Hmmm?

[Onyrlef]

So just finished an 800 mi. round trip from DFW to Louisiana in a 2016 32' Alante. Weight on rear axle was about 1300 lb. below data plate max, front axle about 800 lb. below. TP front and rear, 86lbs, towing a loaded down kayak trailer but only 870 lbs. ( I weighed at a Cat Scale). First 100 miles I kept it to about 60 mph, ecm showed engine temp at a steady 206, stopped and shot the tires and the max on the sunny side rear axle was about 130°, cool side 115° ( outside temp was around 100°)ECM avg highway mpg was 7.2 and ticking up a notch every 25 miles or so. I then dialed her up to 65 mph. and no change in avg mph. other than it may have diminished the rate of gain by a mile or two and engine temp increased to 208-209°. Of course it was flat as a fritter and my expectation is that I'd have likely maxed out at 7.4 -7.5 , but what surprised was the difference in 60 and 65 mph. was negligible. I was however several thousand lbs. below GCWR.

[Grumpy]

Weight is not the issue wind resistance is. The flat front is more of a controlling factor usually.

[Onyrlef]

Originally Posted by Grumpy

Weight is not the issue wind resistance is. The flat front is more of a controlling factor usually.

So then why does it take more power to get up a hill? The shape of the front doesn't change, nor does the coefficient of drag. It's not either/or, wind resistance is not the only factor, it's a factor. The point was the amount of energy required to move the mass (RV) forward at 60 mph ( all other things being equal) was not noticibly different from that required at 65 mph. Moreover, I fully suspect that were I dragging a 5K toad along the same route with a maxed out GCWR of 23k my mileage would vary even though the frontal surface area were the same. How much, who knows?

[Grumpy]

Originally Posted by Onyrlef

So then why does it take more power to get up a hill?

You said it was "FLAT AS A FRITTER"....So now you bring up hills? When I had both motor homes, Toad or no Toad made very little difference in avg MPG and I tracked every gallon on a spread sheet. 50,000 miles on each.

[Onyrlef]

[QUOTE=Grumpy;1062405]You said it was "FLAT AS A FRITTER"....So now you bring up hills? When I had both motor homes, Toad or no Toad made very little difference in avg MPG and I tracked every gallon on a spread sheet. 50,000 miles on each.[/

Raced road bikes for fifty years, there's a reason ain't a fat guy ever won a flat road race and it's not got a thing to do with wind resistance. It's power to weight ratio and that doesn't change whether it's an RV, or a guy on a bike. I think Newton settled this a few hundred years ago, certainly not going to argue it now.

[Ryan B]

Originally Posted by Grumpy

You said it was "FLAT AS A FRITTER"....So now you bring up hills? When I had both motor homes, Toad or no Toad made very little difference in avg MPG and I tracked every gallon on a spread sheet. 50,000 miles on each.

I also track my mileage and see very little difference with Toad and without Toad. I have taken the same trip multiple times from Southern California into the Sierra Mountains with and without toads. It always falls between 7.2 and 7.5 mpg either way.

[Grumpy]

Originally Posted by Onyrlef

Raced road bikes for fifty years, there's a reason ain't a fat guy ever won a flat road race and it's not got a thing to do with wind resistance. It's power to weight ratio and that doesn't change whether it's an RV, or a guy on a bike. I think Newton settled this a few hundred years ago, certainly not going to argue it now.

To get the mass moving and stopped yes, but not to keep it going. Inertia is in your favor.

[Onyrlef]

Originally Posted by Grumpy

To get the mass moving and stopped yes, but not to keep it going. Inertia is in your favor.

Well if that were the case then it wouldn't take fuel to keep it going then would it now? As in perpetual motion? Sometimes you guys are just too funny. Otherwise, as said before, when wrestling with a pig or forum gatekeepers in this case, you both only get muddy and he likes it. Have a good one.

[Learjet]

don't know what you are trying to prove...coming across a little...well...I'll be nice.

[DonBr]

Back in August of 2008 I posted this elsewhere:
------

I crossed the Bonneville salt flats under ideal conditions this morning, with not even a slight breeze when we stopped at the rest stop on the west end. I don't know of anywhere else that you can get so many miles of level straight run without any elevation change.

I ran two miles each with the cruise control at 60, 65, and 70. The GPH for those three were 10, 12.2, and 14.8. That works out:

60=6MPG
65=5.3MPG
70=4.7MPG

A/C was on, so cooling fan was on high.

I did not test 55 because on cruise my rig will downshift to 4th at about 58, and it wouldn't be a good test. Didn't test 75 (the limit) because I rarely drive that fast.

