This article is part of a series on the kinematic simulation and analysis of the Acura RSX suspension. To learn more, feel free to check my other posts:

Front Suspension

Rear Suspension

Full Vehicle

rsx cover photo

Introduction

The suspension system plays an important role in vehicle handling. Manufacturers design their vehicles to achieve a balance of good ride, handling and feeling. This balance is delicate, and modifying your car can therefore have unintended consequences. When preparing your car for on-track use, suspension analysis can help quantify these effects.

With suspension analysis out of reach to the typical enthusiast, an analysis of an ordinary road vehicle would be insightful for anyone looking to improve their vehicle’s on-track performance. We take on this challenge and analyze the suspension from an Acura RSX. In the first iteration, we will focus on the static geometry of the front suspension.

Suspension Pickup Points

The vehicle we will be studying is the Acura RSX. The Acura RSX was Honda’s first modern sports compact to feature a MacPherson strut front suspension. The design was largely seen as a downgrade from the double-wishbone front suspension of the DC2 Integra. To this day, the Acura RSX faces criticism over the performance of its suspension design.

In order to analyze the suspension, we need to obtain the coordinates the attachment points. We used a combination of photography, reference imagery and physical measurements to obtain these points for the Acura RSX.

Using computer analysis, we can build a virtual quarter car and compute design parameters from the suspension points. Below is a comparison between the actual suspension and our model. We are using our own custom analysis routines and using a CAD package to visualize the results.

actual rsx suspension simulation model suspension

Kinematic Instant Axis

The kinematic instant axis in a MacPherson strut is defined by two planes: an upper plane defined by the upper strut mount and damper, and a lower plane defined by the lower control arm. The intersection of these planes is the kinematic instant axis. The instant axis can be analyzed in 2D by projecting it onto the axle plane in front view or the wheel plane in side view.

Front View

Viewing the kinematic instant axis in front view gives us the front view instant centre. This point is important because it influences how the wheel forces are reacted and controls the wheel motion in bump. The front view instant center and the roll center height is shown in the figure below:

ic front

Side View

Similarly to the front view, viewing the instant axis in side view gives us the side view instant centre. This point is important because it influences how the wheel forces are reacted in braking and driving. The side view instant centre is located above the wheel centre in height. This suggests some amount of anti-lift and anti-dive geometry. The side view instant center is shown in the figure below:

ic side

Steer Axis

In a MacPherson strut, the steer axis is defined by the upper strut mount and the lower ball joint. The steer axis is important to study because it controls the wheel motion in steering and will influence the steering feeling and feedback. The front suspension has a substantial amount of kingpin inclination angle and very little caster angle. This suggests there will be negative camber loss as the wheels are turned. Typical of front wheel drive vehicles is a small amount of negative scrub which we can observe in this example. The steering properties can be seen in the figures below:

steering front steering side

Motion Ratios

Update 2020/07/25: the motion ratios have been updated with values derived from a kinematic simulation to correct for an error in the original calculation.

Motion ratios are the ratios of movement relative to the wheel in bump. There are three motion ratios of interest in this design: the spring, the damper and the anti-roll bar. The OEM spring perch is offset from the damper to reduce bending loads in the strut. This is important to note because installing a coil over kit will reduce the motion ratio of the spring and increase the bending loads on the strut.

motion ratios

Conclusion

Obtaining suspension points and analyzing its geometry is a tedious but worthwhile endeavour. We have already gained a lot of insight about the suspension design as it was intended from the factory. There is nothing inherently poor about the Acura RSX front suspension design with most of the performance limitations attributed to the fact it is a MacPherson strut and an early Honda design.

In this installment, we focused our efforts strictly on the suspension geometry but have not yet discussed another important topic: kinematics. In our next installment, we will discuss in more detail what happens when the suspension begins to move in bump and steer.

owt

References

  1. Milliken, William F., and Douglas L. Milliken. Race car vehicle dynamics. Vol. 400. Warrendale: Society of Automotive Engineers, 1995.
  2. Blundell, Michael, and Damian Harty. Multibody systems approach to vehicle dynamics. Elsevier, 2004.

Appendix: table of suspension properties

Property Value Units
Instant centre height - front view 340 mm
Instant centre length - front view 2641 mm
Kinematic roll centre height 96 mm
Virtual swing axle length - front view 2612 mm
Instant centre height - side view 518 mm
Instant centre length - side view 11000 mm
Virtual swing axle length - side view 11002 mm
Caster angle 2 deg
Kingpin inclination angle 17 deg
Scrub radius 19 mm
Mechanical trail 7 mm
Motion ratio - spring 0.92 mm/mm
Motion ratio - damper 0.92 mm/mm
Motion ratio - anti-roll bar displacement 0.41 mm/mm
Motion ratio - anti-roll bar twist 0.10 deg/mm