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PostPosted: Thu Feb 02, 2012 9:32 pm 
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kikiturbo wrote:
I think 5000 N is very realistic in fact..

consider the following.. Let's say upper frame width is about half track width... for sake of argument... that means at that point you see vertical corner load x 2....

with a 500 KG locost, in a corner under braking you could see 250 KG at front outside corner... now multiply that with a normal bump load of 3G, and x2 for the leverage effect I mentioned at the start, and we get... 15000 N....


Maybe I need to be reeducated here because I've been away from the game to long.

My memory tells me the force at the body is the force at the wheel / motion ratio. I am very very confident it has nothing to do with the relative width of the upper frame. For the record I've always thought of motion ratio's greater than 1 first which was the case here as well even though his design might not be that way.

I also would not model a normal braking>cornering> acceleration event with a 3G vertical input. 3G vertical inputs, to me, are reserved for ensuring you're suspension components can physically hold up to a horrible pot hole for example, not a racing condition where you're studying torsional rigidity. I don't care how rigid my frame is if my suspension just got ripped apart. They are different situations that shouldn't be mashed together in my view.

All that being said, if I'm right and you have a motion ratio somewhere around 0.75 or lower and a 1600lb car maybe 1125lb isn't too far of a stretch.


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PostPosted: Thu Feb 02, 2012 10:12 pm 
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A couple of points..

1. The "twisting" forces into the frame come from the outside wheel trough a lever with a length equaling track width/2. That moment is then applied into the frame.. the narrower the frame attachment point, more force it will see for the same moment..

2. As for my 3G load.. When your car settles into a corner, you will see something like a 1 - 1.5 lateral G.. The force trying to twist the frame will, in fact, depend on a multitude of things, like COG location, it's height and difference between front and rear springs (or natural spring frequencies..). However, what you will feel is car reacting to various bumps and curbs, and when you hit that track edge, believe me you will see 3G on the wheel... The whole idea behind a stiff chassis is for it not behave like a loose spring but effectively transfer loads between the corners..

Car frame is more or less a linear spring.. so for FEA it makes no difference if you use 50N or 5000N force, the torsional stiffness will largely be the same.. However, if you use a realistic max load, as your input, you will also have a chance in seeing if your frame bracing is adequate for the job.

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PostPosted: Thu Feb 16, 2012 12:41 am 
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http://thinkfastengineering.com/2012/02 ... fications/

Interesting website this guy has going. What intrigues me is his torsional stiffness numbers for a professionally built FF are in the same ballpark as some of ours - after all a 4 cylinder Se7en is little more than a FF with less sticky tires, higher CG, engine at the wrong end, little extra weight, and an extra seat.

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PostPosted: Thu Feb 16, 2012 2:43 am 
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That was fun to read. I think our locosts can be very similar to the Formula Fords. And the examples like Fred's or Modernbeat's don't give up anything or much in the sticky tire and CG area. Except maybe the driver sitting up more.

Considering how much fun I had in an FF and how fast it was compared to the street car based competition I think we can get a lot more of these locosts on tracks in the USA. Why even bother tracking your Porsche or BMW etc. when the downside is so high if you hit something? If you add up the costs of any mods you might do and the rate of consumption of brake pads, rotors, and tires - the locosts really are low cost.

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PostPosted: Thu May 03, 2012 8:52 pm 
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Catching up on threads in this great new-to-me forum…

Section 8 wrote:
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.


That’s how the tubes are actually loaded, so I’d stay with the ever so simpler beam elements.

And if it’s really a space frame the torsional stresses in the tubes will be small, as they will be borne by the frame at large.

a.moore wrote:
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.


I doubt it would make much difference, since applying pure torsion also applies beaming loads to the opposing faces.

I think of beaming as a subset of torsion, since there’s no way a structure could have torsional stiffness w/o also having beam stiffness, whereas the opposite is not so.

Also, referring to the first picture below of the torsion test I did, load *is* effectively applied to both corners - the reaction force at the jackstand at the front supplies the second one, and will be very similar to an equal and opposite vertical load.

kf2qd wrote:
Ever looked at a railroad car axle? It is not straight. It actually gets smaller in the middle. Years ago I worked for the brother of the guy (Wahl was his last name) who redesigne4d those axles back around WW2. Previously they had been trying to make the axles stronger by making them larger (and stiffer) but they were failing because of the higher loads they were trying to carry. He redesigned the axle with the smaller center so it would have some flex and then axle failures were a thing of the past.

