Car Top Carrier Question
 

I just registered with ecomodder, and I wanted to ask some of you experts a question about car top carriers.

I have a Toyota Sienna, 2004 that I fill up with six kids and two adults. This summer, we plan to drive from Nova Scotia to British Columbia to see my wife's parents. We will be going about 15,000 kms (9,000 miles) in total.

With all those people, we need a car top carrier. We currently have one of the hard shell type-- rather boxy but rounded with a bit of taper to it-- the kind you can get at sears. It is 18" tall and 36" wide and 48" long. I have considered getting a shorter, narrower, longer style to decrease wind drag, but I have no idea how much difference it would make.

Would any of you be able to give me some idea of how much difference it might make in my gas mileage? I don't need to know exactly, just a rough idea.

With the van all loaded up (so much weight that it rides low) on previous trips with the car top carrier in place, we have averaged between 26 and 27 mpg (which interestingly is the same mileage we get driving around town without the carrier or a load). I ordinarily drive about 5 mph over the speed limit with cruise on which often means 70-75 mph. I do look ahead and coast when I can.

I would really appreciate any help you could give us. Finances are tight with such a large family, so every bit helps.

Thanks so much.

must.... suppress.... urges.....

The cost of a new cargo carrier will buy a lot of gas

A longer and narrower cargo carrier would probably be more efficient, but not enough to validate the cost of buying a new one when you already have one that isn't damaged.

If the one you have now ends up breaking or is damaged or something and you need to buy a new one, I would say it would be worth a little extra to get a long narrow one (assuming they might be more expensive?) but not at this point.

http://www.prolineracks.com/sportrac...arrier-lrg.gif

To anyone out there that wants an off the shelf "cargo expander" buy one like the above.


To the OP, picture of the cargo holder?

The cargo rooftop carriers that you get from sears, you should probably mount it backwards, right at the end of your vehicle. It might hurt a little less that way, by helping to train the flow down over the rear hatch of your van.

But, probably not. You'd almost be best off trying to take everyone's crap and making a boat tail out of it on the back window of the van, honestly. Might get a gain out of it!

15,000 miles? How many times are you going to drive there? It's only like 3000 to 4000 miles between the two. Heck, the earth is only 23,000 miles around at the equator. If you want to save gas, recalculate your route.

Can you not fly? (in a plane)

Basically you want to go fast with a big draggy box on your roof. IMHO you've created a worse case scenario for FE. You're going to take a hit no matter what car-top carrier you choose.

As far as FE goes weight is a dominant factor in stop and go city driving, while high speed cruising is mostly about clean aerodynamics and restraint on the go pedal. Hopefully you can quickly detach the carrier off the vehicle when it's not needed.

As a personal object lesson, I had a VW diesel a few years ago and used a ski rack for obvious reasons. At 70 MPH it knocked almost 25% of my FE, so it stayed off the vehicle until needed. Live and learn.

About the detaching ability--yes, I have it rigged so I can take it off very quickly.

Flying is out of the question with eight people. One consolation, however, is that I get better than 200 mpg per person.

Oh yes, thanks for that. It is 15,000 kms or about 10,000 miles. Nova Scotia is way out to the east of Maine, and we are going to Vancouver Island in the West. Also, we are going on a circle trip, down to NC. I did not give the full route, but you are right. It is not quite 10,000 miles. Sorry about that.

Is is interesting what you say about mounting it toward the back. I played with that on previous trips and it did seem to make a difference. I got better mileage when it was mounted as far back as it will go rather than as far forward as it will go.

Backwards? That is interesting. It looks like it is designed to be aerodynamic. Is that just an illusion?

It's stubbed on the back... you want a slope to the rear. The face of an object has a negligible effect on it's aero profile compared to the rear. The idea is rounded front, sharp rear, mild angle between the two. You want the shape of an object to train flow back to a closing point, not leave it opening as the flow trips from the edge of your object.

There are plenty of resources for aerodynamics here and all over the internets.

I corrected the error about the distance in my original post, so thanks again for pointing it out.

The back of those cargo carriers is usually a pretty flat surface. I know people are fond of saying "it would be better backwards".... No it wouldn't, not when a sheer front surface is slamming into the air, causing it to fan out.

