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Why mileage drops if drive the bike beyond 60km/h?

  1. Jun 10, 2013 #1
    Hi;

    I am new in this forum. I was looking for such forum and at last reached to the right one.

    A question is in my mind, if we drive the bike within 60km/h range, we achieve a good mileage but beyond this limit, mileage drops drastically while we cover the distance in short time as compare with slow drive.

    The reason is (for me) the air, which pressure increases with speed? I want to know the graph of this increasing, mean want to know how much power we require to get more speed after 60km/h.
     
  2. jcsd
  3. Jun 11, 2013 #2

    rcgldr

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    If the only factor was aerodynamic drag, then that drag force increases with the square of the speed. If you double the speed, the force is quadrupled, and the power consumed increased by a factor of 8. Since the speed is doubled, the milage decreases by a factor of 8/2 = 4.

    This is not how it works in real life. If this is a high powered motorcycle with a fairing, then the engine tends to be more efficient a producing higher power, so there isn't much change in milage between 60 kph and 100 kph.
     
  4. Jun 11, 2013 #3

    russ_watters

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    Er- but the engine doesn't get 4x more efficient with a doubling of speed, so the drag is a much bigger factor.
     
  5. Jun 11, 2013 #4

    rcgldr

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    Most cars, including econoboxes, will get better milage at 60 kph than 30 kph, partly because they are running in a higher gear, and partly because of the efficiency versus power output of most engines.
     
  6. Jun 11, 2013 #5

    russ_watters

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    You just repeated what you said before, except you changed the speeds...

    If a vehicle can comfortably drive in its top gear at 60kph, it will get substantially better fuel economy than at 100kph.

    What is "substantially"? 20% 40%? There are a lot of sources on the web for this but obviously it is vehicle specific.

    The reason it isn't higher isn't the thermodynamic efficiency of the motor, it is because other friction losses (drive train, rolling resistance) are more linear and make up a larger fraction of the total at lower speed...unless you were including that when you said "efficient".
     
    Last edited: Jun 11, 2013
  7. Jun 11, 2013 #6
    If I recall correctly, part of the aerodynamic drag scales with the cube of the speed. So that grows quite fast.
     
  8. Jun 12, 2013 #7

    rcgldr

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    Which is why I mentioned a high powered motorcycle as a counter example. A high end motorcyle will need to be going 100+ kph to be "comfortable" in it's top gear (redline in top gear corresponds to about 320 kph, but actual top speed is electronically limited to 300 kph).

    A typical medium-small to mid-sized gasoline powered car will get it's best mileage around 70 kph to 80 kph. An econobox might get it's best mileage at lower speeds, and something like a Prius running on it's battery will get it's best "mileage" at even lower speeds.

    The point I was making is that the efficiency of an engine depends on the output power, rpm, and the load (opposing torque), as most gasoline engines are not efficient at producing low amounts of power.

    The drag force increases with the square of the speed. Since power equals force x speed, the power to overcome that drag force increases with the cube of the speed.
     
    Last edited: Jun 12, 2013
  9. Jun 12, 2013 #8

    russ_watters

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    I don't believe that - that's a pretty narrow RPM range: like 2000-6400 rpm. My car's "comfortable" range is wider than that!
    You keep saying it, but it isn't as big of a factor as you are implying because vehicles aren't required to try to generate high power at very low rpm. That's what gearing is for -- and its the gearing that loses most of the extra energy lost at lower speed.

    It is tough to find references for motorcycles, but there are a lot out there for cars. Here's one: http://news.consumerreports.org/cars/2009/09/tested-speed-vs-fuel-economy.html

    For example, it says from 55mph to 65 mph (88-105kph) most cars they test lose about 15%. That's a slightly higher but a much narrower change in speed than we're talking about here.

    You also created a special case that the OP didn't, by bringing in a "high powered motorcycle" and arguing against the reality that the OP has seen. The OP asked "why did I see this" and you said "well if you had something different, you wouldn't have seen that". (paraphrase) Ok, maybe, but so what? Better to just answer the question asked.
     
    Last edited: Jun 12, 2013
  10. Jun 12, 2013 #9

    rcgldr

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    In the case of the 1 liter road racer replicas, in order to acheive high power levels, the sweet spot for the torque band is limited to mostly the upper 1/3rd of the rpm range, from about 9,000 rpm to 13,500 rpm (14,000 rpm redline). With a top gear set to redline around 320 kph, those engines are well below their sweet spot even at 100 kph, and the drop off in engine efficiency at 60 kph is worse than the drop off in power consumed by drag, rolling reistance, and internal losses.

