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Real-world testing : credit: © Alisdair Cusick
Ever wonder how aftermarket products are developed? Alisdair Cusick shares a ride with Terrafirma to find out

We love our Land Rovers, but they didn’t just go from a drawing on a page to a final product. To reach the point of being a production item doesn’t just happen overnight. Many weeks and months are spent designing, prototyping, making, testing – then redesigning and repeating the process. New vehicles undergo complex and rigorous testing programmes, where pre-production test mules go through every type of test, check and verification you can think of before you and I can buy them.

In the late 1960s the process was simple, even for a vehicle such as the Range Rover – just eight initial prototypes were needed for everything from barrier-crash testing to cooling and braking tests – but things moved on, rapidly. By the L322, Range Rover development testing lasted three years, covering 1.5 million miles in 25 countries. Cars were driven not just on the Gaydon test track, but the Nürburgring, in the Sahara and the Arctic Circle. One L322 mule did nearly 10,000 miles towing 3.5 tonnes in Greece in conditions so demanding the trailer needed to be rebuilt daily (but not the Range Rover). Proving tests cover not just dynamic performance (fastest, highest, coldest), but also dust ingress, squeaks and rattles and air conditioning abilities. More recently, the new Defender underwent 62,000 different tests before it was signed off as fit for production.

That’s the high-end world of making a modern Land Rover, but product development is crucial in every area of automotive engineering. Once we buy our vehicle, the first thought for many is customising the car. How do those aftermarket products come about? Well, it isn’t designing a car, but the process isn’t actually that much different. Terrafirma invited LRM along for a ride in its development D4, to get a glimpse into how aftermarket accessories are produced.

Product development can also be fun!

“We’re testing for function and capability first,” says Terrafirma Director, Eddie Priscott, as he deftly crests a rise in the company’s development Discovery 4, “and endurance and repeatability second,” he adds. “The starting point is always the question: ‘What would anybody need to go off-road?’,” he explains as I bounce around clutching my pen and notepad.

Taking a pause from the rough terrain, he runs me through Terrafirma’s process for developing parts for the Disco 3 and 4. “It isn’t just accessorising for accessorising’s sake,” he says. Unlike the Defender, where there are limitless add-ons, the Discovery 3/4 shape has to contain extras within the stylistic silhouette. “The car doesn’t lend itself to lots of bolt-on stuff, so everything has to have a use and a benefit.”

Low-key roof rack lights produce 1800 lumens

LED grille lights are subtly installed, as are the headlight guards

Timescales for the process depend on size and complexity, but can be up to six months. If further testing or multiple prototypes are needed, it can be longer. The Defender rear step winch mount took four months to design, develop and manufacture the first batch, but the Discovery 2 raised air intake took almost 12 months, due to testing multiple complex tooling. Terrafirma uses the latest digital scanning and 3D modelling, which can be shared by engineers and manufacturers around the world.

Where there are market trends, it can affect product development, as ideas are tweaked to fit in with what customers are currently wanting. “There are fewer compromises, though, to enhancing the D3/4,” says Priscott. “We’ve already developed the best, so on rocksliders for example, we offer one set, because they do the job properly.”

New products are tested in the real world for durability and practicality​​​​​​

Development starts at the front and moves rearwards, he tells me, working through what is needed. Take wheels, for example, the perennial desire for any owner. “Everybody asks what is the biggest size tyre I can fit,” reveals Priscott. “Tyre choice is very limited for a 19 inch wheel. The sidewall profile is quite small and prone to damage so 18 inch was the desire, but it was difficult to fit over brakes. We’re now the only people to offer an 8x18 inch steel wheel, which fits the biggest brakes, with a 30mm spacer,” he explains. “There’s some minor wheel arch trimming required to accept the biggest 265/85 tyres – the best all-round compromise – but it still makes the car look right,” he says.

An uprated strut was fitted to the swing-away wheel carrier after tests

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Big wheels are one thing, but if you want a full-size spare in 32/33 inch, then it won’t fit, not to mention being a nightmare to get at, should you suffer a puncture. This lead to Terrafirma designing a rear wheel carrier to cope. Chassis-mounted, with a gas strut, it is a well-designed bit of kit. But this is where development like today comes in. Only when you leave the car slathered in road salt and off-road muck, then use it for daily commuting for weeks, can you spot issues. Priscott tells me that it was precisely this that lead them to change the specification for the gas strut on the wheel carrier. Most struts are used internally, not left exposed for off-roading, so they had to increase the spec to cope with external mounting, including going from 6mm to 8mm on the ball joint.

Even the winch is designed to be as unobtrusive as possible on the D4

Winches and bumpers are a sensitive issue in Terrafirma’s philosophy, but awareness of how the cars are used feeds into development decisions. “We always imagine a D3/4 won’t be used quite as much off-road as a Disco 2 or Defender,” reasons Priscott. “Replacing the bumper as a whole wasn’t as important as being able to recover, so we chose not to do a bumper – and there’s no need when we offer discreet winch mounts,” he points out.

Suspension is an equally complex area, and because of that Terrafirma developed +2 inch lift rod kits for those wanting to keep air suspension, and +2 inch air-to-coil kits, for those wanting ultimate reliability. “The limitation is the amount of dropout the wishbone will tolerate, plus propshaft angle,” explains Priscott. “There is a little more range available, but to keep it reliable we don’t extend the travel.”

Development involves testing in all environments

On days like today we aren’t setting out to get the car stuck. Priscott weaves about, working the car’s talents and simply using it as intended, off-road. Does the kit work? Does it work together as one unit? Those are the overriding questions. There’s no bravado, it’s all about proving the validity of the equipment, and that it then continues to operate over time.

Off-road performance can be easily checked, for if a lift kit fouls on extreme articulation, it’s back to the CAD design screen. But for things such as corrosion resistance, the only way to test that is to fit an item and use it. “As an example, we found that whilst our steel wheels didn’t corrode, the first sets of nuts did,” reveals Priscott. “Development means we catch these issues and rectify them – in that case we changed the plating specification.”

Terrafirma aims to keep all mods within the D4’s silhouette

Sometimes development is an ongoing process, even after the product has gone on the market. “We’re always looking to see if something can be improved, but sometimes the market dictates that,” Priscott explains. He tells me Terrafirma used 3mm wall tube on rockslides for years, but found the US market perception was that it needed to be 4mm to be regarded as heavy-duty. “The reality is that we have the flexibility to increase specifications further,” he tells me.

He also reveals Terrafirma is currently busy developing a range of accessories for the new Defender, all of which are being 3D-modelled as part of the development process. The intention being to be selling them later this year.

That explains Priscott’s out of office auto reply: ‘I am away from my desk working on another exciting Terrafirma project and will respond to your email when I return’.


Terrafirma D4 Spec list:
• 8x18in steel wheels
• 265/65 R18 BFG All Terrain T/A tyres
• 30mm alloy spacers
• D4 stainless steel braided brake hoses
• Uprated brake pads front and rear
• +2in ride height air-to-coil conversion kit
• Polyurethane suspension bush kit
• Discreet winch mount
• A1200 12,000lb electric winch
• Front underbody guard
• Transmission guard
• D4 headlight guards
• Rock sliders with tree bars
• Tow bar security bracket
• Roof rack
• Rear ladder
• Raised air intake
• Door wind deflectors
• Swing-away spare wheel carrier
• 25W 1800lm LED roof rack lights
• Wilderness scene downlighter LED
• D4 LED grille light kit
• Waterproof seat covers
• Boot liner
• Blue silicone intercooler hoses 
• Performance foam air filter


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