True, tires/rolling resistance being the primary source of energy loss at constant speed before aerodynamics supercede it, at what speed that occurs being a function of drag area v weight/rolling resistance.

I like an earlier posted idea of lets all toss a couple hundred extra pounds in our vehicles and see. ;)

I've been driving with ~300 pounds extra for a couple months now. My FE has gone down mainly with the weather and driving changes I've had to make; I don't think I can blame the slight decrease on weight, though I can't prove that (and I can't get rid of the weight to test it, either).

I have observed the same thing, and wondered that too; I think the fundamental underpoweredness of a bicyclist's power source is to blame for extreme sensitivity to wind resistance (and to drafting).

speak for yourself, I'm not underpowered! ;) To work, slight downhill, I can maintain an average, over 3.2miles, of 18mph on my bike. Going home, it's 12mph. That's with the snow tires, heavier, bigger. The summer tires I was 2-3mph faster average.
Anyways.

The valley I live in is flat, so there are no hills on any local commutes. But when leaving, I cross mountain passes, and having the extra 500lbs (exaggeration) of weight only heats up my brakes when I have to slow down the passes. By coasting, I know I'll hit 90-100mph downhill, with a possibility of a ticket; not something I want. So the extra weight actually fails me on both accords for my extreme "hills" up and down. I'll need a SG to prove this of course, with 5 people in the car, and just me.

I do agree that aero drag starts as soon as you move. Property of physics doods. Didn't someone create a wind tunnel that placed the car on a treadmill, so they could incorporate frictional losses into the total drag cd? Yes someone did, for F1 racing though.
Man I'm on tangent today.

I'm sure baasjoos can coast further than I can, even though his car weighs 1000lbs+ less than mine. His drag < mine and that property outweighs frictional rolling losses and mass differences.

Consider it from a ballistic point of view. Sectional density, the amount of mass of a vehicle in relation to its aero drag will make the glide portion extend to a greater distance. Of course the pulse phase will require more power but the additional power is created at the highest BSFC point of the engine as long as that is properly used by the driver.

I think there would not be much difference, especially in situations where elevation changes do not cause losses due to excess downhill speeds. Where elevation changes are severe weight becomes a much more relevant factor.

In my VX with maximum load it makes some difference, and there are no detrimental elevation changes.

Of course with 4 passengers at 51 MPG each person is using only 1/4 of the total fuel, so you could argue that the per passenger mileage would be 204 MPG in this situation.

regards
gary

Generally engine displacement and gearing are far more important than weight, but they kind of go hand in hand. If a car weighs more you have to have a larger displacement engine and/or shorter gearing to make the car not seem like a dog when getting up to speed. By taking a lot of weight out, you will reap some benefits, but you could magnify those gains by going with different gearing and a smaller displacement engine, but that is a lot of work. At that point you would be better of trading in for a lighter, smaller vehicle to use unless you are really attached to the one you have.

thats still more power than a car with the engine off. note i said power, not stored energy(weight);)

I do agree with the whole "weight doesn't make a big difference so leave your extra seats and spare in the car" thing

here is a theory though. my car usually runs between 30 and 40 percent load. if you had a car that ran higher (maybe 50-60 percent load) you would see more benefits from weight reduction. I still think it would take a few hundred pounds to really show any significant gains.

I could get by with a much smaller engine but that is just my opinion.

Adding weight for increasing gas mileage is foolishness I am sorry to say.

Yes it may increase your coasting distance, but there is no free lunch. You invested energy into your car earlier when you accelerated, and since nothing in a car even comes close to 100% efficient you MUST be losing energy.

Additionally, someone mentioned rolling resistance. Rolling resistance is a unitless value and this becomes obvious when calculating the force caused by this. To calculate force due to rolling resistance you do this: Rolling resistance X weight. So if you increase weight, you must be increasing the force of resistance related to rolling resistance.


It is ALWAYs beneficial to reduce weight in vehicle for performance. Only reason weight is useful at all is for downforce so your tires stick, which coincedently the less mass your car has, the less downforce you need.

that is mighty ironic

everything I have done to my car (for better mileage) has added weight to it. grill block, warm air intake, even the scangauge.

most aero mods add some weight just from the materials used. if you are simply adding dead weight in the back seat, then that statement is true. I don't know too many people that do that though.

there is also a guy on here (basjoos) that has added at leas a couple of hundred pounds to his car to get him up to the 70MPG range. his car is " keeps on rollin' " . Oh did I mention that his coefficient drag is 0.17 it makes a difference how you apply that weight.

your use of the word PERFORMANCE is interesting. we care about fuel efficiency which I guess can be called performance but that word is more often used with guys getting around a track as fast as they possibly can which is pretty much none of us (at least not monday through friday).

interesting first post and welcome to the site

*edit* I don't like absolute statements. it is never always anything. but I guess that is an absolute statement within itself.

also, if I wanted more power out of my cavalier, couldn't I add a turbo for more power? that won't work, it's adding weight to the car.

Good point BEEF. Lowering Cd is probably the best you can do to your vehicle. All that weight reduction means nothing if you are still trying to send a 4x8 sheet of plywood through the wind - what it weighs has little bearing on it's Cd.

basjoos is a good example of what lowering the Cd does to FE, even though he has increased the weight with mods.

Size and horsepower of the engine IS a factor, but not so much as how much horsepower you use to get it up to speed. If you have a 55 hp engine, you might use 35 to get to speed, whereas someone with a 555hp engine might use 400hp to get to speed. Who used to most gas? My math has gotten bad over the years.

My left-handed thought is, if you have a 55hp engine or a 555hp engine, it will still take "x" horsepower to maintain given speed with a given vehicle. It's how efficient and how much fuel it uses at that specific horsepower that matters. Somewhere along the line, I'd think there would be an engine to match the vehicle when moving it at 55mph with the least amount of fuel matters, taking in to account Cd, weight, etc. So if you reduce Cd, then it takes less horsepower to maintain given speed with given vehicle - thus increasing FE. My brain hurts...I think I pulled a muscle...need coffee...wife's got me on decaf...just bumped into a wall...