The alloys that vex uses both have about the same modulus of elasticity to weight ratio, so they should be equal structurally. So why do so many teams use aluminum instead of steel? It seems like steel would be the better choice because it takes up less space.
It’s not to have more structural integrity; in fact, I’ve seen aluminum break more than steel on my robots, it seems aluminum is more brittle than steel. The main reason is that aluminum is immensely lighter than steel is, and thus, will help in allowing the robot to be more reliable and faster.
Yep it is all about weight the aluminum is about 40% lighter. My first robot completely steel at the time was about 25lbs and this had no pneumatics on it. My all aluminum robot that had 5 pistons and 9 air tanks that I brought to worlds was about 18lbs without the air tanks it was only about 13lbs. With that decrease in weight it let’s you gear your drive faster.
Also many people may like to build the chassis out of steel. I did mine aluminum because it let me move air tanks where I wanted to adjust the COG. same thing if you want your robot to be 18lbs if you make it all aluminum and say it ends up being to light for your liking. But your COG is not good you can add some weight wherever you want on the robot… its still no compensation for a bad COG design but it can help building it lighter to begin with even if it is a lot more expensive. Just an opinion.
I agree with the above, lighter robots are faster and less likely to stall your motors. Also I think those who plan on hanging without having to use really high gear ratios to lift up with will be looking to keep weight to a minimum.
The robot we went through most of the Sack Attack season up until worlds weighed in at 28lbs, our robot for worlds was all aluminum and weighed around 12-1/2 lbs and moved a lot faster with the same wheels and ratios. The only steel on our Toss up robot is linear slides, axles, stand-offs and screws. Speed is going to be a BIG factor this year with the 3 bucky ball limit, and being able to high hang in a few seconds will be important as well to make hanging worth while.
Steel is much cheaper though.
so much lighter !
In Vex we are not just looking at structural strength, you might need a large flat plate or 5-wide C-channel to make a mechanism that works the way you want, but not need it as strong as steel so go for the lighter option.
The reason people find aluminium better* structurally* is that we are limited in what shapes and sizes the material comes in.
To see how this works, let’s say the aluminium is half the density of the steel, and also half the stiffness. So pick up some 1 hole wide flat bars, one steel, one aluminium. The steel is twice as heavy, but also twice as stiff. Ok, so in a simplified world you could use either one steel bar or two aluminium bars and the only difference would be the cost 
Now lets say you have a 1x1 aluminium L angle. This has twice the cross-sectional area of the flat bars, and therefore weighs about the same as the steel flat bar. But clearly it will be much stronger than either of the flat bars, because of the shape that is difficult to bend in two directions, rather than a flat bar which is easy to bend in one direction as I’m sure you will have noticed.
Similarly, an aluminium 1x2x1 C channel will weigh the same as a steel 1x1 L angle, but again will be stiffer when there is a bending load corresponding to the long side of the C channel.
If you want to know more about the relative strengths of shapes, search for “second moment of area” (it has a couple of other names too) which basically is the sum of the product of each bit of area and the distance that area is from the neutral axis (for symmetric shapes in most materials the neutral axis is the centreline, and to work these things out for any difficult shapes you need to use calculus as all engineering students will know 
). A good example of the use of this concept is an I-beam which has a large proportion of material far from the centreline, and is therefore very stiff for its weight.
Modulus of elasticity is only one property that needs consideration. The other is Ultimate Tensile Stress which basically translates to force required to break for a given cross-sectional area, it would be interesting to compare this data for the Vex materials as well.
Ok, that makes more sense now.
Steel: 200 GPa modulus of elasticity, 7.87 g/cc, 317 MPa UTS
Aluminum: 70 GPa modulus of elasticity, 2.68 g/cc, 228 MPa UTS
the UTS per weight is much better for aluminum, but I’ve never run into parts breaking.
Much of th ebending comes during build time. Be carful cranking a vice with aluminum C Channels. You can bend them in quite easily.
Yeah yet the funny thing is that the aluminum plates do not bend as easy as the steel ones. At least in my experience they are harder to bend. Probably due to the fact of the extra thickness the aluminum has.
Maybe you’re also more cautious, since you feel like it will be easy to bend aluminum.
Yeah, I’m pretty sure that the steel that vex uses is more easy to bend then the vex aluminium (it feels that way, I may be wrong). It is probably because the aluminium is more brittle, but I have never experienced a piece of vex metal (of any kind other then the axles) out right snap. This is why my team only uses steel for something that needs to be heavy, or cheap prototypes, because it doesn’t seem to be stronger (because aluminum hasn’t ever broken on us and it seems to bend more easily).
So which one bends more easily 
Both.
x=y-1
y=x-1
so both have -infinity strength
It seems to me as if steel does, but aluminium is more brittle. The brittleness doesn’t seem to matter though, because I have never even seen a robot snap a piece of aluminium.
Remember that for a part with a rectangular cross-section, like a piece of plate or 1x25 bar, the amount of force required to bend that part a given amount increases in proportion to the cube of the thickness. (the second moment of area of a rectangle is (width)(thickness)^3/12)
This might explain why some people have found the slightly thicker aluminium harder to bend than the steel.
Aluminum definitely bends a lot easier than steel. We just started building or entire robots out of aluminum c channel this year and it is definitely a lot weaker. The point is that it is still strong enough when you build correctly to be perfect for a vex robot; in most case, the steel is far stronger than it needs to be.