Quote:
Originally Posted by swamp2
I think that is pretty accurate. Yes, pure LOSSES, not inertial effects. A rough accounting of it for a RWD vehicle would be:
- Auxiliaries: 1% (will be getting lower with EPS and in general with more electrically and mechanically efficient components along with smart shut downs)
- Manual transmission: 3% (both automatics and dual clutch a bit more, but losses are getting smaller with both in each successive generation)
- Differential: 3% (maybe a bit high but the power does have to make a giant 90 degree turn..., good ones like the M4 have a lot of cooling fins too!)
- Hubs and axles: 5%
That's 12%, thus 10-15% excluding tires to cover a range of more and less quality and efficiency of the components. And again, for clarity these are rpm/speed dependent, as such losses clearly are not a fixed percentage across all gears and speeds. The fundamental (simplest) nature of friction is a non-linear force as a function of velocity. I've provided the formula for tire losses and they grow non-linearly with speed, prob. up to about 30 hp for a car like an M4 at top (delimited speed). That's about 7% of 440 hp, quite significant. |
What about the results from Rototest that show drivetrain losses in the 6-10% range for the E9X?
Aren't "auxilaries" or "ancillaries" losses included in SAE and DIN power ratings nowadays?
Isn't 5% a bit much for drive wheel hubs and axles?
As for tire losses, there seems to be a discrepancy in the number shown here and the one used in the top speed thread. To calculate top speed, the losses of all four tires needs to be considered. However, to establish losses on a 2WD roller type dyno, only the loss of the drive axle needs to be considered (2 wheels). Yet 30hp is used in both scenarios.