06 May 2009 15:49

Pat's Column - Space-frame Chassis

Category: Pat´s Corner

By: Pat Clarke

I have never made any secret of my preference for space-frame chassis in FSG. This preference is not because of some nostalgic feeling, but for very pragmatic reasons.

The thing I like most about a space-frame structure is that it really educates the designer in management of the load paths! It is an educational tool par excellence. Some professors will not permit their teams to consider any other structure for this very reason.

Sure, I understand why teams want to use the latest technology and build carbon composite chassis, but unless the team are very experienced and well funded, the results can be catastrophic.  The reason why I say ‘well funded’ is not because carbon fibre is expensive…the cost these days is very reasonable, but because of the costs in the design and manufacturing stage.  And the cost of the inevitable changes along the way!

Other benefits of a steel structure include ease of repair or modification, access to components and probably an easier time in Tech Inspection.

I know there are teams who will disagree with me, but remember, these cars are being designed and built by students as an educational exercise and as a secondary aim, to compete in a FS or FSAE event.

Teams will argue that a composite chassis can be made stiffer, for a given weight, than a steel space-frame, and that may well be the case, but how stiff does a FS chassis need to be? Dr. Andy Deakin, Tech Advisor to the UK FS competition, authored a paper on this very subject whilst leading the Leeds Fs team about a decade ago (“The Effect of Chassis Stiffness on Race Car Handling Balance”). There is interesting stuff in there even if Andy was operating with a composite chassis!

I would also remind you that the fastest FS car in the world last year was from Stuttgart, and it used a steel space-frame.  Space-frame cars have featured in the Design Finals of every FS/FSAE event and usually are on the podium after the dynamic events.

So, for new teams, considering their options, what is needed to make a good space-frame chassis?

Tube Selection

Firstly, do you use round or square section tubing? A structure made from round section tubing will usually be stiffer in torsion and one of square section may be stiffer in beam. Square section tubing is easier to fabricate (no ‘fishmouth’ joints) but the tube distorts under load by ‘lozenging’ or ‘panting’.This adds to compliance and may lead to fatigue cracking.

Round section tube is preferable, and should a team choose to build a square section chassis, they would need to justify that to the Design Judges. Just stating ‘Easier to fabricate’ will not be enough, after all you could easily have nailed together a wooden chassis if that was sufficient justification. Having said that, there are areas in the chassis where the use of square section tube is preferable.

Incidentally, the difference between the usage of the words ‘tube’ and ‘pipe’ may confuse some teams who do not have English as their first language. Simply stated, tube is described by its outside diameter, pipe is described by its inside diameter, so a 30mm tube is not the same thing as a 30mm pipe and there is no such thing as square pipe!

Many teams will opt for 4130 ‘CroMoly’ tube, but is that necessary? In a word no! Sure, there may be a slight weight saving, but this benefit is not as great as it once was. The rules now stipulate minimum wall thickness in many areas of the chassis, including the heavier components like roll hoops and side impact structures.

An additional complexity when working with 4130 is the (often debated) need to normalise the structure after welding. I have seen too many catastrophic failures in the HAZ (heat affected zone) of 4130 weldments to know this IS necessary! 4130 suould also be either TIG (tungsten inert gas) welded or nickel bronze welded, a form of brazing. But to quote Ron Tauranac,  “The only person who knows it is a good bronze weld is the guy who did the welding”!

I would not recommend ERW (electric resistance welded) tube, I call it ‘furniture tube’,  but CDS (cold drawn seamless) tube is perfect, inexpensive, easy to work with and needs no post weld heat treatment (something to consider when you have to weld on some afterthought bracket or repair damage). CDS can also be MIG (metal inert gas) welded, but I would recommend TIG or nickel bronze welding.

Comparing the properties of CDS and 4130 tube is interesting. The difference in strength is not as great as you might think.
CDS is typically 470mpa, and 4130 is typically 650mpa.
Section for section, 4130 is no lighter than any other steel, and all steel has the same modulus of elasticity anyway and are therefore equally stiff. It is only its ability to resist taking a permanent set and the possibility of using a thinner wall section in some parts of the chassis that sets 4130 apart. Equally interesting is a comparison of tube weights and stiffness when comparing diameters and wall thicknesses.

As an example, a 300mm length of 25.4mm x 2.1mm tube has a weight of 368gm, and its moment of inertia (MOI) is .025.

Going up in wall thickness to 3mm increases the weight to 508gm and the MOI to .033. A 40% increase in weight yields a 30% increase in strength.

Going up to the next tube size whilst retaining the original wall thickness, 28.6mm x 2.1mm increases the weight to 418gm and the MOI is now .037.In this case, the weight has increased by 13% and the strength increased by 48%, showing how important it is to determine what loads the tubes will be subjected to before choosing the diameter and wall thickness.