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PostPosted: April 17, 2008, 6:15 pm 
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Thats excelent information Puk! The one thing I havent figured out how to do with grape yet is to apply masses to the model. I was thinking about defining a imaginary material with a density that could be aplied to known elment lengths to create the weights, but i havent tried it yet, Ill let you know how it goes.

What do you use for allowable stress in the elements of your models? is there a safty factor you like to design to?


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PostPosted: April 18, 2008, 2:36 am 
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Ok another long answer!

First off let me say that I have not created an FEA modeling since leaving university in 1992! But the laws of physics have not changed since then so the loads and reactions ought to be the same, but he way that they are modeled in FEA may be different. There are several people posting on here who do currently model in FEA.

That said what I did way back was introduce material density and a mass for the motor. Michael Costin (Race Car Chassis Design) does something slightly different but then he was doing a hand calc. Basically he turns a dynamic load problem into a static problem by replacing masses with the reactions produced by accelerating them - so no need to introduce any high density objects.

I've sketched a very simple model, to make the concepts clear. Starting simple and getting more complicated: Replace the car chassis with a single beam simply supported at each corner. Then place a mass on the chassis - in this case a purple engine. The engine is bolted to the chassis at each corner.

Image

Now if the whole chassis hits a bump which accelerates both front wheels upwards at 2G then we can calculate the force introduced at the front of the chassis as being 2 times the load born by the front wheels.

If you simplify the effect on the chassis to an upwards acceleration (ignoring the tendency of the chassis to rotate about its CG) then you can find the resulting upwards acceleration of the complete car from F=ma

Knowing the mass of the purple engine you can find the reactions fed into the chassis by the engine at each mounting point. As the whole chassis is assumed to be accelerating upwards the force at by the rear wheels is some value less than 1G times the load born at the rear.

Image

You would then calculate the loads imposed on the chassis beam by the various input forces and reactions. I would think that you could apply this same approach to an FEA model.

If you now replace the beam with a pair of truss frames running down either side of the car from front to back and picking up the engine mounts. Half the vertical loads are taken in each truss frame. So you can calculate the loads in each member - and because his simple model has only considered vertical acceleration all forces are limited to the vertical planes. The FEA model should come to a similar set of loads.

Once you've got the FEA to behave like the hand calc then you can start to make the FEA model more like your actual chassis, so changing its shape and adding the other masses.

The allowable stresses depend on the material - for steel you need to keep the stress under the plastic limit (or you'll have permanent deformation). For aluminium you need to keep the stress beneath the peak stress allowed for the design life that you seek (i.e. 5 million load reversals).

I don't know but suspect that for a steel chassis the desired torsional stiffness will dictate material sections (rather than stress level), but for an aluminium one stresses would be teh limiting factor, rather than stiffness. That is if you use a big enough section to achieve the desired stress levels you'll find that the stiffness is more than adequate.

Let me reiterate; I'm only repeating stuff that I have collated from others. So I'm not saying that if you do it like this it will work. However this is the approach that I'm following and you can see a flaw or a better way please let me know!

Incidentally before some one says you can't make aluminium space frames - yup agreed. If using aluminium you'd design a twin spar (Lotus Elise) or mono because the target stress levels dictate large load bearing sections.

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 Post subject: FEA problems
PostPosted: April 27, 2008, 10:47 pm 
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I played around with grape early in my designs but came to the realization that all the numbers were just theoretical. It assumes that all joints are single point 3 dimsional pinned joints and that the surface area of the welded joint has no factor in affecting the joint stiffness and over all chassis rigidity. If this were true nobody would ever need or use gussets. The continuous nature of our main frame rails contribute greatly to the stifness and I dont believe this is modeled in grape. The main areas of weakness are the open top of the cockpit and the open top of the engine bay. The top of the cockpit is reinforced with a heavy tube roll cage which ties in to all 4 corners of the open cockpit. The engine compartment bracing will also greatly stiffen up the car. But truly. How many pick up points feed direclty into the exact tube joints?. How many parts are mounted in the centers of tubes?. Certainly the beaming effect of the tubes has some importance. I think is only use is when showing the chassis at greatly exagerated loads to point out the really obvious area that need improvement .but that the actual physical tests will be much more accurate
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PostPosted: April 28, 2008, 10:37 am 
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Yes, but it should accurately compare different frame configurations.


