A new product / electronic device my employer is developing is packaged in an aluminum enclosure that is roughly 10'' square. One intended use for this product is a mobile application, mounted to the roof of vehicles like police cars.

I say that the enclosure needs to be sustantially smaller and more aerodynamic, and that it would create substantial aerodynamic drag which could limit top speeds. I also have visions of it breaking loose from the the roof of a vehicle at speeds in excess of 100 MPH.

My co-worker / project manager says that the size of the device is fine and that a 300 HP cruiser will hardly notice the difference.

Who's right?

Light bars are bigger than that. But then they have been getting smaller and more aerodynamic, probably for a reason.

The real answer is anything that adds to the cross sectional area of a car will increase drag, reducing top speed. How many horsepower is irrelavent. The top speed is where you've run out of horsepower, and increasing drag will make you run out sooner.

The real question is: By how much? And how much is OK? How much is too much?

what kind of cop car, where on the roof, type of attachment (suction, magnets, velcro?? )

Put it into dome shaped inclosure. That way the aerodynamic drag will not be a factor. Also will offer a lower profile/less chance on being damaged while the cruiser is parked at a call. It will lessen the blocking of the light bar.

Ever see the size of automatic digital satellite dishs on motorhomes? The first ones were in a pod about 3'round

That's pretty much what I thought. Anybody have a weblink that states something like that, formulas, etc.? My co-worker will probably dispute ''...my bud on the 914club list sez you're wrong..." nanner nanner nanner!

I'd venture a guess that top speed in a crown victoria is probably at about 80, where the suspension gets so mushy that it impossible to steer.....

It will lower the top speed of the car, but probably not enough that anyone will notice. What it might do, is add noise - and possibly get ripped off the roof. Aero forces quadruple as speed doubles, so for the price of something vaguely aerodynamic - it's not something to totally ignore.

Here's what I think the kicker is tho - most police or city departments are in a constant cash crunch - and you will definately make the fuel economy worse with this (anyone with a Yakima rack has experienced this!). So I think that fuel economy is probably the best justification for spending an extra 10 minutes to put your appliance in something vaguely aerodynamic.

why not just mount it somewhere OTHER than on top?

My contention precisely. They want to put all of the electronics in the box, I say put it in something smaller and more tubular shaped and mount the electronics elsewhere.

I used the 'roof rack reducing fuel mileage' example, and he didn't buy it. He's an EE. Any weblinks to back me up? I can probably find the " ..Aero forces quadruple as speed doubles..'' pretty easily tho'.

A:

average variety cop car

facing forward, in the center or on the outer edges pointing at a 45 degree angle

I really hope that the mount will be permanent (they were originally pushing for a magnetic mount and I LOUDLY protested).

Gotta be on the top, it is an imaging device...and customers want it there.

Rob,

This guy doesn't know that 914 owners know everything and are always right?

The guy must be a jerk.

Paul

First of all, a Crown Vic will do way over 80 even my old 88 SSP would do over 115 with a pretty tired 175hp 351. They handle pretty well too, mushy isn't a word you'd use to describe it, take a look under one at the sway bars next time you are near. (I know you were joking, but don’t sell them short)

As for how much it will reduce the top speed probably depends on the particular car (Crown Vic, Charger, Intrepid, Impala, Tahoe?). When slick tops became all the rage, it was tested and pretty universally a light bar took 10-15 mph off the top end. Google should turn up some technical information. I suspect that with the smaller light bars available now and the higher horsepower cars it would be lower.

I think you are going to run up against a good bit of resistance to bolting a box to the top of a police car... I'd suggest your project manager should have some pretty concrete documentation that it will not reduce the top speed or it is going to be a non starter...

JMHO

I can show you the nice scab I have on my forehead from the magnetic mount trailer lights I got this weekend........

Semi passes me and blows the lights off the back of the bus (the rough road helped)....next guy behind the semi pulls up and motions behind....I look in the mirrors and see them dancing along the highway.....I stop and see what I can salvage...... both red lenses gone and one magnetic base.......put lights back up on roof and the one that lost the magnetic swings down and hits me ...blood streaming down my face while standing on the side of I20.......Tell the EE to bolt it to the light bar.............

damn bruce watch out for darwin!

Does your co-worker drive an ass-dragger???

I may be one of the younger memebers here, but I took basic aviation and what not in high school back in the day(a few years ago). I dont remember the #'s or equations, but the smallest things can make a difference.

If you want another example to show/tell him about, tell him about why racers like 935 mirrors and put a pic in his face.

Yeah - I was just kidding. I figured they must be at least midly potent since they are used by nearly all US Law Enforcement Officials...



Fuel economy reduction from roof racks is variously quoted as between 5% and 10%. The 10% came from somone's real experiences with an Audi. The feds claim 5%: http://www.fueleconomy.gov/feg/planning.shtml

Am I the only want curious about what this device is/for? Is this actually being mounted on a police cruiser? Or was that hypothetical? Is it one of those off duty lights which read "out for doughnuts" or somethig like that? If it's some kind of speed enforcement tool, are we letting the fox in the hen house?

