Kammback try on a Honda FIT

Discussion in 'General' started by Die2self, Feb 17, 2014.

  1. Die2self

    Die2self Saving more by using less!

    I have been looking over on EM and other sources for Kammbacks and have decided to try and see what I can do for my FIT to make it more FITTING.

    So I mocked up one this afternoon with some cardboard and tape to see what shape I could get with it. this is what I came up with so far. I left a 2 inch or so flap on the main part and may fill in the side triangles (as seen in the left and right pictures if I want to keep the length.



    I think I have the line/curve from the roof carrying on through so I should have good air flow. Also wanted to use the lines of the lights to do the sides to make it look like it belonged.

    Back view

    Shows the tapering

    From inside

    Even with the extra 2 inches it doesn't intrude to much into the line of sight from the rearview mirror. The third light would be blocked unless I was able to get some clear material (which I would prefer).

    I also have been thinking about completing the kammring on the back of the hatch to help smooth out the whole back end and decrease the overall wake size.

    from the side. (there are some gaps in it but this was just to see what it would look like. I might need a little more length on the tabs.


    From the side with a little angle.

    Thoughts, comments?
  2. MaxxMPG

    MaxxMPG Hasta Lavista AAA-Vee Von't Be Bach

    If I remember correctly, a true Kamm tail tapers until the cutoff where the cross section is about half of the maximum cross section. So if a car body is 50 inches from underbody to top of the roof, and ignoring the taper along the sides, the rear needs to be extended until it's 25 inches vertical. The only way to test for sure is coast-down distance, and that is not easy to duplicate in open air on public roads.

    You have the shape about right, with a 15 degree tangential curve being the upper limit for air at standard temperature and pressure. For the math majors, the Reynolds Number calculator - http://www.grc.nasa.gov/WWW/BGH/reynolds.html - can show how the shape needs to change when fluid viscosity changes.
  3. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Looks good. I love a good application of CAD - cardboard aided design. :p I think the top and sides are just about right - continue the same angle of the roof and side walls.

    I'm not sure the lower piece will do anything. I think it's too far inboard to get any attached flow. I'd just stop above the lights and leave it at that.
  4. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Jason:

    What is your idea for final material and placement? Coroplast and tape or something a little more integrated?

    What Andrew said. Coming off the side rear taillights, it is too far in to keep laminar flow off the rear corners. It might even harm Aero?

  5. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Plexiglass for the top portion?
  6. Die2self

    Die2self Saving more by using less!

    I meant to put on there the final materals.

    I have a friend that has a sign business so I might be able to to get coroplast in any color I want. They have translucent which might help with the third light at the top. However Silver would look better with the rest of the car if I can get a decent match. Not sure the best type of tape to use if I go with the coroplast, but I hope to build it from one piece and use a heat gun to make the bends for the sides.

    If that doesn't work then may look into plexiglass.

    Still playing with how to attached it. I would like to use those metal clips (platic covered) that they use for bake rakes on hatches and sedans that hang off the back. It would be easier with the coroplast to use those and attach them. I have mocked it up to be right on the glass edge of the top of the hatch.

    My think for the side and lower tabs were from an article on semi Kammback designs and how a stepped kammback still provided good effects. However, I see your point that these might not be long enough to allow for the air flow to reattach. I may play around and see later this spring/summer.

    As for the top length, I think what I have on there now is about 18-20 inches. From the pictures I think another 2-3 might be ok to do and could make the connection to the sides meet up from the light angle.

