Weight and Mileage

<div class='quotetop'>QUOTE(efusco @ Mar 8 2007, 11:29 AM) [snapback]402358[/snapback]</div>

This is a very interesting question.

Weight in itself has very small effect on gas mileage. Lets consider a steady state driving condition: you are cruising along on a flat freeway. The weight contributes to bearing and tire friction. This energy loss is fairly small as compared to the energy required to overcome wind resistance. So, driving along on the freeway, if your car almost weighed nothing but had the same wind resistance - it would only get a 10 or 20 percent improvement in gas mileage.

Now, lets look at the other situation: You are in town, with a lot of starts and stops... but never really go faster than about 40 MPH. In that situation, weight is very important and wind resistance is a lot less so. In this scenario, the weight impacts how you accelerate and deccelerate the car. The heavier the car, the more work it takes to bring it up to speed; and the more energy has to be dissipated to slow it down. These issues also apply to going up and down a hill. In the case of the rims, weight is even more important because not only do you have to bring the weight up to speed - but you also have to spin it up (rotational momentum). As a simple rule of thumb, 1 lb of rim weight is equivalent to 10 lbs of vehicle weight.

One thing to consider is that weight reduction is much more important during the design phase of the car versus you trying to slightly reduce the weight of the car that you already own and operate. The reason for this is that the weight of the car is directly related to the size of engine (in terms of peak HP) that will be required to achieve satisfactory acceleration of the car. A smaller engine generally results in better gas mileage.

 

Believe it or not, 50 years ago back seats were optional for Chevies. They were called "Business Coupes." Weight saving was one consideration, I think they saved about 60 pounds total, but cost was the main reason. Back then everybody watched every dollar they had. Also, when they were used for racing the people wanted the lightest car possible.

The website I found, http://www.ftc.gov/bcp/conline/pubs/alerts/fuelalrt.htm states that eliminating 100 lbs is worth an extra 2% for fuel economy. Since it is a government website I know everything there is true too.

An extra 2% at 50 mpg would give you a 1 mpg improvement for a 100 lb reduction, or a 0.1 mpg improvement for a 10 lb reduction. And among the Prius community that could be the difference between ordinary and great, or great and spectacular.

 

<div class='quotetop'>QUOTE(bshef @ Mar 8 2007, 01:54 PM) [snapback]402327[/snapback]</div>

Weight has two direct effects:

• Increases rolling resistance slightly
• Adding acceleration/deceleration energy requirements - some of which is lost in the cycle

Adding weight to the rims (or tires) multiplies the second effect due to the increase energy required to spin the tires which is in addition to the energy needed to linearly accelerate the same weight.

So seat weight has a smaller effect than an equal wheel weight.

JeffD

 

I'm with Jeff.... weight of your tires affect gas milage MUCH more then weight in the cabin. Having to sping those extra two pounds around at 60RPM (or whatever speed tires turn at) will make an impact. Not sure how much.

Not sure I know why, but for motorcycles lighter wheels make quite a difference same on bicycles where having a heavy wheel can be felt

 

<div class='quotetop'>QUOTE(SomervillePrius @ Mar 8 2007, 03:46 PM) [snapback]402400[/snapback]</div>

Tire and wheel weight is unsprung, meaning it goes up and down over every bump. The up and down motion burns energy as heat in form of tire flex and shock absorber heat.

Tom

 

<div class='quotetop'>QUOTE(sub3marathonman @ Mar 8 2007, 03:09 PM) [snapback]402378[/snapback]</div>

My understanding is that a business coupe was marketed to a travelling salesman. The missing seat gave him extra and convenient space for his sample cases.