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Tue Nov 24, 2009 7:27 am
Guys, I need some help in this topic. I am confused as to which cross section to use in a FSAE chassis. On studying, I found out that though a tubular cross section has excellent resistance to torsional loads, a square cross sectiona also provides good resistance to both torsional and bending loads. Also square cross sections are easy to fabricate and easy to mill holes on them. So why should not I use a square cross section. Also can I use a combination of both?
in d event of bending, a tubular cross section will have a more uniform bending process. in the case of a square/rectangular cross section, if it bends then due to the edges there will be a greater chance of total failure.
I have seen fsae chassis using a composite of both square and tubular c/s.
In some motorcycle chassis, oval tubing is used. It can be desirable to have a MC chassis be flexible in one direction, but pliant in another. When leaned over, this can act as an extra source of suspension as MC's suspension are compromised when leaned over. With a car, I've understood as stiff as possible is the best. I'm guessing (IANAE) that proper analysis would lead to a hybrid solution. Both square and round tubing where needed in the chassis. A thought... what about triangular tubing??
The only things i can think about in triangular tubing is the complexions involved in welding different pieces at different angles. Also in static conditions, a triangular tubes either greatly magnifies of nullifies the load distribution due to great difference in the cross sections of two extreme faces. One is a plane and other is a line.
horses for courses your decision will be dominated what you actually are able to afford in terms of recources be it monetary or just how good your welding abilities are. Of course if you startet to think about optimum tube for a frame I cannot think of a constant crosssection tubing as this will inevitably be suboptimum in bending for the amount of material. So not the crossection ,be it square round or triangle will determine stiffness a great deal but the crosssection in length direction should be conical with the widest crossection in the middle of the beam or a single conical shape will also bring a lot of stiffness for weight. next step is of course the thickness of material along the beam worth optimising... just look into bicycle frame production as a starter ,these tubes are readily available but of course they come at a price.
Tubular sections give you the best possible strength to weight ratio. The larger outer diameter thinner tube you can use. Tubes are a bitch to jig though and end preparations requires planning to say the least. Square sections are very easy to jig because they tend to stay in place and not roll around. And chassis tubes hardly take and torsion so any advantages there are pretty much useless.
To jig and weld accurately is important to chassis making so if you are inexperienced the square sections is the way to go. There is nothing wrong with using a combination. In fact a good engineer will use a combination depending on the load that the individual member would take. And also sometimes it is useful to have square sections at the chassis pick up points to put mounts on easily.
I cannot think of producing a chassis without a proper jig ...if you cannot afford to buy a proper jigging table or at least go to a company willing to let you do your work there there is really not much you can do in terms of layout ,which restricts stiffness for weight again . If you start of with a square layout of your chassis crossection ,topped by parallel tubing layouts anywhere i bet you will not see any difference in performance from using square ,round triangle or eliptical,oval shaped tubes... still I insist conical shapes will do more for your individual tube stiffness than special crossections.
Its not too difficult to build your own jigging table. I have made one. Can say the chassis turned out pretty good. And of course conical cross-sections will be nice but then parabolic cross-section will be even better because stress variation is also quadratic. I don't know a FSAE chassis with conical tubing. I'm sure someone must have tried it but I really doubt anyone go through the tedious process.
Cycle companies mass manufacture and a FSAE is a one off so it might not be a best idea to have custom tubes to do into your chassis.
thats exactly my point. If you mention round tubing to be tricky or awkward to hold in position ,then you should think twice if building your own chassis is your project... of course building a jig is not quite rocket science ,but so is tube fitting ,welding and possibly getting tubes into conical shapes. It was not made before in FSAE? I don´t care and if you do you did not understand FSAE .If it helps winning the trophy or getting a new highly paid job it has done the job. Of course you still have to weigh up if the potential weight savings would justify the effort.But with CAD and FEA at least the savings should be possible to quantify and you´d definitely get the attention of the judges ...
come on ,guys ...this is not about what would the average guy use to build his racecar. It#s not a good idea to start your project jumping on the first set of rails -pun intended-you happen to stumble across...