I have been working to create an accurate model of the E82 suspension geometry to allow me to understand its fundamental characteristics (bump steer, camber gain, roll centre, scrub radius, trail) as a function of those characteristics that can be tuned for performance purposes (ride height, camber, caster, toe). I have used a combination of CAD drawing and suspension analysis software (SusProg3D) to document and validate the results against real world geometric and kinematic measurements.
The process begins with establishing an appropriate coordinate system. Chassis measurement were made from an arbitrary point on the front subframe, with the chassis leveled longitudinally and laterally such that the tops of all four wheel wells were at the same level. This provides a repeatable configuration for measuring suspension mounting points in 3-dimensional space.
To translate the chassis datum to ground level, I chose the ground plane as the vertical reference plane, the vehicle centerline as the lateral datum, and the nominal front of the bumper as the longitudinal datum. For modeling purposes, I have assumed a wheel centre ride height (wheel centre to fender lip) of 328 mm front and rear, and a tire rolling radius of 312 mm. Once the suspension model is created, changes in ride height and adding rake can easily be investigated, but starting with a constant ride height front and rear simplifies creating the model.
Measuring of the chassis required removing all suspension arms and then the use of conventional and laser plumb bobs to transcribe the longitudinal (x-axis) and lateral (y-axis) positions to the floor, where they could be measured. The resulting measurement accuracy is expected to be ±1 mm.
For bolted connections, target bolts were manufactured to provide the correct location of the associated ball joint or bushing centre.
Similarly a fixture was built to locate the centre of the top of the strut tower.
Measuring the rear suspension at the same time required fabricating a special fixture to locate the guide rod attachment point.
The centre of the inboard rear camber arm bushing was found by first finding both ends of the bushing using a pointed bolt target and a laser plumb bob, and then finding the midpoint between those points.
Once transcribed to the floor, the locations in x-y space could be measured.
A laser level was used to measure the height coordinate of each point from a horizontal reference plane that could be referenced to the chassis datum height.
This is the general process I used to collect the required dimensional data. I will follow up with separate posts outlining looking at the front and rear suspensions in more detail.