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Discussion Starter #1
I need some help figuring out how much ft/lbs of pressure is created by a light that is 8 3/4" in Diamiter at say 70MPH (Wind or vehicle moving at 70mph)

I realize there are more factors that come into play, but I just need a number that is close.

What I am trying to figure out is the 'max' force applied on the lights by the ~70MPH air speed.

If the mods don't think this qualifies as fabrication.. feel free to move it!

Come on you math minded people! I'm lost on this stuff, and my searches provided me with nuthin!
 

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According to this, you can figure this out by using Stokes' Drag formula: F(little d)=-bV. The article explains what the variables mean, but in order to figure it out, you need to know the "stokes radius" of the object, and the viscosity of the fluid through which it is traveling (in this case, air). Since I have no idea what either of those are, you're probably no better off now than when you started. Sorry! :D
 

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Too many unkown variables.

It would depend on where the light was mounted. And there's no formula on earth that would figure it out. On/under the front bumper vs. on a light bar above and behind the cab would be very different.

Good 'ol real life measurements would be your best bet.
 

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Man, I don't want to have to dig out my theoretical mechanics text book. That said I can at least tell you that what you're looking for is a force expressed in pounds, not a pressure which would be psi, or a energy/torque expressed in foot-lbs.
 

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In open air, assuming it's somewhere between a plate and a prism for drag coefficient, it will create about 6.6 pounds of force, costing you about 1.2 hp, at 70 mph.

If you're mounting this on the front of a light bar and its at least 1-4 inches over the highest point in front of it this is actually not too far off, if it's on the bumper it will probably be pretty close as well, however it will subtract some force the bumper sees, so the net effetc will be small.
 

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I thought about iit a bit more, mounted to the bumper I'd say it's probably closer to about 2/3 the numbers I gave.

I pulled this out of my ass, but looking at tables of drag coefficients, it should be a pretty good estimate.
 

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Discussion Starter #7 (Edited)
The lights are ontop of the truck. The force against them is needed to find out what the max force against them is in pounds...

I'm hoping to use a linear acutator to move the light bar up and down, and to hold it in place. Two bolts with brass washers used as a pivot, an arm of 4.75" long off the light bar pivot, and the linear actuator has a stroke of 12" to achieve 145* of rotation, [lights folded back completely down to lights up past normal angle... pointing almost directly in-front of the truck]. I'm working on various aspects of the design to make it as compact and rugged as possible. I can try to post a drawing of what I have thus far tomorrow if people want. This is just another one of my projects and ideas that I plan to have done... sometime before I completely destroy the truck.

It sounds like 100lbs of holding force is more than enough? I don't care much about HP loss, or MPG loss. Just making sure I don't have too much force at 70MPH to overpower the max holding force of the linear actuator.

I'm probably over thinking this by 10 fold.

This is what I have gathered thus far from your replys:

Togeather the two Hella 4000's should create roughly 13.2 pounds of force. I'll bump the force up to 20 (bigger margin of error, for total height of the light, hight off the roof), and the light bar resistance would be probably close to 5 pounds of force. That being said I am GUESSING I'll have around 25-30 pounds of force with the light bar and lights in the up position... so... the linear actuator has a 70lb holding advantage over the lights.

I think I just confused myself!

Any more information would be helpful. Thank you to everyone who has helped thus far.


This stuff interests me... but the math and is just over my head!
 

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Wait a second: you're making, in essence, a convertable light bar? That sounds either really cool, or really lame, depending on exactly what it is you're doing. :)
 

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Discussion Starter #9
Wait a second: you're making, in essence, a convertable light bar? That sounds either really cool, or really lame, depending on exactly what it is you're doing. :)
I've had the same type of light bar on my truck(s) for the last 4 years... but I have always had to manually raise and lower the light bar. I could never come up with a perfect way to get the lights right where I wanted them for all occasions. A motorized light bar will allow me to do just that, adjust for each situation. IE: Look up on a hill for a trail, adjust where the light output is...

The linear actuators can be had for pretty cheap. 12v... various stroke lengths load ratings, etc.
 

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Oh, OK... that does sound pretty cool. I was picturing a full 90-degree tilt on the entire light bar, or something... like it folded down into the bed. *That* would have been interesting....

What about getting a movable spotlight? Maybe something like this from Go-light? Might be a bit cheaper (actually, probably not... ha), and they make mag mounts for them, too, so you could take it down after you use it. Plus, no worries about wear and tear on an accuator set-up.

-Radar
 

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Discussion Starter #11 (Edited)
Oh, OK... that does sound pretty cool. I was picturing a full 90-degree tilt on the entire light bar, or something... like it folded down into the bed. *That* would have been interesting....

What about getting a movable spotlight? Maybe something like this from Go-light? Might be a bit cheaper (actually, probably not... ha), and they make mag mounts for them, too, so you could take it down after you use it. Plus, no worries about wear and tear on an accuator set-up.

-Radar
The light bar is pretty small, just two tabs welded to the top of my exo cage, tube, and two tabs for the lights.. Nothing crazy. It will be easily removable by pulling 3 bolts or 2 bolts and a pin. I'll figure that one out later. I already have the quick connects for the wires. The actuator probably will not be seeing any movement aside from the shaft moving in and out. Wear and tear on the actuator should be pretty minimal compared to most vehicles. I drive this rig MAYBE 2 times a month, and that is off road.

You can barely see the lights rotated down in this picture. With the new setup they will flip back behind the cab so they are protected better.




I've seen those, but I do want the bar to store out of the way from them pesky rocks should I put the rig on it's roof. (I don't have much need for Euro Beam Rallye 4000's when crawling at 1/2 MPH. They'll be folded down out of the way. I do enjoy driving off road at a decent pace (40-50 mph) on terrain that my rig can handle (IE roads to/from the Trail, and down dry lake beds.)

I would probably try those if I didn't already have the lights, the Rallye 4000's are great lights!
 

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you must be 'tweeking' pretty hard. jk
 

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Be sure to account for any mechanical advantage that may be working for or against the actuator, though I doubt you'll have much of a problem.
 

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Discussion Starter #15
Yer junk going 70 mph on a public road... has got to be a public threat... LOL
Hey fawker, That rig is perfectly stable on the street. It stops better than my Tacoma did! No death wobble, or ill manners!



If anything... I see YOU as a public threat... seeing as your reaction times seem to be slowing down! :flipoff1:
 

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>>I'm hoping to use a linear acutator to move the light bar up and down, and to hold it in place. Two bolts with brass washers used as a pivot, an arm of 4.75" long off the light bar pivot, and the linear actuator has a stroke of 12" to achieve 145* of rotation, [lights folded back completely down to lights up past normal angle... pointing almost directly in-front of the truck]. I'm working on various aspects of the design to make it as compact and rugged as possible.


You guys got wwwwwaaaaayyyy too much time on yer hands!
 

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Where can you find an electric linear acutator?
 

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If you had it flip forward, with the actuator in front, then when the lights are up the actuator wouldn't have to see compression, just extention. I assume the housing of the actuator would be strong enough to handle that.

Hope that made sense:eek:
 

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If you had it flip forward, with the actuator in front, then when the lights are up the actuator wouldn't have to see compression, just extention. I assume the housing of the actuator would be strong enough to handle that.

Hope that made sense:eek:
Adding to his idea...
If you made it so the lights rotated up.
Using a back stop the lights would not but a strain on the actuator. There maybe a problem if you try to bring the lights down while driving at 70mph.
 
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