Sanctifier
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Joined: 07 Apr 2005
Posts: 1794
Location: Good question!
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 Posted: Thu Mar 19, 2009 8:36 am Post subject: ~From The Archives ~ Roadholding II_Choosing Spring Rate! |
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UPDATE: Okay, so we now have basic info on the following...
| Quote: | 1) Suspension definitions... and an idea of the major parts involved...
2) The effect of using different parts on the car's cornering characteristics... (Understeer & Oversteer)
3) The adjustments required for 1- / 2- and 3-way adjustable dampers to refine handling characteristics... |
Link--> ~From The Archives ~ Roadholding!
Now here's info from WORKS on... Choosing the correct spring rate for your vehicle.
| Quote: | From answering numerous questions that WORKS customers have posed and the countless conversations
regarding suspension setup, we�ve come to realize that there is much misconception and misinformation
surrounding the topic of spring rates in particular.
The purpose of this paper is to offer a small glimpse of the thought that goes into properly determining the
appropriate baseline spring rates for a track or road-going vehicle.
1. Suspension Frequency and Determining the Wheel Rate.
The first step in choosing spring rates is to choose desired suspension frequencies for the front and the rear.
Suspension frequency is defined as the undamped natural frequency of the body in ride.
Below is the basic equation for suspension frequency, �:
.............  where... WR = Wheel Rate
.............................................................. SW = Sprung Weight
For example, the desired suspension frequency for a road going EVO that sees occasional track use can be determined to be approximately 1.6Hz. In contrast, vehicles that are dedicated to track use may have a desired suspension frequency in the range of 2.0-2.5Hz.
Generally, vehicles with aerodynamic aids (higher downforce) will require higher natural frequencies.
Lower frequencies result in a softer suspension with more mechanical grip.
However, the transient suspension response will be slower.
Coming up with the optimal frequencies will depend on the type and efficiency of the vehicle�s aerodynamic aids, among many other factors.
It should be noted that the front and rear frequencies are generally different so that the body�s pitching motion is stalled rather than develop into harmonics. (~ sic Front-to-rear "porpoising motion" on bumpy road.)
The frequency split should be chosen based on the car�s desired use and real world testing and development
-- which take into account the effects of damping (which will not be covered in this paper).
Rearranging the above equation and solving for the wheel rate, WR we have:
............. 
2. Finding the Motion Ratio.
The motion ratio is basically the distance the spring moves compared to the distance the wheel moves given the same �bump.� We will choose to find the motion ratio (MR) around static ride height (since the motion ratio is generally not constant over full wheel travel).
To find the motion ratio, the following simple measurements can be made:
Measure the distance from the inner pivot point of the lower control arm to the centerline of the contact patch.
Next, measure the distance from the pivot point of the lower control arm to the point at which the line projected
from the centerline of the spring intersects the line to the center of the contact patch.
Refer to the figure below:
............. 
The motion ratio, MR, is simply the ratio of D1 to D2.
Note: Do NOT assume that a MacPherson strut suspension has a 1:1 motion ratio.
A motion ratio of 0.98:1 or 1.02:1 is more typical.
3. Calculating the Spring Rate.
Knowing the wheel rate, WR, and motion ratio, MR, we can calculate the desired spring rate for that particular corner of the vehicle:
............. 
However, one must take into account the contribution of the strut to the effective spring rate. Thus, we have to calculate the motion ratio of the strut itself. To calculate the MR of the strut itself, measure the angle of inclination of the strut while the vehicle is at static ride height.
This is not your camber or caster angle, it is the angle of the strut body itself.
The equation for the motion ratio, MR, of the strut is:
............. 
This represents the percentage of the force from the spring that is translated into the force that contributes to the spring rate. For example, if by doing this calculation you come up with 98.6%, then that means only 98.6% of the spring force is contributing to the motion of the suspension. So you must add 1.4% to the final calculated spring rate.
Hopefully you have found this suspension information easy to understand and educational. Please stay tuned for more white papers published by WORKS in the future.
Author: Ali Javidan...... Co Author: Eric Shin
Date: 7/14/06................ Copyright WORKS 2006 |
Link--> Choosing the correct spring rate for your vehicle.
Hope this helps...
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Walk softly... and carry a BIG stick!  |
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