designer
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| posted on 10/5/11 at 08:49 AM |
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Straight or curved tubed chassis?
I was always taught, and every book tells you, that chassis frames should be made of straight tubes as a curved tube is already bending. I think this
crash would confirm it as there is very little damage to the wall and the frame has folded with the curve and the damage has entered the leg area.
I know the chassis was professionally designed and welded by a certified welder, but the curved tubes are used for cosmetic reasons, and basic
principles are usually correct!
 
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Peteff
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| posted on 10/5/11 at 09:07 AM |
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That looks more like a concrete plant trough than a wall and the car has ridden up over it. To have lost it like that would suggest he was probably
going a bit faster than advisable but unless you know all the circumstances it is impossible to speculate. Personally I don't think a straight
tube chassis made from 1" square 16g would have faired any better.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Bluemoon
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| posted on 10/5/11 at 09:22 AM |
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No expert, but you could use straight tubes for the passenger./driver area probably including a roll bar ect, then curved tubes to allow crumpling
else were and not in the passenger/driver area.. But I supose you should also think about where the engine may end up...
Dan
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Mike Wood
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| posted on 10/5/11 at 09:23 AM |
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Hi
Was the driver and passenger OK?
I had wondered too about the wisdom of the Atom having curved main chassis tubes. Goes against good racing car and aircraft design principles for
steel tube spaceframes, such as using straight tubes wherever possible as well as to triangulate and don't put loads in the middle of straight
tubes, or ones already bent! Staniforth's book on race and rally car construction is good for this info, as well as Darrell Stinton and the
EAA's aircraft design books.
The Atom chassis reminded me of Cooper single seater racing cars which had curved tubes that were large diameter and gauge, before others - such as
Lotus - came along with lighter stiffer chassis with straighter, smaller diameter tubes and more triangulation (see:
http://www.500race.org/Marques/Cooper.htm)
This was perhaps one of the reasons that Atom chose such large diameter tubes for the main longitudinal chassis members (and of what gauge?), as well
as perhaps giving an assurance in styling terms to buyers who don't think about spaceframe structures. I note that the Atom chassis has more
traingulation than a Cooper.
Mike
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craig1410
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| posted on 10/5/11 at 09:25 AM |
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Firstly, I'd like to say that I hope nobody was seriously injured in this Atom.
Unless you've designed your chassis using FEA techniques or done some crash testing then it is likely that any amateur designed Locost will have
weaknesses which will show up under extreme (crash) stress. Probably the simplest type of crash to predict is a head on into a solid structure and
even here you are likely to see some lightly loaded tubes punching through other tubes and perhaps welds failing etc. On the upside, a typical Locost
has the engine in the front and the driver/passenger are quite far back in the chassis so there is not much mass behind which might crush the tubes
around the cockpit.
When building my chassis I was thinking to myself, "This thing will be mega-strong in the event of a collision", but to be honest, having
seen some bent Locosts since then, I'm not so sure... I think mine will be stronger than most because I employed some of Cymtriks's
structural mods and I have used 2mm tubing instead of 1.6mm (due to availability more than anything) but I'd still rather keep the car between
the hedges because it is very unpredictable what might happen in a crash. In some cases having too much strength in the wrong parts of the car can be
as bad as having too little strength.
Stay safe people! 
[Edited on 10/5/2011 by craig1410]
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Doctor Derek Doctors
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| posted on 10/5/11 at 09:25 AM |
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If you crash hard enough it doesn't matter what you're in, you're gpoing to get hurt (or killed).
If you look at the Atom the curved tubes on the side ar pretty pretty big, which probably compensates for the fact that they are curved, if they were
that size and straight though it would be even stronger.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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britishtrident
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| posted on 10/5/11 at 09:35 AM |
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Just plug some numbers into Euler buckling equations for slender struts subject to compressive loads --- the numbers that come out will show
even a very slight curvature greatly reduces the load to failure in compression.
.
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Alan B
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| posted on 10/5/11 at 10:13 AM |
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quote:
Originally posted by craig1410
......Unless you've designed your chassis using CFD techniques
I'm puzzled by this bit...
I thought CFD was computational fluid dynamics which was for modelling fluid flow, areodynamics etc....is there some new or different meaning for CFD
related to structural design that I'm not aware of?
Alan
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craig1410
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| posted on 10/5/11 at 10:19 AM |
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quote:
Originally posted by Alan B
quote:
Originally posted by craig1410
......Unless you've designed your chassis using CFD techniques
I'm puzzled by this bit...
I thought CFD was computational fluid dynamics which was for modelling fluid flow, areodynamics etc....is there some new or different meaning for CFD
related to structural design that I'm not aware of?
Alan
Sorry Alan, I meant FEA (Finite Element Analysis).
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craig1410
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| posted on 10/5/11 at 10:27 AM |
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quote:
Originally posted by britishtrident
Just plug some numbers into Euler buckling equations for slender struts subject to compressive loads --- the numbers that come out will show
even a very slight curvature greatly reduces the load to failure in compression.
.
Yes but progressive collapse may be preferable to sudden failure. I would far rather have tubes bend a bit than have welds break or tubes punch
through adjacent tubes. Better to have, say, 70% rigidity throughout the event than have 100% and then 10%. Not very scientific I know but
instinctively it makes sense to me at least. 
Also, in the case of the Atom, the large diameter tubes are well supported by triangulation along the length of the car.
