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Author: Clenn Giebenhain, DART Racing, TU Darmstadt
If you take a closer look at the development of motorsport in general over the last decade one can observe the triumphal procession of composite materials. Back in the 90’s Michael Schumachers championship winning Benetton B195 featured steel wishbones, a casted engine block and an aluminium steering wheel which can found in any better tuned streetcar. Today carbon fibre wishbones with integrated flexijoints, metal matrix composite (MMC) engine housings, sophisticated aerodynamic devices and amazingly save carbon monocoques seem to be common in F1.
Composite materials became a state of the art technology which also changed the appearance of FSAE. Its unrestricted regulations compared to other racing categories saw the upcoming of a new breed of racecars which not only adopted F1’s technology in many fields but also made an approach to completely new areas. Recently the most hyped of them all is the carbon fibre wheel.
A short excerpt of history: 1984 saw Honda introducing the first carbon rim on their NSR500 GP machinery but the new device failed catastrophically at high speeds during first qualifying. In 1987 Lancia brought a closed carbon rim prototype on its group B monster which regrettably never raced.   Left: First ever introduced carbon disc rim for autoracing purposes by Lancia in 1987. Right: modern FSAE carbon spoke rim from DART Racing, Darmstadt in 2008.
After those early approaches it became very silent concerning the new technique. A few courageous FSAE teams adopted the carbon disc wheel technology to its cars like UTA FSAE in 1993. Finally the meanwhile matured technology saw an avalanche-like breakthrough in 2005 when Deakin University introduced the first carbon spoke wheel and the idea was soon adopted by other competitors. FSG08 saw no more than six cars using sophisticated full carbon fibre rims and rumours are out for up to ten new solutions for 2009.
So what’s the hype all about? Let’s go into technical details – Truth behind the tales.
Due to the special layout of Formula SAE/Student tracks cars need to be very agile. From a technical view agility is a function of suspension geometry and tyre construction but it is also heavily influenced by mass and mass inertia, especially the masses of rotating and tyre-damped parts with a huge offset related to the centre of gravity. And here the whole wheel including tire has a huge potential.
In terms of lightweight designs carbon fibre has two big advantages. First one is the freedom of shape which allows the design of hollow spokes. Making use of the anisotropic behaviour and an appropriate manufacturing technique will additionally save up to 30% of mass compared to aluminium, in theory.
A common used aluminium 13x6 inch rim, for example the screwed BBS rim, originally designed for racing in Formula BMW, weigh in at 4,3kg. Recent developments have shown weights of 1,1kg in those dimensions in carbon for use in FSAE. So you can save more than 12kg with one set. Great, isn’t it?
Stop! Einstein once said that physics won’t give you an outstanding advantage by chance. So where is the rub? (And yes, the wise man was right. There ARE numerous!)
First of all be aware that a composite wheel is not just another metal part converted to carbon. You rather have chosen one of the most complex parts to do so, as a rim has to withstand massive osciallating forces in all directions and combinations while rotating and being thermally stressed by the brakes.
Where a metal design can be evaluated concerning durability and stiffness with normal FEA-tools and useful loadcases the carbon rim will assign you to new tasks. While the overall stiffness can only be simulated with a complex anisotropic FEA, durability is the most complex part and lies within the borders of the interaction of the fibre with the resin and different carbon layers.
If you really plan to go for it you have to decide between different concepts:
- The hybrid wheel (metal hub and spokes bolted on a carbon bed)
- The full carbon fibre wheel
a) spoke wheel
b) disc wheel Left: Carbon hybrid wheel from Joanneum Graz at FSG 2006. Right: Carbon spoke wheel from Joanneum Graz in 2008.
While a hybrid wheel doesn’t offer the biggest potential (weight < 2kg is hard to achieve) it is surely the easiest and most economic solution to save weight.
The full carbon fibre disc wheel (like the Lancia prototype) is somewhere in the middle. The huge expenditure of carbon spokes can be avoided. Disadvantages are a heat accumulation problem (additional brake and rim cooling is needed) and the composite material itself is poorly used as, due to the rotation of the wheel, forces will be inducted into the discs’ fibres with an angular offset of up to 45 degrees compared to the flow of forces within radial spokes.
The fully carbon fibre spoke wheel surely features the biggest potential but also needs huge efforts and is way more risky.
Technical stumbling blocksStiffness! Perhaps the most critical part of a carbon rim as the advantage in mass can rapidly turn into an overall disadvantage as a slightly flexing rim can destroy the cars performance. 3mm deformation over the complete diameter will result in nearly one degree positive camber gain.
Pressure Resistance! The advantage of a huge tensile strength is put into perspective as compressive forces are solely carried by the plastic resin. With a pretensioning force of 50kN+ for the central lock nut carbon fibre will soon exceed its limits.
Delamination! As mentioned above durability is a function of layer interaction. This will become even harder to calculate as a rim is very sensitive to the separation of different layers due to oscillating stresses.
Tightness! A carbon rim could be theoretically manufactured with an overall wall thickness of less than 1mm. Leakages or preassigned when doing so.
Manufacturing and TestingBeing new to composite designs the worlds of theory and exercise are doomed to collide in a quite unexpected way. On the one hand the lay-up of the prepregs can cause drastic unanticipated problems. On the other hand fibre distortions and the shape of the lay-ups often vary radically from what was planned or unwinded with special software. Demoulding and the autoclave process itself is another critical process which can cause unexpected delays or even worse.
For safety reasons and for the understanding of the design a validation of the rims durability should be performed on a test bench before mounting it to the car. Darmstadts Fraunhofer Institute for Fatigue Strength LBF is a well known specialist in this area. They monitored the cracking of many different light alloy or carbon rim prototypes over the years.
Manufacturing of DART Racings carbon wheel. Left: Lay-up of the single prepregs. Right: Getting ready for the autoclave process.
CostsThe overall costs of a composite wheel can easily blast every prudential calculation. Beneath the material costs you have to consider the costs for mold and die production as those of the chosen manufacturing technique. Especially the dry carbon autoclave technique is involved with a not negligible electricity bill 
ConclusionAll in all everyone has to draw a conclusion for himself if it is worth or not to overcome the temptation to follow this interesting and impressive approach or if it is wiser to spend the time and resources for essential tasks like a well organized driver training or a well functioning data acquisition system.
Perhaps it is even fruitful to come up with the question if carbon fibre as a rim material is really needed or if a perfectly optimized light alloy wheel can save plenty of resources while being a serious alternative. Look at the 13x14 inch F1 wheels which weigh in at about 3,3kg and which have to sustain way bigger loads compared to FSAE rims. In that case look out for the rims DART Racing will introduce on its delta2009.
An in depth analysis of a design cycle of a carbon wheel would be too much for now. But if you plan to do so I recommend the following link "Non-linear simulation of a lightweight composite rim"(press the preview button) dealing with the development of the 15x7 inch carbon fibre rim the Technical University of Darmstadt used in 2008.
Books: - Analysis of Failure in Fiber Polymer Laminates: The Theory of Alfred Puck, Martin Knops, 978-3540757641
- Konstruieren mit Faser-Kunststoff-Verbunden, Helmut Schürmann, 978-3540721894
Software: - Ansys Classic
- Unigraphics NX6
- Unigraphics NX6 Flat Pattern Tool
More carbon rims:
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