Nice summary of mileage stretching

[redpoint5]

Originally Posted by Vman455

I'll rephrase my question: is there a point at which the inertia of a more massive body overcomes the increased rolling resistance of that body such that it will roll downhill faster than an aerodynamically-identical, less massive body? I see people all the time claiming that a heavy car will roll downhill faster than a lighter, but I've never seen any evidence to suggest this is so.

Good question, and I'm sure there is a scenario where increased rolling resistance would outweigh the benefits of having a more dense vehicle. It just isn't likely common.

I know a bicycle has an extremely low rolling resistance, but I can't coast downhill at 55mph, or coast very far up a hill. A car does much better in this case because of the more efficient aero shape and increased density. This, despite having substantially more rolling resistance.

As far as the advise to keep a car in gear on a downgrade... it's the always safe suggestion and sometimes more fuel efficient. No damage will occur to the car by leaving it in gear, and it certainly will save gas on grades steep enough that brakes would need to be used to maintain proper speed.

All that said, I very rarely encounter a grade so steep that coasting will exceed the safe speed.

[niky]

Most heavier cars have more frontal area... and in the case of trucks, worse aero.

If it has the same body, a heavier car should gain momentum more slowly from rolling downhill than a lighter one, but should be able to hit a higher terminal speed (if the terminal speed is drag-limited) and will keep that momentum longer when the road levels out.

I suppose this is the same as EOC... many modders claim that heavier cars are easier to EOC than lighter cars. In my case, I've found small, light cars terrible for neutral coasting... the Yaris sedan I'm driving now (1.1tons) is much easier to coast than the Mirage was (and the Mirage had lower rolling resistance tires and better aero) and the Mirage was easier than the Suzuki Alto or Hyundai Eon... though the Alto has brick-like aero...

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RE: Cars, I typically leave them in gear on downhill grade in mountainous areas here, simply because the roads are relatively steep and are covered in switchbacks and traffic. No sense in building up an 80 mph coast if you've got to burn up the brakes slowing down for a sharp 180 degree turn at the bottom of that particular run. On the highway, where the slopes are gentle, gear out. Definitely gear out.

[UltArc]

To start with, I have not done testing, and these are simply observations I've been noticing.

It appears that the Mustang coasts much further and maintains its speed from ~60-30 much longer than the Insight. When the Mustang gets to ~30, it starts slowing much faster.

The Insight, on the other hand, seems to slow much more quickly from 60~40. But, when it hits the ~40 range it seems to start slowing down very slowly.

Again, these re just observations. I live on a main road (55 mph speed limit) so I have to maintain a high speed for following traffic, but plan my coasts to use no brakes, and not have to restart the engine (Mustang) or put back in gear (Insight).

[user removed]

Think of ballistics and sectional density. The bullet with more mass for its frontal area will take more powder to get to the same velocity but will also maintain that velocity for a longer period of time in flight. The heavier bullet will retain more inertial mass.Think of a cotton ball and a chunk of lead and their differences in fall rates.

Gravity without atmosphere they fall at the same rate of acceleration, but add an atmosphere and you now have to plough through it and the more mass behind the same frontal area the better it will be, because it contains more energy to begin with.

Best bullet design, boat tailed spitzer.

regards
Mech

[Fat Charlie]

My Subaru was quite a bit heavier than my Fit. It did a much better job maintaining speed gliding down hills.

[elhigh]

Originally Posted by Vman455

Has anyone proven empirically that in the case of two cars, with identical Cd and frontal area but different masses, the heavier car will roll faster downhill? Does the increased momentum of a heavy car outweigh (ha!) the increased rolling resistance at some point, and where is that point?

I can answer this one: yes, it absolutely happens.

My truck, empty and just coasting downhill, will v-e-r-y slowly lose speed as it coasts down one particular hill on my way home. I'll hit the top of the hill at 60, turn off the engine and EOC to about the halfway point, bump start and accelerate back to 60, and hold right at 60 all the way to the bottom.

On a few separate occasions I have had firewood in the back, usually 500-1000 pounds. With a half-ton aboard I'm usually only doing about 55 at the top of the hill. Engine off at the top, coast...coast...coast. At the bottom it'll crack 70 if traffic permits, and the engine won't be needed at all to carry me through the flatter part in the middle.

It isn't identical vehicles, it's the same vehicle. I love when I get this result, too - trips like this were part of the first tank I ever had that cracked 35mpg.

[go_slow-save_dough]

Originally Posted by Vman455

Has anyone proven empirically that in the case of two cars, with identical Cd and frontal area but different masses, the heavier car will roll faster downhill? Does the increased momentum of a heavy car outweigh (ha!) the increased rolling resistance at some point, and where is that point?

I have a couple empirical observations: When I have two passengers and two dogs in the Geo, I can scrub 5mph or 300 yds off my EOC starting point vs. just me in the car. The car just rolls and rolls. 300# pounds of extra inertia in the Geo is equivalent to about 40 degrees ambient temperature difference when it comes to EOC roll out. That said, overall mpg is always best when it's just me in the car. Energy wasted accelerating the extra weight is not completely made up for by the longer rolling distance.

Second, my 7L 400hp 7000# 15 passenger van rolls forever. It gets 20mpg vs my Metro's 65mpg which only rolls about half as far on a glide.

[Fat Charlie]

I'm still trying to adjust to the lighter weight of my Fit. Most of my favorite (longest) glides are broken up- my old start and end points are radically changed, and many of them need a bump start somewhere in the middle.

[elhigh]

^^
Aero improvements will bring you some of your distance back, obv. The Fit, though, is probably a fair bit sleeker than what you had, so you might not be able to get it all back.

[Fat Charlie]

I've got a coroplast panel for a lower grille block, and apparently a CR-Zbelly pan is a good enough match for a Fit. That's just about it for stealthy aero on this thing.