View Full Version : Horsepower loss due to driveshaft weight?


htownws6
07-01-2010, 09:33 PM
Has anyone ever calculated what the HP loss would be on a heavier driveshaft over say...a stock driveshaft?

b.d.d.
07-01-2010, 10:44 PM
Depends if you are talking true reciprocating mass where one side is weighted more than the other, or more of what I would call static mass where everything is equal.

I've experimented with heavier flywheels and / or lighter driveshafts with the intent of maintaining torque when a load is aplied or when the gas is eased off and back on again. Pretty much a waste of time.

Some things make sense on paper, but fail to deliver results on the track. In theory a lighter driveshaft would be better, but I have doubled the weight of my DS and not lost or gained anything.

Adrenaline_Z
07-06-2010, 01:24 AM
Are you trying to determine static loss, or losses during acceleration?

I don't think you'll find any difference in steady state RPM, however
accelerating, or decel will be effected by the mass of the driveshaft.

It's such a small diameter that it may be negligible unless you're looking
for that last bit of performance from the driveline?

BlackScreaminMachine
07-06-2010, 08:28 AM
Simple answer would be to say that a Stock Aluminum DS may accelerate quicker or by that thought decel quicker but will break when FORCE is applied at launch.

Consequently a 3" Steel DS may sap some power compariatively but virtually guarrenteed not to break in most situations.

I would rather lose 5hp and not break and have the potential to win the Race or Event.

jimmyblue
07-06-2010, 12:40 PM
Horsepower loss is bullshit. You feel that driveshaft getting
hot? No sir. It's only storing energy in a form you don't
want.

Calculate your rotating kinetic energy at trap speed, divide
it by your ET, and that's how much "horsepower" rolled up
went into rotary rather than forward motion across the 1/4.
Lot of messy detail to get it reasonable but you could call
it a hollow cylinder of 1/8 wall, measured diameter, holding
all the scale weight. I'm sure Wiki would turn up the formula
for moment of inertia and rotating KE for a cylinder on-axis.

FSAE_Junkie
07-07-2010, 06:58 PM
How much torque in what gear? That is the important question. The horsepower that makes it to the wheels from the engine depends upon how quickly you are accelerating the driveline or what gear you are in - along with how much power you have and how much your car weighs.

I can tell you this, changing driveshaft weights or Moment of inertias will have pretty much no effect in any gear other than 1st. Even in 1st gear, I would say its around 1 to 2 lb-ft gained by dropping 20 lbs or so.

Losing weight in the flywheel would be much more advantageous. In 1st, the flywheel will be weighted roughly 10 times more. i.e. removing .1 kg-m^2 of inertia from the flywheel will be like removing 1 kg-m^2 from the driveshaft. Then you have the fact that the drive shaft inertia is only about .05 kg-m^2 worth of inertia and you realize that pulling about 2 lbs out of the flywheel will be like completely removing ALL effect of your driveshaft out of the system.

Here is some hard numerical data for changes in flywheel weights from the following: http://www.ls1gto.com/forums/showthread.php?t=356153

So, I have spent the last few days pondering on what clutch to buy and what flywheel to go with. So I figured I'd compute what the torque gain for each gear would be to see if it'd be worth going with the lightweight flywheel.

So, I drew up a basic flywheel in Solidworks and adjusted the thickness until it weighed 28 lbs and then found the polar moment of inertia. I did the same for the 18 lb flywheel. They were aproximately 0.17 kg-m^2 and 0.11 kg-m^2.

Here are my results (using peak torque of 410 ft-lb):
2nd and 3rd columns are how much torque (lb-ft) the respective flywheel eats up.

Gear | 28 lb Flywheel | 18 lb Flywheel | Torque Gained
1 | 41. | 27. | 14.6
2 | 20. | 13. | 7.1
3 | 9.6 | 6.2 | 3.4
4 | 4.7 | 3.0 | 1.7
5 | 3.3 | 2.1 | 1.2
6 | 1.5 | 1.0 | 0.5

This is assuming 3800 lb race weight (yes this matters). Peak torque of 400 lb-ft. 3.46 final drive.

If the final drive goes up, weight goes down, or you make more torque, the gains from a lightweight flywheel will be more noticable. For example, just 3.91's on the same setup would yield a 2.1 lb-ft gain in 4th (instead of only 1.7 lb-ft).

As you can see, the gains are most noticable in 1st, 2nd, and 3rd. The equivalent hp gains in each gear will be very similar to each torque gain.

For the money ($50), this isn't a bad deal.

Personally, I was just worried about driving on the street with a sticky 6-puck clutch and a healthy cam, so I went with the stock weight flywheel.

Hope this helps other people make their decision on which way to go.