    It is off right now as I didn't want to get it all wet with possible wet weather. I need to get it traced out on another sheet of cardboard to have a reference incase something were to happen with this one.

    thanks every one for your input. I will keep at it and report.
  7. ILAveo

    ILAveo Well-Known Member

    My thought is that the sides below the taillights and the bottom aren't going to help much because they are too far inboard (not much wind there). I'd concentrate on the portion above the lights where there's more action.
  8. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Jason , plexiglass should be more rigid than the coroplast, but it may also be more difficult to shape properly. Sounds like a good project , good luck with it !
  9. RedylC94

    RedylC94 Well-Known Member

    Or how about polycarbonate glazing material? It should have good optical quality.
  10. NeilBlanchard

    NeilBlanchard Well-Known Member


    As ILAveo says, I think the sides and the bottom need to be closer to the air flow - the sides needs to be pushed outward, and the bottom needs to be at the bottom edge of the bumper.

    Have you considered tuft testing? The least difficult way is to have someone in another car video the tufts as you drive.
  11. RedylC94

    RedylC94 Well-Known Member

    What's the plan for keeping the tail lights visible from the rear sides? (We don't see the left light in the photo that's repeated in post #10.) Moving the sides out closer to the air flow might block the nearside light instead.
  12. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Just an idea for aesthetics: Mimic the rear quarter window shape in the side panels of the kammback.
  13. Die2self

    Die2self Saving more by using less!

    Red - The bottom part is just something I am toying with and may not get implemented. But the Top will.

    Andrew after I posted and looked at the same photo from the side, it was like, just follow the side glass profile :p

    I have talked with my friend and they also have plexiglass and heat strips for bending it. So I may drop by within the next week and see what they have laying around that I might be able to use (if nothing then they can order it).

    I am thinking of a plexiglass and then attaching side coroplast (silver), or just going all plexiglass.

    The trick will be attaching it to the glass. I have been thinking of bike straps with the S clamps on them. If I go with the clear plexiglass, I may be able to drill small holes and attach the straps with washers and bolts. Otherwise maybe coroplast tabs (one side of it) super-glued to the plexi then using silicon to attached the other part of the tabs to the glass (with some clamps as well. I don't really want to use tape and want this to be able to removed with out any residue.
  14. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    With my Sport, i'd use the factory lip spoiler's mounting holes on top of the hatch. Any chance you could drill the same holes in yours? I know - ouch!

    Maybe some kind of mounting bar that wraps around the inside of the hatch?
  15. Die2self

    Die2self Saving more by using less!

    I spoke with my friend last night a little bit about what I was planning. She said the Plexi is expensive and get brittle in the cold. She suggested that I might use lexan as it was practically bullet proof (one of its uses) and can be bend with a break. It will yellow in the sun a little but it will take a while to do that.

    Drilling a hole is not removable to me. that would be an absolute last way to do it. My friend stated that they use a silicon adhesive for lexan and it is supper strong. I will have to test it on some glass to see if it cleans up afterwards.

    I wonder what LEDs in the edge would look like. Would the whole edge light up for a third brake light :p
  16. RedylC94

    RedylC94 Well-Known Member

    ... As I already suggested, way back in post #9. It may eventually cloud, just as headlight covers do. The same stuff is often used for eyeglass lenses.
  17. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    Long-term, Lexan (polycarbonate) is not a good idea , at least what they have sold in the past. Especially on Civic headlight covers :). But it is very strong and can withstand "some" impact without breaking. It would be okay for Jason's app , I think.
  18. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    You might try painting a coat of Spar Varnish over it to add UV protection. It's specifically designed for boats and their constant exposure. Next time I refinish some headlights (Ody is getting bad again) I'm doing that.

    (available at lowes and hd)
  19. Die2self

    Die2self Saving more by using less!

    I added some more length the back of the kammback CAD (as Andrew calls it ;)). So this makes it about 19 inches long from front to back.



    From the view on the inside through the mirror, the sides come in more than before, and the top only comes down just a little more than before. It comes down to just about the third defrost line from the top. The shorter one came to the 2nd line.

    Last edited: Feb 23, 2014
  20. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Good on you for going out and doing, instead of perpetually analyzing and never moving to the next step.

    I'd have to go with a clear material. The view blocked by the third brake light annoys me already.

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