Force that a car-mounted flag would produce

  • Context: Automotive 
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Discussion Overview

The discussion revolves around the forces exerted by a car-mounted flag during braking, particularly focusing on the design and safety of the flag mount. Participants explore the implications of drag, torque, and the dynamic behavior of the flag in motion, considering both theoretical calculations and practical design challenges.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks to calculate the force exerted by a 15-lb flag mounted on a car during maximum braking, noting initial calculations were unrealistic.
  • Another participant confirms that using F=ma is appropriate for braking scenarios but suggests that torque may be more critical than force in this context.
  • Concerns are raised about the flag flapping and the shock waves generated, complicating the calculation of maximum instantaneous force on the bracket.
  • A participant emphasizes that drag from the flag will likely induce the worst-case force on the bracket, recommending that the drag be estimated for design purposes.
  • Resources are shared, including a link to a drag force calculator, with a note that the type of drag force (average vs. peak) needs to be clarified.
  • One participant draws an analogy to sizing flagpoles based on flag size and wind speed, suggesting that the height of the flagpole is crucial due to the twisting force created on the bracket.
  • Discussion includes a reference to flagpole wind speed ratings and considerations for scaling down designs for a car-mounted flag.

Areas of Agreement / Disagreement

Participants express various viewpoints on the factors affecting the forces on the flag mount, with no consensus on the best approach to calculate or design for these forces. Multiple competing views remain regarding the significance of drag versus braking forces and the implications of flag dynamics.

Contextual Notes

Limitations include the lack of specific dimensions for the flagpole and flag size, as well as uncertainties regarding the nature of drag forces and their impact on the design. The discussion also highlights the need for further exploration of safety factors in the design of the flag mount.

Who May Find This Useful

Individuals interested in mechanical design, automotive engineering, or those involved in flag design and mounting systems may find this discussion relevant.

kspabo
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Let's say I have a 15-lb flag that is mounted to a car on a welded bracket. I want to know the general force the flag will exert on the bracket (neglecting flex in the flag, assuming uniform weight throughout) in the scenario of let's say braking at max speed (as this is the most simple situation to calculate this). You're going 75 MPH, slam on the brakes and stop within 3 seconds.

I tried doing some really simple hand calcs myself (f=ma=m*dv/dt) but I realize this is the incorrect method (as my numbers were unrealistic)

To give you a better idea of why, I'm trying to design a flag mount for a car that pivots and has a lock pin the keeps it in position. I'm looking to see the chance of failure in this pin.
 
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F=ma is fine for braking. Use SI units if you want neat unit conversions.

Torque will probably be more important than the force.

A 15 pound flag? What is the expected drag of this massive flag?
 
The worst-case force on the bracket will be induced by drag on the flag, not the stopping force during deceleration. I recommend estimating the drag of the flag and pole and then design your bracket with a strong safety factor from there.
 
This page looks useful..

https://www.lmnoeng.com/Force/DragForce.php

..but you would have to review the paper on which its based. For example I don't know if it gives you an average drag force or a peak drag force due to the fluttering of the flag.
 
Your problem is analogous to sizing a flagpole based on flag size and wind speed.

You didn't say how high your flagpole is that is key because you create a twisting force on the bracket with a long lever arm.. I'm guessing a 6x10 flag weighs about 15 pounds. It would have been better for you to give the flag size.

Here is a table of flagpole wind speed ratings that may be helpful. https://www.eaglemountainflag.com/content/flagpolewind

The shortest one in the table for a 6x10 flag is a 7 inch diameter0 pole with 0.156" walls. In other words huge. But that's for 40 feet pole rated for 120 mph and bedded in the ground with cement.

How to scale that down to a 6 foot pole rated 75 mph welded to the car? Multiply by ##(6/40)*(75/120)^2=0.06##? I'm not sure.

Maybe you could search for banner bracket ratings. The kinds of brackets used to attach banners to utility poles.

For a completely different approach that suggests much thinner flag poles, consider this crazy. The forces he's holding look pretty modest.

 

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