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Bladder control

Motor racing is dangerous. This is printed on the back of every motorsport venue entrance ticket and emphasised whenever possible to all involved. The risks of colliding with either track scenery or other competitors on the track is of course well known and accepted, but the next greatest risk to life and limb is surely that of fire. The biggest hazard on board the vehicle is therefore the fuel and its method of storage. And if you realise that the flashpoint (the lowest temperature at which the application of an ignition source causes the vapour to ignite) of gasoline is so low (below -40 C) that it cannot even be determined using normal techniques, then it is wise to be cautious.

So while simple aluminium fuels tanks are still sold in their thousands, such is their inability to prevent or even control the spillage of fuel after a damaging impact that the only option is to use some form of safety fuel bladder. Mandatory in Formula One for many years, these have been adopted by virtually all forms of racing these days, such that the FIA has introduced controls and homologation to ensure standards. The standards cover the inclusion of baffles (to reduce fuel slosh), fittings and connections as well as limiting the age of the tank, but the biggest section is reserved for the design and performance of the materials and construction.

Thus for instance the bladder itself should be made from “a reinforced material in polyamide, polyester, aramid or equivalent” and be impregnated with and coated both internally and externally with some form of fuel-resistant elastomer. When woven, polyamide and polyester fibres are well known for their use in clothing. In the case of aramid fibre the most famous brand name of a material is Kevlar; as well as being known for its use in carbon fibre composites and anti-stab vests in military applications, its resistance to heat and puncture make it perhaps the ideal fuel tank material. Because of cost and manufacturing issues, however, bladder tank manufacturers tend to use their own versions of woven materials to meet the FIA specification.

The tensile strength test, for instance, goes into quite significant detail, and while specifying the size of the test specimens (25 mm in width, a minimum of 150 mm in length) and how they are to be selected and cut from the bladder material to be tested, the test procedure also asks to show that the material doesn’t have tensile properties biased in any particular direction. While tensile strength testing is rigorous, perhaps even more so is the testing of a material’s puncture strength and the resistance of the seams or material to tearing in the event of impact.

The puncture resistance test is similar to that of the tensile strength test, differing only in the use of a piercing tool that looks much like the blade of a screwdriver. An interesting choice of puncture tool tip, you might add, but as some of us may know, a weak bladder can be more than just an inconvenience.

Fig. 1 - Touring car fuel tank

Fig. 2 - Hillclimb single-seater. Holding only enough fuel to complete the course, I would hope this tank will have some kind of fuel tank bladder installed, even though it’s still packaged away behind the driver 

Written by John Coxon

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