The Claytex team was posed a question recently; “How easy would it be to create an articulated truck model with what we’ve got already?” My immediate thought was that it should be easy. The dynamics are no different from a conventional car and trailer and apart from a couple of extra axles, wheels and a different hitch it’s a matter of parameterisation. But I wanted to check that so I spent a day creating one.
The Basis
Basing it on the VeSyMA and Suspensions libraries gave me a great structure for creating a tractor-trailer model, as there was pretty much everything I needed to get a simple artic going:
- 2 Axle, rear wheel drive vehicle
- Fifth wheel type hitch
- Trailer model with 2 axle, independent suspension
There were a couple of things that were missing that needed to be added to get a basic model working:
- 3 Axle trailer
- “Dual” wheels
- Dependent trailer suspension
- Dependent tractor suspension
The rest of the differences could be achieved by re-parameterising existing models.
3 Axle trailer
The 3 axle’d trailer layout was created by duplicating the 2 axle trailer template then:
- swapping the brakes from 4 wheel to a 6 wheel interface model (VeSyMA.Brakes.Templates.SixWheelBrakes)
- Adding a 3rd axle
- Adding the 2 extra wheels
Figure 1: Modified trailer template
Dependent Suspension
The trailer models that exist in the suspensions libraries contain independent torque bar suspension as found on small car trailers. The suspension on articulated trailers is varied but the most common design is dependent suspension controlled through 2 trailing arms. This is slightly different from the truck arm suspension found in the Motorsports library that uses a panhard rod to control lateral movement.
An ideal model was created with trailing arms and a fixed axle. To not over constrain the axle, it had a cylindrical joint at the centre to allow the 2 trailing arms to move semi-independently and allow roll.
Video 1: Trailing arm suspension in a 2 post rig test
The front suspension also requires a new template, duplicating the dependent rear suspension and changing the extends statement to extend from VeSyMA.Suspensions.Interfaces.AxleS instead of VeSyMA.Suspensions.Interfaces.Axle to ensure that it could be used in the front suspension. Then add the steering interface to provide the steering rack.
The most common suspension used in the front axle are leaf springs, which are an undergoing development for us. So instead I used a trailing arm design that pivots around the front leaf spring hanger. This would serve as a placeholder and approximation until a full leaf spring model is developed. Instead the spring force model can follow a leaf spring force profile to mimic the effects on the vertical load.
The steering rack is attached to the axle to allow for proper steering dynamics. For simplicity, the 1D ARB model (VeSyMA.Suspensions.Components.AntiRoll1D) is used in each axle.
Dual Wheel models
The dual wheel models were simple to implement. They could either be added to the vehicle model with an offset from the suspension hub and then have multiple wheels per corner.
But to make the vehicle and trailer model more simple I created a dual wheel template, extending from the wheel interface and adding 2 wheel interfaces within that. This means a single wheel redeclaration is made in the vehicle as normal but introduces 2 wheels per corner.
Full Model
The half car and dual wheel models were used on multiple axles both on the tractor and trailer to form a 2 axled tractor and 3 axled trailer.
The mass, inertia, wheelbase and engine were re-parameterised to the appropriate values but it did inherit the body visualisation from the default VeSyMA vehicles.
As it’s extended from the VeSyMA vehicle model, the animation is the same so it looks a little odd but the body is purely for animation purposes and we’ll update it once we get our hands on a representative CAD rendering of the tractor unit! As mentioned, parameterisation (dimensions, performance, inertias and masses) is in line to that of a truck.
Conclusions
Creating an articulated lorry model did require a few extra templates and specialised suspension models. But a majority of the vehicle systems were from the existing libraries, and all specially made systems used common parts such as masses, springs, dampers and wheel models in a different layout.
Written by: David Briant – Project Engineer
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