Would you say a Prius is more aerodynamic going backward? No, so why say it about any other Kamm shape?

http://auto.sears.com/staticcontent/...9322G_CL_1.jpg

This one might not be so bad backward
http://i45.photobucket.com/albums/f9...Carrier003.jpg

Slowing down would save you the most.

A hitch rack box would make a negligible difference on a van, it might even help a bit.

A roof box and high speed is a disaster.

Might I suggest a small trailer instead? One with cargo box capacity has negligible drag - I used to pull one behind an 850 cc bike on long camping trips, and it was barely noticible. Other advantages:

1) It's not producing any extra aero drag, and might even help a van's aerodynamics.

2) It takes most of the weight off the van's suspension, so better/safer ride & handling.

3) It's a lot easier to load & unload :-)

4) If you want to make side trips, you can leave it behind.

Do you mean to say that it would be better? Mine is more or less shaped like the second one, although taller and wider and not nearly as long.

This is unquestionably the route you should take if you have the option.

Yes, it makes parking harder, and hatch access more difficult.

But it's by far the best way to to improve efficiency vs. a roof carrier.

Have you done the math to calculate the MPG penalty of your rooftop carrier? If I had to guess, I'd say it's in the range of 10-20% at typical freeway speeds.

http://www.piggypack.com/website/art...adle_large.jpg

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.

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?

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.

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".)

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.

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.

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

Car Top Carrier Test
 

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.

Great job quantifying that data for us! I know that careful testing takes time (and gas) so it's not always easy to really spell out the cost savings like you have done.

So what will you do...slow down with no carrier and save $368?

So what will you do...slow down with no carrier and save $368?

Right now, I am looking at getting a Sport-rack type of device that mounts on the back, but I have to look into the cost of getting a hitch. On a trip that long with 3 daughters who are 16, 18, and 21, and 3 sons (3, 6 & 14), I definitely need the extra space (especially since we are picking up one of them from university enroute). I think I will slow down a little (5 mph), but not 15 mph as that would add 4 days of travel which we can't afford in more ways than one!

you should be able to find a trailer hitch online for under $200 shipped, installation in very easy. Most new vehicles have the holes there (threaded too), it just requires you to crawl under the car and attach 6 bolts. then you are done. I did this with my Matrix 3 years ago and it took about 10 min.

once you have a hitch get a rear basket thing recommenced by others (or borrow one from a friend). both of these options will pay for them selves in the first tip.

Just to add a caution. You have stated that your van is already overloaded (sagging) and putting too much weight on the very rear could create a very bad stability problem if a gust of wind hit you or you needed to make a sudded move. I don't think I need to get graphic. Something to think about though.

I'm confused. What do you mean between these two? (bold ones)

I second the trailer hitch buy/self install.

I've had mine for 2 years and I couldn't live with out it.

Trailers, carriers, tow hooks. It's the ultimate in utility.

Sorry, I can see how that might not be clear.

The first two show the difference a change in removing the carrier makes assuming I was already going to drive the speed indicated.

The second bold one is the difference in cutting speed alone when I am not taking the carrier anyway.

The guy at work who has a car backpack thinks it is an Aerosoft brand. He mounted it so the bottom sat on the rear bumper, than moved his license plate into the rear window. He said it worked well until the trans went out. He ended up with a bigger van to finish the trip & didn't need the cargo room.

You may want to investigate shipping some of your cargo to destination if possible. This would cut down on drag, weight & passenger complaining.

Don

Thanks for sharing the calculations. The math makes this a compelling argument, especially when you consider the fact that's savings from one trip, and you do it each year.

Look around a bit and you might find a deal on a hitch. I found one on the local craigslist for $60 for my sienna. Make sure you buy a tap and run it through the welded nuts on the van body before installing a hitch. They get rusty and can easily cross-thread which is a big problem.

A lot of sienna owners who tow buy rubber air bladders that go inside the rear springs. They can be pumped up and help with excessive sag when loaded. They are pretty cheap and easy to install and might be worth it for this situation - especially if you go with the hitch route.