jonas2307
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| posted on 7/9/04 at 11:18 AM |
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Strenghts vs weight?
Hi!
I'm from Sweden and I'm building a locost that started at as an average, if there is one...
Please be patient with my spelling as well...
I have a question regarding the strenght of the chassie, will it be half if I use 25*25*1 mm instead of the normal 25*25*2 ?
I guess it isn't that simple, but I would appreciate if someone had done some calculations that they would like to share.
Thanks!
--- Jonas
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stressy
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| posted on 7/9/04 at 11:23 AM |
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simple answer...
bad idea, i would not reccomend going that thin at all. Why do you want to do this, is it just to save weight...
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jonas2307
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| posted on 7/9/04 at 11:34 AM |
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Yes, weight is one reason...
Another is that for the moment I'm looking att making atleast parts of the chassis as a sandwich construction.
I have done some testing where I made framework with both 2 and 1 mm material, normal steel.
Then, on the thinner construction, I placed "distancematerial", divinycell, between all the steelbares and then covered the whole thing
with carbonfibre.
From the rather simple tests that I can perform in my garage, this sandwich part is stronger then the thicker all steel part.
Maybe this could be a solution for the whole chassi?
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stressy
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| posted on 7/9/04 at 03:04 PM |
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Interesting
Sandwich construction is in no way comparable to spaceframeconstruction. A sandwich panel filled sapce frame does not prfrom anything like you may
expect.
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jonas2307
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| posted on 7/9/04 at 03:14 PM |
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Ok, so you don't believe that it would work?
In what directions are the the sandwich construction stronger or weaker?
I guess there is a reason that I haven't seen any locost built like this yet.
And there might be more issues than cost related to this....
But, there has to be a way of making the construction lighter.
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ned
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| posted on 7/9/04 at 03:15 PM |
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I know on some racecars where regulations allow carbon fibre or honeycomb panels are bonded into the spaceframe to further strengthen the chassis.
This really makes it a semi monocoque i would have though. I'd be curious to know how this affects welded joints int eh frame etc as these must
be the most loaded points when the chassis is being loaded, whether thinner tube would cause any problems with strength or implications to the welding
I don't know.
Ned.
beware, I've got yellow skin
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stephen_gusterson
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| posted on 7/9/04 at 03:30 PM |
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a locost is already pretty light and minimalist.
Just pick up a wheel and tyre and see what kinda basic weights you are stuck with! Saving a few kg on the chassis isnt going to make a ballistic
difference at the end of the day. You would be better off adding 10hp to the engine than spending time, and money, and adding risk with
'trick' construction techniques.
atb
steve
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jonas2307
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| posted on 7/9/04 at 03:36 PM |
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Yea, I guess.
But, in my mind this is all part of something that I like to call development!
If there wasn't any, we all would still be looking att those big creatures that we call horses and wonder why they existed...
Don't get me wrong, I just like to test things.
I have a book chassis near completion and now I'm thinking of buildning another one with this sandwichconstruction just to be able to
compare.
The hard trick, one of them, is how to perform testing to make me feel safe....
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ned
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| posted on 7/9/04 at 03:45 PM |
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goes back to the age old 'tried and tested' arguement! though if someone has the time to test and develop, then why not, as long as it can
be proven to be strong/safe enough!
Ned.
beware, I've got yellow skin
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pbura
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| posted on 7/9/04 at 04:23 PM |
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Here's a car with very similar construction (funny how this guy keeps popping up):
http://www.geocities.com/MotorCity/7630/index.html
In his case, he used aluminum tube as a framework to hang the panels on. Very important to have plates at stress points to spread the loads.
Certainly very rigid at 9000 Nm/deg (over 12000 ft-lbs/deg) 
Pete
Pete
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stephen_gusterson
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| posted on 7/9/04 at 05:17 PM |
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problem is, you will be testing on the road.
if something fails cos you went all adventurous, it will be your ass all over the road in in the hospital if something fails 'under
test'.
atb
steve
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jonas2307
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| posted on 7/9/04 at 05:29 PM |
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True, it is ofcourse a risk.
But, I intend to do the testing on the chassis in the garage. Not dynamical but it should be possible to test it for torsional stiffness and compare
it with the normal framework.
More ideas are welcome...
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leto
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| posted on 7/9/04 at 07:58 PM |
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If anyone remember and/or care about the original question I believe the answer is no
Skål!
“I'm gonna ride around in style
I'm gonna drive everybody wild
'Cause I'll have the only one there is a round”. (J. Cash)
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stressy
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| posted on 7/9/04 at 08:04 PM |
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Jonus,
Its not that a sandwich design chassis is no good, it can be very very good indeed, its that you need to fully understand what you are trying to
achive with it and how you will achive it.
As ned i think suggested a number of people use sandwich panels for additional strength, generally the best effect of this is with impacts where there
may otherwise be hole in the spaceframe for example.
Sandwich panels on thier own are only really effective when there are out of plane forces acting upon them. The core in the panel is used to stabalise
the thin skins. What this means is that by offsetting the skins from each other using a core the bending stiffness will be much much higher than the
equivelent thickness of plate to the two skins. The buckling resistance will also be greater.
Building a locost frame and packing it full of sandwich panels will not result in a chassis which acts as a sandwich panel monocoque design, it will
be a stronger in some aspects but HUGELY expensive.
If you really do fancy a monocoque first consider just using alloy sheet, or as you will find is very common, an alloy box frame alloy skins and alloy
core.
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silex
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| posted on 8/9/04 at 07:03 AM |
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The answer to the original question is yes and no.
If the member is in pure tension (a bit rare on a chassis) then - yes
Under bending and tortional stress - no the strength will be more than half, but not fantastic, maybe 55~60% (without calculation.
A good way to make a judgement without doing full bending calcs is to calculate the 'I' value of the section. The higher the
'I' Value the more resistant to bending.
B or b = breadth
D or d = depth
For a hollow section you need to take the inner "empty" square away fom the outer square.
so I = ((B(D^3))/12) - ((b(d^3))/12)
This works for any box type section.
Do go ahead and play with the figures it works.
I personally have a chassis made from 48mm diameter thin wall tube - its much lighter and way stronger than the standard chassis.
Murphy's 2 laws
1. If it can go wrong it will
2. In case of emergency - refer to rule 1.
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jonas2307
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| posted on 8/9/04 at 07:08 AM |
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Thank you for the answer and the figures.
Fairly easy to do some spreadsheet calculations.
Regarding your chassis, have you done it by the Book but replaced the tubes or is it another design?
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silex
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| posted on 8/9/04 at 11:25 AM |
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I designed my own chassis from scratch because there is no way I can fit my engine and gearbox into it. But even if you stick to the basic design you
would be supprised at what you can achieve.
Example a 25 x 50 x 1mm section is approx the same weight as 25 x 25 x2mm section, but under bending the 25 x 50 section will take 5 times the load of
the 25 x 25 when loaded on the 50mm section. thus the strength of the chassis is increased and to is the torsional rigidity for very little weight
penalty. Or it is possible to use this to reduce the weight of the chassis.
Good Luck........and keep up the development.
Murphy's 2 laws
1. If it can go wrong it will
2. In case of emergency - refer to rule 1.
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