Aerodynamic Mods to Reduce Drag

[NeilBlanchard]

Right, that is essentially a Kamm back with a recessed cavity that greatly lowers the drag of these trucks.

Drag reduction definitely works!

 

[Llecentaur]

Well I am really tempted to make one for the i.

Plexiglass fixed by suction cups ?

Anyway of calculating the ideal dimension ? Intuitively it should not be longer than one foot or so ?

 

[Don]

I *think* ideally it should taper to a point and the longer the better . . . . like the back end of an airplane - This is based on my understanding of the hydrodynamics of boat hulls operating in water, but I believe the aerodynamic principles are similar

I think you'll be disappointed in the performance gains though - On a larger vehicle (like a semi trailer) operating at freeway speeds, I'm sure the gains could easily be measured, but on an iMiEV which doesn't do most of it's miles at freeway speeds, it might be hard to actually measure any gain - But, the proof is in the pudding, so to speak and if you have the data for use both with it and without it, you may indeed see a small difference

How far anything can legally sick out behind the tail and side marker lights would be something to check on before you begin though - Good luck and we'll all be interested in your results, I'm sure

Don

 

[Llecentaur]

Thank you Don. I understand that the gains will be limited to highway use and that was indeed my intend, reason why it would have to be dismentable.

As for the tes, I have now made several trips from my house to Geneva with detailed results. Actually very easy to measure because with a precision of a few 100 yards, I loose one step on gauge for every highway exit and end up with exactly 50% at the very entrance of the parking!

Therefore as long as I can run a test with present temperatures and avoid wind, we should have a some data easily.

Now need to find a transparent material to keep visibility also for brake lights and check the legal aspect for the protuberance, I think it us about one meter.

Will post if I get around to do that test.

 

[jray3]

The best forum I've found for planning such modifications is ecomodder.com They have an aero template tool to plan with. Here's an earlier thread on this forum http://myimiev.com/forum/viewtopic.php?f=14&t=326
And one on ecomodder with the template
http://ecomodder.com/forum/showthread.php/aerodynamic-streamlining-template-part-c-9287.html#post116392

 

[MLucas]

I've been thinking of the same thing and over the winter plan to do some designs first with cardboard or foamcore board. My commute is mostly highway, there is no way around it unless I want to make my 40 minute commute into an hour and a half. I keep my speed between 80 and 90 kmh. My thoughts on the lights was to make the box between the tail lights instead of outside of it.

My final design will be with balsa wood wrapped with fiberglass and painting it black to match the car. Mounting it will be another matter. Not a fan of drilling into the metal. Haven't thought of this part yet. Edit* - just came to me, big honken' magnets will work. If I embed them in the balsa wood and fiberglass, I can mount the box using magnets and felt to prevent damage to the finish.

I've been looking at the ecomodder site and getting some ideas there.

 

[Llecentaur]

You are way ahead with the project. For fixing it, have you thought of high grade suction cups, I saw some in a military shop that looked very sturdy and probably capable of holding such piece tight against the rear glass

 

[NeilBlanchard]

At a mere 28-30MPH, aerodynamic drag is about HALF of the load on the drivetrain. Think of when you ride a bicycle - 20MPH feels like a *lot* of air drag to me. I've ridden a motorcycle for about 100,000 miles and even with a fairing, the force of the air is very, very strong. Case in point: the EV1 had the lowest drag of any production car, and it has the lowest consumption of any production EV. It uses almost half as much at highway speeds as any EV we have today - about 150Wh/mile at 55MPH.

A Kamm back if done correctly acts almost as well as a full boattail.

There may be ways to have temporary attachment, like flat hooks that hold onto the gap on the hatch door? I don't think that suction cups will do it, no matter how good.

 

[Don]

I think you'll be disappointed in the performance gains though - On a larger vehicle (like a semi trailer) operating at freeway speeds, I'm sure the gains could easily be measured, but on an iMiEV which doesn't do most of it's miles at freeway speeds, it might be hard to actually measure any gain

I *hate it* when I'm proven wrong (but it still seems to happen all the time :lol: ) but it looks like with the big 'barn door' backside the iMiEV has, a proper tailpiece might make much more difference than I thought

http://www.gizmag.com/aerodynamic-long-tail-improves-fuel-efficiency-15/13678/

How practical it would be remains to be seen, but it looks like the improvement in freeway range might be as much as 15% or maybe even a little more

Don

 

[Llecentaur]

Well the challenge sounds like building a kamm back with transparent material to accommodate rear visibility while being easily dismount able and if possible foldable to have it in the boot

 

[MLucas]

Well the challenge sounds like building a kamm back with transparent material to accommodate rear visibility while being easily dismount able and if possible foldable to have it in the boot

One of my first ideas was plexiglass. If you want visibility you could use plexiglass and some type of hinge at the corners and on the sides for foldability. I'm going more for the semi-permanent mount with magnets. Magnets are used for motorcycle tank bags and they hold very well. If you don't have it tied down or bolted down - it's coming off on a motorcycle.

I'll share with you what I come up with and vice-versa, we might make something that will actually work very well. Maybe, Neil will give us some pointers as well. I've got a few more projects on my i-MiEV before I plan to get started. Almost finished with the interior modifications.

