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Shape is everything. Metals resist tension better than they do pure buckling.
And think of the keystone bridge. Put this all together and you have an
intuitive explanation for why those ramps are not perfect cubes.
Unfortunately, this IS what I studied in college. :/
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>> Does that really work? I mean, can you really just say "oh, this thing
>> has 4 wheels, so each one only takes 1/4th of the load"?
>
> Well from experience most cars are pretty equal for left/right weight
> distribution, and as I said worst case is probably 70% on the front
> wheels. So yes, probably 35% of the total weight is the maximum on any
> single wheel.
Hmm, interesting. I wouldn't have expected that to work.
> Usually when you buy a pair of those ramps they will be marked as
> supporting a certain load.
Sure. I just meant that they probably design them to easily support more
weight than any common car that somebody might try to put on them.
> Of course, I was simplifying to get a rough estimate. In reality you
> would have to take the worst case loading condition, which is probably
> with the tyre directly on top of a single column.
Isn't the worst-case when you drive the car onto the ramp and the
suspension jiggles it up and down over one support column?
>> And what about the horizontal elements? They need to not bend at the
>> points where they're unsupported as well.
>
> You would obviously check for this if you were designing it, but I
> assume this wouldn't happen as a tyre usually spreads out the load
> across an area. The loading would be concentrated at the tops of the
> support columns as these won't budge.
Looks to me like the tire would usually sit between the two horizontal
struts. One is directly over an upright, but the other is on the middle
of the beam. (Obviously that isn't a problem or they'd have added
another upright...)
>> Looks thinner tham 5mm to me - but then again, it isn't cylindrical...
>
> Yes, something like an I beam or a hollow cylinder is a more efficient
> use of the metal.
A few of those struts seem to be angled. But most of them are just flat.
>> Just how strong is steel?
>
> As an example, my steel
> ruler has a cross section of 1mm x 25mm, you'd need to pull on it with
> between 6-18 kN of force, that's the weight of a car.
Jesus, that's strong! o_O
So... what the hell is the thickness of a tin can then? Those seem to
deform pretty easily.
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>> Well from experience most cars are pretty equal for left/right weight
>> distribution, and as I said worst case is probably 70% on the front
>> wheels. So yes, probably 35% of the total weight is the maximum on any
>> single wheel.
>
> Hmm, interesting. I wouldn't have expected that to work.
What, that a car has more weight on the front wheels than the back? Most
front wheel drive cars on the roads are configured like this. Some rear
wheel drive cars have more 50/50 weight distribution, others (if they have
the engine in the back) can have more weight on the rear wheels than the
front. Of course the car's suspension and so-on is designed for this, so
you probably won't notice much when driving normally.
> Sure. I just meant that they probably design them to easily support more
> weight than any common car that somebody might try to put on them.
You would think so.
> Isn't the worst-case when you drive the car onto the ramp and the
> suspension jiggles it up and down over one support column?
Probably. But I really suspect in the design of these things that they just
make a few and test them. I highly doubt they do some computer simulation
or really complex calculation to figure out what size metal to use - it just
wouldn't be worth it because it's so quick to just make a few and test them.
> So... what the hell is the thickness of a tin can then?
0.2 mm or thereabouts.
> Those seem to deform pretty easily.
You mean by squeezing them on the sides? Well yes, I can bend my ruler too
pretty easily and that's 1mm thick! What you're doing there is essentially
using a huge lever, you are moving your fingers a few cm to cause a
contraction/expansion of a few microns in the surface of the material,
generating a huge stress which causes it to permanently distort. Now try to
permanently stretch a tin can by pulling on each end :-)
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>> Hmm, interesting. I wouldn't have expected that to work.
>
> What, that a car has more weight on the front wheels than the back?
Possibly, once you've got the back wheels up on a ramp. ;-)
I meant more that I wouldn't have expected to be able to just completely
disregard 3/4 the weight of the object just because I'm only looking at
one wheel.
>> Sure. I just meant that they probably design them to easily support
>> more weight than any common car that somebody might try to put on them.
>
> You would think so.
So it's probably rated to 5 tonnes or something.
>> Isn't the worst-case when you drive the car onto the ramp and the
>> suspension jiggles it up and down over one support column?
>
> Probably. But I really suspect in the design of these things that they
> just make a few and test them.
Oh, yeah, probably. More like they figure out approximately how much
load it's supposed to take, then design it to withstand 80% more or
something, and then go check whether it breaks or not.
>> So... what the hell is the thickness of a tin can then?
>
> 0.2 mm or thereabouts.
...my God. You can make metal that thin?? o_O
>> Those seem to deform pretty easily.
