I was thinking about this on the commute home the other day. Why is it that a gas car weighing 4000#'s can hit 40 mpg but a 200# scooter only get 100mpg? I mean, I understand the mechanics and everything, just seems weird to me that the MPG difference isn't really that much.
The aerodynamics of the human body moving above about 25mph simply suck.
Aerodynamics.
Motorcycles have less frontal area, but they have terrible aerodynamics. They're all rough edges and turbulent wakes...
Read up on Craig Vetter, he explains motorcycle aerodynamics quite well
http://craigvetter.com/index.html
470mpg contest winner: http://craigvetter.com/pages/470MPG/Vetter%20Fuel%20Economy%20Contests.html
Now, take that same 4000 lb car, downshift it a few gears, and drive it floored at redline and see what the gas mileage is like.
That's how a lot of very tiny engined scooters and mopeds are run.
Not discounting aerodynamics, just bringing up another factor that can very much come into play.
Shhhh! If the EPA finds out that motorcycles could get better mileage at the expense of performance, they'll muck that up too! 
Also, Miles Per Gallon is a bizarre way of measuring efficiency. Gallons per One Hundred Miles makes a lot more sense.
Like this:
10 MPG = 10.0 GPHM
20 MPG = 5.0 GPHM
30 MPG = 3.3 GPHM
40 MPG = 2.5 GPHM
50 MPG = 2.0 GPHM
75 MPG = 1.3 GPHM
100 MPG = 1.0 GPHM
So each MPG jump seems like a lot, but in actual gallons used to travel the same distance, it's a diminishing return.
The rolling resistance of tires is the main drag until 50-60 mph, then effects from wind drag dominate in some kind of exponential fashion. Where as the rolling resistance kind of plateaus after so many miles per hour (15 - 25 mph, IIRC).
Like people have mentioned, aerodynamics have a great role to play, but there is something else. The size of the engine as a role to play in its efficiency. As a general rule, the larger the engine, the more efficient it becomes (with all else being equal). Some of the most efficient (power generated in relation to fuel used) are the giant marine engines. It's a scale thing (like you can't make a functionnal glow-plug engine if its too large, etc.).
foxtrapper wrote: Now, take that same 4000 lb car, downshift it a few gears, and drive it floored at redline and see what the gas mileage is like. That's how a lot of very tiny engined scooters and mopeds are run.
Try an open class 2 cycle MX bike in a Hare Scramble with deep sand and mud trail conditions! I think I've been as low as 10 mpg! I also know that on more than one occasion I've sucked the carb's float bowl dry climbing a long steep sandy hill. That is to say, the engine was consuming the fuel faster than gravity could feed it to the carb!
But for most bikes, aerodynamics are what prevent them from getting better mileage .
.
RossD wrote: The rolling resistance of tires is the main drag until 50-60 mph, then effects from wind drag dominate in some kind of exponential fashion. Where as the rolling resistance kind of plateaus after so many miles per hour (15 - 25 mph, IIRC).
Drag comes in two forms, induced and parasitic with each having very different change values relative to velocity.
A reasonable approximation though is that drag increases with the cube of velocity. So, if you’re experiencing 100 Lbs of drag at 50 mph, you’ll experience 800 Lbs of drag at 100 mph.
Beyond drag, I think a big factor is the engine’s “BSFC” best specific fuel consumption which is measured in terms of pounds of fuel burnt per hour per horse power. A really efficient piston engine such as a high compression V-Tec, will get you down around 0.45 and at the other end of the spectrum, say my Rensis rotary, you’ll be up around 0.53.
Bikes typically throw away BSFC in the interest of getting the maximum amount of power relative to displacement possible while being compact and simple. In other words, throw enough fuel at a problem and it’ll get solved.
when I ran my ZX2SR at a track day, it would get 8-10 mpg.
On a road trip, 35-36 was nomal, even got 40 a couple of times.
Does this equate to motror cycle mpg ? I don;tknow
Javelin wrote: Also, Miles Per Gallon is a bizarre way of measuring efficiency. Gallons per One Hundred Miles makes a lot more sense. Like this: 10 MPG = 10.0 GPHM 20 MPG = 5.0 GPHM 30 MPG = 3.3 GPHM 40 MPG = 2.5 GPHM 50 MPG = 2.0 GPHM 75 MPG = 1.3 GPHM 100 MPG = 1.0 GPHM So each MPG jump *seems* like a lot, but in actual gallons used to travel the same distance, it's a diminishing return.
We use litres per 100 kms in Canada.
Javelin wrote: Also, Miles Per Gallon is a bizarre way of measuring efficiency. Gallons per One Hundred Miles makes a lot more sense. Like this: 10 MPG = 10.0 GPHM 20 MPG = 5.0 GPHM 30 MPG = 3.3 GPHM 40 MPG = 2.5 GPHM 50 MPG = 2.0 GPHM 75 MPG = 1.3 GPHM 100 MPG = 1.0 GPHM So each MPG jump *seems* like a lot, but in actual gallons used to travel the same distance, it's a diminishing return.
It's.... just another way of showing the same thing.
In reply to Swank Force One:
Well, yeah, of course it is. But it shows that increasing efficiency is actually diminishing returns. Nearly every other country on the planet besides us use some form of "units of energy per a standard distance" as their official auto efficiency measure.
1988RedT2 wrote: Shhhh! If the EPA finds out that motorcycles could get better mileage at the expense of performance, they'll muck that up too!
