Why not aluminum for the chassis?

[PCC]

I was thinking about it today. Electric cars are typically aluminum bulkheads or plastic bulkheads with an aluminum motor plate mounted to a plastic or carbon fiber chassis. Then we add heatsinks to control the heat that the motors make. Why not just make the chassis aluminum and tie the motor plate/bulkhead into the chassis so the heat is transferred into the chassis? No heatsink needed as the car is a rolling heatsink.

I know, aluminum is heavier and will bend in a crash. In the case of the lighter touring cars out there (TC3, XRay T1, HPI Pro2 come to mind) the cars are so light that you need to add weight to them to make them race legal anyway. With a good alloy (7075 T6) it'll be strong and light.

Ideas?

[SteveK]

Some cars use aluminum because it's easier/cheaper to manufacture than molded plastic, or at least not so much more expensive that you won't put it in for the 'ooh' factor.

Aluminum is not as rigid as graphite, and it's heavier. Lightness, that's obvious, but the rigidity is a key factor in consistent handling, and it can be bent or tweaked.

The heatsink facotr isn't a real valid arguement: A properly geared car shouldn't need much in the way of heatsinks.

[HauntedMyst]

The reason is weight, plain and simple. In order to make it strong enough not to bend in a crash, it would be way to heavy. Even though most cars are under weight, that isn't a bad thing. That just means the driver can add the weight where its needed.

[crank throw wei]

Actually,magnesium would be a better choice than aluminum;it's much stronger and lighter,abeit very expensive.I agree with Steve K that you really shouldn't need the heat sink qualities,however,BEND? It's a rare thing to see a bent pan from an 1/8 scale buggy,and they are driven much harder(jumps,etc.).A properly engineered pan(one with some shape,not just flat!) would be damn near impossible to bend. I also disagree that graphite is stiffer than aluminum(a T6 grade anyway). Take a simular size piece of each,under simular load,the graphite will deflect more.:)

[InspGadgt]

Graphite is much stiffer then aluminum within the same shape. To get aluminum to the same amount of rigidity as a flat graphite you must make it a tub or other bent form. Doin that raises the CG of the car. Plus it's much easier to design and work on a flat surface then it is a bent one.

[cobra81li200]

No, no, in some way, crank throw wei is right.

What is very rigid is not the graphite, but the carbon fibers in it. So the carbone fibers are much more rigid than aluminium, that's right. But they are only fibers, so they're rigid in only one direction (strongly isotropic), ie traction, whereas aluminium is anisotropic.

So what do we have in a chassis ?

In my left hand, a graphite chassis, made with a matrix of carbon fibers + a resin to get them stiffed together. As carbon fibers can only get stressed in one direction, there are different directions for one matrix, and the matrix have different orientations, so that the material is nearly anisotropic.

Due also to the fact that there are different matrix glued one one the other, we must considerate the mechanical caracteristics of the resin used, which are way worse than the ones of the aluminium.

So, because of all of that, the bending properties of carbon fibers are not as good as what you think, that's why most of the chassis use a upper deck, so that instead of bending, the main stress is traction/compression, and the thickness can be very low, with still a rigid chassis (the chassis of my TRF 414M is 2mm and VERY strong).

In my right hand, is an aluminium chassis, it's made of ONE matrix of aluminium, with some defaults (which determinates its mechanical caracteristics). The bending of the chassis depends on them, but also on the quadratic moment, which is directly linked with the thickness. This way, it's much more rigid (I mean under a bending moment) and that's why the aluminium chassis in gas are always such thick.

[HauntedMyst]

I saw that movie The Matrix. It was pretty good. I'm waiting for the sequel.

[cobra81li200]

Lol, it's reloading at the moment :D

[InspGadgt]

Which is why most graphites we use is a weave now to provide more rigidity in more then one direction. Also the component of the resin used depends on the quality of the graphite. When made under high pressure less resin is used and the graphite layers are closer together making them even more rigid. Size for size a good quality graphite like what we see in RC today is lighter and more rigid then the same sized piece of aluminum. Also under impact graphite is more likely to return to it's original shape where aluminum stays bent. Notice on 1/8th off-road cars and former RC10's in order to be rigid the chassis are tub or bent around the sides and also braced. 1/8th on-road cars and 1/10th TC's that run flat aluminum chassis also run a graphite upper deck and their aluminum is thicker then the graphite on a 1/10th scale electric double deck chassis. DA even went as far to make a graphite laminated aluminum chassis for some on-road nitros so that it would still have the heat sink properties but was much stiffer and lighter then the aluminum alone.

[cobra81li200]

Oh, talking about aluminium and graphite, I'm looking after more infos on SSG (Silver Surface Graphite). I've heard that it has aluminium so that it's more resistant to crashes, but I don't have anything else.

So, if anyone has further information, I'm interested !!!

[InspGadgt]

I wonder if the silver coated graphite Yokomo uses for it's 4 cell conversion of the RC12L3 is the same stuff. I know Yokomo went this route because the coating protects the graphite from sunlight. Unfortunately graphite does break down over time in direct sunlight. My guess is this is a drawback due to the degredation of the resin used.

[losixxx213]

why don't they make chassis out of carbon fiber? In bmx racing, some of use carbon fiber forks that are strong and light, and they don't flex that badly(they flex when you case the jumps). I think that'd be the best way to go. It's lighter than aluminum and stiff and lasts for a long time.

[cobra81li200]

We are already using carbon fiber chassis (it's the same as graphite). But on BMX's forks, the structure (a cylinder) is better suited to have a very strong component.

[crank throw wei]

Cobra: I looked up aluminum reinforced composites in "Engineering to Win" by Carroll Smith.The following is a brief exerpt-

A great deal of research has been done in this area,particularly with alloys of aluminum and copper(the 7xxx series).So far as I can determine,to date there has been no practical success.In fact metallic filaments(or wiskers)have generally proven,so far at least,to be unsatisfactory materials cheifly because they are very suseptible to surface oxidation which causes dislocations to appear in their crystal structure.And they are very intolerant of structural imperfections.


This book is a few years old,so things may have changed,.....or maybe not.

[PCC]

Good information. Learned something today. Thanks for the replies.

[cobra81li200]

Yes and no, in fact aluminium is highly oxydable. Indeed, you've never seen unoxydated aluminium, because as far as an atom of aluminium gets in contact with oxygen, it gets oxydated and become what we call "alumine" in french (I guess it's something like Al-O but I don't remember the exact formula).

This is a ceramic, very very hard, but it's also very thin, so it can't be used as a surface treatment.

Why I said yes and no, is that I don't know if it's permeable or not, but I don't think, otherwise the thickness of this component would change with time.

What I'm sure of, is that aluminium is highly reliable, as it's used in the aeronautical industry, but maybe they use another alloy, less oxydable...