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post #1 of 5 Old 11-22-2010, 12:15 PM Thread Starter
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Suspension Frequencies

I am having some issues with calculating suspension frequencies on my fox. Car is using a 3-link rear with coilovers on both ends. I calculate my front frequency to be 1.6xx. I then calculate my ideal rear frequency (based off the front) to be 1.8xx. To attain this rear frequency I must use a much stiffer rear spring then the common rule of thumb of 100lbs/in less than the front rate. Using my 275 front springs, I calculate needing a 225 rear spring.

Formulas that I am using:
TWB= (0.0568 x WB) / speed (I used 70mph)
P = 1/FN
FN(R) = 1/P(R)
P(R) = P(F) ‚Äď T
FN = (sqrt (386.088xWR)/CSW) / 2PI
.9 x spring rate = front wheel rate
1.1 x spring rate = rear wheel rate

I had to make educated guesses on some of the variables. Am I messing up in the calcs. somewhere? Anyone have some insight?

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post #2 of 5 Old 11-24-2010, 08:35 AM
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What sort of corner weights are you assuming? Are you deducting the unsprung weights (around 100 lbs/corner up front, and about 150 lbs/corner out back)?

Keep in mind that the undamped "flat ride speed" will be slightly different than what you get when you do consider the effect of shock damping on frequency once this damping gets much past 'minimal'.


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post #3 of 5 Old 11-27-2010, 08:53 PM Thread Starter
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No, Norm I neglected to take unsprung weight into consideration. For corner weights, I really just took a wild guess. Based off prior cars I believe my car to weigh around 3250 lbs. I then assumed a 55/45 spilt divided equally across the sides. I know that is not real accurate but I figured that it was better to do this then just buy something with no real info. to go on. The car is different from most foxes in the suspension mix/match along with softer springs then most people use with 3-links, etc.
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post #4 of 5 Old 11-28-2010, 09:51 PM
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I just read through the notes on my car and this is what I have for spring rates and frequencies for various set-ups:

Front/Rear Springs; Front/Rear Frequencies => Ride ratio
600conv/200conv; 1.286/1.573 => 1.223
650conv/220conv; 1.326/1.636 => 1.233 (FRPP "C" Springs)
400c-o/325c-o; 1.927/2.396 => 1.244
275c-o/225c-o; 1.665/2.065 => 1.240 (Your Calculated Values)
275c-o/175c-o; 1.665/1.861 => 1.118 (Your "Rule of Thumb" Values)
350c-o/400conv; 1.830/2.096 => 1.145 (My Current Set-Up)
400c-o/400conv; 1.927/2.096 => 1.088
400c-o/500conv; 1.927/2.295 => 1.191

Hopefully this helps you a little bit. I can plug in different rates if you want me to check anything in particular.

I will add that my car rides "nicer" with the 350c-o/400conv set-up than it did with the FRPP "C" Springs.


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post #5 of 5 Old 11-29-2010, 08:13 AM Thread Starter
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Thanks, Vince. I need to soak that info. In a little but it is very useful.
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