Oh, the EPA has been into bikes for a while now. Catalytic converters, noise, etc
actually two reasons
1 Aerodynamics (not rehashing, it has been explained here well) 2 It has been mentioned with little attention, tuning. Bikes are tuned for performance.
Until the Honda NT700V
There was a gentlemen that made a 3 wheeled hypermiling car with a Honda VFR Interceptor engine in it and it was getting around 100 mpg with the stock sport tuning.
Aerodynamics. Modern cars have a Cd around .3, with the stuff EPA rated at 40mpg or more usually being in the high .2s. A bike is around 0.7. HUGE difference. And remember that while the weight may be 1/10th the weight of a car, the frontal area is more like 1/3.
I doubt BSFC is really a contributing factor for scooters. Many modern engines are very close in peak BSFC... just the better ones have a larger sweet spot. The scooter is way more likely to be operating at BSFC under cruise than most cars are (except the Prius, and probably Fusion).
Remember that many of those scooters have a really inefficient CVT or slipper-clutch setup.
Javelin wrote: Well, yeah, of course it is. But it shows that increasing efficiency is actually diminishing returns.
I don't get this. Going from 25mpg to 50mpg means your car will use half the fuel. Going from 50mpg to 100mpg means your car will use half the fuel. Each 100% improvement in efficiency, is a.... 100% improvement in efficiency. Where is the diminishing return?
I prefer the US method because its easier to do math backward to see how many gallons of gas I will use. Going on a 360 mile trip, and your car gets 30mpg? thats 12 gallons. Going on a 360mile trip, but your car gets 3.3gallons/100 miles? thats, uh... (3.3/100)*360
ProDarwin wrote:Javelin wrote: Well, yeah, of course it is. But it shows that increasing efficiency is actually diminishing returns.I don't get this. Going from 25mpg to 50mpg means your car will use half the fuel. Going from 50mpg to 100mpg means your car will use half the fuel. Each 100% improvement in efficiency, is a.... 100% improvement in efficiency. Where is the diminishing return? I prefer the US method because its easier to do math backward to see how many gallons of gas I will use. Going on a 360 mile trip, and your car gets 30mpg? thats 12 gallons. Going on a 360mile trip, but your car gets 3.3gallons/100 miles? thats, uh... (3.3/100)*360
1) Your second jump is twice as big (an increase of 50mpg instead of an increase of 25mpg)
2) and it still only gets you half the fuel savings.
How is that not diminishing returns? For every MPG improvement in your second jump you get 1/4 the benefit that you got in the first. That's textbook diminishing returns.
Writing this the other way (fuel per fixed distance) while the actually same measurement emphasizes that focusing on improving already high numbers is mostly just wanking and that the meaningful gains are almost exclusively at the other end.
Javelin wrote: Also, Miles Per Gallon is a bizarre way of measuring efficiency. Gallons per One Hundred Miles makes a lot more sense. Like this: 10 MPG = 10.0 GPHM 20 MPG = 5.0 GPHM 30 MPG = 3.3 GPHM 40 MPG = 2.5 GPHM 50 MPG = 2.0 GPHM 75 MPG = 1.3 GPHM 100 MPG = 1.0 GPHM So each MPG jump *seems* like a lot, but in actual gallons used to travel the same distance, it's a diminishing return.
using different numbers to say the same thing... i prefer to know how many miles each gallon is going to take (or has already taken) me without doing any complicated math..
Javelin wrote: Also, Miles Per Gallon is a bizarre way of measuring efficiency. Gallons per One Hundred Miles makes a lot more sense. Like this: 10 MPG = 10.0 GPHM 20 MPG = 5.0 GPHM 30 MPG = 3.3 GPHM 40 MPG = 2.5 GPHM 50 MPG = 2.0 GPHM 75 MPG = 1.3 GPHM 100 MPG = 1.0 GPHM So each MPG jump *seems* like a lot, but in actual gallons used to travel the same distance, it's a diminishing return.
Clearly.
I can also see the value of using the mpg figure, but some people seem not to get the fact that taking small car fuel economy from 30 to 40 mpg is not nearly so important as improving large car/truck/SUV mileage from say, the low teens to the mid 20's.
Folks, Javelin is right. For the sake of simplicity and ease of understanding, I'd MUCH rather view things on a linear graph than an exponential/logarthimic one.
Think about it like this (Yes, I've rounded some so that I can use round numbers. No, it is not exact. It is close enough for an example):
You have a 15 mile trip to work. Your car gets 14 MPG. You use 1.07 gallons to get there.
Now, you've traded in your gas guzzling 14 MPG car for a 15 MPG car. You use 1 gallon to get to work. You will get roughly 215 trips to work for the same fuel that only took your old car to work 214 times.
Now, do the same test with a 40 and 50 MPG car. At 40 MPG, you use .37 gallons. At 50, you use .3. Again, in 214 days you've saved enough gas for a free trip to work.
14-15 is a one unit difference. 40 to 50 is a 10 unit difference. However, they are effectively the same increase.
Gallons/100 Miles makes a lot more sense. We just don't like it because it is different.
Got into a discussion with a friend eco-modder who was taking of how driving slower gets better mpg.
I brought up the idea that driving at much higher speed, you are on the road for less time , the distance being the same.
So is gallons per hour the way to go.
We never resolved the question.
That's also why buying that 2nd lottery ticket to double your chances of winning is a better investment than buying the 3rd ticket in order to tripple your chances.
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