You have the number and name right. The motion ratio (MR) is indeed about 2.0. (It's actually 1.886). But the wheel rate is MR^2 * spring rate. If you state that the motion ratio is ~2.0 then you must agree that the wheel rate is is not MR*(spring rate), because that doesn't take into account the reduced spring travel.The Dark Side of Will wrote: Based on checking things out with a tape measure, I thought that the mechanical advantage for both early and '88 suspensions was 1.4, which puts the motion ratio at 2.0.
I'm not sure where you got the 1.4 mechanical advantage measurement from. The mechanical advantage of a second-class lever is the ratio of the distance from fulcrum to effort to the distance from fulcrum to load.. The spring is ~200 mm and the balljoint is 350mm from the control arm pivot. That's 1.75. The extra 8% of the motion ratio comes from the installation angle of the spring.
Discussion of the actual motion ratio measurements for the '88 Fiero begin in this post of this thread.
I'm sorry, but Dixon is wrong in your print of that book. I have the same author's other book (The Shock Absorber Handbook) and it has the correct definition which is used by the E30M3 guy and the Kaz Engineering source I cited (as well as dozens of others I can pull up if you'd like). Here are some excerpts from Chapter 4, page 135 of The Shock Absorber Handbook by John C. Dixon:The Dark Side of Will wrote: I've read some of the E30M3 Project pages before. He uses the term "motion ratio" differently than the way it was introduced to me. What he calls motion ratio is really mechanical advantage. (I got my def here: http://www.amazon.com/dp/1560918314/ so I'll stick with it...)
You can find a better explanation of the same information in his newer book, Suspension Analysis and Computational Geometry, but I don't have a copy of it. Here's the motion ratio definition from Amazon's excerpt:4.2 Motion ratio
At a given suspension bump position Zs from normal ride height, the damper compression is ZD. A small further suspension bump motion (delta)Zs results in a corresponding further damper compression deltaZD, Figure 4.2.1. The ratio of these is the displacement motion ratio for the damper at position Zs.
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Often the spring and damper are fitted coaxially and have the same motion ratio.
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4.3 Displacement Method
One way to obtain the motion ratio value for one nominal bump position is by analysis of a pair of slightly different suspension positions. If the position analysis is undertaken by a drawing method it is prone to inaccuracy because of the relatively small difference of positions. Hence the drawing must be undertaken by an experienced draughtsman at a large scale. With less epmhasis on accuracy, a wide spread of positions will give an average ratio over the movement which may be useful in some cases. Also a sequence of, say, eight to ten positions may be drawn through the bump motion, and the damper compression plotted at a graph against suspension position, with the curve smoothed through the points. This helps to reveal any errors. The motion ratio for any particular position is then the gradient dZd/dZs of the curve.
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The dynamic behavior of a vehicle depends on the on the effect of the spring as seen at the wheel, commonly called the 'wheel rate' or 'effective spring stiffness', and on the damping coefficient effective at the wheel. However, the spring and damper are not installed at this point, but elsewhere on the suspension. The effective spring and damper characteristic at the wheel must therefore be related to the characteristics of the actual spring and damper. On a double-wishbone suspension they often operate abotu half-way out on the bottom suspension arm. On a racing car, often they operate through a linkage, including various forms of intermediate rocker. Basically the effect of the spring and damper depends on the ratio of velocities spring-to-wheel and damper-to-wheel when the wheel is displaced in its bumpa ction. In general this velocity ratio, also known as the 'motion ratio' or 'installation ratio', is not constant, but varies with the wheel bump position.
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One of the ways to obtain the motion ratio value for a given nominal suspension bump position is by analysis of a pair of slightly different suspension positions. [...] The spring motion ratio for any particular position is then the gradient dxz/dzs of the curve at the particular suspension bump position.
Here's another excellent source about suspension design including ride frequencies and spring rate: Powerpoint presentation on suspension design from FSAE Lead Design Judge, Steve Lyman from DaimlerChrysler
(motion ratio information on slide 8 )
The coolest thing about that article is actually slide 6, which has a chart of front and rear suspension ride rates, corner weights, unsprung weights, sprung weights, and frequencies. It includes vehicles of all types including the E36 M3, E320 AWD, '02 Grand Cherokee, VW Passat, and so on. Everything from SUVs to hatchbacks.
EDIT: Merged my recent posts