Adding Weight to improve FE

[Varn]

Sean Costin a record holder in bicycle racing told me a story about how they added 70 pounds of weight to their streamliner for battle mountain a number of years ago.

Battle mtn has a 200 foot drop in the 5 miles of run up. About the limit of what the rules allow. They were hoping that the increase in weight would improve their top speed on the flat section 1km and 200m that follows. The gist of it was that their top speed was the same. No free lunch.

[SentraSE-R]

The only way you could consistently improve your mpg by adding weight would be adding the weight at the top of a long downhill grade, and driving back up the hill without the weight. Of course, rivers do the same thing, for free.

[comptiger5000]

It might help on the highway in a hilly area if it adds enough momentum to reduce the number of times you downshift on a hill, allowing the engine to be kept in a better BSFC range.

[rbrowning]

The problem in comparing a bullet and a car adding weight is aerodynamics. A heavier bullet will be longer, for a given diameter, than a lighter bullet. Adding weight to your car is internal and doesn't make it longer and more aerodynamic. At long range competitions they are shooting very heavy-for-caliber bullets, but it is more to reduce wind drift than anything else.

In town with the stop and go weight will hurt you. On the highway it doesn't make much difference, just a little on the rolling resistance. That is somewhat offset by increasing the momentum and allowing you to coast a little further.

[kgwedi]

I finally decided to some A-B-A tests with different weights in my Metro.
Because I do P&G it would be very hard for me to be consistent with regards to acceleration rates and shifting gears. I opted for increasing the distance to help even out the inconsistencies.
I did my testing using LOD of 34 to accelerate up to 55 MPH then coast engine off till 40 then accelerate again. I drive this road often and have specific markers to start coasting and to start accelerating when it is more efficient. I started outside of one town and stopped before the next town so there were no restrictions from stop signs or lights. I drove totally in the emergency lane so traffic was not a factor at all.
I got some canvas bags, and filled them with sand (454 pds). With the spare tire and other thing in the car the weight I removed between tests 510 pds. My two places (E and M) to stop and turn around were 22 miles apart, and had a 200 hundred foot difference in altitude. The wind was blowing from E to M at 8-10 MPH, and remained steady throughout the test. I filled the gas tank, and put all tires at 44 PSI. Here are my weight load, average speeds and MPG for each run.

E -> M Light-- 46 MPH 81.6 MPG
M -> E Light-- 43 MPH 54.8 MPG
E -> M Heavy- 44 MPH 87.4 MPG
M -> E Heavy- 43 MPH 53.2 MPG
E -> M Light-- 45 MPH 86.5 MPG
M -> E Light-- 43 MPH 54.6 MPG

This shows almost no advantage to either being light or heavy. I tried another test that was A-B-A going on a more level road only in the one direction. This leg was 9.7 miles.

E -> A Heavy- 43 MPH 73.5 MPG
E -> A Light-- 44 MPH 75.0 MPG
E -> A Heavy- 44 MPH 76.8 MPG

I conclude that with my driving style, it doesn't matter whether I am heavy or light. Being light makes it easier to P&G because less shifting is required. When heavy, I used third gear to accelerate from 40 MPH.
Only with large altitude differences will I be concerned about weight in the future.

[slogfilet]

We use a similar concept in rocketry to determine the optimum weight to achive maximum altitude... often, adding weight will increase the final altitude. However, I think the system is different enoguh from automobile FE that it is difficult to draw any conclusions. For one, there is only one pulse and one glide on a rocket, rather than a repeating cycle.

I guess the takeaway is that there is no hard and fast rule about weight. In general, lighter will be better... however, there may be cicumstances under which you may benefit from an increase in weight. My thought is that these conditions will be highly specific, difficult to indentify, and unlikely to be extrapolated to "general use."

A contest may be an area where enough of the variables are known that you can make some improvements, but daily driving is such a crapshoot that it'd be a heroic effort. Not that that should stop us from trying.


(Random thoughts... would a lighter bullet undergo increased acceleration from the same amount of propellant, therefore exiting the barrel more quickly and experiencing less total force than a heavier bullet? Thus, a lighter bullet may actually have less total energy for a given propellant mass and barrel length than a heavier bullet. As can't think of a corrolary of this with respect to cars, the bullet analogy might not work.)

[fusion210]

Originally Posted by kgwedi

I conclude that with my driving style, it doesn't matter whether I am heavy or light. Being light makes it easier to P&G because less shifting is required. When heavy, I used third gear to accelerate from 40 MPH.
Only with large altitude differences will I be concerned about weight in the future.

Thanks for taking the time to do the test. You said that you used third gear when heavy, does that mean you didn't use third gear when you were light?

[euromodder]

500+ lbs more or less is a serious difference in weight for a car the size & weight of a Metro.
I'm a bit surprised that it doesn't show more clearly than just 1.5 mpg going from M to E.

Originally Posted by kgwedi

This shows almost no advantage to either being light or heavy.

It does however show the effect of wind and - more importantly - the change in altitude.
A whopping 30 mpg


It also shows that variation in this kind of real-world testing can be quite high - 5 mpg for the same situation.

IMO this kind of real-world testing simply cannot bring out the minor effect of small mods, as the testing environment isn't stable enough.

[jamesqf]

Originally Posted by rbrowning

A heavier bullet will be longer, for a given diameter, than a lighter bullet.

Not necessarily. If you have three bullets, one lead, one depleted uranium, and one lead free, all the same size & shape, they're going to have different weights.

[rbrowning]

Originally Posted by jamesqf

Not necessarily. If you have three bullets, one lead, one depleted uranium, and one lead free, all the same size & shape, they're going to have different weights.

You are absolutely right. But considering that 90+% of the bullets used in the world are basically lead (DU is real hard to come by for some reason!), with some small amount of copper on jacketing them, they are basically considered lead.

Changing density but not changing shape is what kgwedi did by adding weigh inside his car but not effecting it's aerodynamics. And I applaud his diligence in his testing, no small effort that. And predictably, on a highway run, the weight mattered little. In town I am sure he would have seen a much greater effect, and the weight would show a big disadvantage in most situations. The only place I see weight coming in as an advantage is if you encounter intermittent resistance such as small hills, or snow and slush, that the weight helps you plow through without having to power through, but rather you use the kinetic energy you have already input into you car to get you where you need to be.