[Tambohamilton]

Originally Posted by fe1rx

The principal dimensions for the chassis related to the front suspension are shown in the figure below. For design purposes, front camber of -3.0° was selected, and a caster angle of 7.3°. These settings establish the strut top position for design purposes, which is assumed to be adjustable by means of a camber/caster plate.

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To complete the picture up front we also need the geometry of the steering knuckle, strut and suspension arms.

The true length of the suspension arms (ball-to-ball) requires some careful setup to measure.

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Measuring the steering knuckle must be done with particular precision due to the close proximity of the various pivot points. This required disassembly of the steering knuckle, installation of spherical targets (recovered from actual suspension components to ensure proper geometry).

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A spherical target makes it easy to find the exact centre of rotation of each pivot location. I used a dial indicator to centre on each ball, and a digital readout on the vertical mill to find the location of each ball in x-x-z space. Desired accuracy is ±0.2 mm.

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The front strut has its own coordinate system (as specified in SusProg3D), centered on the wheel hub mounting surface. The assembly is measured with no camber or caster.

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With this data we can create both a CAD model and a SusProg3D model to visualize and characterize the suspension. The dual ball joint design of our front suspensions actually steers about a virtual lower ball joint that moves continuously with respect to the knuckle during both steering and vertical motion. Using CAD to find the virtual ball joint location for the one design condition is difficult but not impossible. Using it to solve for the location at a variety of suspension height is definitely impractical. Luckily SusProg3D handles this task with ease. Having both the CAD and SusProg3D models is useful to provide a cross check for logical consistency.

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To complete the model, the following additional data is required:

Initial Wheel and Tire data: I chose 18” x 8.5” ET45 wheels with tires with a rolling radius of 312 mm. These can easily be changed later in the SusProg3D model if desired.

Initial ride height: I chose 640 mm from ground to wheel well datum (corresponding to 328 mm wheel centre plus 312 mm rolling radius). Again once the model is established, ride height can be altered at will.

fe1rx, firstly a massive thank you for sharing so much of your hard work with us! I'm using your data to create a CAD model of my E91 suspension - I don't have the time, facilities, tools or even patience to get all these measurements myself, and your work is clearly so much better than mine could be.

Not meaning to look a gift horse in the mouth, I think I've found an error somewhere in your front suspension measurements:

Firstly, entering the coordinates for the front upright/strut into cad, I find that the angle between the strut and the hub flange is actually 8.38deg. I've gone through the coordinates multiple times, but can't find my error.

Inserting this component into an assembly with the front arms and chassis pickup points, along with the -3deg of camber, I find that the top strut point is some 21.25mm shy of the chassis coordinate in that location. The strut top point is nearly vertically below the chassis point; I aligned the plane of the front axle/strut with the chassis point, so that the strut/hub was constrained from rotating. I get 6.95deg caster. These measurements assume 0deg toe, though that can't account for all the discrepancy.

I'm sure the issue is just a typo somewhere, otherwise the diagrams you've posted would look significantly different, but I thought I should share at least this part of my findings in case it helps you or anyone else fix an error.

EDIT: I think the error is in the strut top coordinate, in the hub/strut model; I think this should be 0.0, 222.1, 442.5 instead of 0.0, 222.1, 422.5.