The aerodynamics of an open top box at speed

[Mtneers]

I hate the looks but it’s tough to beat the functionality.


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[Gone in 60]

By the way, I do need to credit the California Freeway Service Patrol. When I stopped on the shoulder this morning, got off the bike and was rummaging through my trunk to see if anything was missing, one of their tow trucks stopped behind me. I gave him a thumbs-up, and he gave me a thumbs-up back, and we both went our ways.

Nice to know he was lookin' out for me!

[Cormanus]

It’s good that no one has answered your question about the aerodynamics, Gone, as it allows me to join yet another queue. I can’t help either. Like you, I find it curious. You’d expect it to slam shut.

I’ve two theories: first, it’s another application of whatever law of nature it is that says things that will embarrass or inconvenience you will occur in spite of all logical expectations to the contrary. The second is that it must have to do with the airflow around the the bike. You could take everything out, leave the lid unlatched and go for a ride with a mate astern of you to see whether it happens again. You may find you have to put something important in there so you can lose it to repeat the result.

[Gone in 60]

I have to think that the air passing over me, as the rider, must create a swirling motion that results in a back-current that is pushing up at that spot.

Sort of like my hatchback, which has a large spoiler on the back. If I drive through dust, it will stick to the back window.

Most likely it's the law of nature that is just trying to embarrass me, as you say.

[noroomtomove]

I'll have an attempt with this.

As you are riding along at 80 miles per hour air is rushing all about you now just for a minute don't think of air as a gas but as a fluid. When water flows down a river the volume of water moving stays the same but when the depth of the water in the river is reduced the water speed is increased. Now air unlike water is not confined by the banks of a river but as a gas moves at different speeds as it moves over parts of you and the bike this creates areas of low and high pressure so as you rode down the freeway at 80 miles the area behind you back became a low pressure zone this once it over came gravity lifted the lid due to the latch being what it was. In a similar manner the air passing over the wing of an airliner is forced to travel further on the top of the wing than the bottom side due to the wings shape thus creating a low pressure area above the wing relatve to the area below, this is sufficient to lift the aircraft of the runway on takeoff.


regards NRTM

[Gone in 60]

So then what you're saying is that if I left the trunk open... I should be able to fly?

[Lord Popgun]

I have a Cortech bag mounted on the seat with the top opening from back to front (and yes a top box on the rack). If I forget and leave it unzipped it will lift up and flap against by back telling me there is a low pressure area behind me.

Hey Noroomtomove, I learned everything I needed to know about aerodynamics from Sister Bertrille—When lift plus thrust is greater than weight plus drag...

[Houtman_imp]

So then what you're saying is that if I left the trunk open... I should be able to fly?

Yes , and living in California you would as high as a kite !

[GoldOxide_imp]

You want bulbous? Here's bulbous. The V40 is my normal go-to, since it's smaller and lighter and holds the stuff I usually take back and forth from the office. Looks like a Weber barbecue on the back of either bike, but when I get to work, it clicks off and I carry it in like a briefcase.

And yes, I get a lot of grief from the Triumph purists who say I ruined a perfectly good Bonneville. But, the rack is held on with six screws, and if it bugs me enough, it comes off with one Allen wrench and ten minutes.

Givi makes a trim piece for the V40 which I'm thinking of buying to break up the black bulb look, but I don't think fifty bucks worth of additional plastic can polish this very practical turd.

Weber BBQ is just about spot on.

I hate the looks but it’s tough to beat the functionality.


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We hear ya!

[tod.branko_imp]

I'm not an expert in aerodynamics but i know that the bikes with screens tend to create an airflow that rolls forward from the back of the bike (basically turbulence) that was one of major quirks of that old Honda 800, what was it's name... Pacific Coast, yes. It would create this bubble that would keep smacking you in the back of the head constantly. I think it has something to do with creating a vacuum behind the windscreen and then the air would rush forward to fill up the negative pressure (but don't quote me on that), that's one of the reasons modern screens have some vents at the bottom to allow some air to fill up the bubble of negative pressure and reduce turbulence.
Anyways I think that could effectively keep the top box open.

I've only ever had one bike with a top box, it was a maxi scooter - Aprilia Atlantic 500. It was a Givi box, and fortunately I haven't had any issues with it, and the Mrs enjoyed having a backrest. The bike, on the other hand, was utter garbage, and I couldn't get rid of it soon enough.

I did forget to close my tank bag on a few occasions and I think it was only pure luck that saved me from losing all my stuff

'14 CB1100 STD 5 speed