------
This was with a 46,000 lb bus (very flat front) pulling a Jeep Liberty, Detroit 8V92 500HP, numbers taken with VMSpc off the engine computer.

[Grumpy]

Originally Posted by DonBr

Back in August of 2008 I posted this elsewhere:
-----

I ran two miles each with the cruise control at 60, 65, and 70. The GPH for those three were 10, 12.2, and 14.8. That works out:

60=6MPG
65=5.3MPG
70=4.7MPG

Which reinforces my original comment. Right from a web search.

What happens to wind resistance as speed increases?
Explanation: Wind resistance increases dramatically as the speed increases. Resistance tends to prevent the motion of a body. When speed increases, the particles of air are set into motion and this energizes them.

[Midnightmoon]

This is an interesting read. It is written with over the road trucks in mind. But, may be applicable in this discussion.

https://www.fleetowner.com/for-the-d...-truck-drivers

[DonBr]

Originally Posted by Grumpy

Which reinforces my original comment.

Exactly. Now with the same rig I got 10MPG once for a half hour or more coming down I-80 from Tahoe! Gravity overcame drag in that circumstance.

[Grumpy]

Originally Posted by Midnightmoon

This is an interesting read. It is written with over the road trucks in mind. But, may be applicable in this discussion.

https://www.fleetowner.com/for-the-d...-truck-drivers

It's been an age old RV discussion too!

Here is something that was pushed years ago for RV's as well although designed for trucks.

https://www.airtab.com/how-do-they-work.htm

https://www.airtab.com/application-rv.htm

[ARoamer]

Being a aircraft nut, the following applies to anything moving through air. Not taking rolling resistance or gravity (or lack of !) into the equation.

"Doubling speed makes the surface encounter twice as much air moving twice as fast, causing drag to quadruple."

If you want to go fast there will be a price to pay. Streamlining can help, but it can be costly or impractical. With the front end of a class A MH basically being a 8' x12' wall it's gonna be draggy. Vortices at the rear also cause drag which the air tabs are supposed to help reduce.

[Onyrlef]

Originally Posted by DonBr

Exactly. Now with the same rig I got 10MPG once for a half hour or more coming down I-80 from Tahoe! Gravity overcame drag in that circumstance.

Drag is not the primary issue when climbing, weight is. Again I'll go back to a road biking example. When you're in the flats you can tuck in on a wheel (draft) and save 20-30% in energy expenditure, on a climb it's mostly every man for himself, drafting when climbing will save about 3% in energy usage but the killer there is moving your weight uphill. You'll even see pro riders throwing water bottles off at the foot of a climb. That's how critical even a pound or so is when you're fighting gravity. That being said riders, do form up on climbs because even a 3-5% reduction in drag can be crucial, however the reason there is rarely a sprint finish on a climb is at some point a rider is going to break away and hoof it up the rest of the climb on their own and at that point being in a group in the chase is irrelevant. Weight is the primary factor in the mtns. but not the only factor. Same as the coefficient of drag is a major factor in determining energy consumption on a flat run but it's certainly not the only factor (issue). Momentum is the product of mass and velocity (p=mv).

[DonBr]

Originally Posted by Onyrlef

Drag is not the primary issue when climbing, weight is. Again I'll go back to a road biking example.

While I don't disagree that it is likely that weight matters more than drag when climbing, I don't think it is easily provable, and your examples mix apples and oranges. Bikers do not have 100 sq feet of non-aerodynamic surface.

When a bus goes up a hill, both weight and drag are holding it back. How do you measure the amount for each to see which matters more? I made my measurement on the flattest, windless, piece of road I possibly could in order to rule out the weight factor.

[Grumpy]

DonBr said apples and oranges, I say apples and watermelons.

This whole conversation started because you questioned why you didn't see a major difference in AVERAGE mpg while on a road trip. No where did anyone talk about what happens when you hit a hill for a short spurt in an RV. Why you don't want to accept the documented facts that when going on a long road trip, wind resistance and speed variations, not weight, are your biggest enemy is beyond me.

Enjoy your rig...

[CAG]

Originally Posted by Grumpy

DonBr said apples and oranges, I say apples and watermelons.

This whole conversation started because you questioned why you didn't see a major difference in AVERAGE mpg while on a road trip. No where did anyone talk about what happens when you hit a hill for a short spurt in an RV. Why you don't want to accept the documented facts that when going on a long road trip, wind resistance and speed variations, not weight, are your biggest enemy is beyond me.

Enjoy your rig...

Yep!
And if the OP keeps changing the question it is pretty hard to come up with an answer.

[Onyrlef]

Well it did get muddy anyway.