Car Top Carrier Question

[MetroMPG]

Pack-Right Cargo Saddlebag Car Back Carrier by Piggy Pack

[CROSS CANADA WITH 8]

Originally Posted by MetroMPG

Pack-Right Cargo Saddlebag Car Back Carrier by Piggy Pack

Yes, I thought about one of those saddlebags, but I am wondering how it is legal as it covers up my license plate and safe as it covers up a good portion of my tail lights. The Sienna tail lights are partly on the tail gate.

[brucey]

Those saddlebags look scary and expensive.

You can pick up a hitch carrier for 70$ brand new. Throw in another 5 for some plastic storage containers to keep the weather out.

That's going for double that.

I wonder if the makers are claiming an increase in MPG like another bag I've seen running around?

[MetroMPG]

Wouldn't want to obscure the lights, no.

And I have no idea about pricing of either this or the hitch style. (Do you have a hitch?)

Either way, carrying stuff on the rear is definitely the way to go from an aero perspective.

[Christ]

winkosmosis -

I say that because the largest percent of them that I've seen AREN'T flat on the back. They've got some stupid rounded shape that forces the air up, making an attempt at clean separation, but leaving an obviously larger wake. If the cargo carrier had a true downward taper at the rear, it would still be better off backward in some cases, because the front of them almost always has a stagnation point lower than the horizontal center of the box itself. A flat "pig nose" with faired/rounded edges tapering back into the same shape as the front of those cargo boxes would be the best solution, I believe.

Of course, I've never done any testing on this, because I carry everything inside/behind my vehicles. I'm not aware of any wind tunnel testing by Thule, either, but they can still make claims that their boxes are aerodynamic, right?

*(Of course, they're protected by semantics in their claims, because the word aerodynamic is abstracted to mean so many different things, and if they could prove something so stupid as the box providing added lift - negative or not - they could be safe in saying that it's "aerodynamic".)

[CROSS CANADA WITH 8]

Thanks for the suggestions-- all of you who have made them.

Unfortunately, I don't have a hitch, but I suppose that is better than having the wrong kind of hitch.

I will look into something like this, because we make this trip every other year (we have done it twice now) because my wife's parents are too old to fly out to see us like they used to. We also go to North Carolina every year where my parents are and where I have a meeting I am required to go to with my work, so it is not just for this year that I am looking into a better way.

[MadisonMPG]

Look around craigslist/ebay and buy a hitch i'm sure the van has a place to mount one. It should pay for itself on the first trip. You could eve Kammback from the sides to the thing to help with aerodynamics more. I would not suggest slowing down. It's not "worth" it.

1 way:
57 hours @ 70
61 hours @ 65
72 hours @ 55

Well, dropping to 65 may be worth it. It is only 4 hours more. Check your economy @ 65 and 70 and report back to us, that will help us/you decide if slowing down is worth it.
Now those numbers may be wrong, I have stayed up too long. :P

[Christ]

Originally Posted by MadisonMPG

Look around craigslist/ebay and buy a hitch i'm sure the van has a place to mount one. It should pay for itself on the first trip. You could eve Kammback from the sides to the thing to help with aerodynamics more. I would not suggest slowing down. It's not "worth" it.

1 way:
57 hours @ 70
61 hours @ 65
72 hours @ 55

Well, dropping to 65 may be worth it. It is only 4 hours more. Check your economy @ 65 and 70 and report back to us, that will help us/you decide if slowing down is worth it.
Now those numbers may be wrong, I have stayed up too long. :P

Aero mods and driver mods seem like they're going to be your best friends here... Unfortunately, I still don't recommend EOC for automatic transmissions, because there just hasn't been enough data retrieved on the subject to verify that it's OK for all instances, but that would really rock the MPG world for you, especially over such a long distance.

A few simple things you can do is try to time your fuel/piss stops so that you're driving during times where traffic isn't at a peak or a valley. Middle of the line traffic, and basically following their speed creates a nice wall of air effect, especially if you're in a closed highway (sound walls). That helps to divide the aero burden of moving air over all the vehicles around you, instead of squarely on your nose.

Distance drafting may be your friend, as well. Get close enough to draft, but make sure the driver can still see you. I've found that the only time drivers really get mad about me drafting is when they can't see me behind them, but know I'm there. I keep a headlight poked out to the side for that purpose alone.

Check under your rear bumper and make sure it's not a parachute... there's a quick mod you can do that should help out a bit. If it's a parachute, you can add a piece of coroplast from the underbody down to the lower edge of the bumper, and that should keep the flow from being obstructed there.

[DonR]

Someone I know got a saddlebag for his Sienna (2000 I think) He was able to arrange it so the license plate & lights were showing. Drove it to Wisconson & transmission took a crap in Chicago. Ended up with a different vehicle on the trip. I will ask what brand how it worked.

How about making aerodynamic modifications to the Big Mac you already have? Tapering the front more & adding a boattail/kammback onto the rear. I know this suggestion was made about a roof mounted ladder at one point.

Don

[CROSS CANADA WITH 8]

Thanks to all of you for your helpful suggestions.

I went out today and did a fairly controlled test with the car top carrier on at various speeds, and then off at exactly the same speeds. I thought you would all be interested to hear the results.

It was about 50 degrees, cloudy, with no rain and lower than average wind (for NS). I had to drive to the testing area, so I was able to fully heat up the engine before I began. I drove between two exits that are about 7 miles apart. I set my cruise (which stays dead on) at a starting point and never touched the gas or the brake once until I reached the ending point. Then I turned around at the exit and did the same thing in the other direction (getting a very consistently different result because the return trip was uphill). Then I averaged to get a round trip result.

Here are the results:

Average MPG.

Speed -----Without ----- With --- Percent
----------- Carrier ----- Carrier --- Change
60 mph --- 35.4 mpg -- 32.2 mpg -- 9.0%
65 mph
70 mph --------------- 27.5 mpg
75 mph --- 28.25 mpg - 25.3 mpg---10.4%

Comments:
1. As you would expect, at higher speeds, the
carrier has a greater effect at higher speeds.
2. Speed is a bigger factor than the carrier.
- Slow 15 mph w/out carrier improves mpg 20.0%
- Slow 15 mph with carrier improves mpg 21.4%
- Slow 5 mph with carrier improves mpg 8.0%

Cost for 10,000 miles with gas at $3.25/gallon

Speed -- Carrier -- MPG -- Gallons -- Cost
- 75 ----- Yes ---- 25.3 --- 395 --- $1284
- 70 ----- Yes ---- 27.5 --- 363 --- $1180
- 60 ----- Yes ---- 32.2 --- 310 --- $1007
- 75 ------ No ---- 28.25 -- 353 --- $1147
- 60 ------ No ---- 35.4 --- 282 --- $916


So…
What difference does it make to remove the carrier?
- At 75 mph, it saves $137
- At 60 mph, it saves $91

What difference does it make to cut speed if I use the carrier?
- Cut speed from 75 to 70 mph; save $104
- Cut speed from 75 to 60 mph; save $277

What difference does it make to cut speed if I am not using a carrier?
- Cut speed from 75 to 60mph, save $231

What difference does it make to cut speed and to remove the carrier?
- Remove carrier and cut speed from 75 to 60mph; save $368
- Remove carrier and cut speed from 75 to 70mph; save $230*

*The last of these is a reasonable estimate as 70 with no carrier was never measured.