Aerocap Performance (Why are we seeing wide variation in reported MPG savings?)

[Big Dave]

Airplane people have a saying: "If it looks right, it probably is right."

Viz: The LanceAir.

But this is not always true at the lower Reynolds numbers that cars and trucks develop. Viz: The M-B "Boxfish."

[Fubeca]

I have to say I'm a little sad that my topper is on the home page with bad pictures and bad testing (but that is just my pride speaking). It does make me happy that a person can make it in a day +/- with cheap materials and basic tools.

I'm also pleased that a person can get real MPG improvement. I'd just like to be able to clarify how much and at what speeds.

I really need to do some more rigorous testing at a higher speed (typical freeway speeds). I would also like to do better tuft testing and perhaps make a few tweaks to clean up some airflow.

[round.boater]

My butt-o-meter says ~15% is probably about right for my cap. I'll do A-B-A if anyone in Denver would help me lift the thing on and off. One of the big benefits for me is that I can take it up to 75 without watching the mpg's completely crater. (And FWIW, I'm still the slowest vehicle on the road where I do that)

Bondo, when are you going to design an underpanel that will bolt into the stock running board / step bar holes and fill the gap between the frame and the body?

Still waiting for someone to build a 50 mpg car that can tow 5000lbs and has room for me to sleep in the back...

[MetroMPG]

Originally Posted by Fubeca

I have to say I'm a little sad that my topper is on the home page with bad pictures and bad testing

I agreed when Big Dave said testing in winter conditions is challenging... but I also didn't see anything in the description you wrote up of your test that jumped out as being "bad".

(Or I wouldn't have posted it on the home page!)

[Fubeca]

I really wish I had time to duplicate that test and do some more testing at a higher speed. With the temperature and the crosswinds, I just don't know how valid the test was.

Of course it could just be the sour grapes of my dashed hopes

[RobertSmalls]

Originally Posted by round.boater

Still waiting for someone to build a 50 mpg car that can tow 5000lbs and has room for me to sleep in the back...

That's a very tall order, and part of me wonders if it's really necessary. I'm pretty sure it would not be possible with aerodynamic improvements alone.

It might be possible with a small donor truck, a large budget, ride height reduction, an aerodynamic rebody, and an open mind about acceleration and the appearance of the finished product. I'm picturing a chopped and teardropped Ranger with a low, feet-forwards seating position and an imported 2.0L Ford diesel.

However, the more reasonable thing to do would be own a 20mpg truck for towing your bulldozer, and a Prius for commuting, sleeping in, and towing 1000lbs.

[round.boater]

Originally Posted by RobertSmalls

That's a very tall order, and part of me wonders if it's really necessary. I'm pretty sure it would not be possible with aerodynamic improvements alone.

It might be possible with a small donor truck, a large budget, ride height reduction, an aerodynamic rebody, and an open mind about acceleration and the appearance of the finished product. I'm picturing a chopped and teardropped Ranger with a low, feet-forwards seating position and an imported 2.0L Ford diesel.

However, the more reasonable thing to do would be own a 20mpg truck for towing your bulldozer, and a Prius for commuting, sleeping in, and towing 1000lbs.

How about a Prius, which already gets 50mpg, slightly basjoos'd, with an adjustable rear suspension, tougher frame / hitch, and just enough power for the load? It doesn't seem that far off. In the mean time I'll do what I can with my truck. I finally found a coroplast supply here in Denver so I can start working on the underside in addition to the top. With the Aerolid already on, I'm not sure there is much to be gained north of the wheel wells...

[3-Wheeler]

Something that AeroHead alluded to in his post, is that wind tunnel testing numbers do not relate exactly to real world numbers.

Let me give you an example...

I have been performing coast down testing for the last two months. One thing that gave way to the research was that:

At 55mph, 65% of the drag is due to air resistance
35% of the drag is due to Crr

The faster you go, the more the ratio shifts to air drag.

If one sees a 20% air drag benefit in the wind tunnel, then the only way that this will show a 20% benefit on a test track is to be going about 200 mph were the air drag portion swaps out the Crr portion.

As you slow down from this speed then Crr starts to become are larger factor again, and reduces the affect of the aero gains from the mods.

Jim

[KamperBob]

Originally Posted by Fubeca

I really wish I had time to duplicate that test and do some more testing at a higher speed. With the temperature and the crosswinds, I just don't know how valid the test was.

Of course it could just be the sour grapes of my dashed hopes

Crosswinds? Which direction? That could at least change the symmetry of the flow field.

I agree with Darin's apples-oranges point. Not only different shaped caps, they are sitting on different trucks. I would expect the latter to have the bigger effect. My 2c.

Cheers
KB

[tasdrouille]

I've read people saying that rear wheel skirts did HURT their FE. I've read people saying that adding side skirts and/or a belly pan didn't change a thing. But I've never seen anyone not reporting FE gains with an aerocap. I think that fact alone speaks for itself. There is a concensus behind such a product, but of course YMMV depending on how you drive.