Swingarm analysis

[PRuggiero]

hello,

I am working on doing this analysis of a swing arm, which is the rear suspension of a vehicle shown below. I am trying to simulate a worst case scenerio for a FEA which would be the vehicle falling and landing on just the rear wheels.
Im not quite sure how to calculate the loads that it will see, but I put the assembly in Solidworks Motion works and applied a rotational acceleration to see what the loads would be where the shock/damper attach.

What I don't are how to calculate what acceleration the swing arm would see in the above scenerio, and after I have the forces from the shock/damper at the swingarm, should I place the swingarm in an FEA constrain the rear where the wheels would be and apply the forces from Motion Works? Any info is appreciated.

Thanks,
Peter Ruggiero

 

[GregLocock]

Well, how is the load transmitted to the swingarm from the ground?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PRuggiero]

The holes that you see in the rear is where an axle goes through and the wheels attach to the axle. So the wheels hit the ground which loads the axle and the damper/spring provide the restraint and it all rotates about the long forward shaft. I guess the problem is that it doesn't necessarily fail in static. I was originally doing the FEA by applying a load where the axle goes and restraining it at the damper/spring hook ups, but even in that method I wasn't sure what loads should be applied.

thank you,
pete

 

[cbrn]

Hi,
as far as I understand it, this is a typical case of "crash test". You could calculate the impulse force knowing the mass of the vehicle falling and the acceleration, but I seem to know that this accel is not really "g" but a greater one which takes into account the fact that the load is impulsive. Probably more expert people in this field will give you further details.

Regards

 

[GregLocock]

OK, it wasn't obvious that you knew that.

Establishing the maximum load you see in a suspension is a bit of a black art, several times the weight of the bike would be a good guess.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PRuggiero]

Yeah it seems as though it is hard to figure out what load to put down. Like I have mentioned I have applied a force and a direct motion on the part. Since the force on the dampers is velocity dependent, when I apply a harmonic force I input a maximum force and decide how fast it dies down which changes the force from the damper since its velocity dependent. If I just apply a constant force that is proportional to the weight of the vehicle then the max force seen is just how far the spring is compressed times the spring constant. When I apply a motion to the swing arm I am just guessing values which doesn't do me any good. I'm not sure how to go about calculating the impulse, which I know is just the change in momentum over time, momentum is easy, how long is takes to stop the motion of the swing arm is tough and would depend on the spring/damper. Any sites/books to point me in the right direction, a vibrations book I would assume would, but I don't happen to have one. Thanks so far for the help.

 

[GregLocock]

If you know the mass of the bike, the height of the jump, the stiffness of the tire and the spring and damper characteristics then it is a one degree of freedom non linear model.

Any book on simulating multi body dynamics would be a good start but I don't have one.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

Any chance you have access to some type of Event Simulation software? Algor, NEiNastran, LS-Dyna, AMPS, COSMOS Motion, are the ones that come to mind. I would suggest that any of these could probably do a pretty good job on a simple "drop". If this is a superbike playing X-Games, those dynamics may be a little harder since it would depend on whether you are still hitting the down side of the ramp, or are you missing altogether.

Garland E. Borowski, PE
Borowski Engineering & Analytical Services, Inc.
Lower Alabama SolidWorks Users Group
Magnitude The Finite Element Analysis Magazine for the Engineering Community

 

[PRuggiero]

Greg, thanks for the advice, I kind of forgot about the tire's play in the whole thing since it wasn't modeled. I would imagine the modulus of the rubber would reduce the impact force on the swing arm by increasing the time that it is in motion. I am on the search to finding these equations, no luck yet. GBOR I am using Cosmos Motion Works now but don't see anyway that would simulate a fall, although I am new to the motion works, but it seems like a pretty cool tool to figure out the forces involved once I know a little more info. The other thing I wan't to know is how you guys would do a FEA on it once I have the forces since its not really a static case but I don't have access to dynamic software. I have to restrain the swing arm from moving somewhere.

Thanks again
Pete

 

[GregLocock]

restrain it via pin joints at the frame end and the shock mounting points.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GBor]

Well, I have to admit that I threw COSMOS Motion out there out of ignorance. I don't really have any experience with it, but there is a COSMOS forum that would probably help.

One caution: A drop test FEA is very dependent on the impact surface. Between the unmodeled tires and the fact that you may not be landing on an infinitely rigid impact surface will DRAMATICALLY affect your results. Generally, these packages offer some type of "impact surface" or "motion stop" that doesn't allow nodes to penetrate beyond a certain x-, y-, or z-value. If you are landing on thick concrete, probably not a bad assumption, but in the case of what I am picturing as a soil mechanics problem, your impact plane is likely no where NEAR infinitely rigid. You may be conservative with an infinitely rigid assumption, but you may also overdesign as a result.

Just a warning...

 

[GregLocock]

Oh, and my suggested constraints are excessive. you should apply an axial constraint at the shock absorber points, and think carefully about what to do at the swingarm to body joint.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ksaw]

You may also want to consider the swing arm loading that occurs when hitting a large bump at max speed. Especially if the damper is progressive and relatively stiff at high shaft velocity, the tire and axle compliance could play a major roll in limiting the load seen by the swing arm.

Kent Sawyer

 

[chicopee]

Estimate your impact landing with the basics introduced in"Satics and Dynamics" then you can apply equations developed in "Vibration Ananlysis" to size Spring and Damper constants so that you have an idea of the numbers before using FEA.