What I am trying to say is this - if you make the frame too stiff then you are transferring all the loads from bumps(impacts) at the tire into a smaller and smaller area of the frame and will probably have to deal with failures at the shock mounts and a-arm mounts. Bumps(impacts) are very short duration, but very high forces. If you stop them over too short a distance then you have to make the frame stronger so that it doesn't fail at the point of impact - the shock mounts and the a-arm connection points. Given - there is some compliance in the shock, but if you have gone to rod ends for your suspension there is very little compliance there. By having some flex in the frame you are reducing the total force that the frame is experiencing. Force = Mass * Acceleration - if you impact the frame with some force (tire in pothole - extreme) and the frame has a minute amount of give (flex) then the total acceleration will be lower so the force the frame has to disipate will be lower. There is some practical limit to what you will be able to gain by stiffeneing the frame, above which you will have to add more weight because the suspension attachment pooints will have to be improved so you won't destroy the frame at those points.


This is true where a more or less fixed deformation (usually bending) is enforced, in which case lower stiffness means lower stress.

But in our case where most of the compliance comes from the suspension and tires, and frame loads are intended to be resolved into tension/compression, more material will mean lower stress in direct proportion.

It may or may not be true (I’d have to analyze it) for lateral loads fed in by the suspension, but it would make more sense to decrease impact loads on the frame by increased bushing compliance than by frame compliance.

a.moore wrote:
Engine bays are such a royal pains in the butt. I may revisit it one of these days - a bolt in brace always seemed appealing. The kicker will be getting the bolted joint stiff enough so the brace is effective and not just ballast.


Third pic below shows how I did it on my senior project 3-wheeler frame, which was my first time doing FEA.

The frame weighs 108 lb and stiffness is ~4800 lb-ft/deg.

Lonnie-S wrote:
However, LISA does load and mesh simpler components and does a really cool job of meshing some complex parts. Here is the original Haynes Roadster pedal box meshed by LISA from a STEP file and a separate rendering of it so people can get clear on the geometry, which is hard to see just from the LISA mesh file. This is a 3D, volume mesh.


This part would be much better modeled with 2-D plate/shell elements for both efficiency (run time) and accuracy.
To give reasonable accuracy there should be several layers thick of 3-D elements.

Also they have only 3 DOF per node (the translations) and have no rotational stiffness; every node behaves like a balljoint, which often results in unintended mechanisms in the model.

You can think of 3-D finite elements as “struxels” – structural elements, same as pixels are picture elements; to paint a reasonably accurate picture of the structure, it takes a lot of them.

Whereas the only property assigned to 3-D elements is the material, beam and plate elements are assigned materials and dimensions from which behavior can is calculated using appropriate equations, so you get a whole lot more from each one; it could take hundreds of solid elements to give the same accuracy as one beam element, except at the attach points, where all bets are off..

The key word here is vehicle mass... The lower the vehicle weight, less torsionall stifness is nedeed... so it is a bt pointless (but informative) to compare to a RR... :)[/quote]

The proper way to normalize stiffness vs. mass is vibration frequency, as it depends on only those two things.

Decades ago I remember reading in a Road and Track test of a Mercedes that its chassis had achieved 26 Hz.

I presume that the boundary conditions are with the chassis resting on the suspension.

This would be pretty close to what is called free-free, i.e. no restraints, since the suspension is so much more compliant; probably 1 Hz or so, reflecting a relative stiffness of ~1:600.


Attachments:
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sp1.JPG
sp1.JPG [ 24.92 KiB | Viewed 1806 times ]
sp0.JPG
sp0.JPG [ 48.72 KiB | Viewed 1806 times ]

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Last edited by NoahKatz on Fri May 04, 2012 1:51 am, edited 1 time in total.
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PostPosted: Thu May 03, 2012 10:09 pm 
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Quote:
That’s how the tubes are actually loaded, so I’d stay with the ever so simpler beam elements.

And if it’s really a space frame the torsional stresses in the tubes will be small, as they will be borne by the frame at large.


FWIW, I found some substantial torsion loads in tubes in my frame design. There were some very high loads in the book Locost model in this thread that were not axial also. My frame shows high torsion loads in the roll bar hoop above the bracing, 2 or 3 times as high as any axial load. I was surprised.

I think what happens is that steel is a stiff material and it takes very little strain in the frame before loads start appearing that are not strictly axial tension/compression. I'm not an ME, so I may not understand this stuff well though. I am using Grape for my FEA.

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PostPosted: Thu May 03, 2012 11:05 pm 
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Anything other than perfect triangulation will start introducing bending loads. A roll hoop is not a triangulated member and from what I remember of book frames I've seen they aren't even close so bending should be expected.