I just loves conspiracy theories It's from all those years I spent listening to people's phone conversations without their knowledge.

BenT

You are selling so you got to look at your selling points!
1)if it’s big & ugly cops wont like it
2)if its not aerodynamic its bound to make whistling noises at high speed - cops wont like it
3)if its big & heavy when car is being ruff driven on a chase bumping other car...
The object will pick allot of momentum - would brake loose - cops wont like it.
4)fuel economy? I don’t know, do cops pay it out of their on pocket??? Not a selling point.
5)top speed? is this cops racing? do cops use their cars top speed often?
not a big selling point.

Do you happen to work for G2 Tactics?

No, but I am the Business Development Manager for one of their competitors...

Since this is a 'prototype/development product' I can't disclose very much, as I might incurr the wrath of the "Sales Prevention Department" AKA Contracts staff.

Interfering bodies. (the proper name is eluding me at this moment) Door mirrors, roof lights, etc. When two objects are close to each other in the same airstream, they create a tremendous amount of drag. When Ordnance is loaded up externally on say F/A-18's, F-16's, Strike Eagles, the Flight Crew has to recompute increased fuel burn rates due to the increased drag of not only the shape of the ordnance, but also the turbulence created by the interference of the airflow about each shape. The increase in weight is a minor factor in the increased fuel burn. Another example. When the Air Force introduced the F-15, they stated that it had less drag than three bicyclists. Arms, legs, spokes and such cause all that drag.

Ask your coworker to look at the roof racks on the latest SUV's. They have almost all gone to side rails that are integrated into the roof structure. Oh, and why Yakima roof racks have that front "wedge", to lower drag.

I hate roof racks, which is one of the reasons I bought my BMW 530i Touring (a wagon) as the roof has integral t-slots in the roof to mount a rack. Keeps the profile clean. I always smirk when I see an obviously environmentally friendly driver (as stated by the bumper stickers) with an aftermarket rack for bikes that never gets taken off. A big increase in gas usage.

In summary, mount it as a bump, not on a stand-off mounting. I don't suppose that you would like to cut a hole in the roof of the vehicles so that the electronics are inside the cool air conditioned car and only the working piece is outside? Tough to sell the car when it's older, but the cabbies might just put lexan over it and call it a sunroof!

Trying to help,

FIN

Hard to hide a chain gun in a little aerodynamic package. And whaddya do with all the spent brass? The Cap'n

LOL ...that would be the "Enforcement" option. Kidding.

You're not selling to the cop though. You're selling to the police force - which is a government entity. Whilst having the cop on your side is a good thing - if you're trying to get something bolted to the entire fleet, stuff like fuel economy start to make a big difference - think about having any number of V8 powered large sedans rolling around a city/district 24/7 (and making donut runs!) and you're talking about a lot of fuel. Adding another 5% on top of that bill isn't going to look good to a publicly funded entity.

My point exactly. There are loads of articles telling how recent gas price hikes have affected police departments: less patrols, curtailing or eliminating 'drive home' priveleges, etc.

What I need is calculation that shows how different device profiles / designs affect either top speed and/or fuel economy. I found some weblinks that give me pieces of it
(Cd's for different profiles) Click to view attachment

and a drag equation from insideracingtechnology.com (gotta love racers!):

For real body shapes, air at standard conditions, V in mph, and drag in pounds of force, this equation becomes:

Drag = 1/391 x Cd x A x Vsquared

Sooo... if my math is correct, at 70 MPH I come up with about 10.7 lbs of force for a cube vs 7.1 lbs for a cube turned 45 degrees vs 4.5 lbs for a half circle vs .355 for a streamlined shape. How would this force expressed in lbs. equate to loss of MPG, extra HP needed to maintain 70MPH, etc? Also, how do you determine the Cd for combined shapes (a half circle on the front edge of a long cylinder)?

Look at the chart; The lowest drag coefficient was 0.04 for the airfoil shape. Make them mount their lights in an airfoil shaped pod raised above the roof. Or use an airfoil shape placed at a right angle to the roof, like the dorsal fin of a shark.

Have you noticed how in-car cameras are mounted on racing cars? In F1 they use a horizontal airfoil shaped enclosure at the top of the roll-over structure while CART, IRL, Sportscars, and stock cars use an airfoil shaped enclosure in a dorsal fin configuration.

Don't forget that drag alone is not the only consideration. If the shape, even areodynamic, is not mounted correctly it can induce high speed instability. Does the device add lift or downforce at various speeds, and how does it affect handling.

It doesn't matter that a cop car seldom sees top speed, just one, once, is enough.

I would hazzard a guess that the increase in drag (cd x area) over the original drag figure would relate closely to the increase in fuel consumption.

any way they will have a fild test with cops of coarse and what they have to say about it will be concidered much more than any 5% that will or will not be even calculated.
as far as i know wind factor is not real high below 60 mph.

Real simple - no way your gonna calculate it. This is the reason they invented the wind tunnel
Got a friend in the Air force? NASA is good too