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Old 11-30-2012, 06:52 PM   #61 (permalink)
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Originally Posted by goingtoscotland View Post
Man you guys are getting terrible gas mileage.

I average 32mpg
Since then I have really only been averaging 28mpg.

I did see 35 on one pure highway leg, though, and I was averaging 85-90mph
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Old 11-30-2012, 07:28 PM   #62 (permalink)
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Why is 80 mpg needed in the first place? 45 is plenty for me and that's what I get.
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Old 11-30-2012, 07:48 PM   #63 (permalink)
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You know speaking of aerodynamics , the Myth Busters actually created a coating for a car that had the form of golf ball shaped holes on it , sure the paint made the car really heavy so they started messuring the MPG once the car averaged a certain speed since weight at certain speed does not affect MPG if maintained .
Uh, weight most definitely affects the mpg even at a unchanging speed. At a steady speed an engine only needs to generate enough force to equal the friction applied to the vehicle. The two types that matter are air resistance and rolling resistance, the equations are, respectively:

.5(Cd)(A)(rho)(velocity^3)=Pd
Where:
Cd=coefficient of drag
A=frontal area in cubic meters
Rho=density of air in grams/cubic meter
Pd=Power of drag, in Watts

RRC(m)(g)(v)=Prr
Where:
RRC=coefficient of rolling resistance of the tires
m=mass in kg
g=specific gravity in meters per sec per sec
v=velocity
Prr=power of frictional rolling resistance in Watts

If you care to you can convert Watts to hp pretty easily, and see the minimum horsepower the engine must use to keep the vehicle at that speed.

But, the second equation has mass in it, meaning the heavier the vehicle, the more rolling resistance it has, meaning the engine uses more fuel to keep it at speed. Think about rolling our front wheel vs a truck wheel, which is easier? Why?

So what variables can we change to get better gas mileage? Surprisingly pretty much everything. Lets start at the top:
Cd- Bikes suck compared to cars for this number, the busa, which was highly wind tunnel tested comes in at .561, expect .6-.7 for our bikes. What can we do? Smooth everything out. Get mirrors like pig spotters instead of ones that stick out into air, get rid of any lip that sticks out or minimize it, this would be bolts, screws, hardware, pegs, etc, etc. The object is to make the bike as slick as possible, it's currently awful.

Frontal area (A)- well, this is related to Cd, but different. Removing the mirrors will reduce the frontal area, so will dropping the bike 2 inches, and you guessed it, being in full tuck. Having smooth integrated turn signals will reduce this as well.

RRC- surprisingly a lot of tire manufacturers have these numbers available to the public, especially those who have the best tire out there. Michelin prides themselves in their low RRC on their tires. For a bike, it might not be worth changing tires for this though, low RRC tires have very stiff sidewalls, are a hard compound, and usually have very little tread for water. On a car there is at least 4x as much tire surface touching the ground, on the bike, bad tires means a wreck. Your call.

Mass- lose some damn weight, whether it's on you or the bike. This ends up conflicting with Cd eventually, does going naked mean more or less total drag, will the equations collide? and at what speed?

Velocity- the most important factor! Yes, going faster means you end up running the engine for a shorter amount of time, but the frictional force increases as you speed up so you use more gas at a higher speed then at a lower, again, these equations collide, and at that collision, that's the point of your best gas mileage.

Do some math if you really want to get into it.
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Old 11-30-2012, 09:43 PM   #64 (permalink)
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Oh, and I didn't even mention the mechanical aspect of the bike. High flowing filter is the most simple thing. Going more in depth, well, it gets a little silly very quickly. To reduce bike on bike parasitic loss you can get ceramic bearings in the wheels and transmission. Woodcraft sells both of these things. Past that you can go into porting the heads for better volumetric efficiency, changing the cam, and well, all sorts of shit like that.

One thing to make sure though, is that you aren't incurring loss due to brake drag.
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