The rough and the smooth

Land Rover had to overcome some design challenges to produce a sport utility model with the expected standard of off-road performance. David Fowler reports

Land Rover’s decision to move into the growing `sport utility vehicle’ sector presented it with an unprecedented challenge. The new vehicle, appealing to a younger audience than traditional Land Rover and Range Rover buyers, had to have a ride quality, driving characteristics and fuel economy good enough to woo customers from a hatchback.

Yet the new Freelander had to be capable of upholding the Land Rover tradition of serious, off-road ability.

The first requirement meant ditching two fundamentals to previous Land Rover designs: the ladder chassis, and a beam axle suspension/drive-train arrangement. Instead, in came unitary construction and independent suspension: the first to reduce weight and help fuel economy, and the second to improve driving characteristics in normal on-road use.

But did this also mean ditching the off-road ability? Yes, said the traditionalists. It meant a tense time for the design team, of which about half was drafted in from Rover to supplement Land Rover’s permanent design staff – at least at first.

`The Land Rover diehards said it wouldn’t work without a separate chassis,’ says Mike Galley, chassis project leader . Good reasons, born of 40 years’ experience, back this view.

A beam axle provides constant ground clearance and more articulation over rough surfaces than can be achieved with independent suspension, though the amount of usable articulation may be limited by getting a driveshaft to the wheel.

The disadvantages lie in ride and steering capability. Compliance steer, resulting from changes in the steering geometry as the wheel moves up and down, for instance, cannot be controlled.

Independent suspension reduces unsprung weight, and gives the ability to control wheel geometry more effectively. But providing enough wheel travel and maintaining ground clearance off-road present problems.

A ladder chassis copes with flexing induced by travelling over rough terrain, but is heavier than a monocoque construction. A monocoque is stiffer than a separate chassis, but stress concentrations occur at corners and apertures, with the risk of fatigue cracking. The stresses are exacerbated in off-road use.

The argument was settled by a prototype with independent suspension and a Maestro van body, which showed the arrangement could work.

The result uses MacPherson struts on each wheel. The suspension links were analysed so that each could withstand abuse while keeping the weight down. Each individual suspension component can support the car’s whole weight if it is `beached’ on uneven ground. `It’s designed not to get into a position where something breaks, or if it does, the least expensive thing breaks,’ says Galley.

With the vehicle’s high centre of gravity, stable steering is essential. Using a simulation on Adams software, Rover devised the orientation of the bushes in the lower control arm to make compliance steer `consistent and tunable’. It has applied for a patent on this.

Struts at the rear provide a good balance between front and rear. For off-road use there is almost as much usable wheel travel as a beam axle could provide, says Galley, but the Freelander is good on-road as well.

A primary concern for Mark Burniston, body structure manager, was fuel efficiency. `We were trying to produce a car which would not be too much of a shock to someone coming from, say, a VW Golf in both fuel economy and the way it drives.’

But it was decided that the ride quality and fuel economy could not be achieved with a chassised vehicle. `We had to convince ourselves we could build a car worthy of the Land Rover name using a technique not previously used.’

The resulting design used heavier gauge steels than a normal monocoque, and high strength steels, where appropriate. But the steel is a lighter gauge than would have been needed for a separate chassis. Laser welded blanks, in which strength can be built in by welding in heavier or stronger steel where needed, were not used in the Freelander, but Burniston feels this technique is certain to be used in future designs.

Preventing undue load concentrations leading to fatigue was a main concern. Design work has gone into dissipating loads at critical points, for instance by the use of diaphragm plates to spread the load at junctions between window pillars and roof.

The design is the result of considerable finite element analysis, coupled with animation to show where problems were likely to occur. The need for strengthening complicates the manufacture of the body.

`It’s not as manufacturing friendly as you can get,’ says Burniston. `We made ourselves unpopular with the manufacturing people by doing things that are inefficient from the manufacturing point of view to provide local strength.’

Power is provided by the well-proven L-series two-litre diesel and the 1.8 litre K-series petrol engine as used in the MGF. For the Freelander, however, the K-series gets a different starter motor and alternator developed, jointly with Denso, to resist mud and water. Torque at lower revs has been boosted by fine tuning of the inlet and exhaust system and air intake. Power and torque are the same as for the MGF but the torque peak of 165Nm is developed at 2750 rather than 4500rpm.

Power from the transversely mounted engine goes via a standard Rover PG1 five-speed gearbox to a water-cooled intermediate reduction drive made by Steyr Daimler Puch. Housed behind the engine this incorporates the front axle differential, a 1.4:1 step-down drive and a 90 degrees bevel drive to the prop shaft driving the rear axle.

Between the centre bearings of the propshaft is a viscous coupling. This allows small variations in speed between front and rear wheels, such as occur when travelling around a curve. But if a significant variation in wheel speeds between the front and the back occurs, as when the front wheels lose grip and spin, the unit `stiffens’, increasing drive torque to the rear wheels to compensate.

There is a 1% mismatch between front and rear final drive ratios to preload the viscous coupling and give an element of permanent four wheel drive, with 1-2% of torque being distributed to the wheels in normal circumstances.

But perhaps the most innovative feature of the Freelander is its patented system for descending steep off-road slopes.

Previous Land Rovers have coped with this by having a transfer gearbox to provide a set of extra low ratios, allowing the vehicle to be slowed by engine braking in the lower gears.

For the Freelander this was considered unnecessary complication and weight. (The Freelander’s first gear is lower than low second in a normal Land Rover, the ratio which would normally be used for moving off in off-road conditions.)

Instead, a collar on the gear lever puts the anti-lock braking/traction control system into hill descent mode when first or reverse is selected. With the driver’s foot off the accelerator, the system applies the brakes automatically under ABS control to limit the speed to 9km/h (or 7km/h if the system detects that the descent is twisting or uneven). Using the accelerator – normal off-road technique is to use neither clutch or brakes – the driver can increase the speed to up to 50km/h if wished.

If, in slippery conditions, the system is unable to maintain the set speed, it goes into normal ABS mode to prevent the wheels locking.