01974 kg.m^2
wstart = 0 RPM, wfinal = 5600 RPM
5600 RPM = 35185.8 radians per minute = 586 radians per second
The energy stored in the driveshaft = E = 1/2 * .01974*586^2 = 3389 joules
If it took 16.2 seconds to spin the driveshaft from zero to 5600 RPM, then the 3389 joules were spread out over 16.2 seconds so the power (energy per unit time) is: 3389 joules/16.2 seconds = 209.2 watts.
209 watts = .28 horsepower.
Side note: So since the headlights consume more than 200 watts, I would argue that driving with your headlights off vs On, would make more of a difference than removing the entire 30 pounds from the driveshaft (NOT accounting for the reduction in overall vehicle weight). SO I am sure to start whole new flurry of naysayers with that one.....
So to be clear which of the following experiments would make the bigger difference:
Replace the stock driveshaft with a 15 pound lighter one (shed 15 pounds from vehicle weight AND 15 lbs from rotational mass at the driveshaft) and do a 1/4 mile run with the headlights ON
AND
Remove 15 pounds from the spare tire (shed 15 pounds of vehicle weight) and do a 1/4 mile run with the headlights OFF.
Which run is faster?
The one where you removed the weight from the spare tire, and drove with headlights off.
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Can't seem to step away since this insanity keeps going.
Can you please tell me why you are absolutely fixated on 0-100mph acceleration test? Of all the tests to try and prove your opinion that is the best test (0-120mph would be better or better yet, a standing mile?) Who is talking about drag racing here except everyone that is trying to say driveshafts aren't a good mod for drag racing? Are we seriously still having this discussion? What you are doing is akin to determining the worth of an aerodynamic aid based on drag at 15mph. It just doesn't make sense. Sure your numbers aren't wrong but you are making the wrong comparison. Please tell me why you aren't seeking to disprove your own hypothesis before rattling off these inane examples? It's seriously annoying. You are myotically fixated on this singular example and that is horrible science.
All these false choices you present are ridiculous too. Why can't someone remove the spare, turn off their headlights and have a lighter driveshaft? Do you go into every exhaust thread and tell people they are wasting their money, that mathematically, a turbo kit is the only way to go? $1000 for 5-10whp or $4000 for 100whp? Easy! Science! Drive on a cold day and make more power than that $300 intake. Science! One nerd to rule them all! Nanu nanu!