Worth the weight

The use of titanium suspension springs to save weight and improve performance reaches new heights on Ferrari’s Challenge Stradale. Jon Excell reports.

At £134,000 the Ferrari Challenge Stradale, a street-legal racing car, may be out of most drivers’ price range – but it is technically the world’s first production vehicle to feature titanium springs on all four corners of its suspension.

In the constant struggle to improve fuel efficiency, meet EU emissions targets and satisfy consumer demands for more and more onboard technology, carmakers are facing an uphill battle to reduce the weight of their models.

One way that large weight savings can be made is through the use of ‘exotic’ lightweight materials, and the traditionally conservative industry is finally beginning to make meaningful strides in this direction.

The performance benefits of titanium in automotive applications have been widely reported for a few years now, and the alloy is already used in a number of high performance vehicle components, particularly within the valvetrain. Mitsubishi, for instance, uses titanium valve spring retainers on all of its 1.8 litre engines, while a large number of VW vehicles use titanium washers to seal the line to the brake caliper. However, owing to its high-strength, low-shear modulus and low density, the space age alloy is particularly suitable as a spring material.

The Stradale is a production streetcar derived from the Ferrari 360 Modena, although it weighs 110kg less. The Modena, itself something of an exercise in weight reduction, is the first production Ferrari in which the chassis, primary suspension components and body were built primarily from aluminium. This weight-saving philosophy has been taken one step further with the Stradale, which as well as using titanium for the front and rear suspension springs, also uses titanium wheel bolts.

The Stradale is not, in fact, the first production car to use titanium suspension springs. That honour belongs to VW’s Lupo 1.4FSI, which had two fitted to the rear of the vehicle.

Kurt Faller, president of Timet Automotive, which supplies titanium for almost every automotive application, explained the full significance of the Ferrari story.

Although the Lupo was a production vehicle, it was sold only in Europe in relatively small volumes, and was largely viewed by VW as a public test-bed to evaluate the impact of a number of new technologies. As well as being the first vehicle of its kind to use titanium suspension springs, it was also the first manufactured in Europe with fuel stratified injection for a petrol engine.

‘The Lupo wasn’t a big marketing job, and the technology wasn’t really flagged up,’ said Faller. However, the Stradale, he stressed, is not a test bed. Titanium is used throughout the suspension, and its presence as a significant enhancement is being heavily marketed.

He is keen to stress just how significant this enhancement is. ‘When you look at bolt-on, weight reducing technologies, a titanium spring represents the most mass you can take off a car without making big engineering changes to it,’ he said.

To bring about comparable weight reductions he explained that it would be necessary to either look into making major engine changes or to employ weight-saving technologies such as carbon fibre body panels which, from a tooling standpoint, are extremely expensive. By contrast, the tooling for titanium springs is unproblematic, as the springs can be wound on the same equipment that is used to wind traditional steel springs. In this sense, said Faller, despite the relatively high costs of the raw material, a comparable weight reduction in other areas would incur considerable tooling costs. So using titanium springs is a cost-effective way for smaller volume applications to get a lot of mass off the car, he added.

Titanium’s high cost is further tempered by the fact that, unlike steel (the traditional spring material) it doesn’t require expensive anti-corrosion coatings to protect it. Steel springs, he explained, are highly prone to corrosion if the coating is damaged.

The weight savings, while impressive, are somewhat difficult to quantify. The front springs yield a 39 per cent saving over the Modena, while the rear ones yield 28 per cent. However, Faller said that it’s hard to talk about the savings in terms of mass, because Ferrari also designed the titanium springs to be 20 per cent stiffer than the steel ones on the Modena 360. ‘Ferrari has never manufactured a steel spring with the same spring rate of the Stradale,’ he said.

To illustrate the scale of the weight savings made possible he drew attention to the density of titanium – roughly half that of steel. This means that a 1-tonne titanium spring could show the same performance characteristics as a 2-tonne steel spring. Faller pointed to one trial application in range rover and land rover SUVs, where using titanium has shaved 50 per cent off the previous steel springs that weighed 8kg each. ‘Multiply this figure by four and the savings are really significant,’ he said.

While titanium springs are making an important contribution to overall weight reduction they also improve the handling and ride characteristics of the car. By reducing the unsprung weight of a car in proportion to its total weight, the ride and roadholding characteristics can be enhanced. The unsprung weight is typically defined as the total weight of the wheels and tyres, wheel bolts, hubs, brake disks and calipers, springs, shock absorbers and suspension arms.

However, because suspension springs are usually considered half unsprung weight, any reduction in spring weight can translate directly into improved ride and roadholding performance. Thus, the value of titanium springs to a car manufacturer must be measured with these performance benefits in mind as well as the ‘cost of weight saved’ measurements. ‘When the total benefits of weight reduction, both total weight and unsprung weight are considered, selecting titanium springs may be one of the most cost effective decisions a manufacturer can make,’ said Faller.

In the immediate future, perhaps the biggest market is motorcycles. Perhaps even more weight sensitive and certainly more adventurous in terms of new design technologies, motorcycle manufacturers already use titanium widely, with titanium exhaust systems, for example, now more or less standard on all Japanese high-performance sports bikes.

However, cost is still an issue – and while there is already evidence that price is being driven down by demand, titanium will, said Faller, always be a more expensive raw material. Despite this, many manufacturers are evaluating prototypes. Titanium suspension springs are, said Faller, being successfully tested on SUVs, luxury and high-performance cars. But at this stage it’s hard to say who out of these will fully embrace the technology as standard on production vehicles.

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