CHASSIS AND SUSPENSION
Simon Johnson, lead engineer on Raptor vehicle dynamics, was given the challenging task of creating a chassis both capable of handling off-road abuse and one that could provide acceptable on-road behaviour. And all this while staying within a tight budget. The fact Johnson had also worked on the standard Ranger and Everest’s dynamic abilities was useful.
The Raptor’s ladder-frame chassis is based on the Ranger front-end and Everest rear, with the main reason being the addition of a Watt’s linkage axle, bene cial for axle control at speed (with close to straight-line vertical axial movement but no lateral movement allowed). This is where the similarities end, though. The Raptor’s track width is 150 mm greater (in addition to a 50 mm increased ride height) with extra bracing added to elevate rigidity in torsion and bending moments. This meant the front double-wishbone suspension received new A-arms with optimised geometry (20% more travel than on the Ranger) and Fox doubletube racing shocks (more about the shocks later).
At the rear, Johnson removed the leaf springs and altered the ladder frame so the Fox coil-over suspension could be mounted outside the ladder frame (30% more travel than on the Ranger) connected to the wider rear axle. There is no pathway for the exhaust to exit at the rear and it is therefore cut off short under the body. According to Johnson, the rear section of the chassis was a more expensive solution than was initially planned but, after senior management drove a prototype vehicle with this setup, he got the go-ahead.
The Fox suspension is state of the art, with the double-tube arrangement sporting oil-bleeding holes in the inner sleeve. When the shock is compressed, these holes get sequentially closed as the piston moves down; this essentially increases the damping rate in steps as the shock is compressed, resulting in a compliant ride during small movements but, because of the increased damping, it is almost impossible to bottom out the shock at maximum travel.
According to Johnson, who calibrated the spring-and-damper settings, the bump stops are seldom reached, preventing the mounting points from breaking. The sharp spikes in loads when the end-stops are touched are usually chassis breakers.
On rebound, the damping has a “hold zone” keeping the suspension compressed for longer after full compression to prevent bucking behaviour (also known as porpoising) after a big jump.
Simulations predicted suspension loads and these forces were validated by vehicles equipped with strain gauges recording loads on critical components. It was impressive to witness rst-hand the kind of loads this suspension can cope with without showing strain. Interestingly, even the spare wheel’s mounting points were strengthened to cope with the added load strain.