Gas Mileage And Sprocket Teeth

[skymasteres]

I totally agree that the single most moniating factor in fuel economy when riding a motorcyle is the style in which the rider applies the throttle and their average speed.
There are several factors that have an effect on fuel economy and their dominance can shuffle depending on where you are in the performance envelope.

Case in point, the interternal friction of the engine increases with the square of the RPM, but this loss is dwarfed by the cubic function of power required to overcome aerodynamic drag. (Power= 1/2*(rho)*(V^3)*A*Cd) Where Rho is the air density, V is how fast you're going, A is your cross sectional area, and Cd is your drag ceofficient.

Brisk acceleration from a stop uses more fuel as you are demanding more of the engine. From a pure efficiancy standpoint, you're maxium steady state fuel economy will generally be acheived if you are operating the engine at it's torgue peak. This is usually where your break specific fuel consumption is lowest with respect to the power produced.  The trick, is gearing so that your typical cruising speed is in that range.

The real question becomes, if you do all of the figuring to gear for max economy, is it going to matter when over 70 or so the aero dynamic drag is the dominating factor?...

[Capn Ron]

"Riding an FJ for economy is like going to a prostitute for a hug"

Noel

But some of them are REALLY good huggers!   

Cap'n Ron. . .

Is that what Ron White called a "mouth hug"?   

Ewwww...I'm with BMinder...wouldn't ever try that out with a pro!   

Cap'n Ron. . .

[Capn Ron]

There are several factors that have an effect on fuel economy and their dominance can shuffle depending on where you are in the performance envelope.

(Power= 1/2*(rho)*(V^3)*A*Cd) Where Rho is the air density, V is how fast you're going, A is your cross sectional area, and Cd is your drag ceofficient.

Looking at that equation...  Although air density isn't anything you can control, I felt a marked increased in air drag in Death Valley!!! Steady-state speed is by far the biggest factor...slowing down a bit helps the mileage.  You can't do much about A except duck your head behind the windscreen...which will likely lower your Cd.

My "road trip" car is a GEN 1 Honda Insight.  It has (or had when it was last produced in 2006) the lowest Cd of any production car at .25 and I can regularly squeeze 75MPG tank averages out of it with some driving tricks....my best tank was 81MPG...I went from Los Angeles to Northern Utah without having to stop for fuel.  Some owners of these cars have replaced the side-view mirrors with rear-looking bullet cams and an LCD screen in the dash to reduce drag even further!

This is all good "bench racing", but I don't ride the FJ for fuel mileage...at 45+ that's close enough for me!

Cap'n Ron. . .

[FJ_Hooligan]

I remember reading a few articles in the mags back in the '80s on motorcycle aerodynamics.
The things that stuck in my mind:
At 180mph, the fairing on the Yamaha Venture would have enough lift to pull the front end off the ground (I said it was an old article).
Motorcycles have the areodynamic coefficient of drag of a brick.
The rider and exposed wheels are the largest contributors to drag.
It only takes about 12 horsepower to drive the mechanicals (engine and drivetrain friction) of the motorcycle at a speed of 150mph.
It takes over 120 horsepower to push the motorcycle through the air at 150mph.

[Capn Ron]

The Hooligan speaks the truth.  I found this chart that was compiled from the specs out of old Car & Driver and Road & Track magazines.

     Vehicle           Drag Coefficient (cd)
     Description       Low     Medium   High
     ----------------------------------------
     Experimental      0.17    0.21     0.23
     Sports              0.27    0.31     0.38
     Performance      0.32    0.34     0.38
     60's Muscle       0.38    0.44     0.50
     Sedan              0.34    0.39     0.50
     Motorcycle        0.50    0.90     1.00
     Truck               0.60    0.90     1.00
     Tractor-Trailer   0.60    0.77     1.20

Motorcycles can have more Cd than a tractor trailer!  Fortunately, we only have a fraction of the frontal area.  In the case of my Insight, the FJ is about 1/2 the frontal area with about 2 - 3.6x the Cd...so the total drag on the FJ is about even to DOUBLE that of my car!

Cap'n Ron. . .

[movenon]

"operating the engine at it's torque peak"

This is why I think higher gearing doesn't really help the MPG deal. It sure helps with noise and comfort level though.
On an engine test stand (not real world) running at peak TQ is usually max fuel efficiency or  "Brake specific fuel consumption (BSFC)". So that's running at 7500 to 8000 RPM....... ? What do I do with the rest of the gears.....

George

[Pat Conlon]

.......From a pure efficiancy standpoint, you're maxium steady state fuel economy will generally be acheived if you are operating the engine at it's torgue peak. This is usually where your break specific fuel consumption is lowest with respect to the power produced.  The trick, is gearing so that your typical cruising speed is in that range. ......

I don't think this is a correct statement.
Operating a engine at its torque peak for the best 'mechanical' efficiency....yes, I can buy that.

But...
Operating a engine at its torque peak for the best 'fuel' efficiency?

Nope, I have a problem with that.

The best peak torque numbers on the FJ engine comes in at what....6800-7400 rpm?
My bike guzzles gas in the kookaloo zone.

[andyb]

Pat, the BSFC is almost always at the torque peak.  I somehow doubt that you're cruising at your torque peak in a lower gear (thus running a rational cruising speed, as higher speeds would increase the drag quickly as described) for any length of time.  I'd wager that you tend to be sitting near WOT when the needle takes a walk past the torque peak, in fact.....

Reducing frontal area noticably isn't that difficult.  Lower the bike at both ends, remove the seat, and put your chin on the gas tank.  If you're not gearing limited, your top speed will increase a surprising amount.


More importantly...  Guys, c'mon.  If you're that worried about mileage, let's worry about tire wear, chain wear, brakes, and so on.  Gas isn't the only expense to think of here.  But more importantly, why exactly are you using an inline 4 of well past a liter displacement?  Sell the bike, buy a small diesel hatchback.  Or trade down to an EX250.  I mean, come on!

[ribbert]

Lower the bike at both ends, remove the seat, and put your chin on the gas tank. 

This bloke's nailed the technique.