LocostUSA.com

Learning how to build Lotus Seven replicas...together!
It is currently Sat Nov 01, 2014 7:00 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 350 posts ]  Go to page Previous  1 ... 14, 15, 16, 17, 18, 19, 20 ... 24  Next
Author Message
PostPosted: Wed Jan 04, 2012 10:29 pm 
Offline

Joined: Sun Apr 06, 2008 11:40 pm
Posts: 48
Location: Emerald, Melbourne, Australia.
Lonnie-S wrote:
One of the questions I've wondered about is how stiff is stiff enough?


Hi Lonnie,
This isn't a direct reply to your question, but in Victoria each new chassis gets tested for torsional stiffness.
We have to meet:
1) At least 4,000 Nm/degree (~2,950 lb-ft/degree in your money)
&
2) A "somewhat" linear graph of stiffness vs distance down the chassis.
This is so that fatigue between different sections with different stiffness is avoided.
From memory, this applies to a 4 cylinder engine, and the figure becomes 6,000Nm/degree for 6s & 8s.
Cheers - Gavin.


Top
 Profile  
 
PostPosted: Thu Jan 05, 2012 12:38 am 
Offline
The voice of reason
User avatar

Joined: Thu Jan 10, 2008 4:47 pm
Posts: 5808
Location: Massachusetts
krepus wrote:
Horizon, I'm really liking your design over the typical locost chassis... If you have dimensions available, it may be the way I go...


Krepus, thanks, I'm flattered. It is a big help if you can download the files and run sketchup, that was my idea anyway. It might help you make changes etc. Also I'm hoping to get people to help with the models of parts so that this gets easier and easier.

Then it occurred to me that the files that are used for Grape give the location of every tube junction and a description of every tube. It's very easy to read. So you just need to encourage me to keep the FEA files up to date. That seems like a good idea anyway.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
 
PostPosted: Thu Jan 05, 2012 2:28 am 
Offline
The voice of reason
User avatar

Joined: Thu Jan 10, 2008 4:47 pm
Posts: 5808
Location: Massachusetts
I've started looking at the loads our suspensions put into the frame. Starting with cornering loads on the front lower wishbone. I am using wishbones with one leg perpendicular to the chassis. So I expect all my lateral force to basically go thru that leg.

I am planning on using 1"x2" rectangular tube some places and this is one place I am considering this. So I chose to compare numbers for the rectangular tube in both orientations and also 1" square and 1.25" square because they are the commonly used choices. My frame design does not feed these loads directly into a tube junction for practical considerations of ride height and typical frame construction. My early designs did not have a flat floor, but now I am opting to start out simple.

For this test I simply chose to put the load into the tube 1/4 of the way up from the floor to the top tube. This is higher then the natural height for a Mustang II type spindle, but probably lower then a Miata front spindle. The load chosen is 1000 lbs. because that's an easy number to work with.

Deflection Stress Element
0.011" 23,000 1"x1
0.008" 15,213 1.25"x1.25"
0.009" 13,377 1"x2" ( flat side towards load )
0.004" 10,977 1"x2" ( 1" side towards load )

The first result concerns me a bit. that would be close to the yield for hot rolled tubing.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
 
PostPosted: Thu Jan 05, 2012 2:56 am 
Offline
User avatar

Joined: Sat Oct 23, 2010 2:40 am
Posts: 577
Marcus, I just returned from some "offline" holiday travel and am impressed with the progress you've made. Looks like I have a lot to catch up on.
Two items caught my eye from your latest post:
Quote:
I am planning on using 1"x2" rectangular tube some places and this is one place I am considering this.
Quote:
Deflection Stress Element ... 0.004" 10,977 1"x2" ( 1" side towards load )
Given your previously mentioned thought to bolt suspension mounting brackets thru sleeves welded into the verticle chassis tubes, wouldn't the stronger "1" side towards the load" with 2" to mount the brackets be a natural choice?

_________________
Cheers, Tom

My Car9 build: viewtopic.php?f=35&t=14613
"It's the construction of the car-the sheer lunacy and joy of making diverse parts come together and work as one-that counts."

Ultima Spyder, Northstar 4.0, Porsche G50/52


Top
 Profile  
 
PostPosted: Thu Jan 05, 2012 10:11 am 
Offline
User avatar

Joined: Fri Oct 24, 2008 2:13 pm
Posts: 1679
Location: Carlsbad, California, USA
gavin_eakins wrote:
Hi Lonnie,
This isn't a direct reply to your question, but in Victoria each new chassis gets tested for torsional stiffness.
We have to meet:
1) At least 4,000 Nm/degree (~2,950 lb-ft/degree in your money)
&
2) A "somewhat" linear graph of stiffness vs distance down the chassis.
This is so that fatigue between different sections with different stiffness is avoided.
From memory, this applies to a 4 cylinder engine, and the figure becomes 6,000Nm/degree for 6s & 8s.
Cheers - Gavin.


Thanks, Gavin.

