How bad is this, Aerodynamically? (rear-mounted bike rack on SUV)
Hard to pinpoint exactly but when my bike is on the back of my Forester I seem to take a 1-2MPG hit on the highway. Doesn't seem like it should affect the MPG (one of the reasons I got a hitch rack rather than a roof rack). Would the factory rear 'spoiler' (2nd pic) do any good?
http://www.subaruforester.org/vbulle...fc6b90be0a.jpg http://www.trucktastic.com/images/D/TT-SUFR0910.jpg |
Can you easily remove the handlebars ? I would think you could, but you would have the brake lines holding it on.
You could simply zip-tie it to the bike frame and re-attach it once at your destination. The handle bars are right inside where your wake spills over the rear of the vehicle ( unfortunately, so are the edges of the tires, but I would see if the handle bar removal helps any. ) |
Much better than the roof rack obviously but I have been told by a fellow van engineer that the aero concept over those is that they want the wake to somehow "push" the vehicle.
If the concept is true, then a bike would affect it. And let's be honest, your car is more van sized than car sized ... |
It's far, far better than a roof rack.
If the bike's tires extend past the sides of the vehicle, then it's worse than not having anything at all back there. In fact, even if the tires only approach the sides of the vehicle, it'll be worse than nothing. That rear spoiler will help if extends the downward "taper" of the roof line at the rear (should be easy to eyeball). If there's no obvious downward taper, it may still help, but only careful testing would confirm. Regardless, it would not undo the effect of having the bike's tires extend wider than the rear of the vehicle though. |
PS: delete your roof racks. :)
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The tires do, in fact, extend a few inches past the car on both sides. This is probably where most of the MPG hit is occurring. Not much I can do about that, but on long trips I may go ahead and remove the bars and velcro to the frame. Sometimes I use an extender for a 2nd bike and I'll bet the bars (and seat for that matter) would have even more effect.
Roof rail delete is on the list of things to do! |
Just a suggestion...
Remove the front wheel, turn the handlebars parallel to the frame, center bike on rack. Use motorcycle tie down straps to secure the bike to the rack. > |
Have you tried transporting it inside ?
With the front wheel out and the seats partially folded down, it looks like it'll fit inside. |
It easily fits inside .... if I fold down the rear seats. I don't particularly like to do that because mo matter how careful I am I end up scuffing up the inside of the vehicle (the headliner, sides) and chain lube and dirt gets everywhere. If I remove the front wheel it becomes easier. I like to just throw it on the rack which takes less than 30 seconds no exaggeration and be on my way. I guess it's a choice I'm making. Yeah, I know I'm whining.
For long trips I have lots of other stuff inside not to mention the wife and her bike. For those long trips I may do as you suggest and use a tiedown to hold the fronts to the rack. This would allow me to get the rear wheels out of the side airstream of the vehicle. |
Quote:
They come in all sorts and sizes. Sure, it's a bit more work than putting the bikes on the rack. Quote:
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Pulling one wheel and moving over should get most of your loss back, as suggested above. At some point it should be inside a "bubble" that will form in any case. That is the "money" bubble we try to eliminate :D
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A 1-2mpg loss with a bike on back isn't much. Go ahead and remove the front wheel, turn handlebars flat and recenter as suggested it can't hurt. But don't expect to get back to baseline. The bubble behind your tailgate isn't static. A pair of eddy currents rotate in 3D with mirror symmetry down the vertical center plane of the vehicle. Balance dictates forward flow up the center then outwards towards each side. A smooth tailgate facilitates that flow bubble. A bike impedes that flow. Satisfaction follows expectations. :)
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I have had a new Kia Rio for the last week. Most of my hiway driving up to this point has returned mid thirties mpg. Today I drove to Burlington, VT. Brought the wife and both our mountainbikes on the trunk mounted rack. It absolutely killed my mileage. I saw about 26 mpg. I could definitely feel it. Like I was dragging a parachute.
I would guess that the SUV wasn't hurt as badly as the roofline is high all the way back, so the bike is tucked down in the wake. Not so a fairly aero sedan. I could have done as well bringing the wife's Odyssey with the bikes inside! |
Are you sure? 1-2mpg on 30? How much raw data collected in what way?
It could be head/tail winds or even barometric pressure. Without the bike is without the rack as well? Adding a diverter below the back bumper might do as much or more than the 'spoiler'. |
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