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Kinematic Model on a Reasonable Budget

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Creating a useful kinematic model of your suspension can be done with very careful measurement of component and vehicle geometry. 2-D CAD can solve some simple suspensions, but the multilink rear suspensions now common on performance vehicles need kinematic software to solve. So too do the double ball joint front struts used in my BMW. SusProg3D provides an economical software solution that appears to support virtually every imaginable suspension type.

Kinematic models are very susceptible to garbage-in/garbage-out, so I found it very useful to validate/document everthing with a CAD model as a consitency check.

My primary motivation for wanting a good kinematic model was to understand the locations of the roll centres, and to understand how they migrate with roll and ride. That is another story. As with all these "science experiments" some unexpected insights emerge. A case in point: caster is generally considered not to have any significance for a rear suspension because it doesn't steer (except that it does bump and compliance steer). A cursury look at vehicles with strut rear suspensions will show that they have negative caster. I have seen this explained away as a packaging decision - it isn't. For the answer we need to look at compliance steer, but I am getting ahead of myself.

If you are interested, here is what went into creating my kinematic model, and what I learned from it:


Hi James, nice work. I really like your method with all the fixtures, plumb bob and tape, it's great. My own experience with kinematic modelling is more limited to complex and expensive software. However, I am aware of Susprog in general and seems to be a useful and affordable tool. I think I'll look into it for our upcoming suspension course.

As you know, however, there's usually quite a lot of work involved in measuring all the hardpoints for yourself before you even get to the simulation, as your post proves! We actually went through this process a while ago with one of our shop cars, but we had it 3d scanned. For sure it can save you some time at the front-end, but a lot of post-processing is required from the raw data and there are still so many places for errors to stack up, I'm not sure if it's actually a better method than what you went through in the end.

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