Building a Scania V8 R500 - Part 4. Steering

I`m building a Scania R500 - from a 1:24 scale kit produced by ITALERI.

I`ll be looking at aspects of truck construction as I go along, hopefully highlighting some basic truck technology.  It`s easy to think that modern vehicles bear no resemblance to those of the past, but that`s not true.  Suspension and steering; engine, transmission and final drive; and tyres are all there to maximise the laws of physics, and have retained the same configuration since diesel replaced steam.  Cost and natural performance limits have meant that the chassis abandoned long ago in car design is still used in truck manufacture.   Yes, your truck is computer controlled and a modern marvel;  but so is the modern cruise liner - which is still a Titanic underneath.

Building a Scania  R500    Part 4. Steering



The steering for the rigid beam, non-independent suspension system is a simple, time honoured layout. Brake backplates are mounted on stub-axles that swivel on king pins at each end of the axle. Wheels rotate on bearings on each stub axle. 




The steering column from the driver`s wheel works the integral PAS (power assisted steering) box, mounted on the chassis.  The box converts this rotating motion through 90 degrees, working a drop arm.  The drop arm works the drag link, via a ball joint.  This then operates onto a steering arm (upper), which forms part of the stub axle fitted to the end of the axle behind the position of the steering wheel.  The stub axles (left side and right side) are connected to one another by lower steering arms and a track rod

 



Steered wheel angles are important.  KPI (king pin inclination) reduces scrub by reducing drag on the steering and bending and sheer forces on the king pin.  True centre point steering (not always used) is created by ensuring that an imaginary line passing through the centre of the king pin and the centre of the wheel meet at ground level.  Wheel scrub radius is further reduced by using dished wheels and wheel camber.  Caster creates steering self centreing.





The Ackerman set-up  is needed to reduce scrub as the vehicle turns, and is created by angling the steering arms at each end of the track rod.  As the steering is turned, each wheel describes a different angle of turn, ensuring that the tyres travel in different directions according to the distances they are covering (the radius of the inner tyre is different than that of the outer tyre). 



The steering box is mounted on the left side chassis rail (LHD) and a drag link connects it to the left side steering arm, which forms part of the stub axle.

Inside the steering box a piston acts as the power ram for PAS.  A control valve directs fluid under pressure to either side, depending on which way the steering wheel is turned.  The piston rides on a worm gear, driven directly (through the control valve) from the steering column. Recirculating ball bearings reduce friction in the box.  The piston operates onto a sector shaft that is connected to the drop arm.  The control valve uses a cam system, or a torsion bar, to sense the driver`s requirements and regulate the amount of effort given by the PAS.

Next.  Part 5  Drive Train