Aluminum driveshaft - Prove me right- please!
#21
"They" probably are. The math is a bit more complicated than that. Even a lb at the wheels is less than about 1.5 lbs deadweight in most cases. I'd guess that a "gain" at the wheels due to reduced driveshaft rotational inertia might be closer to half of one percent (measured in 4th gear). Different U-joint operating angles could account for more than that.
Norm
Norm
So ... I was wanting to look into how to make my ride quicker and with all the 'opinions' on both sides, I found this writeup. Which makes the absolute most sense:
http://www.w8ji.com/rotating_mass_acceleration.htm
Basically, drive shaft weight means close to nothing in terms of rwhp. Same with quite a few other 'rotational mass' changes ... except wheels, apparently wheels with less weight on the outer edge, either by tire or rim weight a benefit.
Nice.
so, Norm, so it seems you're near dead-on, it's hard to argue with physics. :-)
Last edited by ryan4096; 12-27-2009 at 01:13 PM.
#25
Less MOI = less torque lost rotationally accelerating it. BUT . . . at top speed, both forward vehicle acceleration and powertrain rotational acceleration are by definition zero. At zero rotational accelerations, everything that rotates is now at constant speed, so the rotational MOI effect also drops out. IOW, the MOI effects will not affect top speed.
Less weight = minutely less rolling drag = a theoretical gain in top speed. But it'll be tiny. 10 lbs weight difference times a rolling drag coefficient of something like 0.02 is 0.2 lbs less drag (out of several hundred lbs or so total drag @ 140+).
Friction loss effects from U-joints that are either more or less efficient and/or operate at either slightly greater or slightly smaller angularities. I'm betting on this being potentially the greatest effect, and in the absence of reliable U-joint data this could work either way.
Norm
Last edited by Norm Peterson; 12-27-2009 at 03:11 PM.
#29
That article deals only with the MOI effects, which are finite but quite small as far as acceleration is concerned (and depend somewhat on axle gearing and tire size, actually) and are nonexistent for top speed.
Norm
Norm
#30
I am going to recall my physics class, but that rule of thumb of about one pound lost from rotational mass is 3 rwhp is a complete lie.
If you ever sat in a spinny chair and spun with your legs and arms out, then brought them in your angular velocity will increase. The closer you bring that weight (of your arms and legs) towards the center axis you are spinning around the faster you will spin.
In other words to build the most successful drive shaft, locating all the mass of the shaft closer to the center axis (of which the shaft spins about) will reduce rotational inertia.
This means that the shaft could weigh as much as the stock shaft, but if it was denser it would inevitably give you more power to the wheels.
In case anyone cares, Force = mass x acceleration which is equivalent to Torque = inertia x angular acceleration. Inertia is complicated to calculate but basically it is a function of mass and radius. If radius is zero then the inertia zero, in other words the closer the weight is to the center axis the less the rotational inertia.
also why a crank shaft has such a goofy shape.
I hope this statement to end all debates but I doubt it will
If you ever sat in a spinny chair and spun with your legs and arms out, then brought them in your angular velocity will increase. The closer you bring that weight (of your arms and legs) towards the center axis you are spinning around the faster you will spin.
In other words to build the most successful drive shaft, locating all the mass of the shaft closer to the center axis (of which the shaft spins about) will reduce rotational inertia.
This means that the shaft could weigh as much as the stock shaft, but if it was denser it would inevitably give you more power to the wheels.
In case anyone cares, Force = mass x acceleration which is equivalent to Torque = inertia x angular acceleration. Inertia is complicated to calculate but basically it is a function of mass and radius. If radius is zero then the inertia zero, in other words the closer the weight is to the center axis the less the rotational inertia.
also why a crank shaft has such a goofy shape.
I hope this statement to end all debates but I doubt it will