Forces on radius arms....

OK, dumb question. I have my radius arms drawn up in Solidworks and I am trying to figure out if they are strong enough or if I need to build some from scratch. Any guesses at what I should use if I want to simulate coming down hard on a rock? Maybe the wieght of the front of the vehichle?

I think worst case would be be about half the weight accelerated over 5 to 10 inches. This would simulate droping a tire off a rock ledge and coming down on the radius arm. I don't remember how to calculate the force but any physics book should have the equatin in it.

well, i think that this is more of an impulse rather than a force. assuming the mass landing on the top of the arm over these 5" is 0.5 the total vehicle mass and modeling this situation requires the solution of the maximum force transmitted over these 5". this model should include the damper and spring attached between the ground(arm) and the mass. subject the mass to a 5" displacement and calculate the maximum force that the ground(long arm) sees.
This is a function of the spring rate ,damping coefficient of the shock, initial displacement(5", and velocity of the mass.
this is differential calculus.

take that force and apply it to the center of the arm and solve for the maximum stress...compare it to the yield stress of the material and get your factor of safety. then change the material/geometry/cross-section accordingly.
not sure if that makes sense.

I see you have drawn it up in SW, do you have access to a mechanical modeling software....like Workingmodel. otherwise, you could write out the analytical solution by hand


-J

Well I ran it through with cosmosexpress with the vehichle wieght on the lower back of the radius arm and it looks ok. But, I am just learning this software so my rendition of the radius arm is stronger becasuse I can't figure out how to take material out of the middle. Any way this is what I came up with.
http://www.picturevillage.com/photo/showpic.php?aid=9120&pid=237684&uuid=3022

Do you think it looks close to what would be expected? I am to tired to figure it out on paper right now. And I guess I should take into account the spring compression also. This program is so cool.

Simple version of what jjvande said:

the radius arm itself does not carry the weight of the vehicle unless the arm is the only thing touching the ground, that is the job of the spring. theoretically, if the XJ was dropped straight down, the arm wouldsee very little force on it.

The arm does, however, carry the weight of the axle, so if you were were trying to figure out if the arm whill bend if you drop it on a rock, then just use +- half the weight of the axle (driver/passenger side will not be equal because of the pumpkin) times the distance from the axle connection end (because of torque)

If the arm is what is holding the vehicle up, the weight on it would be determinate of how many other tires were on the ground.

if you were trying to figure the force on the arm when the tire hits the ground it becomes a little more complicated. If the arm itself does not contact a surface, then only the component of force along the arm will have any effect on it. I am assuming that since you are doing stuctural testing that you know how to calculate that.

the above is a "spherical horse in a vacuum" approach to finding forces but for what you need i think it will work. :greensmok

you're way overthinking it. use what has worked for others, just build it with 2x2 .25 wall square.

Yeah I like to overthink though. I guess I wans't clear, I am just curious about how strong stock ford radius arms are, and from what I modeled their descent but not perfect.

CW said:

Yeah I like to overthink though. I guess I wans't clear, I am just curious about how strong stock ford radius arms are, and from what I modeled their descent but not perfect.

Click to expand...

What is the peak stress compared to the modulus for steel (what is the safety factor)?

Did you add sideloads and twist?

Stock Ford arms are good for slow rockcrawling, but have been known to bend hitting rocks at speed (angle impacts in a turn at speed turn them into tacos).

yea....ther's a lot of complicated loading on the long arms and the Champaign of beers was talkin:anon:. What I would suggest is to do a comparison between a common proven design (say 2" x .25" square tubing) and the ford RA design for several different loading conditions.

by "remove material from the middle" do you mean the ford arms are hollow? i'm not familiar. use the "Shell" feature and play with the wall thickness if that's the case. or use the "extruded cut" feature.