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PostPosted: Fri May 04, 2012 1:45 am 
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RbBugBitMe wrote:
Anything other than perfect triangulation will start introducing bending loads. A roll hoop is not a triangulated member and from what I remember of book frames I've seen they aren't even close so bending should be expected.


Yep

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PostPosted: Fri May 04, 2012 12:20 pm 
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NoahKatz wrote:
Lonnie-S wrote:
However, LISA does load and mesh simpler components and does a really cool job of meshing some complex parts. Here is the original Haynes Roadster pedal box meshed by LISA from a STEP file and a separate rendering of it so people can get clear on the geometry, which is hard to see just from the LISA mesh file. This is a 3D, volume mesh.


This part would be much better modeled with 2-D plate/shell elements for both efficiency (run time) and accuracy.
To give reasonable accuracy there should be several layers thick of 3-D elements.


While I'm still at the "more than slightly dangerous" stage with respect to FEA, I've also made some progress. Yes, I have subsequently learned that the thin materials prevalent in a Locost require some particular FEM models (as you point out) for thin plates and shells. I just took the default option in LISA, which is not a good choice here and way too complicated for this particular application.

The 3D modeling software I've switched to, Alibre Design, has a working partnership with a company called FEMdesigner (http://www.femdesigner.com/) and through forum discussions with them and some actual FEA practitioners, I've learned of an excellent book that others here could benefit from. It's only available used, but is worth buying if you're not an ME and would like to understand the actual rationale and best choices for particular situations. It is a very practical book.

Book = "Building Better Products with Finite Element Analysis"
Vince Adams and Abraham Askenazi
OnWord Press, Santa Fe, NM, USA 87505-4835
First Edition, 1999
ISBN 1-56690-160X

I'm planning on purchasing FEMdesigner AD when that particular version supports thin plates and shells (which their standard version has done for some time) because it is integrated into Alibre Design and would make life very simple for me. Also, since I'm actually about 1/3 the way through building my Locost now, I don't have the time to backtrack and design/redesign using FEA. That will have to wait until my next project.

Cheers,

Lonnie

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: Sat May 05, 2012 3:33 pm 
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Quote:
RbBugBitMe wrote:
Anything other than perfect triangulation will start introducing bending loads. A roll hoop is not a triangulated member and from what I remember of book frames I've seen they aren't even close so bending should be expected.


Yep


Well I'm not sure I understand the point here. It was commented that "if it's really a space frame the torsional loads will be small" ( in reference to individual tubes ). Is the complaint here about the use of the term space frame to generally apply to these car frame designs? Many of these designs contain significant compromises for various practical considerations.

I was trying, and perhaps poorly, to make the point to our general readership that may choose to improve their designs with FEA, that they should remember to examine their designs for all the possible loads that the tubes may see. I am not embarrassed that I failed to recognize, without these tools, that I would have large loads in the roll bar above the bracing. Observing that it is not a perfect space frame in the area of the required rollbar may be true, but not so helpful as saying that as a practical matter one should examine all the possible loads.

Furthermore, a perfect space frame will become imperfect when submitted to loads because it will generally flex. This is particularly true where weight has been a design consideration and compromises have been made. Modeling the frame with beam elements instead of as a truss acknowledges this and also allows the examination of these other loads.

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PostPosted: Sat May 05, 2012 3:59 pm 
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Not sure why you're getting defensive. The point was to support your assumptions and observations with facts. Most of the designs do have many compromises and they will see bending loads because they are not triangulated properly.


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PostPosted: Fri Jan 18, 2013 1:06 am 
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Not sure if I should post this here, but maybe some guys here can take a look at what I've got a suggest some improvements. Thanks!

First the model:
Image
Now the FEA:
Image
Image


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PostPosted: Fri Jan 18, 2013 9:21 am 
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I would constrain the model where the shocks attach to the frame; it will give you a more accurate result.

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PostPosted: Fri Jan 18, 2013 9:32 am 
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That is a fantastic model and has some very interesting elements. The "X" structure over the engine is novel and the design is obviously very well triangulated. I can't comment about the FEA, but would point out that some of those joints will be very difficult to make in real life. The joint where the driver's footwell meets the lower firewall piece along with the triangulation of from the engine mount is the most obvious to me.

I wonder what would happen to the FEA analysis if they were joined through a less than perfect flange or two (like 1/4" or 5/16" plate) that allowed the centroids of the members to meet in space, but permitted a more realistic welding situation. It would be a very interesting comparison to make and not too expensive in time or money using FEA.

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: Fri Jan 18, 2013 11:46 am 
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Great model. I'm not convinced you need everything you've got to be sufficiently stiff. What is your target and why? I agree welding will be a complete nightmare.


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