Building a Scania V8 R500 - Part 3. Suspension, front axle

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 3. Suspension (front axle)

A front spring.  The suspension spring assembly comprises two leafs (main and Secondary), saddle and U bolts, centre pin and bump stop, swinging (rear - right in picture) and fixed shackles. 



Leaf springs come in different shapes.  In the days of horse drawn carriages, full elliptical springs were sometimes used.  Today, we see parabolic, semi-elliptical and quarter-elliptical springs (see Pt 2  drive axle). 





Leaf springs.  The Scania`s front axle is a non-independent dead beam setup.With front axle weights typically 7.5t there is no real need for air springs. This system is the most efficient way to achieve steering axle geometry.  A main spring is wound into eyes at each end and attached to the chassis, a secondary spring is attached to the main spring.  The main spring is very important as it attaches the axle to the chassis and provides a chassis connection for the secondary spring to work on - if the main spring breaks, axle location and suspension will be lost. 






Although Scania use a parabolic spring, this term refers to a particular shape, the mathematical description of which is quite complex.  Sometimes it`s best to think visually, in this case how curves are formed in cones.

 



Some leaf spring shapes are derived from ellipsis





The Scania`s front leaf spring shape is parabolic.  If in doubt, they are simply leaf springs.

 

The rearmost shackle (the chassis seen here inverted) attaches the rear end of the spring assembly to the chassis.  It uses a swinging device that allows for spring (longitudinal) extension when the suspension deflects.  A fixed shackle will always be at the front end of the spring, as the vehicle drives and brakes through this rigid connection. 

A `C` anti roll (stabilizer) bar is used on the front axle.



Anti-roll bar in action.  Anti-roll bars reduce body roll but also increase spring rate in roll, transferring weight across the vehicle.  This will effect handling.  Body roll is important on trucks as increased roll can reduce stability (rollover).  The anti roll bar will attempt to level the body by lifting the inner wheel (on a bend) or the vehicle`s body itself.  It can only do this up to a certain point, after which the bar begins to twist more and there`s an increase in weight transfer.





The front axle is an I beam (because its cross section resembles the capital letter I).  The depth of the beam and the width of its flanges deal with bending loads and twisting (torque).



Next:  Part 4  Steering