Aero Standie Build Log

[ibbuckshot]

rgambord,
I would be very interested on weather you could give me some idea of what kind of wind resistance I would be facing with my standie design which is an altered cargo trailer. I would be pulling it with a
small GMC Sonoma pickup truck. The total length would be about 18 ft long including the tongue. The main body would be 14 ft long, 61/2 wide and 61/2 high. At the center of the body (7ft) the roof line
would slope downward to be 4ft at the nose of the trailer. This, I hope, would help make the unit much more aerodynamic. Also the wheels would be designed to be under the body rather then using
finders that would stick out right and left creating wind drag. I am putting my parts list together for this build when I ran across your thread. I use to drive tractor trailer cross country and have always
been interested in fuel mileage and wind resistance. Any information would be very helpful.
Have fun,
buckshot

[rgambord]

rgambord,
I would be very interested on weather you could give me some idea of what kind of wind resistance I would be facing with my standie design which is an altered cargo trailer. I would be pulling it with a
small GMC Sonoma pickup truck. The total length would be about 18 ft long including the tongue. The main body would be 14 ft long, 61/2 wide and 61/2 high. At the center of the body (7ft) the roof line
would slope downward to be 4ft at the nose of the trailer. This, I hope, would help make the unit much more aerodynamic. Also the wheels would be designed to be under the body rather then using
finders that would stick out right and left creating wind drag. I am putting my parts list together for this build when I ran across your thread. I use to drive tractor trailer cross country and have always
been interested in fuel mileage and wind resistance. Any information would be very helpful.
Have fun,
buckshot

Gonna cut your mpg in half. Pretty much the same result I'm expecting. It's cause the trailer is tall. Drag is proportional to Frontal Area * cD, so even if you maintain aero (and you take a slight hit with a boxy trailer), doubling the height of the vehicle will double drag.

[rgambord]

Progress update: The welding place that I ordered a new axle from apparently forgot about me, so screw em. I decided to stick with the standard harbor freight axle/wheel combo, and add wheel wells. I simply cut 7"x18" slots for the wheels, and then used 1x2 pine spacers and 3/8" exterior treated plywood for the top. I also bolted the original wheel wells directly to the plywood to provide rock and water protection. Hopefully it holds up well. I also installed some outdoor carpet I had laying around on the front 2/3 of the trailer. I really like the way it looks:




I had to go back and redo all of the floor attachment, because I discovered that the left wheel is 1" farther from the frame than the right wheel, so I recentered the body of the trailer over the axle, rather than the frame. I'm planning on solid plywood or OSB walls, but I haven't decided exactly how I will attach them. I'm thinking I will screw a length of wood to the inside of the wall and have that rest on the edge of the floor (and cover up the carpet staples). Then, to hold the walls to the trailer, I'll have several small diameter (1/4") bolts through the 2x4 subfloor frame (with large washers to distribute the clamp pressure). This way, the walls will be supported by the lip, which will evenly distribute the load with glue and fasteners, and then the bolts will hold them up against the sides, allowing me to easily remove them for storage/maintenance.

As for the roof, I plan on having it be several pieces of FRP which will be wider than the trailer by 1-2". Each section will have several slats running across it (across the trailer). The slats will be screwed to the frp where it extends past the walls, which will prevent water from leaking in, since there will be no through fasteners between the walls. Finally, the individual pieces will just rest on top of the walls, which will have slots cut out for the slats to slip into. Either the slats will have grooves to accept the walls, which will keep the walls apart, or each slat will have a bracket on each side, and a bolt to attach to the top of the wall. Sketches to follow tomorrow when I have more time to work on it.

[ibbuckshot]

rgambord,
Thanks for your thoughts. I guess I will have to deal with what ever I can get as to the mileage. I have a thing against motels. I would rather be able to pull over some place and take a nap then
try to find motel and be disappointed. I know my little trailer will be much cleaner and more convenient.
Thanks again.

Have fun,
buckshot

[lrrowe]

That is what I have come to accept. I am just not going to worry about the wind resistance and resulting gas mileage. It is what it is. I am just going to enjoy the fun from the trip.

[capnTelescope]

This is an interesting subject, rgambord. You are contributing valuable information that obviously interests a lot of us. But I wonder if we aren't barking up a minimally productive tree.