 

[Llecentaur]

Yes let's share the dev ideas.

I agree with plexiglas.

Re hinges, I have not thought it through but I concerned about the fact that the main horizontal piece has to gently dive down, that means it will be supported by the two side ones. Now, can a long flexible piano type of hinge work despite that curvature ? Otherwise it would have to be several hinges.

From posts it seems that the main piece would be between two and three feet long and have an sloping angle of less than 15deg.

Would you agree that we should start by deciding that length and angle. Maybe there is a compromise to accommodate the hinges...

Two more thoughts:

Should we think of a last spoiler type of shape at the rear end of the main piece ? I guess it would be valid if the kamm is on the short side.

What about flaps to cover the back wheels. Maybe a transparent easy to fit flap would fit also from an esthetic point of view many iMievs. Mine is white, therefore white plexiglass would be OK. But no idea about how to fix it...

 

[MLucas]

Yes let's share the dev ideas.

I agree with plexiglas.

Re hinges, I have not thought it through but I concerned about the fact that the main horizontal piece has to gently dive down, that means it will be supported by the two side ones. Now, can a long flexible piano type of hinge work despite that curvature ? Otherwise it would have to be several hinges.

From posts it seems that the main piece would be between two and three feet long and have an sloping angle of less than 15deg.

Would you agree that we should start by deciding that length and angle. Maybe there is a compromise to accommodate the hinges...

Two more thoughts:

Should we think of a last spoiler type of shape at the rear end of the main piece ? I guess it would be valid if the kamm is on the short side.

What about flaps to cover the back wheels. Maybe a transparent easy to fit flap would fit also from an esthetic point of view many iMievs. Mine is white, therefore white plexiglass would be OK. But no idea about how to fix it...

It depends on how compact you want to make it. If you wanted just three sections, you could mount them independently and forget the hinge. I'll put some drawings together and scan them in so you can see what I'm thinking about.

I plan to make this into a rigid three sided box that will stick on the back of my i-MiEV using magnets and possibly using hooks like Neil suggested. The angle of the top part of the box will need to be calculated, not sure what that should be. I plan to make it quite large and extending out and down, possible 6-8 inches. The sides will come out 4-6 inches. But, I won't know for sure until I start making the model.

Edit* -- I remembered ecomodder had an aerodynamic template. After writing the first response I went out to look for it and found it. http://www.weirdcalculator.com/test/aero_template/

I think I got it lined up according to the directions. According to this templage our slope above the hatch is about 7.5 degrees (aprx) The slope of the kammback should be about 12 degrees by way of this calculator. I'd like not to go beyond the rear bumber for the top part of the box.

 

[NeilBlanchard]

That looks about right. Remember, too, that the sides can/should taper about like the top. Even just extending the Kamm back to the 12 degree line would be a huge improvement. And remember that the trailing edges have to be "crisp" and if there is a slight recess on the back, this also helps.

 

[Don]

Just a thought - If the tail piece were mounted on the rear hatch, you could then add another switch to open the hatch with the tail still attached, since the hatch latch is electric. Just make sure you have enough available headroom for the then near vertical tail. I *think* you're also going to need lights nearer the end of the tail piece, so those and the hatch switch could be wired to a new 'accessory plug/socket' similar to what you'd use for trailer lights. Should still be easily removable for city driving

Don

 

[MLucas]

This is what I'm thinking about. Except I'm not going to screw it into my hatchback like they did. There is a fair bit of metal around the 3rd brake light. I'll put in a cross brace to hold down the top wing and mount my magnets to that.

http://www.importtuner.com/tech/1310_1992_honda_civic_vx/photo_40.html

Don - putting LED lights on the trailing edge would very trick, I'll think about it when I put this together.

 

[JoeS]

If nothing else, it should certainly keep the rear window cleaner. :roll:

 

[jray3]

Hey, no need for the screws or cross-bracing. My lexan duckbill stayed put for over a month with 3M Command strips till I pulled 'em off. (clear tape that you stretch perpendicularly to the bond for a clean release, but I've since found that it loses adhesion in summer heat). The first try pictured here with white foam tape pulled the tape apart (weak foam, strong bond). For an opaque duckbill where you can't see the adhesive, I'd bet on VHB tape.
The curved roofline kept the sheet stiff, but the Lexan always wanted to pull back to flat, so the rear corners curled up slightly. I cut the Lexan on a curve to match the top edge of the hatch, with half-circle cutouts for the two bulges. The hatch opened without any problems.
https://silverslipperimiev.shutterfly.com/pictures/15

I'm about ready to try again with better documentation before and afterwards of noise n dirt. Don't have the patience for a day's worth of A-B-A coastdown testing. Btw- mudflaps didn't do diddly to reduce mud, even when mounted low enough to scrape the ground at full suspension compression, so I'm not making wheel strakes yet.

 

[alohart]

jray, is there a way to post your photos so that someone without a ShutterFly account could view them?

 

[jray3]

jray, is there a way to post your photos so that someone without a ShutterFly account could view them?

Dunno- I'm not a geek, just look like one, and haven't been successful in posting photos to this forum or my weblog for many months now..
:?
So, per others' suggestion, I'm resorting to IMGUR.com
http://imgur.com/39VJ5PA