Originally Posted by
Tim.Wright
In the interests of a balanced discussion, I will counter some of Z's points.
The methods outlined in RCVD are simplifications and yes simplifcations require some cutting of corners. In this case, following the "traditional" method, you are largely constrained to end up with a traditional suspension with 4 springs and 2 anti roll bars. For passenger cars, this arrangement is a very good compromise between performance and complexity. Packaging anti roll bars is very simple. Packaging a longitudinal Z bar is a nightmare, and for what exactly?
At the end of the day, ANY model or method (including Z's) is necessarily a simplification and the traditional methods are quite a reasonable way to arrive at an adequate (but not optimum) solution without a lot of time or large amounts of input data. As you have mentioned, you don't have the time to do something out of the box so following a well established (and sucessful) route is a good choice from a risk management point of view. Even Z would agree, its more important to get the car built 1 month earlier than to agonise for 1 month more in the design phase about suspension rates.
The main problem though, from an educational point of view, is that often the traditional methods are taught as though they are the first principles of a suspension system whereas they absolutely are not. They are a (quite heavily reduced) simplification of how a suspension operates in roll and pitch. Additionally, they only have a limited range where they are valid (and this unfortunately does not include limit behaviour) and some suspension geometries cause the calculations to crash.
Your intuition should be telling you this anyway since these methods simply a complex system of 2 wheels + elastic tyres, 12-20 links located in 3D space, connected by a compliant chassis by compliant joints down into an equation with only 9 or so parameters (hrcF, hrcR, KwhlF, KwhlR, KrollF, KrollR, mass, mass_dist, cgh).
Regarding the book, there is too much good solid theory in RCVD (particularly the chapter on steady state stability and control) to simply brush it off as codswallop. My advice would be to follow whats in RCVD or in the Optimum G tips by all means but recognise you are cutting some corners by doing so.