Article Number: 101 | VC6 | VC5 | VC4 | VC3 | Post Date: September 8, 2020 | Updated: September 8, 2020
When I add additional weight to my trailer, or move the trailer cg, my tractor’s rear suspension is bottoming out. What can I do to fix this? How can I position my vehicle doesn’t compress the suspension when my simulation starts?
In some cases, adding additional cargo weight, or moving a trailer cg forward may cause the rear suspension of the leader vehicle to compress downward beyond an acceptable tolerance. This problem can be resolved in a few steps. First, let’s look at an example tractor-trailer system, where both the trailer weight has been increased and the cg position has been moved forward of the rear axles.
In contours view mode, it’s easy to see that the rear axle suspension on the tractor is bottoming out. Another undesirable feature that is visible below is that the polygons of the mudflaps are making direct contact with the terrain mesh – this is something we never want to happen.
Before we discuss how to solve the issue of bottomed out suspension, let’s first review how the suspension model works. In Virtual CRASH, by default the specific suspension stiffness and damping values are determined based on the “normal”, “stiff”, or “soft” settings. For the selected vehicle, Virtual CRASH places the vehicle on a flat plane and solves for the amount of suspension force required to keep the vehicle in static equilibrium at each wheel (without regard to couplings to any other objects). With this known force, the spring constant is calculated using:
$$ k = F_{0,z} / d_0 $$
where \(F_ {0,z}\) is the force needed for a given wheel for static equilibrium, and \(d_0\) is the initial spring compression. For “soft” \(d_0 = 20~cm\), for “normal” \(d_0 = 15~cm\), and for “stiff” \(d_0 = 10~cm\). Note, as the vehicle curb weight increases, the spring constant automatically increases.
If the “user” option is used, then one can directly specify the spring constant and damping coefficient. In this case, Virtual CRASH will also perform the static equilibrium calculation to determine \(F_{0,z}\) for each wheel, but the initial spring compression will be determined by \(d_0 = {F_{0,z} \over k}\).
If additional weight is effectively added to the vehicle through joint coupling (such as a tractor-trailer), occupant loading, or using the add cargo weight option in the “mass properties” menu, the initial spring compression will not be sufficient to start the vehicle static equilibrium, and you will likely notice the suspension compression after the simulation starts. This was illustrated in the first video above. In such cases, Virtual CRASH assumes the vehicle starts with an equilibrium force of \(F_{0,z}\). After the simulation starts, due to the additional weight, the suspension will compress by the amount \(\delta z\) thereby increasing the suspension force by \(\delta F\) until a new equilibrium is reached, or until the suspension bottoms out when limits-upper is reached.
The “user” option
By increasing the spring constant, this will naturally reduce the needed additional compression to achieve a new equilibrium force value at each wheel.
The “normal”, “stiff”, or “soft” options
Another option to prevent bottoming out, is to increase “limits-upper” to accommodate the necessary additional compression to yield static equilibrium (\(\delta z\)). In the video below, we increase the “limits-upper” values to allow the suspension to compress without bottoming out. Note, we also increase the ground clearance to prevent unwanted contact with the terrain (we will adjust this again later).
If your vehicle’s pitch angle settles to an equilibrium value that is too large, this can be easily fixed. First, we will adjust our tractor model mesh object so that it begins with an initial pitch angle equal to the negative of the final pitch angle. The net effect of this will be to rotate the tractor model to pitch = 0 after the simulation begins. To do this, we start by creating dynamics report and noting the final pitch of our tractor as well as the z-displacement of the tractor’s cg.
Now, we clone our tractor, remove physics, and simply set the mesh’s pitch to the final pitch from our report multiplied by -1. Once this is done, we select our mesh and use “Export Selected” to create our custom vcm file. Then, we drag and drop the vcm on our tractor to replace the mesh with the rotated version (this is the same workflow shown in Chapter 7 of the User’s Guide). With this done, we should now see the tractor rotate to zero pitch after the simulation begins.
Next, we adjust our tractor’s ground clearance and height to match up with our specifications. Here we’re using simple box objects for reference height. Note, we want to match the ground clearance and overall height after the tractor reaches static equilibrium.
This process is repeated for the trailer.
Note, you may want to adjust your cg height setting (in the “size” menu) upward to account for the initial downward displacement at the start of the simulation due to the extra weight. To make this adjustment, simply add the difference between initial and final cg z positions from the dynamics report (shown above) to the target cg height parameter.
Finally, you may wish to begin your tractor-trailer system with its initial pitch and height equal to the equilibrium conditions so that your system isn’t settling down at the start of the simulation. To do this, sample the final pitch angles and cg heights from the dynamics report, and use these values as the initial values.
If you adjust the initial conditions as shown above, remember to disable “auto align to plane” or else the cg heights and pitch angles will snap back to their original values.
Here we see our tractor-trailer traveling over uneven terrain.
Initial Suspension Compression
As noted above, Virtual CRASH will use the vehicle’s curb weight to either calculate (a) the needed spring constant to provide the necessary equilibrium force at each wheel given a known initial spring compression, or (b) calculate the needed amount of initial equilibrium spring compression to produce the equilibrium force at each wheel given the user input spring constant.
When the “auto align to plane” option is used (this is enabled by default), Virtual CRASH will automatically position and orient the vehicle so that the ground clearance is maintained. This helps ensure that your vehicle doesn’t start with some portion of the terrain penetrating into the bottom of the vehicle’s mesh, which could result in unwanted effects. Here for example, our sedan’s initial position is moved along a flat plane object. Notice, as the simulation plays, there’s no change in the sedan’s height. That’s because Virtual CRASH places the vehicle above the plane so as to maintain ground clearance, which in this case, means the wheels start already in contact with the terrain and the suspension springs are already compressed with the value needed for static equilibrium.
When the terrain is no longer a flat plane, Virtual CRASH continues to search for an initial vehicle position and orientation that respects the ground clearance for the vehicle (if auto align to plane is enabled). Notice in the example below, as the initial (x,y) position of the sedan is changed, the Virtual CRASH automatically changes the initial height as well as yaw, pitch, and roll angles. When auto align to plane is disabled, only the initial (x,y) position changes.
In this example, when auto align the plane is used, Virtual CRASH positions the sedan so that the middle portion of its body is above the curved portion of the mesh by a distance equal to the ground clearance. By doing so, the suspension on the front and rear wheels must extend downward until they are in contact with the terrain (to the limits of the suspension).
By extending the suspension, the force at each wheel is no longer large enough to keep the sedan in static equilibrium, and so it moves downward. To counter this, as we did above with the tractor-trailer, simply read off the final position and orientation values from the dynamics report and use these final values as the initial values. This same method works for occupant loading or when using the add cargo weight option.
Tags: Adjusting suspension, spring rate, damping, bottoming out, stiffer suspension, vehicle sags, vehicle moves down, suspension compresses.
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