Weight, where is the loss?

[2009Prius]

Originally Posted by RobertSmalls

Rolling resistance increases linearly with weight, ....

Originally Posted by AeroModder

Increased mass just makes the effects of inertia more apparent. Good for highway cruising, ....

I have to say the increased rolling resistance due to increased mass is bad for highway cruising; bad for any cruising for that matter.

[Thymeclock]

Originally Posted by 2009Prius

I have to say the increased rolling resistance due to increased mass is bad for highway cruising; bad for any cruising for that matter.

Usually. You are right under most circumstances, but not all.

Let's say you live at sea level. And you drive to a place that is 100 or more miles away that is at a much higher elevation, more than 1000 feet. You will burn more fuel going there than coming back, assuming that all other factors are equal.

Now, if you are fortunate enough that you leave with your vehicle empty (including a half-empty gas tank) and return with a full tank of gas and a 1000 pound load in it from that higher elevation, it will work to your advantage. It's all downhill from there, essentially, no matter how you slice it.

But if you have to leave with a full load, driving to a higher elevation, and you must return with it empty, the prevailing forces will be working against you.

[AeroModder]

Originally Posted by 2009Prius

I have to say the increased rolling resistance due to increased mass is bad for highway cruising; bad for any cruising for that matter.

Now, is the increase in rolling resistance enough to counteract inertia? Or does the increased resistance to change speed overcome the rolling resistance?

I'm pretty sure that the slight increase in rolling resistance is not enough to overcome inertia.



Thymeclock: Of course, increased mass means that the pull of gravity will be stronger. The added weight comes at a disadvantage for uphills. However, my initial post assumed level roads.

[RobertSmalls]

Originally Posted by AeroModder

Now, is the increase in rolling resistance enough to counteract inertia? Or does the increased resistance to change speed overcome the rolling resistance?

http://ecomodder.com/forum/tool-aero...ToStep=5-200-5

About a third of the load at 60mph is rolling resistance for the above-linked car. So a 3% increase in mass (here, 90lbs) should bring a 1% increase in fuel consumption on a flat road with no stops or hills. It's a small impact, but every little bit helps (or hinders).

Also, inertia is bad for fuel economy. It makes DWB harder, it moves your RPMs upward during acceleration, raises the stakes every time you brake, and it makes you downshift on too steep of a hill climb. The only good thing about it is that it makes P&G easier with longer pulses and longer glides.

[ATaylorRacing]

I use my 96 Geo 3 banger as a delivery truck. I usually carry from a min of 100 lbs to a max of 400 lbs. Over nearly 3 years I have found that every 100 lbs of extra wt in my car at the normal 60-65 mph costs me an extra one mpg.

[brucey]

Originally Posted by RobertSmalls

The only good thing about it is that it makes P&G easier with longer pulses and longer glides.

I think that's what is happening on this route. Since it's all either uphill or downhill. It somehow averages out.

Or maybe I'm just silly and that was a fluke.

[AeroModder]

Originally Posted by RobertSmalls

The only good thing about it is that it makes P&G easier with longer pulses and longer glides.

Right there shows the added mass overcomes the added rolling resistance and makes it easier to keep it moving. However, added weight does make it harder to get it moving from a stop, as you said.

Hence,

Increased mass just makes the effects of inertia more apparent. Good for highway cruising, bad for stop-and-go in the city.

You also need to take in consideration the placement of the load, how much and where it compresses the suspension. Driving with the front lifted and the rear lowered will increase drag by forcing more air under the car, and compressing it as it moves to the rear.

[2009Prius]

Originally Posted by Thymeclock

....

Let's say you live at sea level. And you drive to a place that is 100 or more miles away that is at a much higher elevation, more than 1000 feet. You will burn more fuel going there than coming back, assuming that all other factors are equal.

Now, if you are fortunate enough that you leave with your vehicle empty (including a half-empty gas tank) and return with a full tank of gas and a 1000 pound load in it from that higher elevation, it will work to your advantage. It's all downhill from there, essentially, no matter how you slice it.

But if you have to leave with a full load, driving to a higher elevation, and you must return with it empty, the prevailing forces will be working against you.

So in brief the argument is that the gain in potential energy difference may offset the loss due to increased rolling resistance. I suppose that may be possible in steep grades. I would be interested in seeing a calculation of the example of 1000 feet drop over 100 miles.

Originally Posted by AeroModder

Now, is the increase in rolling resistance enough to counteract inertia? Or does the increased resistance to change speed overcome the rolling resistance?

I'm pretty sure that the slight increase in rolling resistance is not enough to overcome inertia.....

Remember the "inertia", or more precisely, the kinetic energy, comes from burning fuel. More fuel will be burnt to reach the same speed with more mass. There is no "free lunch", just net loss due to increased rolling resistance. On a flat road at least.

[2009Prius]

Originally Posted by ATaylorRacing

I use my 96 Geo 3 banger as a delivery truck. I usually carry from a min of 100 lbs to a max of 400 lbs. Over nearly 3 years I have found that every 100 lbs of extra wt in my car at the normal 60-65 mph costs me an extra one mpg.

Nothing beats hard data.

[RobertSmalls]

Originally Posted by 2009Prius

I would be interested in seeing a calculation of the example of 1000 feet drop over 100 miles.

Sure, that's real easy to do.

A car with 300lbs extra cargo converts .4MJ of extra gravitational potential energy from a 1000 foot descent. 300lb*9.81m/s*1000ft - Wolfram|Alpha

With some fair LRR tires, CRR=0.010, the cargo burns up that much energy through rolling resistance every 20 miles. .01*300lb*9.81m/s*20mi - Wolfram|Alpha

If you were descending an endless slope, it must be a 1% grade for extra cargo to pull its weight at CRR=0.01. This is independent of factors like the weight of the car, cargo, aerodynamics. It depends solely on CRR.