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Get the TJ weighed when it is loaded for trail riding to get the correct F/R weight bias. Then subtract the axle and tire weights from the measured front/rear TJ weights to get an estimate of the sprung weight.

Estimate the stock front axle at about 265 lb and rear at about 225 lb. The tire and wheel combination is estimated at somewhere between 125-150 lbs each.

Using the estimated sprung weight, and the desired up/down travel of the coilover, calculate the effective spring rate that achieves the loaded down travel.

Since the force applied to the two springs is exactly the same, the effective spring rate is found from the following equation:

Effective_Rate = Rate1 x Rate2 / (Rate1 + Rate2)

Application of this equation leaves endless options all of which achieve the desired value – which generally is not a good thing.

If the equation is analyzed, the options can be limited based on the 2 rates provided by the coilover, the effective rate and the final rate.

The final rate which is reserved for some portion of the up travel is determined by the bottom firmer spring and the effective rate as calculated above. So these values become important when trying to tune the shock.

It is interesting to note that the delta between the effective and final rate is minimized when the individual spring rates are the same and that this delta increases non-linearly as the delta between the individual spring rate difference increases.

If the final rate and effective rate are known, which they should be, a better equation that calculates the softer spring rate is given below:

Rate2 = Effective_Rate x Rate1 / (Rate1 – Effective_Rate)

Round the rate 2 value up from ideal calculated value to allow some room for height adjustment as needed.

Now all of these calculations assume the shock is traveling in a completely vertical motion, or some simple trigonometry will need to be applied to the force vector to adjust for these values.

Please post up your rates, how you calculated them, and if you are happy with the results. Or anything else that you think is relevent.

Thanks,

Ken