TECHNOLOGY – TUBULAR FRAMES
Perfecting the spaceframe chassis It's been written that the spaceframe chassis has been optimised. It hasn’t, says one former Coventry University student
The hybrid spaceframe has a torsional rigidity of 4,450Nm, which is over four times greater than the steel spaceframe that it was designed to replace. There were negligible weight savings due to constraints on material dimensions, although the specific strength of a hybrid spaceframe is significantly higher
T
he tubular steel chassis is not fashionable any more. These days almost every young engineer wants to use advanced composite materials like carbon fibre to design their new car. They even turn their noses up at the use of a tubular chassis. They argue that steel chassis are heavy, lack rigidity and are not as safe as composite monocoques. Those in favour of the tubular steel chassis argue that the ‘spaceframe’ is cheap, easy to manufacture, easy to repair and highly versatile. But one student at Coventry University in England wondered if it would be possible to get the best of both worlds. Replicating the thoughts of many low-cost formula car designers over the years, such as those
behind the British-built Rossalini FV391 Formula Vee, could he replace at least some of the steel with readily available composite tubing? Giorgio Demetriou took his university’s 2012 Formula Student car design (a steel tube frame) and investigated converting the design into what he calls a hybrid spaceframe. The results are relevant to anyone designing or building cars in an open rules environment, such as Formula Ford, Vee or a number of SCCA classes. ‘I aimed to develop a chassis which is competitive with composite monocoques used in Formula Student, but which could be manufactured for substantially less cost in a shorter timeframe,’ explains Demetriou. ‘I realised quickly
that there were two key points to address: the use of tubular carbon fibre in spaceframe construction and the application of direct metal laser sintering as a manufacturing process to create connection nodes.’ One of the key points of the project was that Demetriou and the other Coventry students did not want to modify the existing design in any way, leaving the hard points and geometries as they were. ‘Spaceframes have been used
for many decades, and as a result much research and testing has gone into the optimised design. An ideal framework would consist of only struts and ties, pin jointed and loaded at the joints,’ adds Demetriou. ‘It could be argued that tubular chassis design has been completely optimised, and literature has been written to this effect. The design of a spaceframe chassis is unique to each vehicle and each will pose a new set of packaging constraints. A good spaceframe will receive
Table 1: carbon tube selection Tube
Material
OD
Wall Thickness
Baseline 1
Steel
25.4mm
1.6mm
Baseline 2
Steel
25.4mm
1.25mm
Replacement 1
CFRP
28mm
1.84mm
Replacement 2
CFRP
26mm
1.56mm
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