The real benefits of lightweight pulleys (quantified)

[Subsonic]

Well, that has been a fun weekend, thanks all for your contributions to this discussion.

Originally Posted by Ultramaroon

@Subsonic, your units for inertial moment are incorrect. Should be in kg*m^2. It's only in notation, though. The numbers converting from imperial to SI are good so it must've been a typo. I appreciate your statement to the effect of neglecting the contribution from the other rotating masses. I did the same in my calcs which I never finished.

Thanks for that, I’ve corrected the first post to change to kg.m^2. I think the numbers stay the same though ;-)

Neglecting the other masses and all the other variables is what makes physics possible to understand. If you’re OK with the concept that you can leave out all those variables that only make the case worse for the lightweight pulley, and you still find that you only have an improvement that is too small to measure, I think it’s safe to draw a conclusion if the numbers are small and only get smaller the more maths you do. I really liked 86TOYO2k17’s Atlas stone analogy as well, thanks for that.

Actually, when measuring my crank pulley the other night when measuring mine in response to nickypoo’s argument, I was surprised how hidden it is normally. A more useless place to add bling for bling’s sake I can’t imagine.

I don’t really want to poke any trolls further, but I also read in here that someone feels their car revs “much” quicker with lightweight pulleys. Assume when clutch is in. This doesn’t feel like it would bear up to a simple test either. If we knew the moment of inertia of the Flywheel, clutch pressure plate, crank, rods and valve gear including chains, cams, belts and factory pulley and added them together, we could take off 0.0072 kg.m^2, the difference between the two types of pulleys and see if it means much. Something tells me the flywheel alone is going to be pretty significant. It is a flywheel after all, whose job it is to retain energy.

Please excuse me if I don’t follow this thread for a couple of weeks, Heading over to the USA tomorrow for holidays. Woo hoo!

[Ultramaroon]

Originally Posted by Subsonic

Please excuse me if I don’t follow this thread for a couple of weeks, Heading over to the USA tomorrow for holidays. Woo hoo!

Have a wonderful and safe time traveling!


Please don't hold our Douchebag in Chief against us. There are still many nice things in the U.S. that he hasn't shit on.

[Kodename47]

Originally Posted by nikitopo

I'll say it again and for the last time. If you do a dyno on 5th gear you won't see any gains. I mentioned this already many times. The rate of the rpm increase is much slower and you'll lose all the advantage of the lighter rotating mass.

Is that even a thing? How is rate of acceleration going to change the power output? Power output is the input into change of acceleration, not the other way around.

[nikitopo]

Originally Posted by Kodename47

Is that even a thing? How is rate of acceleration going to change the power output? Power output is the input into change of acceleration, not the other way around.

Yes, the rotational inertia (RI) of an object is not fixed and it depends of how fast you spin this object on a fixed period of time. You can check here a site that calculates the RI (flywheel section) and what are the needed parameters: http://hpwizard.com/rotational-inertia.html

[Kodename47]

Originally Posted by nikitopo

Yes, the rotational inertia (RI) of an object is not fixed and it depends of how fast you spin this object on a fixed period of time. You can check here a site that calculates the RI (flywheel section) and what are the needed parameters: http://hpwizard.com/rotational-inertia.html

Put in the calculations for the type of values you'd expect for a crank pulley and the change in values in gear are minimal.

[nikitopo]

Originally Posted by Kodename47

Put in the calculations for the type of values you'd expect for a crank pulley and the change in values in gear are minimal.

Yes, I am aware of this. Thank you.

[tomm.brz]

But you still think 8whp is a reasonable and possible gain for lighter pulleys?