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Old 07-13-2007, 10:27 PM   #11
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I've made up a spreadsheet that allows you to play with weight at Cd and see the theoretical changes. It also has coast down input fields to get the experimental equivalents... then you can correlate to get a pretty good picture of your car... once you have that, varying things like weight should be pretty close. It doesn't do mpg directly, just force instead, but from that you can get to work and power pretty easily. If anyone wants to try the spreadsheet, drop me a mail with your email and I will send.
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Old 07-13-2007, 10:31 PM   #12
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>>Mass only affects acceleration not constant speed driving.
>>Force=Mass X Acceleration

Only a theoretical truth. In practice, once is constantly accelerating and decelerating around your target speed.
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Old 07-14-2007, 12:08 AM   #13
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Look here :

http://www.gassavers.org/showthread....hlight=rolling

It is a simple spreadsheet I made - the original is from the link at the top of the thread. It lets you add certain details about your car, and then work out the power being wasted by aero and rolling resistance (the two main sources of drag for a car).

It is very useful for deciding what speed to pulse & glide around, and so on.

One proviso - the engine efficiency drops off as speed decreases, so, the slower you go, the more pulse/glide you need to do. I find my car is best doing P&G from 30 - 40 (because below 30, I can't use 5th gear, and above 40, engine enrichments move to a more extreme mapping).
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Old 07-14-2007, 07:44 AM   #14
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Quote:
Originally Posted by lca13 View Post
>>Mass only affects acceleration not constant speed driving.
>>Force=Mass X Acceleration

Only a theoretical truth. In practice, once is constantly accelerating and decelerating around your target speed.
I think that one usually multiplies mass by the Crr and some kind of velocity term to get rolling resistance. So, it does matter at a steady speed.
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Old 07-14-2007, 04:47 PM   #15
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Friction

I really don't think that adding weight could ever improve mpg, even considering that you could EOC further. From the physics I remember, it would make sense that less weight means less acceleration needed, but less glide. More weight needs more acceleration, but can glide further. However, with more weight, there will only be more friction. IF you had no friction, than EOC would help immensely, but that can't be the case. I would vote for a lighter weight always having better mpg.
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Old 07-14-2007, 05:17 PM   #16
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I would be inclined to vote for that as well. Why else would a company like Honda make the VX as light as they could to make their fuel sipping model?
This whole heavier/lighter question is in the same league I think as what is better for FE, hilly country or flat land...
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Old 07-14-2007, 07:51 PM   #17
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The reason for lighter weight generally having better fuel economy is that people use brakes.

The added RR from weight is negligible, the problem that weight has is that you can't use as much of the momentum as with light weight.

With a light weight car, when you coast to a stop, you slow down quicker, and therefore when you finally apply the brakes, you're wasting little forward energy, because you wouldn't have had much past that anyway.

With a heavy car, when you apply the brakes, you waste far more of your forward energy. Therefore that extra energy that you spent getting that car up to speed cannot be reclaimed with it's longer coasting.

Heck, with my current 3500+ pound t-bird, I've been highway cruising at 60mph before, came to the "1 mile to rest stop" sign and right there popped it in neutral and turned off the engine. When I finally got to the rest stop, I was still doing almost 30mph (On level highway). I could never have done that with my old 2300 pound 89 Corolla coupe.

So not necessarily in a world without friction (Because then FE would almost be infinite. Get to 60 mph and coast for the whole trip across the country), but in a world without braking, weight wouldn't have any real effect on FE.

Edit: Heck, when I just went on vacation, I was loaded down with 2 extra people plus at least 200 pounds of luggage, bringing my total weight to almost 4500 pounds, and I was still getting the 35-45 MPG highway (at 65+ mph) that was my ideal before I just changed my O2 sensor.
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Old 07-14-2007, 07:58 PM   #18
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I think saying extra weight never helps is a bit of an oversimplification. Even if you were pulsing and gliding on a flat track with no wind, it STILL takes a certain amount of energy to restart your engine (an mph or so), which will be again lost when you kill it, so there is definitely some return in cycling the engine less (pushing longer and gliding farther).

Having enjoyed RC sailplanes for a few years, I can tell you that I was making a mistake when I was thinking everything had to be as light as possible. My first longest flight happened after I decided to add a couple OZ to my 1.5 meter RES. Prior to that it was like trying to fly a piece of tissue paper. After the extra weight, I could penetrate the wind and cover more ground quickly and "coast" to the next bit of lift. Most sailplanes come with adjustable ballast to adjust to current conditions, don't want too much or too little ballast.
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Old 07-14-2007, 10:06 PM   #19
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There are so many variables in this that I dont think it could be experimented properly. The grade of incline/decline, weather, traffic, everything would throw this way off...

Extra weight WOULD help you glide further and EOC further, but this small gain would be overwhelmed by the effort it would take to accelerate, especially from a stop. And, so, you can glide further downhill, but you still have to go back up it. The mpg gained coming down will not overrule the mpg lost going up..

Either way, lighter is the way to go. Why else would every prototype and race car be light?
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Old 07-14-2007, 10:40 PM   #20
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Race cars are always light because rate of acceleration is more important than FE. Prototypes because they're cheap and don't put everything a production car needs into it. Plus they need to be able to push them out onto the showroom floor. Hehe!

But I'm not at all denying that lighter cars get better FE. That's proven time and time again. But that better FE is in city driving. On highways, or any situation where you wont be touching the brakes, weight doesn't make a difference.

But honestly, you'll never make a trip where you don't touch your brakes. It's only on segments of the trip you don't touch your brakes. Such as P&G. In P&G, weight doesn't matter. In nearly all other driving, it does.
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