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The nerd in me has reared it's head again

Spaceclam

Spaceclam

Cruisin' Guzzisti
Joined
Nov 8, 2008
Messages
472
Location
Chatsworth, CA
So i read once that motorcycle news clocked a stock G11, with a 180 lb/ish rider, 0-60 in 3.9s, i think it was. In real world terms, that sounds reasonable. Slow for a bike, but gives a carrera a run for it's money.

But i got to thinking. 70 hp at the rear wheel, how is this possible, from a physics standpoint. So, i whipped out my calculator, and did some math.

520 lbs + 180 lbs rider =700 lbs, or 317.5kg

60mph=26.82m/s

if V(final)=V(initial) +AT, then 26.82=0+3.9A, acceleration = 6.8776 m/s^2. (That's about 2/3 of a G force laterally)

Distance=.5*A*T^2 so the distance traveled in 3.9s is 52.3m. So total joules = 52.3*317.5*6.8776, =114207.3J,

spread over 3.9s is 29283.9 joules/second (watts)

1hp is 740 watts, so that's an average of 39.5 hp. Basically, only 57% the energy measured at the rear on a dyno wheel turns into actual motion in this instance.

I know a tremendous amount of energy goes to squat the suspension, there's a slight delay for shifting gears and starting off, and the tires flex a lot, and some wind resistance towards the end. But are these things really THAT inefficient?

Just to put things into perspective. If the engine is running at 10% efficiency (90% loss) through the drag, there is a 10% loss in the drive train, (90% efficiency) and there is a 43% loss (57% efficiency) of power to the rear wheel, which leaves 39.5hp, then the transformation from gasoline to movenemt has an efficiency of about 5.1%.

Of course these numbers are too low, because while the clutch is being operated, power is NOT being transmitted, but time continues to elapse, lowering the average power.

But still,
Astounding. :ugeek:
-Clam
 
Clam: The guy who pulls the trigger has a huge impact on how quickly something accelerates. One who knows what he's doing will be much quicker than one who doesn't.
FBC
 
I read somewhere a Guzzi racer wrote that there is a 16-18% drivetrain loss on shaft drives, depending on its wear level, but a chain drive loses 8% with a correctly tensioned chain, going up to 25% before re-tensioning/renewal.

It could always be that Guzzi make up a number and add 20% for the engine output, much like the Japanese do :D
 
Another thing to consider is that the engine spends a good amount of time (when you're only talking 3.9 seconds) at much lower rpms than where the peak HP is. Also, how well the rider picked their shift points near the peak. I had a friend who raced on a drag racing circuit and he regularly hopped in my car and would beat my best time by 2 tenths. It would piss me off all the time :)
 
You are averaging it out over the 3.9 seconds.
While it is not that simple, it does make sense that the average power would be 1/2 of the maximum power.
Oh, and drive train losses are already accounted for in the rear wheel dyno numbers. My Griso posted about 80 hp at the rear wheel (before the pipe and PCV), I know those numbers can vary depending on multiple things but if the motor makes about 100 hp or so then 80 at the wheel makes sense.
 
I only used the drivetrain efficiency when calculating the efficiency of the fuel's transformation into kinetic energy.

It's not part of the equasion for acceleration, as we are SOLVING for the total power output, based upon acceleration, distance, and time.
 
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