I doubt that any aerodynamic blunder short of towing a parachute is going to halve your overall gas mileage. Doubling the drag would double the aerodynamic drag un-contribution to overall mileage, but not halve overall mileage

Let's not forget that there are other factors affecting gas mileage besides aerodynamics. Most notable are rolling resistance, weight (mass) and driving style.

Rolling resistance will be reduced most easily by keeping your tires properly inflated. (Sometimes I'm called CapnObvious) Even better would be solid steel wheels--think railroad cars. They'd just be unacceptably noisy, ride rough and have poor traction. The flywheel effect of such a heavy wheel would also shorten the life of your brakes.

Weight is the real mileage killer. Sadly, we can't drive anywhere without having to accelerate or climb a hill. (How is it that going downhill never seems to balance out going uphill?) More weight translates into more work for the TV. More work becomes more burned fuel.

Driving style. I notice that when I'm towing, I have to put my foot in it more than when I'm not. This includes driving away from a full stop where drag isn't much of a factor, as well as accelerating at freeway speeds where drag is significant.

So, keep your tires properly inflated, ruthlessly avoid adding weight to your trailer, and don't drive so fast. But if I had a lower cD than you, I'd be sure to brag about it.

Thanks for an interesting thread!

[rgambord]

This is an interesting subject, rgambord. You are contributing valuable information that obviously interests a lot of us. But I wonder if we aren't barking up a minimally productive tree.

I doubt that any aerodynamic blunder short of towing a parachute is going to halve your overall gas mileage. Doubling the drag would double the aerodynamic drag un-contribution to overall mileage, but not halve overall mileage

Let's not forget that there are other factors affecting gas mileage besides aerodynamics. Most notable are rolling resistance, weight (mass) and driving style.

Rolling resistance will be reduced most easily by keeping your tires properly inflated. (Sometimes I'm called CapnObvious) Even better would be solid steel wheels--think railroad cars. They'd just be unacceptably noisy, ride rough and have poor traction. The flywheel effect of such a heavy wheel would also shorten the life of your brakes.

Weight is the real mileage killer. Sadly, we can't drive anywhere without having to accelerate or climb a hill. (How is it that going downhill never seems to balance out going uphill?) More weight translates into more work for the TV. More work becomes more burned fuel.

Driving style. I notice that when I'm towing, I have to put my foot in it more than when I'm not. This includes driving away from a full stop where drag isn't much of a factor, as well as accelerating at freeway speeds where drag is significant.

So, keep your tires properly inflated, ruthlessly avoid adding weight to your trailer, and don't drive so fast. But if I had a lower cD than you, I'd be sure to brag about it.

Thanks for an interesting thread!

You're correct about air resistance not being the only contributor to fuel economy. Rolling resistance for my car is something like 30 lbs, whereas at 60 mph my car experiences ~85 lbs of drag force, so rolling resistance accounts for about 26% of fuel usage.


Believe it or not, it's actually more efficient to drive uphill and coast downhill than it is to drive along a flat road. The reason for this is that an ICE engine is more efficient under full loads than it is when nearly idling (such as when in 5th gear and only exerting the 115 lbs of force needed to drive at 60 mph). That is, not accounting for difference in actual distance traveled between the two, since going up and down a hill is necessarily a longer distance than straight through it. Interesting thing to think about.


Sadly, this build log may be ending. I am seriously considering just using this trailer as a flat-bed trailer for my stuff, and putting a tarp/ratchet straps over everything. The reason for the build was a cross-country move, but the fuel required to haul what is essentially air is not really going to end up being worth having a travel trailer. I believe I may be better off spending the money on campsites/warm showers and food.

[capnTelescope]

Sadly, this build log may be ending.

Oh nooooo! You gotta do what you gotta do. Hopefully, you'll be back here after the move.

Believe it or not, it's actually more efficient to drive uphill and coast downhill than it is to drive along a flat road.

Here's one to chew on: I drive from the Austin area to Houston and back fairly regularly. 200 miles, 700-ish feet elevation here, down to sea level there. Prevailing cross-winds from the south, Same route each way, no differences in anything else that I can tell. The trip computer says I consistently get about 1 mpg better on the uphill trip than the downhill one. Probably it's Houston traffic/time of day when going to, but otherwise it's totally counterintuitive.

The trip computer does show "instantaneous" mileage goes up much farther when going down that it goes down when driving up.

Again, thanks for an interesting thread. Be safe on the move and good luck in the new hometown.