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PostPosted: April 28, 2008, 11:29 am 
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I think the comments about grape are inaccurate. It would be possible to model the way that "pmatolcsy" talks about. But from the appearance of the pictures, the modeling is taking into account the weld. I say this because the tubes are not uniformly colored from end to end. If they were treated as pinned they would only load in tension / compression and therefor should be a single color.

It's possible I am getting photos from different FEA programs confused here, but I thought they all showed this in at least some examples. I do see some models treating tubes as single elements, but many seem to show tubes made up of small lengthwise increments.

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PostPosted: September 24, 2008, 2:05 pm 
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First of all I get element, node and whatever the third one is mixed up so bear with me.

A truss in 2 dimensions can be modeled with single elements and get accurate results ie tension and compression with pinned elements. With the world as it is, you can do the same with solid joints as well, it changes the forces a little, but being that it is static, and shouldn't move (only deflections) it works out OK.

With 3 dimensions in a space frame modeling with single elements isn't the best way because the added dimension brings twisting moments that put combined loads into the member. no longer is it just tension and compression it is torsion, shear with the tension and compression.

Now, I don't know the GRAPE software, I have only used Cosmosworks, so it is possible that GRAPE is infact taking that into account and just putting the maximum value along it as the "color" of the whole item.

A counter point: you are measuring torsional ridgidity, so how can you do that with a model that doesn't transfer that force? ==>A cylinder will be infinitly ridgid. Model an "L" shape and fix one end and put a force at the other and see what the result is. It should be easey to see if GRAPE deals with combined loading or not.


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 Post subject: 62000 lbs degree???
PostPosted: October 1, 2008, 1:59 pm 
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can i get someone to check me on this model for torsional stiffness. I also need help with some smaller tube size geometry data because most of the tubes in the model are too big, at 1.125 dia by .049 wall. does anyone have smaller tube geometry data? and did I do the math right for the torsional stiffness.

point 200... loaded with 2000lbs vertical in +y direction. 66.8 inches from point 199, wich is constrined in y only. (points simulate tire contact patch of suspension). point 200 displaces 0.20934 inches. / 66.8 and arctan-ed gives .179554778 degrees force applied is 2000lbs with a moment of 5.566... feet is 11133.33 lbs.ft of torque / .1795.. degrees = 62005.21 lbs/degree right???? or am i way off?

see attached picture!!!
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PostPosted: March 6, 2009, 2:21 am 
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I have a quick question? I'm testing my frame and just want to confirm that I calc it right. I'm using solidworks with weldments and a beam anaylsis in cosmos. I'm gettting 1400lb/deg by fixing three corners and applying a load on the last one. Like I said stupid question but I believe I mulitiply the 1400 by the length to the centerline. In this case it is above 6000 ft*lb/deg with a rollcage. This is for a bike build and solidworks is saying my frame weighs ~300lb's is that reasionable for this app? I'm trying to finalize the frame this week, order parts next and spend springbreak building :)

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PostPosted: March 6, 2009, 2:27 am 
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predatorlt1 wrote:
can i get someone to check me on this model for torsional stiffness. I also need help with some smaller tube size geometry data because most of the tubes in the model are too big, at 1.125 dia by .049 wall. does anyone have smaller tube geometry data? and did I do the math right for the torsional stiffness.



point 200... loaded with 2000lbs vertical in +y direction. 66.8 inches from point 199, wich is constrined in y only. (points simulate tire contact patch of suspension). point 200 displaces 0.20934 inches. / 66.8 and arctan-ed gives .179554778 degrees force applied is 2000lbs with a moment of 5.566... feet is <a href="tel:11133.33">11133.33</a> lbs.ft of torque / .1795.. degrees = <a href="tel:62005.21">62005.21</a> lbs/degree right???? or am i way off?

? did u convert in to feet? That sounds like in*lb/deg?

see attached picture!!!

Image

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PostPosted: March 6, 2009, 2:28 pm 
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nkw8181 wrote:
I have a quick question? I'm testing my frame and just want to confirm that I calc it right. I'm using solidworks with weldments and a beam anaylsis in cosmos. I'm gettting 1400lb/deg by fixing three corners and applying a load on the last one. Like I said stupid question but I believe I mulitiply the 1400 by the length to the centerline. In this case it is above 6000 ft*lb/deg with a rollcage. This is for a bike build and solidworks is saying my frame weighs ~300lb's is that reasionable for this app? I'm trying to finalize the frame this week, order parts next and spend springbreak building :)


Are you fixing the corners of the frame, suspension mounts, or shock mounts?

