Drag and rolling resistance formula

The whole site is worthwhile. As are the forums. Those of you with an interest might consider contributing.

There are interesting cases. A set of threads by "Orbywan" detailed how he took his 1995 Ford E350 Class C (7.3L) to 15-mpg with an attractively made boat tail.

Nice as are reductions in fuel burn, so is the reduced strain on the drivetrain. And far fewer steering corrections. Easier up the grades.

wilber1 wrote:
.34 is the CD of a Camaro. Looking at CD's for 1500's, our HD's should be .5 or more and with a barn door fiver behind .8 might even be conservative.

What you're saying sounds reasonable at first but, if the coefficient of drag were .8 it would be impossible to get better than about 7 mpg driving 60 mph pulling a big 5th wheel. Because we generally get about 10 mpg and we know our diesel engines can't achieve an efficiency beyond 40% nor can the drivetrain efficiency be any more than 85% the very highest value that seems to make sense for the CD is .47.

With a CD of 4.7 for my RV I could still get 10 mpg towing at 60 mph yet at 120 mph I would be needing 605 HP and be getting 3.4 mpg.

According to the formula, the HP required to overcome drag at 120 mph with my RV, assuming a CD of .47, is 531HP and the HP required to overcome drag in a Camaro at 120 mph with a CD value of .34 is 88 HP. This sounds reasonable.

Turtle n Peeps wrote:
The problem with these formulas is you don't know what numbers to plug into it.

Here is a real example of what little change can make a big change in the numbers.
Back in the day I used to work on airplanes. I guy came into my buddies shop and wanted to change all the button head rivets on his plane to flush mount rivets. It was a big job but my buddy did it. The plane picked up 7 or 8 MPH. I would never have believed such a little change could make such a big difference in drag.

My point is, you can plug in any numbers you want on the front end and the program will spit out any numbers you want on the other end. Cool to play with but of no real use unless you have a wind tunnel at your disposal.

This kind of thing is not uncommon. For people that own sailboats that race real competitively, they will use 3-4' long hand sanders to make sure the bottom is really smooth. Along with remove the typical thru hull fitting which is a mushroom with 1/4-1/2" off the hull to a flush thru hull. Take and figure out the shape of the keel, again fair it to the correct shape with molds....gain can be upwards of .2-.5 knots of boat speed. When you figure some boats have a max speed of 5-6 knots, that is a 5-10% gain on a competitor. Enough to put you from middle to the front of a pack!Or back to middle, then with a few more improvements, you are at the front.

I have some charts that show HP needed to move a given load, but, notes say, the HP needed can vary by plus minus 30% based on many factors. Some include tire type, design, rubber of tires, road type, aerodynamics of the rig itself. The info these charts come from, have a formula that has upwards of 15 variables to add or subtract HP needed from a given rig or load.

At the end of the day, these things can be fun to play with, and for one to realize, you can get better mpg with an 18K load, vs a 15K load, as it is pulling my bobcat trailer vs my old TT. The bobcat trailer weighs 3K lbs more, but due to a lower aerodynamic, it got better mpg being pulled than the travel trailer. Many have commented that going from a TT to a 5w that weighs more, gained mpg too, do to a better aerodynamics of the setup.

Marty

The problem with these formulas is you don't know what numbers to plug into it.

Here is a real example of what little change can make a big change in the numbers.
Back in the day I used to work on airplanes. I guy came into my buddies shop and wanted to change all the button head rivets on his plane to flush mount rivets. It was a big job but my buddy did it. The plane picked up 7 or 8 MPH. I would never have believed such a little change could make such a big difference in drag.

My point is, you can plug in any numbers you want on the front end and the program will spit out any numbers you want on the other end. Cool to play with but of no real use unless you have a wind tunnel at your disposal.

Realistically you need something that takes into effect the combination of TV and Trailer. The truck obviously has a major affect on the air that gets to the trailer. You would think that the area in between would be turbulent and the bottom of the trailer would be shielded to some extent by the truck ahead of it.

You probably need to do wind tunnel measurement to get a somewhat real number.

.34 is the CD of a Camaro. Looking at CD's for 1500's, our HD's should be .5 or more and with a barn door fiver behind .8 might even be conservative.

I used a CR of .012, a CD of .4, an engine efficiency if .4 and a driveline efficiency of .8 and got some numbers that seemed pretty believable.

Looks like you need to do it twice. Once for the tow vehicle and again for the trailer. Ignoring the TV it looks like I should be able to tow my 5ver at 115 mph before I run out of hp.