Lonnie

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

Build Log: viewtopic.php?f=35&t=5886


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 4:11 pm 
Offline

Joined: Tue Jan 31, 2012 4:04 pm
Posts: 3
Hello,

I've just read the entire thread and It was very intersting to see there are many other people ot there are dealing with FEA and chassis rigidity.

I'm new to this topic, so I would like to know if anybody can validate my math. From my calculation I should have about 1126 Nm/degree of torsional rigidity.

Here the brief results of my FEA on the suspension mounts on the front bulkhead:

Image


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 4:47 pm 
Offline
Always Moore!
User avatar

Joined: Fri Nov 09, 2007 3:40 pm
Posts: 3353
Location: Pittsburgh, PA
Your math looks right to me. ;)

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 4:54 pm 
Offline

Joined: Tue Jan 31, 2012 4:04 pm
Posts: 3
a.moore wrote:
Your math looks right to me. ;)


if it so, my design is pretty weak...


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 4:58 pm 
Offline

Joined: Mon Jul 26, 2010 8:16 am
Posts: 94
Seems like an unrealistically high load though.


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 5:51 pm 
Offline
The voice of reason
User avatar

Joined: Thu Jan 10, 2008 4:47 pm
Posts: 5808
Location: Massachusetts
Quote:
Seems like an unrealistically high load though.


The size of the load will not affect the stiffness though....

Quote:
if it so, my design is pretty weak...


I was somewhat appalled after my first run too. If you are using the same software we are, you can download Andrew's Locost model and that will likely make you feel better.

Then just keep at it. You can post a screenshot of your frame for comments and also study the frame during animation to find weak spots...

Good luck.

_________________
Marcus Barrow - Car9 an open design community supported sports car for home builders!
SketchUp collection for LocostUSA: "Dream it, Build it, Drive it!"
Car9 Roadster information - models, drawings, resources etc.


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 6:05 pm 
Offline

Joined: Mon Jul 26, 2010 8:16 am
Posts: 94
horizenjob wrote:

The size of the load will not affect the stiffness though....


I have no idea what his chassis or model looks like so take this for what it is worth. I'm speaking from the suspension mount design standpoint and not whether that load could actually occur in a bad situation on the road because it could occur. Will the area around the input deflect significantly? Real world testing could show his deflection is way off the FEA model simply because it is a very large local load.

Essentially he is modeling for a catastrophic situation, not what his chassis will react to in a road racing/autoxing situation when you actually care about torsional rigidity.


Top
 Profile  
 
PostPosted: Tue Jan 31, 2012 7:19 pm 
Offline
Always Moore!
User avatar

Joined: Fri Nov 09, 2007 3:40 pm
Posts: 3353
Location: Pittsburgh, PA
It all goes back to knowing what you are testing. "Garbage in, garbage out" as they say. Not to say analyzing all chassis loads doesn't have some merit but checking torsional stiffness is a pretty simple test.

If you are measuring torsional stiffness, any reasonable load will give you the stiffness. It is the same theory as a linear spring - whether you apply 100 lbs or 10,000 lbs, it will have the same spring rate (lbs/in, N/m, etc).

I won't disagree that 5,000N is high. If anything I'd lower it to 250N or 500N and see if the torsional stiffness is the same - it should be close

There are at least 10-20 more loads that would have to be placed on the chassis to accurately analyze some sort of catastrophic situation. I'd be nervous if only a reasonable torque applied the shock mounts did a chassis in.

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
 
PostPosted: Thu Feb 02, 2012 7:21 pm 
Offline
User avatar

Joined: Sun Jul 22, 2007 10:58 pm
Posts: 387
Location: Zagreb, Croatia
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....

_________________
Building a single seater middy BEC with GSXR 1000 power :)
build log: http://locostusa.com/forums/viewtopic.php?f=36&t=5899

day job: http://smg.photobucket.com/albums/v723/turbolimac/portfolio/


Top
 Profile  
 
PostPosted: Thu Feb 02, 2012 8:33 pm 
Offline
Always Moore!
User avatar

Joined: Fri Nov 09, 2007 3:40 pm
Posts: 3353
Location: Pittsburgh, PA
Ooops...I messed up the conversion before. :(

I don't know what I was looking at since 5,000 N = 1,125 lbs. Why can't the rest of the world use a normal method of measuring? I've heard inch-pound-second is good. ;)

_________________
-Andrew
Build Log
Youtube


Top
 Profile  
 
PostPosted: Thu Feb 02, 2012 8:44 pm 
Offline
User avatar

Joined: Sun Jul 22, 2007 10:58 pm
Posts: 387
Location: Zagreb, Croatia
funnily enough.. I went to high school in the US, and I do not remember having to deal with inches/pounds at all... Gosh that was too long ago.. :)

_________________
Building a single seater middy BEC with GSXR 1000 power :)
build log: http://locostusa.com/forums/viewtopic.php?f=36&t=5899

day job: http://smg.photobucket.com/albums/v723/turbolimac/portfolio/


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 350 posts ]  Go to page Previous  1 ... 14, 15, 16, 17, 18, 19, 20 ... 24  Next

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
POWERED_BY