Introduction .

In this example of a short-long arm suspension system, there are several modeling tools to note.

Tools Described

♦ How to use the IMPACT function to model tire contact

♦ How to create bushings (coming soon!)

♦ How to impose a motion that simulates ‘rumble-strip’ test (coming soon!)

♦ How to create a spring-damper (coming soon!)

IMPACT Function to Model Tire Contact

To more accurately simulate the suspension reaction to road surface irregularities, a one-sided IMPACT functioncan be used.

The IMPACT function does not constrain the tire contact patch to be coincident with the road surface. Since theIMPACT function does not remove this degree of freedom, the tire contact patch is able to lift-off the road surface.

The IMPACT function also allows for elementary modeling of vertical tire compliance. Basically, the IMPACTfunction acts as a compression-only spring.

To actually implement the IMPACT ADAMS/Solver function, a custom 3-component force was created that acts onthe spindle with the reaction force acting on the contact surface. In this model such effects as lateral camber thrustforce are not accounted for. Therefore, the 3-component force has values of zero for the x and y directions. The z-direction (vertical direction) contains the IMPACT function to model the tire compliance.

(For additional info on the 3-component force object and the IMPACT function, please see VFORCE and IMPACTin the ADAMS/View guide in the function builder section.)

3-Component Force on Spindle

In this example we have the following properties for the elasticity of the tire:

unloaded tire radius = 290 mm

stiffness coefficient = 375.0 N/mm

deformation force exponent = 1.01

damping coefficient = 0.525 N-s/mm

To create an object that will model these parameters, the force-create menu is utilized by right clicking on the springicon from the Main Toolbox. Clicking on the icon shown creates a three-component VFORCE. The followingcharacteristics are specified:

construction = 2 Bodies – 2 Locations

direction vector =Pick Feature

characteristic = Custom

2 Bodies – 2 Locations because we want the force to act on the spindle.vertical_ref point and react on thepatch.contact_patch point.

Pick Feature because we want to specify the reference frame to define the z-direction that the force will be appliedalong.

Custom because we would like to use the function builder to specify the IMPACT function to define the magnitudedefinition of the force.

- From the Modify Force Vector window that appears after specifying the above information, right click in the Zforce definition box and select Function Builder.

- From the Function categories pull-down menu, select contact.

- Clear all text from the Function work area and double click One-sided Impact (the IMPACT function). Thisplaces the IMPACT function in the work area with the function properties listed but undefined.

- ‘Disp’ specifies a distance variable used to compute deformation. Delete the text, ‘Disp’, and replace it by adistance measure. From the Function category pull-down menu again, select ‘Displacement’. In this case, wewant to measure a distance in the Z-direction of the ground.contact_patch reference frame betweenspindle.vert_ref and patch.contact_surface. These markers are added by utilizing the Getting predefined datasection of the Function Builder.

- ‘Vel’ specifies the time derivative of ‘Disp’. Using the same procedure as for ‘Disp’ above, select ‘Velocity’from the category pull-down menu and specify the same markers for To_Marker, From_Marker, andAlong_Marker. Reference_frame can be left blank since it defaults to the global reference frame.

- Replace ‘Disp_trigger’ with the unloaded tire radius specified above.

- Replace ‘K’ with the tire stiffness.

- Replace ‘Exp’ with the deformation force exponent.

- Replace ‘C’ with the damping coefficient.

- ‘Ramp_dist’ requires a positive real variable to specify the boundary penetration distance at whichADAMS/Solver applies full damping. In this case we choose to specify 3mm as the ramp distance. NOTE:specifying to small of a distance for the ramp will cause Solver to have problems converging on a solution.

The function now appears as the following:

The actual equation that ADAMS/Solver uses to calculate the force is:

MAX (0 , K*(Disp_trigger – Disp) ^ Exp – STEP (Disp , Disp_trigger – Ramp_dist , C , Disp_trigger , 0)*Vel)

-when Disp < Disp_trigger, zero otherwise

Notice that Solver never will return a negative value of force. This is prevents the creation of an unstable,energizing force instead of a damping force. This also relates to why it is best practice to choose your distance andvelocity measure markers such that you are confident the marker will not be coincident at any point during thesimulation. From the above equation definition, you can see that if the markers become coincident and continuemoving in that direction, the IMPACT function will return a force value of zero.

Requesting information on the 3-Component Force will return the following window of information: