Car roof cargo box

[pmark]

Hi,

I'm looking to get a car roof cargo box, what are some things that I should know about when trying to pick one up? Does anyone have any experience with the cargo bags instead of the hard box?

Thanks!

 

[NutBucket]

I figure hard boxes are more aerodynamic but either one kills fuel economy. I don't like soft bags as they usually sit on the roof and are likely to scratch the paint. An alternative may be a cargo basket. Of course, that won't offer any weather protection but it's more versatile. The real downside is cost. Hard carriers and baskets tend to be pricey. Soft bags are relatively cheap.

 

[Yuriman]

If you can get a hitch-mounted cargo box, it won't hurt your fuel economy in the same way. If you must go with a roof-mounted box, longer, narrower and shorter are better. The more you have facing the wind, the more it'll hurt your economy.

 

[pmark]

I ended up getting one from Pep Boys: http://www.pepboys.com/product/details/9511536/00818

The price was much cheaper than a brand name one and people say that is it made by Thule. I figure I can't justify the 2/3x cost of a real Thule/Yakima since I'm only going to be using it for a hand full of times a year. Those read mounted cargo boxes are pretty expensive too.

 

[NutBucket]

If you can get a hitch-mounted cargo box, it won't hurt your fuel economy in the same way. If you must go with a roof-mounted box, longer, narrower and shorter are better. The more you have facing the wind, the more it'll hurt your economy.

This is a great option if it works for you. Truck or suv no problem. Anything smaller and the tongue weight is too low to be useful.

 

[Yuriman]

This is a great option if it works for you. Truck or suv no problem. Anything smaller and the tongue weight is too low to be useful.

I opted for a trailer behind my G1 Insight instead, because of tongue weight. I'm getting close to 10,000 miles on the trailer.

 

[Zorba]

I ended up getting one from Pep Boys: http://www.pepboys.com/product/details/9511536/00818

The price was much cheaper than a brand name one and people say that is it made by Thule. I figure I can't justify the 2/3x cost of a real Thule/Yakima since I'm only going to be using it for a hand full of times a year. Those read mounted cargo boxes are pretty expensive too.

It is hard to tell the shape from the picture, but if it looks like most with a pointy end and a flat end, put the flat end facing forward and the pointy end backwards. This is a much more aerodynamically streamlined shape.

Since it looks like it opens on the flatter end, make sure you get it latched .

 

[pmark]

It is hard to tell the shape from the picture, but if it looks like most with a pointy end and a flat end, put the flat end facing forward and the pointy end backwards. This is a much more aerodynamically streamlined shape.

Since it looks like it opens on the flatter end, make sure you get it latched .

Really? How come almost all manufacturers make it so the pointy end is the front?

 

[LTC8K6]

Yeah, the pointy end is the front.

 

[Yuriman]

Really? How come almost all manufacturers make it so the pointy end is the front?

He's right, though. Manufacturers put them with the pointy end forward because they "look" more aerodynamic that way, but they're not. It's counter-intuitive, but consider how an airplane is shaped. Big, bulbous nose, and the tail comes to a point. Or, a water droplet, falling through the air. The pointy end is at the tail.

^ Take a look at the two teardrop shapes. It's surprising, but the one with the point forward has 7x more drag than the one with the point in the rear.

EDIT: The reason for this is that turbulence in the wake is what causes most drag. Having a taper in the back reduces your turbulent wake.

Here are some of the most aerodynamic cars:

 

[Midwayman]

I figure hard boxes are more aerodynamic but either one kills fuel economy. I don't like soft bags as they usually sit on the roof and are likely to scratch the paint. An alternative may be a cargo basket. Of course, that won't offer any weather protection but it's more versatile. The real downside is cost. Hard carriers and baskets tend to be pricey. Soft bags are relatively cheap.

The real upside of a hard carrier is its lockable. Sure they're not that difficult to break into, but I'd worry a whole lot less about them be rifled through while I'm away from the car for a couple hours.

 

[LTC8K6]

Yuriman, the equivalent to putting the cargo box on "flat end first" would be having the Insight be more aerodynamic going backwards.

So, I don't think it's more aerodynamic to put the cargo box on opposite of the mfgs instructions.

 

[Zorba]

Really? How come almost all manufacturers make it so the pointy end is the front?

Because manufacturers market to people that don't understand aerodynamics and believe the pointy end should be in front from "intuition," instead of actual physics knowledge.

 

[Murloc]

fuel economy apart, putting it in the centre of the roof and then pulling it to the side to take stuff out looks like a pain.

Yuriman, the equivalent to putting the cargo box on "flat end first" would be having the Insight be more aerodynamic going backwards.

So, I don't think it's more aerodynamic to put the cargo box on opposite of the mfgs instructions.

cars have practical considerations to them as well. Like fitting stuff in the car trunk.

Still, it's better to mount the cargo box by following the instructions as you risk it opening up while traveling or having other issues, and the aerodynamic advantage is not that big from looks of it.