>
> You mean by squeezing them on the sides? Well yes, I can bend my ruler
> too pretty easily and that's 1mm thick! What you're doing there is
> essentially using a huge lever, you are moving your fingers a few cm to
> cause a contraction/expansion of a few microns in the surface of the
> material, generating a huge stress which causes it to permanently
> distort. Now try to permanently stretch a tin can by pulling on each
> end :-)
Heh, yeah, well, those horizontal beams may only be 5cm long, but they
have up to 2 tonnes pushing them sideways. That's a lot of force...
PS. Apparently human bones have a higher tensile strength than solid
copper. WTF?
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On Tue, 09 Feb 2010 15:37:05 +0100, Invisible <voi### [at] dev null> wrote:
>
> I meant more that I wouldn't have expected to be able to just completely
> disregard 3/4 the weight of the object just because I'm only looking at
> one wheel.
Why not? Try doing some one-handed push-ups and see if they are not harder
to do than two-handed ones.
>>> So... what the hell is the thickness of a tin can then?
>> 0.2 mm or thereabouts.
>
> ...my God. You can make metal that thin?? o_O
Surely you are joking.
http://en.wikipedia.org/wiki/Aluminium_foil
--
FE
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>> I meant more that I wouldn't have expected to be able to just
>> completely disregard 3/4 the weight of the object just because I'm
>> only looking at one wheel.
>
> Why not? Try doing some one-handed push-ups and see if they are not
> harder to do than two-handed ones.
When I become able to do two-handed push-ups, I'll let you know. ;-)
>>>> So... what the hell is the thickness of a tin can then?
>>> 0.2 mm or thereabouts.
>>
>> ...my God. You can make metal that thin?? o_O
>
> Surely you are joking.
>
> http://en.wikipedia.org/wiki/Aluminium_foil
Damn. I thought that stuff was plastic with a metal-powder coating...
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On Tue, 09 Feb 2010 15:49:55 +0100, Invisible <voi### [at] devnull> wrote:
>
> When I become able to do two-handed push-ups, I'll let you know. ;-)
Oh, come on. Even I can do push-ups, and I have arms like a little girl.
I have seen pictures of you; I know you are not obese. You can do push-ups.
>> http://en.wikipedia.org/wiki/Aluminium_foil
>
> Damn. I thought that stuff was plastic with a metal-powder coating...
http://en.wikipedia.org/wiki/Metallised_film
--
FE
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> I meant more that I wouldn't have expected to be able to just completely
> disregard 3/4 the weight of the object just because I'm only looking at
> one wheel.
Why not? A car has 4 wheels, the weight is (roughly) spread evenly between
the 4 wheels. If you want something to support one wheel it need only
support 1/4 of the total weight, plus a bit for safety.
> So it's probably rated to 5 tonnes or something.
As I said, it seems most common ones are rated to 2 ton for the pair, so
that would be 1000 kg per ramp/wheel. That should cater for most normal
passenger cars, plus quite a lot to spare.
>> 0.2 mm or thereabouts.
>
> ...my God. You can make metal that thin?? o_O
You've seen aluminium foil haven't you? That's probably 10x thinner. You
just force it through a pair of rollers that are very close together.
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>> When I become able to do two-handed push-ups, I'll let you know. ;-)
>
> Oh, come on. Even I can do push-ups, and I have arms like a little girl.
>
> I have seen pictures of you; I know you are not obese. You can do push-ups.
Not obese, no. Also not fit. My arms contain very little muscle, and
attempting to lift 100 Kg using only my arms operating in an unusual
direction is pretty much beyond my current capabilities.
>>> http://en.wikipedia.org/wiki/Aluminium_foil
>>
>> Damn. I thought that stuff was plastic with a metal-powder coating...
>
> http://en.wikipedia.org/wiki/Metallised_film
Oh, I see... So *that's* why that stuff is so indestructable.
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Invisible wrote:
> Not obese, no. Also not fit. My arms contain very little muscle, and
> attempting to lift 100 Kg using only my arms operating in an unusual
> direction is pretty much beyond my current capabilities.
You make it sound harder than it is :-)
First off you are probably not lifting 100kg when you do pushups,
because a good part of your weight is supported by your feet. I think
less than 70% of your weight is more accurate. Even less if you do them
on your knees (which you should probably try).
And the arms are not in an unusual position when you do that exercise.
They are very well made to operate in front of your torso :-) For a more
unusual position try the reverse push ups, with the belly facing up
instead of down...
Read this book for more fun :-D
http://www.lulu.com/product/t%C3%A9l%C3%A9chargement/the-little-book-of-push-ups/6102671
--
Vincent
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