    In the case of the hyperbikes, like the Hayabusa or ZX-14, with 1.3+ liter engines, the engines make good torque for 2/3rds of their rpm range, but still they get better gas mileage at 100 kph than 60 kph.

    Even in the case of a common 600cc sport bike, the milage is better at 100 kph than at 60 kph.

    As to cars, page 2 of this article explains the issue of engine efficiency versus power consumed versus speed:

    http://auto.howstuffworks.com/fuel-efficiency/fuel-economy/question477.htm

    My point is that 60 kph or 37.5 mph is too slow to be efficient with most cars. Most cars get their best milage between 45 mph to 60 mph.

    True, but 600cc sport bikes are at the low end of the spectrum in the USA (except for Harleys), and they also get better milage at 100 kph than 60 kph. Their best mileage would be somewhere between 60 kph and 100 kph. My guess is that even a relatively low powered bike like a Harley gets better mileage at 45 mph than it does at 37.5 mph.

    On the low end of the scale, like a moped, then the ideal speed is much lower, but those generally aren't considered to be "bikes" in the USA.
     
    Last edited: Jun 12, 2013
  11. Jun 12, 2013 #10

    I'm no physicist or economist but I think it's because "time is money" :smile:

    Joking aside I've seen on BBC Top Gear a top speed run for the Bugatti Vayron. James May went though a very good description of what happens on the way to those speeds of 400km/h.

    One of the descriptions was about the comparative "thickness" or "viscosity" of the air. At 400km/h he likened it to Molasses.

    Another neat comparative was the amount of additional horsepower to go from say 360km/h to 400km/h. Just spitting out what I remember but it was along the lines of an additional 100 hp for an extra 40km/h, no kidding like molasses!
     
    Last edited: Jun 12, 2013
  12. Jun 12, 2013 #11
    A source for these figures would be appreciated.
     
  13. Jun 12, 2013 #12

    rcgldr

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    This will take a while. 2010 GSXR specs (13,750 rpm redline). Rear tire circumference is 76.91 inches (based on forums and the gearingcommander site mentioned below).

    update Recent 1 liter road racer replicas are not as peaky as the early models. Looking at some dyno charts, it seems that most of them acheive 75% to 80% or more of peak torque for about 1/2 of the rpm range. The current hyperbikes with 1.3+ liter engines acheive 80% or more of peak torque for about 2/3 of the rpm range (this hasn't changed much since 1999, when the Hayabusa was first released).

    http://aperaceparts.com/tech/2010 Suzuki GSXR1000.html

    More info on speeds versus gearing for various bikes can be obtained here. Note you do not have to allow the "add-on" to run, in order to use this web site. Note that MP rpm is max power rpm, not redline. It seems you'll need to get this information else where. Also missing is the data for the two main "hyperbikes", the Suzuki Hayabusa and the Kawasaki ZX-14.

    http://gearingcommander.com

    Based on this data, the GSXR 1000 would redline at 197.5 mph at 13750 rpm, and the engine would be running at 2610 rpm at 60 kph (37.5 mph), well below the "sweet spot" and possibly lugging the rev happy engine of the GSXR 1000.

    Hayabusa speed (mph) in gears at redline (the bikes can't reach redline in top gear):

    Use this line as 17/40 is the stock number of teeth on the chain drive sprockets:

    17/40 80 108 137 163 184 201

    hayabusa_gearing.html

    The 600cc racer replicas are still peaky. The 2011 GSXR 600 has a red line around 15,200 rpm, and according to this review, the sweet spot for the torque curve doesn't kick in until 12,000 rpm.

    2011_gsxr_600_review.html


    The chart at the bottom of this web page shows the issue of engine efficiency versus load versus rpm (the low spots on the curves represent the best efficiency):

    http://www.bmwmotorcycletech.info/FuelMileage.htm

    The main issue for me is the comparason using 60 kph == 37.5 mph. This is too slow a speed for many vehicles to be able to run in top gear, at least in the USA. Most of the articles I've read mention best gas mileage occurs between 70 kph (about 45 mph) to 100 kph (about 62 mph), depending on the aerodynamics, weight, and power to weight ratio of a typical car sold in the USA.

    On the other extreme, as suspected, a Prius get's it's best milage at very low speeds, assuming the chart on this forum can be trusted:

    prius_mileage_chart.html
     
    Last edited: Jun 12, 2013
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