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blakep82
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| posted on 10/5/11 at 10:30 AM |
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i'm sure curved tubes are supposed to be stronger. forget cars, but think about an egg, if you try to press down on the top and bottom with your
fingers, they're near impossible to break, if eggs were cubes it would be very different.
thats how i see it anyway, but i'm no chassis designer
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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scootz
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| posted on 10/5/11 at 10:36 AM |
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I have no idea about these things, but so far, Blake is making the most sense!
We should be driving egg-shaped cars!
It's Evolution Baby!
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craig1410
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| posted on 10/5/11 at 10:37 AM |
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quote:
Originally posted by blakep82
i'm sure curved tubes are supposed to be stronger. forget cars, but think about an egg, if you try to press down on the top and bottom with your
fingers, they're near impossible to break, if eggs were cubes it would be very different.
thats how i see it anyway, but i'm no chassis designer
I understand what you are saying and it's a fair point but think of it another way:
If you have a drinking straw and you start to apply load along its length, it will take a lot of load until it starts to bend and then it will
collapse rapidly. If the straw was bent to start with then it would not take anywhere near as much load although the failure would be less sudden due
to the fact it doesn't take as much load as the straight straw did to begin with.
The strongest shape is a triangle but only if you apply loads to the vertices only. If you took an equilateral triangle and squeezed it between your
fingers with one finger on the top vertex and the other finger in the middle of the bottom edge then it would not be strong because you would be
applying bending loads to the bottom edge.
An egg is strong because the load you are applying is distributed evenly through the egg's structure and is not localised. However, it will also
fail suddenly if a more concentrated load is applied and you will usually end up with egg on your face...
[Edited on 10/5/2011 by craig1410]
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scootz
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| posted on 10/5/11 at 10:38 AM |
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Something like this...
It's Evolution Baby!
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scootz
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| posted on 10/5/11 at 10:40 AM |
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Or this...
It's Evolution Baby!
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blakep82
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| posted on 10/5/11 at 10:44 AM |
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^ tasty AND practical 
thats true craig, hadn't thought of it like that
with any chassis though theres going to be curves in it. whether it be one rolled tube, or 2 tubes welded at an angle, like on a 7 chassis at the back
of the engine compartment. hit that slightly off centre, won't that still do the same? it all depends really on whats inside the outside
supporting it
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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scootz
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| posted on 10/5/11 at 10:45 AM |
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I think I agree with Craig now... Blake - you're a fanny!
It's Evolution Baby!
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craig1410
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| posted on 10/5/11 at 10:47 AM |
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quote:
Originally posted by blakep82
^ tasty AND practical
thats true craig, hadn't thought of it like that
with any chassis though theres going to be curves in it. whether it be one rolled tube, or 2 tubes welded at an angle, like on a 7 chassis at the back
of the engine compartment. hit that slightly off centre, won't that still do the same? it all depends really on whats inside the outside
supporting it
Yes you are right, if you have two tubes joined at an angle then you need a third tube to create the triangle but again this is only strong if the
path of the load is into a vertex. If a tube is hit in the middle (good chance) then it will bend immediately and compromise the rest of the
structure. This is where theory ends and the real world begins...
Like the egg cars scooz!!
[Edited on 10/5/2011 by craig1410]
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blakep82
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| posted on 10/5/11 at 10:50 AM |
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quote:
Originally posted by scootz
I think I agree with Craig now... Blake - you're a fanny!
yeah, i know! and?
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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scootz
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| posted on 10/5/11 at 10:55 AM |
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... imagine thinking that egg-cars were the way forward!
It's Evolution Baby!
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Liam
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| posted on 10/5/11 at 11:29 AM |
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Blake your reasoning applies to round tube vs square tube (i.e. round tube better resists buckling), but not to the use of curved members in
tension/compression i.e. in a chassis. As has been pointed out using curved members in that way dramatically reduces their resistance to buckling. Or,
to paraphrase a certain Mr Chapman: curved tubes are pre-failed
I remember that photo from a good while ago. If I recall, the car went straight on at a t-junction apparently due to a stuck throttle (or was it a
prod of the wrong pedal?). Can't remember any other details, particularly regarding the survival or not of the driver, unfortunately.
As for the Atom's chassis, whilst curved tubes may not be optimum from a pure engineering point of view, if they are over-engineered enough it
really doesn't matter. In other words you can design a chassis to a certain strength using curved tubes by all means, it just wont be as
light/stiff as it could have been with straight tubes. I dare say the Atom chassis is well designed and fit for purpose. Remember also that some
deformation in a crash would likely be designed in, and maybe the curved tubes are part of the mechanism that achieves that in the Atom design? Also,
I believe that was a particularly high speed crash - any tubes will deform if the crash is fast enough.
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Minicooper
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| posted on 10/5/11 at 02:36 PM |
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I remember reading the thread on this, it was posted by his friend, he survived but had severe leg injuries, that was the last I heard
Cheers
David
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iank
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| posted on 10/5/11 at 05:27 PM |
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quote:
Originally posted by Minicooper
I remember reading the thread on this, it was posted by his friend, he survived but had severe leg injuries, that was the last I heard
Cheers
David
This thread, with lots more pictures.
http://www.locostbuilders.co.uk/viewthread.php?tid=91300
ETA: I think it held up pretty well given the speed it was estimated to have hit at (120kph).
[Edited on 10/5/11 by iank]
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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Bare
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| posted on 11/5/11 at 06:21 PM |
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At least the brickwork remained unscathed .. seemingly unmarked as well ;-)
C'mon these cars (all of them) are 'safer' than a Moto.... but not a helluva lot safer, clearly, they also require as much skill to
operate.
No one should be surprised at the results of a head on into a genuinely solid object. Happily it wasn't a fatal crash.
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