Power Calculation BS Tech by amateurs, you've been warned.

HP Loss is
@Torspd Not static. Depends on the drivetrain loss per drivetrain of whichever vehicle. The percentage loss to go from 30 hp at the crank to 4 hp at the wheels is roughly a 90% loss. That isn't happening unless your e-brake is pulled or your foot is on the brake pedal.

Average of 15% loss, equates to roughly 25.5 whp, gain.

So whichever fan you start with, there will be a power notice at the wheel. Having a before and after dyno always helps.

Dragons Taco: I'm curious here. Why is there a percentage loss at all? I mean, I understand that the drivetrain will rob more power at speed, but that should be linear for a given speed right?
If at 100mph the drive train takes 50hp, if you make 10 more on the crank, we should see 10 more at the wheels since the 50hp has already been robbed and the drivetrain hasn't taken any more power?
Teach me!



Torspd: They are called parasitic losses. Due to the energy it take to move the masses of all of the parts going to, and including, the wheels. Overcoming the frictions of fluids, gears, and tires, takes energy (power). Depending on how efficient(gears & fluids) and how light (mass) all of those components are, can increase or decrease the drivetrain losses.

So you always have to start out with how much the BHP is, to then compare against a chassis dyno. Finding what your % loss is, at a 1:1 (4th gear usually) ratio.

The number is all based off of acceleration. Takes more energy to get the vehicle moving, (fighting against inertia) than it does to maintain a speed or course.

Dragons Taco OK, just one more since I'm stuck on this in my head.
What I'm asking is why is it calculated as a percentage? Maybe there is a physics answer that is beyond me here, but bare with me.
My confusion is with the running gear itself, and how much HP it takes to simply accelerate it to say 100mph with the wheels jacked off the ground.
The power required would be the same regardless of the HP of the engine with all other things being the same. Same lubricant, same moving mass, same friction of bearings and air.
A dyno itself has mass and friction (force) itself to overcome, which is what is used to calculate the power created during a run. It must always be the same and accounted for in order to provide an accurate result.
The amount of power a vehicle or motor on a dyno doesn't influence the opposite force of the dyno (except for the weight of the vehicle , but I digress) so why should it influence the drivetrain?

Now maybe I can answer it myself. Since this changes with speed, is it just easier to measure it as a percentage instead of developing a curve for every drive train / temperature / humidity ?
My entire thought process is a well sorted vehicle with a clutch or locking torque converter, so there again the number of potential variations may make it impractical to figure it otherwise.

@Samboats Because Math...

@yota243 I kinda see both sides and it should start to plateau eventually but at the same time it would never just be a flat number in any given circumstance i.e. if it were a flat 25 hp and you had a 20 hp motor it would never move. Friction and inertia and other physicsie words apply.

Dragons Taco: I'm gonna have to make a thread for this.

 

Its should be asymtoptical (thats prolly not an actual form of that word but in math if u divid something in half and keep doing that to infinity it will never reach 0 but it will get really close) the driveline power loss curve should be similar. This does not take air drag into account this would be strickly for a dyno, if air drag was factored in the curve would most likely grow exponetially.

 

@Lucario Runner

 

@kuffer14

 

All drivetrain gears in our trucks(aside from the reverse gear) are helical cut gears, and exert force in multiple directions. When the gears are loaded, it will exert rotational as well as an axial force on the driven gears. This is the reason for thrust bearings in the transmission, and the conical style carrier bearings in the rear diff. While they are always the same parts, lubricated with the same oil, force exerted will translate to force being wasted in axial torque, leaving a smaller percentage of the available torque to be translated into rotation.


It's late, and I hope this makes some sense

 

Here is one dyno chart where you can see the transmission/wheels power loss (the lowest graph). So the crank hp (kW) is the wheel hp + loss hp.

It's 2011 Skoda Octavia 1.6 TDI (77 kW) with some mods.