Stonefield breaks
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new ground... by Bill Brock
HIS MONTH at Stonefield's sctory at Cumnock, ,yrshire, Scotland, full prouction will begin of 4x4 and x4 cross-country vehicles rhich up until now have only een available in preroduction form.
Recently I had the opporinity to try out the fourheeled version at an army tesng ground in the South of 'gland. Norman Watson, the an who has seen the vehicle 'rough each stage of its devepment, was on hand to explain le finer points of construction. Norman told me that soon ter .Stonefield Vehicles was arted in 1974 by the late Jim IcKelvie jointly with the Scot;h Development Agency, work arted on the chassis frame. le conventional ladder type mstruction was considered isuitable for a cross-country !hide application, as it relies e body structure to provide the Ara strength required. Here, e chassis is built on the iaceframe design principle, ing square and rectangular in-walled tubular welded sec
An important feature of the onefield design, which is esntially'a base frame built up ch side to waist level and inTporating a cross-coupling sembly at the rear of the cab th cut outs for the wheels, is at the conventional use of dianal stress members has been deliberately avoided. Instead the strength of each individual frame member is designed to withstand the loads imposed.
The body panels, riveted to the frame, are not intended to be load bearing other than for the purpose of containing the cargo. A frame which retains its flexibility but at the same time is torsionally strong, simplified the calculations involved in the suspension design. To reduce fore and aft pitch (easily induced with a short-wheelbase vehicle operating over rough terrain). suspension frequencies front to
rear differ greatly.
Ten-leafed springs are employed on the first axle and eight-leafed springs on the second, both located by rubber bushes at the front with slippers to the rear. Surplus capacity of the 279mm (11 in) stroke shock absorbers helps to keep the units cool even when the full 200mm (Sin) of movement, limited by check strap and bump stops, is regularly used.
Two power units are offered in the standard specification — the Chrysler 5.2-litre (318cuin) V8 petrol engine with a power
unit of 110kW (150bhp-) coupled to a manual box; and the Ford 2614E V6 four-stroke petrol engine producing 102kw (138bhp) driving through a Ford C3 automatic gearbox to both axles via a transposing box which divides the effort, using an epicyclic differential, 37/63 per cent front and rear. This was the version we tested.
If any of the wheels be,oi., to, slip in either braking or drive an automatic clutch comes into operation, transferring the wasted effort to its diagonally
opposite number. The transposing box also houses the drive for two ptos and a drive line lock which can be engaged when either are in use.
Large movements in propshaft angles, accommodating the axle articulation, is provided by double-hook cv joints forwards but only a single hook is needed to the rear. Convoluted gaiters protect the shafts at the points at which the splines enter the axles. Steering is connected to a 1 to 1, 90 angle box which continues downwards via a knuckle-jointed shaft to a cen-' trally mounted, 22 to 1 ratio, steering box, then onto the front wheels using two half -track rods.
Tyres on the test vehicle were 900 x 16, 6-ply with NATO cross-country/ road pattern with grooved bars, compromising between good off-road grip and comfortable highway use.
At the front, a tubular bumper, positioned at 50' to the chassis line, protects the cab which is a separate assembly bolted to the main frame. At one stage in the vehicle's development, the possibilities of a tilt cab were investigated, but for safety reasons it was decided that for this application a more permanent fixture should be used.
The cab is basic but functional, providing seating for three people. Controls are confined to a gear shift lever separating the high and low range selector from the power take-off lever. The steering wheel is padded and slightly dished while the foot controls, limited to an accelerator pedal and foot brake, are conveniently placed. Driving it was quite an experience, as I was to find out when I took it over the humps and bumps of the test circuit.
Deep water-filled pot-holes, heavy, slippery cambers and
even loose gravel on the steep est inclines didn't stop it. With only a 250kg (5cwt) payload or board, the lower range of gears was not needed, but I took thE opportunity to use them on thE steeper gradients and founc them very efficient in containinc its speed on the descents.
It is practically possible tc transfer from high ratio to lovs while on the move, but in thE interest of preserving the Mors( chain and sprockets within th( transposing box, it is recom mended that the vehicle b( brought to rest and neutral gea selected before doing so.
Automatic transmissior made negotiating the difficul hairpin bends on the summits o 1 in 3 test hills child's play, a my hands were left free to con centrate on turning thl steering-wheel quickly, whil the drive to the wheels look& after itself.
Ground clearance of 50 E the front and 45 to the reE allowed the short 15 ft high teE humps to be driven over wit the ground clearance betwee the axles allowing easy passag at the peak. Over the uneve terrain, axle articulation allowe the wheels to remain in contai with the ground for most of th time, but even when it didn' the drive remained constant.
Again along slippery bank and through the water holes, th
automatic transmission toc care of itself, leaving my hanc free for the steering, which ga■ a slight kick-back through 0wheel at times.
On the road a speed of abol 70mph was reached but it m much more comfortable at cruising speed of about 65mp The opportunity to measui acceleration times did not ariE but Stonefield claims it ca reach 50mph in under 2 seconds, which is one of th requirements it must meet if it to be considered for use as E airfield fire appliance.
• Fuel consumption is difficL to assess as it will obvious depend upon the type of terrai but 8mpg has been suggestE a typical for cross-country wor improving to around 12mpg f road work with a full payload 1.5 tonnes, grossing at just ov 3.5 tonnes.
Basic price is C10,500 whil will make it competitive, as fills a vacant slot betwet models offered by other Briti manufacturers.