[nanard289]

Sorry, I had understood that 13" was the total distance when the wheel was in the lower position (out of the floor). If 13" distance refers to the normal position, you have to measure the minimum distance when the shock will be fully compressed. If for example you measure 8" you will obtain a compression stroke of 5". Then you apply the 1/3 - 2/3 rule and you will obtain 2.5" for the extension stroke (note that in this case the track rule is not applicable due to the large suspension stroke). It means that when the wheel will be fully extended the shock length will be 15.5". So you have to provide a coil over shock 15.5" fully extended and 8" fully compressed. In order to have the correct 13" distance in normal position, the spring will be define as previously indicated. In this case, the spring length shall be about 10". If the suspended load at the shock axle is 600lbs (for example) you have to calculate the spring as follow: 600:2.5= 240lbs per inch. It means that when the coil over will be installed the load on the spring will be 600lbs and the spring length will reduce of 2.5" accordingly. To have the complete shock stroke (7.5") the relevant load will be 240 x 7.5 = 1800lbs (about 3G for vertical acceleration). Remember that you can always adjust (as final action) the trim by using the lower spring retainer to modify the spring load.