Auto suspension design made easy

Charles Clarke

Lotus Engineering Suspension Analysis (LSA), the commercial version of software from Lotus Engineering in the U.K., quickly creates 2D and 3D kinematic models of standard suspensions using templates. These define how suspensions connect to vehicles and can be run with a library of default values.

LSA handles configurations such as double wishbones, Macpherson struts, trailing arms, semitrailing arms, pull-and-push rod-damper actuation, and "A" and "H" frames. Users can also create and distribute their own templates. Even without extensive suspension-design knowledge, designers can be looking at something workable in about 10 min.

Selecting a particular template brings up a suspension in 3D wire frame. Tires and wheels are assumed rigid. (In kinematics terms, the tire contact is a point, not a patch.) A shaded 3D model clearly shows suspension details, at the cost of some animation speed. In wire-frame mode, however, changes are instantaneous. Users can work on one or two corners of a suspension, although most concentrate on one corner. Hard points are defined in a global ( automotive) coordinate system while bushings are oriented in a local coordinate system.

Templates show hard points as a list of coordinates. The change-coordinates function modifies default values which update the model. A "joggle" feature steps through changes for particular coordinates. These changes can be set as "fine" or "coarse." Hard-point coordinates for suspensions can be picked on screen and edited. Points can be dragged in 3D or constraining the dragging along one or more axes. Graphical results are updated continuously as points move.

When geometric relationships are fixed, points can move as a group to maintain dimensional relationships in, for example, a wishbone. Tolerance Analysis lets designers simply select a point and specify the tolerance.

Users assign "point limits" by putting a box round each point. So if a particular motorsport regulation specifies that hard points should be within a certain envelope, users can define that envelope in the system. The software won't let anyone violate the regulations as they modify the suspension.

Motor-sport rules often call for maintaining existing parts. To do so, users specify "retain part lengths" so when hard points are moved, the suspension is rebuilt with the same geometry for the particular part.

The software even puts a body on the suspension. If engineers were examining the suspension's roll mode they could see animations of the suspension as the vehicle goes through a roll sequence. And the animation shows items such as the roll center and migration of the center of gravity. Changes can be made while the suspension is being animated.

Outputs include all significant kinematic-suspension parameters such as camber and castor angles, toe, kingpin angle, spring and damper ratios, CG and roll-center position, and brake steer. The software shows either graphs or numbers for specified bumps, rebounds, rolls, and steering articulations. External forces can be applied to suspensions along with suspension-spring loads, antiroll-bar forces, and the influence of steering-rack coupling and compliance. Calculations instantaneously update graphical displays after each design change so users instantly see the significance of their changes.

Results are presented as a series of curves showing how suspension geometry changes with variations in a particular element. There can be as many curves open as necessary to examine the consequences of particular modifications. Curves are for parameters such as bump-steer characteristics, camber, toe, castor angle, roll-center heights, and tire vertical forces.

The software produces graphs and numerical listings which can be fed into other software. Engineers previously had to write out wheel-center coordinates for others to do wheel-envelope work. Now if they want to know the coordinates at a certain position, they just request them. For example, the software will list all hard points every 30 mm of bump travel. A user would then cut and paste that data to other applications.

Changes to graphs aren't reflected in the geometry, but the target setting is sensitive enough to let users readily identify trends. For a particular response, set up a number of runs with varying geometry until it converges to a required result.

A few details such as spring forces and roll bars can be turned on and off. The software will also find unconventional roll bars such as blades — as long as they can be represent as a mechanism with stiffness.

An optional module calculates compliant displacements and bushing forces by adding stiffness properties to compliant bushings. As in the kinematics module, users pick and drag points around, and actually manipulate the bushing axis for a required response. Bushing rate, position, and orientation are easily modified using the same interactive editing environment to include effects such as lateral force steer due to compliance.

Compliant deformation can be animated either throughout the suspension travel or as a scaled deformed geometry sequence at a particular suspension position. Designers can also plot bushing distortion beneath the animation to examine whether a particular bushing axis would shorten its life.

In addition, external forces can be defined to see how compliance affects the suspension geometry. Users see graphical feedback on how the system deforms together with numerical updates to the analysis. Any number of forces can be applied to any parts and the system recalculates when any are moved.

Slider bars in the software modify parameters such as the stiffness of a bushing. But there are many ways of varying properties.

LSA's 3D viewing, editing, and rapid computation make it ideal for designers and analysts in a project's concept stage. The software includes libraries of common suspensions while user-definable templates construct unique suspension configurations.

An additional handling module uses the MSC.Adams solver. It imports data from the Lotus Engineering Suspension Kinematics and Compliance Measurement System (SKCMS) or other kinematics module. The software runs a virtual compliance rig with a full vehicle suspension modeled to see what happens on the rig. This shows lateral forces in particular load cases and generates data for the handling module. This module models vehicle properties such as unsprung mass and track and tire data.

The handling module is beneficial to existing MSC.Adams users as a preprocessor. The handling module simulates wheel bumps, road undulations, steering motions, step changes, and acceleration and braking maneuvers. The vehicle model is driven by splines populated with kinematic data or data taken from the SKCMS rig. The model provides accurate data on suspension and steering behaviour in response to applied quasistatic forces approximating those at the tire-contact patch during cornering, braking, and accelerating. Additional forces can be applied at the wheel center if required. A full set of vehicle tests on the SKCMS takes about 16 hr.

The software works comfortably on an Intel P4 (or equivalent) PC with 512-Mbytes RAM and a reasonable graphics card. Users don't need specific solvers for kinematics and compliance modules. Everything is contained in the code. LSA is available from Lotus Engineering Software, (www.lesoft.co.uk), starting from as little as about $637/month (£350/month) up to a perpetual licence price of about $14,560 (£8,000), small beer when users consider the wealth of development in this code. —