Designing a tail extension for the first-gen Insight

[Christ]

Jim -

Not answering for Robert here, but the vent behind the plate may work insofar as being able to vent air into a low pressure zone... however, if the smallest cross section is only as tall as the plate itself, you'd want to prevent any flow from occurring top or bottom of the plate, and only allowing it to occur at the sides, which could train the separation at the sides of the rear transom.

Easiest way to do that would be to mount the plate about 1/2" away from the transom with bar-type spacers top and bottom.

Placing a vent anywhere that the pressure difference across the vent is higher than OE will improve the performance of thru-flow heating/cooling and defrosting, with the usual diminishing returns clause.

EDIT - Woops, I forgot to say why you don't want flow above and below the plate...

When the rear transom is only the size of the height of the plate, in this case, six inches, the flow will separate at that cross section. Any air vent which causes flow to exit above or below the plate will increase the effective (dynamic) cross section of area that the flow sees.

[RobertSmalls]

I get it, but if you have an inch-wide channel of air flowing above the plate, you're making your wake an inch taller than it needs to be.

My answer was going to be that the illumination goes to the right and left of the plate. However, I just measured my CAD model, and the transom will be roughly 6"x23". A US plate is 6"x12", which leaves 6"x5" for each taillight cluster. I probably won't find three-color clusters that small, so your solution does sound pretty good after all.

[RobertSmalls]

@Christ: Aerodynamically, the ideal location for the vents is on the underside of the tail extension. We may have to compromise attached flow on the bottom, in order to give the car a usable departure angle, and adding extra air down there can only help.

[Christ]

Originally Posted by RobertSmalls

I get it, but if you have an inch-wide channel of air flowing above the plate, you're making your wake an inch taller than it needs to be.

My answer was going to be that the illumination goes to the right and left of the plate. However, I just measured my CAD model, and the transom will be roughly 6"x23". A US plate is 6"x12", which leaves 6"x5" for each taillight cluster. I probably won't find three-color clusters that small, so your solution does sound pretty good after all.

Look at trailer lights... there are a set that work for tail/turn/clearance. They have white/amber lights on the sides of the casings, so that if you mount those with the lights facing inside rather than out, they will illuminate the plate for you.

Also, you may be able to get away with using an illuminated license frame in that case, as long as it's a white or dull yellow light and it doesn't flash, flicker, or otherwise modulate.

Like this:


Except on those, they may just be reflectors.

My personal option would be the illuminated frame.

There are also bolts which have directionally lighted heads, normally used on motorcycles, etc.

[3-Wheeler]

Originally Posted by RobertSmalls

...However, I just measured my CAD model, and the transom will be roughly 6"x23"....

Hi Robert,

How did you end up with a tail width of 23 inches???

Just curious.

__________________________________________________ ________

The departing angle of the bodywork next to the bumper, which will be removed, is 12.2°.

If I extend this angle and include the "template" curvature, that leaves my tail width of 26.5 inches. This is with a length of 48 inches.

Actually, 26.5 or so inches is probably about the minimum anyways:

1 inch gap + CJ light width (6) + 1 inch gap + plate width (12) + 1 inch gap + CJ light width (6) + 1 inch gap = 28.0 inches

The gap between the outer edge of the tail lights and bodywork kammback edge can be made slightly smaller than 1 inch to get the total width down to 26.5 inches.

__________________________________________________ _______

If the plate and lights are that close to each other, it may be that the air circulation vent has to be under the edge of the tail anyways just to make room for it.

Jim.

[RobertSmalls]

Originally Posted by 3-Wheeler

Hi Robert,

How did you end up with a tail width of 23 inches???

Just curious.

Probably using similar math to yours. I overlaid aerohead's template + its relfection on the top view of the car, scaled it up to the width of the car, and slid it forwards to match the curvature where the extension begins. This was done by eyeballing it. I did something similar to obtain the shape of the top of the extension. The bottom was done by eye, as a compromise between what I think will be good aerodynamics and what I think will give good clearance for exiting parking lots.

I truncated the tail where it was 6.5" high, then used the Measure function to find a width of 23".

The bottom of the transom will be 18" off the ground, and 77" behind the rear wheel center. That gives a departure angle of 13°. Will that be sufficient?

[Wonderboy]

I have those same (or very similar) trailer lights that Christ posted on my car trailer. They have two bolts in the back, making them very easy to mount. Another bonus is that these lights are all LED. The white lights that are designed to point downward from the corners of a trailer are arrays of maybe 6 LEDs that point all in slightly different directions. This feature would easily lend itself to adequately illuminating a license plate, however you'd need to modify the housing because I think you need to have the directional blinkers pointing sideways. IIRC, the stuff inside the housing is pretty movable and it's easy to get in to. Nice lights.

Edit: I couldn't find anything explicit about rear-side direcitonal visibility, just rear directionals. http://public.leginfo.state.ny.us/me...MMONQUERY=LAWS

[jime57]

Originally Posted by RobertSmalls

The bottom of the transom will be 18" off the ground, and 77" behind the rear wheel center. That gives a departure angle of 13°. Will that be sufficient?

Robert,

I think you are going to have a problem with clearance. I did a few quick back of the envelop calculations. If the back of the boattail is 18" off the ground and extends 77", then a road dip of a mere 9.9 inches in 14 feet is going to cause the tail to drag. There are lots of dips at intersections and curbs which are this great, particularly in older cities.

This is one of the things which has slowed me down a bit on the boattail question. Gotta be able to drive the thing on the street! I'm still interested in doing something, but I want something shorter or find a way to get the tail higher or both

[3-Wheeler]

Originally Posted by RobertSmalls

...The bottom of the transom will be 18" off the ground, and 77" behind the rear wheel center. That gives a departure angle of 13°. Will that be sufficient?...

Hi Robert,

My end plate will be 16.5 inches off the ground at a distance of 72.5 inches from the wheel center, so your tail is slightly longer.

I'm not sure in what context you meant sufficient?? 1) Did you provide enough information or 2) is 18 inches enough clearance to keep from hitting the ground.

I would think that 13° is enough clearance for most driveway aprons.



Jim.

[jime57]

Originally Posted by 3-Wheeler

I would think that 13° is enough clearance for most driveway aprons.



Jim.

I'm not Robert, but................

The clearance isn't just a rear issue. It is an issue with the whole car. The correct way to analyze the clearance is by use of triginometric methods which consider both the wheelbase of the car, 94.5", and the boattail extension.

The sum of the wheelbase, and Robert's boattail extension forms the long side of a right triangle, whose length is 171". The short side of the triangle is the proposed tail clearance of 18". A similar triangle can be drawn, with the same apex angle, inside the first triangle. The second triangle has a long side of 94.5 inches and an short side of unknown length. The short unknow side is the amount of allowable drop of the rear wheel before the tail drags. This unknow length can be found by using the law of proportions and in Robert's case is 9.9 inches. This is the amount within a 171" span that the rear wheel can drop before the tail drags.

The clearance issue is not just one's driveway. There are numerous other hazards encountered by street vehicles. There are numerous other driveways to consider. There are entrances to malls, gas stations, and other business locations. And perhaps the most devious of all, there are some intersections in older cities which have been repaved so many times that the crown of the road alone is enough to drag a long extension. I don't say these things out of lack of experience. For many years I have driven motorhomes which have significant rear overhangs. It is very common to drag the rear, but fortunately motorhomes invariably have a massive steel trailer hitch to take the abuse.

I'm not against boattails - in fact I would like to have one myself. And, I'm finding the discussion very, very interesting. But I just don't want to see you nice folks put lots of money and time into something which is going to be easy to damage. We can all learn to drive with constraints, like learning not to pull the first gen up to a curb and damage the front strakes, but a long overhang is gonna take even more "training".