Try fixing one of the rear shock mounts in the X, Y, and Z directions, constrain the other rear shock mount so it cannot move vertically or fore/aft but it needs to be able to move sideways so the chassis is not over-constrained. Finally, apply equal and opposite vertical loads to the front shock mounts - if the program will let you, displace each front mount equal but opposite amounts that equal 1 degree of chassis twist and use the load to achieve this displacement to determine the stiffness. If you cannot displace the mounts, apply equal but opposite vertical loads to each forward chassis mount and use 2*force*(distance to chassis centerline) to determine your torque/displacement in inches - convert this to torque/degree.

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PostPosted: March 7, 2009, 10:57 pm 
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I have fixed the uprights at 2 points on all corners but the front left where the force is applied at the bottom of the upright.
rear left - constrained form the foward and vertical
rear right - constrained in all directions
front right - constrained vertical

the angle of rotation is seen in the following pic with 1500 lbs applied. the frame is 1x1x 1/16 and the roll cage is 1.5 with .125 wall thickness. I'm waiting to here back from NASA because i want to use a smaller size. i will also try taking away tubes. the wieght of the frame is 241 lbs but since i shortened the engine bay and lengthened the distance from the rear wheels to the cab and added the engine, i now have a values really high. my rigidity is 25126 ft*lbs/deg. yes it is high but i beliew my setup is correct because before i had values around 6000 before the changes. any feedback?


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1500 lbs1.JPG [ 127.43 KiB | Viewed 3324 times ]
1500 lbs.JPG
1500 lbs.JPG [ 125.13 KiB | Viewed 3322 times ]

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PostPosted: March 7, 2009, 11:26 pm 
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when i apply the load to the rear my numbers are 4700 ft*lbs/deg


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PostPosted: March 8, 2009, 12:10 pm 
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You're getting there but keep in mind you aren't getting a torsional reading with that number. By applying the load to only one corner, you are getting a combined torsional and beaming stiffness. Keep in mind the torsional rigidity number is to measure the stiffness of the "torsional spring" connecting the front and rear suspensions so you have to twist (not bend) the chassis. Your torsional stiffness number should be practically identical if you are running the test correctly whether you fix the rear and apply the loads to the front or visa-versa.

Try applying two vertical 1000 lbf loads to the front uprights. Make the one on the left front up (+ direction) and the one on the right front down (- direction).

Also, only apply the load to the lower ball joint - think about how the load is transmitted in the real car; if your spring is attached to the lower arm, the upper arm is not carrying any sort of vertical load aside from a small amount from friction in the upper arm's bearings. If you fix or apply a load to the UBJ, it won't be an accurate simulation either unless you should happen to be running a pull-rod suspension.

Lastly, unless you shocks are attaching to the lower corners of your "boot" support frame, fix the rear of the frame where the springs will actually attach to the frame. Randomly fixing points will also give you inaccurate results.

It is extremely important to load and constrain the model exactly as it will be in real life, otherwise your numbers will not be accurate.

Keep it up..you're getting there.

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PostPosted: March 8, 2009, 12:34 pm 
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ok thanks, i didn't really think about the fact that i was getting torsional and beaming stiffness. i'm currently rerunning everything with smaller tubing. i'll post it up when i'm done with the constraints the way you have stated

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PostPosted: March 8, 2009, 12:40 pm 
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nkw8181 wrote:
I'm waiting to here back from NASA because i want to use a smaller size.
any feedback?

Yes, did you check the rulebook?
You're definitely not required to use 1.5x0.125, it it much heavier than the requirement for such light vehicle.
I'm using 1.5x0.095 for my cage and I'm _still_ a weight class above my requirement.

According to NASA's rulebook (as of the date of writing this message), a vehicle weighing 1500 lbs or less (with NO driver or fuel) is required to use a minimum tube of 1.375x0.095 DOM or Chromoly.
A vehicle weighing 1500-2500 is require to use a minimum tube of 1.5x0.095 DOM or Choromoly.

And just one more point, I presume that you're looking into racing your car since you're looking into a cage and waiting to hear back from NASA, in that case you should consider extending the side protection bars beyond the width of the frame, I didn't like the idea that the first thing to get hit from the side is my elbow.

Moti

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