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Old 10-28-2014, 09:12 AM   #11 (permalink)
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Quote:
Originally Posted by palmer_md View Post
For this drafting test, do you mean finding a friend with a car and following very closely, or just following somewhat close to any vehicle, or are you talking about following trucks?
Its a simple and quick test to do if you have an instantaneous mpg gauge on board. Find a flat stretch of road on a windless day, pull up behind a truck and draft him long enough (30 sec to a minute) to get a good idea of your mpg in that mode and speed, then slow down and back well out of his turbulence zone, then speed up to the same speed you were drafting at and note what your mpg is. It only takes a couple of miles of highway and a few minutes of your time. How close you get, consistent with safety, on the drafting portion of the test is up to you, obviously closer is better as far as showing you the ultimate low aeromodded mpg you could get. But you don't need to get all that close, as this test is just for giving you a ballpark figure of the mpg improvement you could expect to get via aeromods and to see if aeromodding is worth doing on your vehicle. When drafting, engine rpm, drivetrain losses, and rolling resistance are unchanged. The only thing that has changed is a reduction of the wind load on the car, which is exactly what you are doing when you lower your Cd via aeromods.

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Old 10-28-2014, 11:02 AM   #12 (permalink)
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Quote:
Originally Posted by freebeard View Post
It wasn't documented, but as basjoos says, it probably about doubled my mileage. But I picked up a lot of rock chips in the paint and I was a nervous wreck, so I don't do it anymore.

Instead of rear wheel spats, maybe something like this:



Instead of a tab running across the tread, divergent fences that force the wheelwell turbulence outward where it can be sheared off by the ambient flow, instead of under the car. Adjusted for adequate departure angle of course.
Yea those rock chips...I try to draft vans and smaller trucks instead for this reason.

+1 to the wheel air fences, but you have to make them pretty low and may get ground clearance issues. In some racing magazine they wind tunnel tested a Lotus Exige, and the drag reduction from making fences that go nearly all the way down to the ground was enormous, even though that car has a short tail.

It's way cheaper than lowering the car though, so it's probably worth a shot. I noticed Tesla Model S have these too but they are small and stubby.
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Old 10-28-2014, 11:06 AM   #13 (permalink)
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Quote:
Originally Posted by basjoos View Post
When drafting, engine rpm, drivetrain losses, and rolling resistance are unchanged. The only thing that has changed is a reduction of the wind load on the car, which is exactly what you are doing when you lower your Cd via aeromods.
Actually with a Prius this is incorrect. The engine RPM is adjusted to the load, so when drafting, I note the RPM drop as the load drops. It is quite interesting to watch the RPM when you have cruise control on and driving on level ground. If you are passing a large truck, you can see the RPM drop and the mpg rise as you go past in the passing lane. There is a nice spot with the nose of my car even with the tail of the truck that puts me in their "wake". There is a bad spot when my nose is just about at the trucks door and I'm on the high pressure point on the trucks wake. RPM goes up and mpg goes down.

Anyhow, I know this will make a difference, so now it is a matter of how far I go with the design. I really do like this tail that is in the thread linked in my first post.

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Old 10-28-2014, 11:12 AM   #14 (permalink)
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^ This is a good point.

Cars with continually variable transmissions should theoretically benefit more than other transmission types when reducing aerodynamic load (or any other load), as the engine RPM drops for a given road speed.
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Old 10-28-2014, 11:16 AM   #15 (permalink)
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Quote:
Originally Posted by freebeard View Post
Be aware that the boattail that MetroMPG posted has a bad mismatch at the trailing edge of the hatch. The whole top or at least 50% will be in turbulent flow.
Yes, there will be separation/reversal immediately aft of the factory spoiler, with flow probably re-attaching somewhere further down the tail.

I recall the builder saying this was his quick 'n' easy compromise for preserving the view of the center mounted brake light.
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Old 10-28-2014, 03:55 PM   #16 (permalink)
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Quote:
Originally Posted by serialk11r
+1 to the wheel air fences, but you have to make them pretty low and may get ground clearance issues. In some racing magazine they wind tunnel tested a Lotus Exige, and the drag reduction from making fences that go nearly all the way down to the ground was enormous, even though that car has a short tail.

It's way cheaper than lowering the car though, so it's probably worth a shot. I noticed Tesla Model S have these too but they are small and stubby.
I see the fences as being two walls of an exponential horn, the third wall would be the diffuser. Exponential horns are used for outdoor public address systems because they couple so well with the ambient atmosphere.



Here's an example on a Lotus that uses straight, rather than curved, fences.

palmer_md -- That mid-height boattail would work well with this. The fences could be attached to the rear suspension and submerge into the boattail with suspension travel. Using the same curve horizontally, and held in close to the inner side of the tire, they could cross the width of the tire with much less length than the [full] boattail.

For the rear skirts, consider the '57 mercury turnpike cruiser style skirt. It would eliminate the vertical seam at the back of the wheelwell. For the front skirt, here's a picture I did for aerohead's T-100 in another thread.



The program locked up at that point so I just took a screen grab. I don't know if you can read the illustration. The blue/yellow part represents the car body and tire. The wireframed part is the skirt. The cylindrical shapes represent a 4-bar linkage, with the two top bars combined. They are shown on the outside of the body with a bubble skirt, but depending on the clearances, they might be in the wheelwell with a flush skirt.
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Old 10-29-2014, 09:53 PM   #17 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
^ This is a good point.

Cars with continually variable transmissions should theoretically benefit more than other transmission types when reducing aerodynamic load (or any other load), as the engine RPM drops for a given road speed.
That's only true if the minimum gear reduction ratio isn't being used. A Prius doesn't have much horsepower so the engine does in fact spin up a bit faster than it's minimum possible speed on the freeway, but for gasoline-only cars with CVT, the minimum ratio is attained well before highway cruise speed.
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Old 10-30-2014, 08:05 AM   #18 (permalink)
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I like CVT's as well as the dual clutch 6 (and more) speed transmissions. Manual operation of both is possible.

Generally the maximum ratio "spread" is increasing in both, probably beyond 10 to 1, which is beyond the dimensional capabilities of conventional manuals.

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Old 10-30-2014, 08:59 AM   #19 (permalink)
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Quote:
Originally Posted by serialk11r View Post
That's only true if the minimum gear reduction ratio isn't being used. A Prius doesn't have much horsepower so the engine does in fact spin up a bit faster than it's minimum possible speed on the freeway, but for gasoline-only cars with CVT, the minimum ratio is attained well before highway cruise speed.
True. But remember aero isn't only a highway speed issue.

It's typical that half a vehicle's fuel is being burned to overcome aero drag at speeds as low as 25-30 mph.
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Old 04-08-2020, 04:12 AM   #20 (permalink)
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saw this on reddit. pretty bad execution, but I bet it works.

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