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Old 06-21-2007, 06:31 AM   #1
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Weight reduction - not always good?

I was just thinking about it...

Pulse and glide is the best way to increase engine efficiency (versus driving at a constant speed in gear).

If your car had a mass of 1 gram, but was the same size, and you also weighted 0 grams, then...

If you drove at 60mph in gear, you would put out 20% less energy as there would be no rolling resistance - however, the engine efficiency would decrease due to using it at less load.

If you came out of gear, the car would decelerate to 2 mph in about 2 seconds, as the aero drag would completely overwhelm the momentum of the car.

If your car weighed 100,000 tons, then your FE would be about the same at 1 or 1000 mph, but, would again be very low due to low RR.

So, in reality, weight reduction is not necessarily good - in reality, there will be a point of maximum efficiency, which will change based on average speed.

I have no idea where this point would be, but it would be interesting to find a way to work it out!. I guess, you would need the RR, CdA, engine efficiency at load, engine efficiency while cruising, engine efficiency while idling, mass of the car, and do some complex maths to work it all out!
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Old 06-21-2007, 06:45 AM   #2
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Ummm... If your car is super heavy you need more force to accelerate it. Yes it can hold momentum better which is what I think your getting at, but the over all picture of mass is miscalculated here. The energy requried to accelerate a 1 gram car is minimal. It'd take my whole tank of gas to accelerate the 100,000 ton car to 60mph.
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Old 06-21-2007, 06:59 AM   #3
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When I had my BMW 540i, rear wheel drive, 280hp V-8, with a custom automatic racing tranny, I found out really quickly that weight reduction was a bad thing in the winter. It also had a safety feature that was really annoying. The rear wheels would stop moving if it felt the slightest slip. So I'd be sitting there at a green light in light snow, with the little warning light flashing, and the car wouldn't move an inch. That was really annoying in rush hour, for me and everybody behind me.

Finally, a mechanic friend gave me two 5 gallon buckets of misc parts. I kept one above each rear wheel so I could still drive in the snow. I also added bags of salt. That probably hurt my FE a bit. LOL.

It was a great summer car, though. Here it is in downtown Salt Lake. See TRAX in the background, the light rail. That saved me tons in gas and parking. It was free for students!


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Old 06-21-2007, 07:10 AM   #4
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I don't think that the extreme examples are really applicable to the real world. I can't quantify it, but I think that relative to engine displacement, there is a sweet spot in weight that is in modern car design, without fail, less than the car weighs. This is because based on the gross mass of cars and trucks, aerodynamic drag is really next to nil relative to momentum below 40mph. It would take a huge reduction in mass to find the point where returns are significantly diminished, barring a corresponding reduction in engine displacement to match.
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Old 06-21-2007, 07:26 AM   #5
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I believe weight reduction would only hinder you (based on your theory) if you are coasting more than you are accelerating or cruising, which I don't believe is the case with most of us. And unless you are taking out a great deal of weight, I don't believe it helps much at all. I drove my Civic with full interior for a tank and got about 47mpg. Then I gutted everything out but the driver's seat and dash for a tank. MPG ended up about the same. So I will probably be putting the seats back in this weekend. All that stuff equals up to 100 lbs or so. Didn't make much difference in my case, based on a couple fill-up's.
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Old 06-21-2007, 07:59 AM   #6
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Quote:
Originally Posted by bbgobie View Post
Ummm... If your car is super heavy you need more force to accelerate it. Yes it can hold momentum better which is what I think your getting at, but the over all picture of mass is miscalculated here. The energy requried to accelerate a 1 gram car is minimal. It'd take my whole tank of gas to accelerate the 100,000 ton car to 60mph.
(I have also read the other posts above).

What I was getting at is a 'sweet spot'. In fact, the idea is that there is a sweet spot for economy. The 1 gram car will actually be worse (aero drag stops you coasting at all), and the 100,000 ton car would be rubbish (due to massive rolling resistance).

The question is, where is this 'sweet spot' - it will be different for each car, and it is pretty unlikely that the car designers will try to achieve it.

The best idea would probably be to reduce aero drag as much as possible, and fit low RR tyres, keeping momentum, but using low RR tyres, and spending most of the time working on Aero rather than weight. Although in stop-start driving the other way round (work on RR *and* momentum) would be more appropriate.

Interestingly.... If RR was the same on the 1 gram and 100,000 ton car, then, the 100,000 ton car would get the best economy if driven on a very very long straight road. You could put the energy into it, at 40% efficiency, then coast the rest of the way, while the 1 gram car would need the engine to be on all the time, so it would be maybe 22% efficient, so only get half the MPGs
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Old 06-21-2007, 09:06 AM   #7
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Drag would be the same on heavy and light car.
Assuming no rolling resistance the force to keep both cars at a constant speed is the same isn't?

So the variable is acceleration, and F=ma. Maybe I'm not getting it, but I dont think there is a sweet spot, or at least not a realistic one that can be reached. If you could strip 10% off the mass of your car, I dont think it would be less efficient, except the winter driving case.
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Old 06-21-2007, 11:53 AM   #8
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On the gently rolling hills north and west here, a really heavy car could P&G with the flow of traffic, since a heavy car will keep its speed up well on a slight hill (as compared to a light one).

But, your mileage would get killed in traffic...
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Old 06-21-2007, 12:03 PM   #9
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Hmm...this brings back memories of when I used to drive a bus. We were on a charter trip, and after fueling my bus, I found that the batteries were dead. We needed to move the bus away from the fuel pump since it would be a few hours before the repair crew would arrive. One of the older drivers told me to pop it in neutral (this is a Greyhound size bus!) and laughed when I looked at him like he was crazy when he said we would just push the bus a few hundred feet away on level ground.

Well...we did push it, and I had to RUN once it was up to speed to get back in the driver's seat. It was MUCH MUCH easier to push that bus than ANY car I have ever owned. Talk about a lack of rolling resistance...and this is a 28000lb bus! This tells me that it was less than a horsepower to overcome RR on this vehicle, but on the interstate, it is almost 100% air resistance (shaped like a beveled brick).
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Old 06-21-2007, 12:34 PM   #10
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Quote:
Originally Posted by bbgobie View Post
Drag would be the same on heavy and light car.
Assuming no rolling resistance the force to keep both cars at a constant speed is the same isn't?

So the variable is acceleration, and F=ma. Maybe I'm not getting it, but I dont think there is a sweet spot, or at least not a realistic one that can be reached. If you could strip 10% off the mass of your car, I dont think it would be less efficient, except the winter driving case.
The three variables are:

Rolling resistance ( = weight * rolling_resistance_coefficient)
Momentum (is proportional to weight) (related to kinetic energy)
Aero drag.

So, the heavy car means you can 'charge' up the momentum, and then coast for ages, whereas the light car has no momentum to charge.

I suppose what I'm saying is that....

If you could half the RR coefficient on the tyres *OR* half the weight of the car,

then you should half the RR coefficient on the tyres - because then you will have half the RR but the same kinetic energy which you can use in a 'hybrid' fashion.
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