I wonder why an university team built a low-power car with this shape

if it's all bs.

The original insight was also one of the best cars when it comes to drag and it follows this idea quite closely

 

[Yuriman]

Yuriman, the equivalent to putting the cargo box on "flat end first" would be having the Insight be more aerodynamic going backwards.

So, I don't think it's more aerodynamic to put the cargo box on opposite of the mfgs instructions.

The Insight is shaped such that it has a taper identical to an airfoil in the back. Blunt nose, taper in the rear:

I built my trailer so that it would not be in the wind, and continue (at a shallower angle) the airfoil shape. Pulling the trailer on the highway, I get cose to 70mpg.

Here's another drag chart:

EDIT: Most of Thule's boxes do actually have some taper at the rear, but they also have to be able to market them, and few people would buy a cargo box with a blunt nose (even if it has relatively little aerodynamic impact).

 

[Zorba]

Yuriman, the equivalent to putting the cargo box on "flat end first" would be having the Insight be more aerodynamic going backwards.

So, I don't think it's more aerodynamic to put the cargo box on opposite of the mfgs instructions.

Look at his image, drag is reduced 7x when the pointy end is backwards. Car designers have to make several compromises when designed a car, and therefore can do a proper aerodynamic closure on the back end. I think what he was trying to show was that more aerodynamic cars taper their backends much more than say an SUV.

Here is another image that shows the wake behind a streamlined object and a more blunt object.

Notice that the flow separates at the rear of the more blunt end. But the flow doesn't separate at the front of either object. This is because when the air is being pushed out of the way in the front there is a favorable pressure gradient so it is able to follow much more blunt shapes. However, as the object passes and takes up less volume the air must fill this void, but it does so under an unfavorable pressure gradient, so the flow is much more likely to separate and cause a low pressure wake zone. This low pressure wake zone literally "sucks" the backside of object, causing pressure drag. This is why the back of semi trailers and SUV get so dirty, the low pressure area literally sucks dirt up there.

Also look at aerodynamic helmets:

Or wheel pants on a Cessna:

Manufacturers recommend a certain direction because that is what people think it should look like, not because they did extensive wind tunnel testing and provided the entire aerospace industry has been wrong for the last 100 years.

Now if you were driving supersonic, the pointed end should go in the front.

 

[Zorba]

Still, it's better to mount the cargo box by following the instructions as you risk it opening up while traveling or having other issues, and the aerodynamic advantage is not that big from looks of it.

I agree with this if the box opens on the front or back, like the box linked by the OP. Most of the boxes I've seen open on the side, so you there shouldn't be much difference, unless the front had extra sealing.

 

[LTC8K6]

Patented AeroNose Design reduces drag and noise making it the most aerodynamic box available.

Thule must be using a wind tunnel.

But then it would be even better mounted backwards.

This sounds like a new Mythbusters episode.

 

[Zorba]

Thule must be using a wind tunnel.

But then it would be even better mounted backwards.

This sounds like a new Mythbusters episode.

Don't confuse a patent with proof for anything, or trust marketing claims without data. Now if they publish the CD, I'll believe that it went through wind tunnel testing (Out of all the brands, it wouldn't surprise me if Thule did do real testing)

This may be the real reason they put the flat end on the back: "Patented Rear-Angled Base and expanded vehicle mounting points help maximize trunk and hatch clearance"

So again, everything in life is a compromise.

Notice they do have a nice gently slope on the back, before they truncate it. This box turned around may perform worse since it would be sloped against the airstream and there would definitely be separation at the corner.

Now if you flipped it around and made a little round nose for it, it'd do great backwards .

Edit: Speaking of Myth Busters, this is sort of like the one were they showed how the truck tailgate up was much better than down because it created a more streamlined shape. Since then truck manufactures have started adding cab "spoilers" and tailgate spoilers to help make the airflow follow the streamlined shape. I saw somewhere that Ford claimed the tailgate spoiler was worth 1 MPG by itself.

 

[Yuriman]

Edit: Speaking of Myth Busters, this is sort of like the one were they showed how the truck tailgate up was much better than down because it created a more streamlined shape. Since then truck manufactures have started adding cab "spoilers" and tailgate spoilers to help make the airflow follow the streamlined shape. I saw somewhere that Ford claimed the tailgate spoiler was worth 1 MPG by itself.

You might be interested in this:

A Practical Guide to Pickup Truck Aero-mods

 

[LTC8K6]

IIRC, Mythbusters found that using a mesh or net tailgate gave the best fuel economy for a pickup truck. Better than tailgate up or down or off, or a tonneau cover.

Reminds me of the Mythbusters Porsche 928 episode about whether it was more aerodynamic going backwards. It wasn't.

 

[cabri]

IIRC, Mythbusters found that using a mesh or net tailgate gave the best fuel economy for a pickup truck. Better than tailgate up or down or off, or a tonneau cover.

Reminds me of the Mythbusters Porsche 928 episode about whether it was more aerodynamic going backwards. It wasn't.

Reduce turbulance