A Tesla Model 3 owner figured out that his electric car is much more efficient with a roof-rack box gets if he flips the box around.

Philpug

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graham418

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Any aerofoil works like that. Think of a wing. Im guessing his efficiency is a result of reduced ground force
 

teejaywhy

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I still remember the two Freds who came into our shop with their cycling aero helmets on backwards... Looked like giant woodpeckers...
Wish you would have taken a photo of that!
 

LegacyGT

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Any aerofoil works like that. Think of a wing. Im guessing his efficiency is a result of reduced ground force
Agreed, the profile looks more like a wing in this direction but I don't know that you get more lift one way or the other. Lift is generated by the pressure differential caused by the air flowing over the top traveling a great distance than the air flowing under the bottom. In this case those distances are the same in either direction.
 

Started at 53

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Tape measure form leading edge over the top and the bottom and it is longer over the top it looks to me. Basic aviation lift theories should provide lift in that circumstanc.

Still looks silly
 

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I expect this has more to do with drag than lift. From my limited understanding, the narrow trailing edge of the box should reduce turbulence produced by the roof rack, which results in drag.

Edit: Also, looking at the pictures, the leading edge sticks way out over the windshield when installed the "correct" way. All that air flowing up the windshield is going to have to go somewhere, and most of it won't be under the box.

I bet this would be more efficient on just about any smaller car.
 

cantunamunch

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If you mean by that link to show that conventional explanations of wings are nothing other than mythology - I agree with you. There's a detailed lecture on the topic here:


I expect this has more to do with drag than lift.
If by "this" you mean 'the effect this person has observed' - again I agree with you. It has more to do with drag than lift BUT the lift that does occur can make the car unstable both by lateral deflection (the yaw angle is not always zero) and by vertical forces.


Edit: Also, looking at the pictures, the leading edge sticks way out over the windshield when installed the "correct" way. All that air flowing up the windshield is going to have to go somewhere, and most of it won't be under the box.
You are absolutely right to consider that - but don't forget that for all the air coming up from the windscreen there is an equal volume of air coming from straight ahead, and there is a 'mixing bowl' of airstreams right where the fat part of the box pictured currently sits. The only part of that airstream that the box is correctly teardrop-shaped for is the direct ahead portion.

Have another look at Thule and Yakima boxes - in their conventional orientation. Look for an added volume like a fat proto-teardrop on the front, that dips in towards the windscreen. There is no way that was designed without considering that mixing area.


I bet this would be more efficient on just about any smaller car.
I can tell you from direct experience reversing the box isn't more efficient if the box is a Thule Pulse with that little dropper front and the car is a VW Beetle or Honda Civic or a GLC 300.
 

anders_nor

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I managed to fit 10 skis, 2 snowboards, and 3 people in my model 3 for a weeks ski vacation.

I have an order in for the Y... we will see!

the ski rack attaches to the GLASS ROOF and has me SCARED, so noop on skiboks here.
 

Tim Hodgson

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I would say that if you have strong racks and crossbars that, with the box in that orientation, lift will lighten the vehicle weight on the road.

And if you don't have strong welds on the roof, strong racks and strong crossbars, when you are driving 80-95 mph on I80 into a headwind on your way to Alta, UT, lift will rip your box off the roof and you will see it bouncing end to end on the road behind you in the rear view mirror.
 
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cantunamunch

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And if you don't have strong welds on the roof, strong racks and strong crossbars, when you are driving 80-95 mph on I80 into a headwind on your way to Alta, UT, lift rip your box off the roof and you will see it bouncing end to end on the road behind you in the rear view mirror.
I call fake news. That's a cross-sectional area of what, 3-5 leaf blowers tops. Borrow some of your neighbours' blowers and try it with the car standing still. Betcha it doesn't even load up the clamps much.
 

mdf

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I call fake news. That's a cross-sectional area of what, 3-5 leaf blowers tops. Borrow some of your neighbours' blowers and try it with the car standing still. Betcha it doesn't even load up the clamps much.
Maybe we can get the whole car to lift off!
 
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