Car Top Carrier Question

[CROSS CANADA WITH 8]

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

[CROSS CANADA WITH 8]

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.

[CROSS CANADA WITH 8]

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?

[Christ]

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.

[CROSS CANADA WITH 8]

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

[winkosmosis]

Originally Posted by Christ

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!

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?



This one might not be so bad backward

[brucey]

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.

[jamesqf]

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.

[CROSS CANADA WITH 8]

Originally Posted by winkosmosis

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?



This one might not be so bad backward

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.

[MetroMPG]

Originally Posted by MadisonMPG

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.