rear coil spring options (lengthy)

[Geoff B]

I was looking for some JGC coils to lift the rear of my '87 Pathy, and had a good deal of trouble finding any in the salvage yards in my area. The only place that had a pair wanted $100 and would not verify whether it was a 6 cyl or V8, Laredo or whatever.

 

So I checked with several of the off road shops in my area, all of which were more than willing to let me sort through and measure what they had lying around. While I didn't find any JGC coils (not that common for building up as trail rigs), I found that Jeep TJ front coils can be another option for the rear of the Pathys. I found a set that I was told were stock TJ Rubicon coils, but I think they may be a pair of lift coils for front of the TJ (no Mopar part # tags). I snagged them for $40 for the pair. They measure as follows: 17.25" long, 5" coil outside diameter, .595" wire size, 10 coils (without the curly end). After calculating the spring rate of these TJ coils at approx 220lb/in, compared to 146 lb/in for stock, I estimated these to be about 2.5 - 3" of lift, assuming about 1600 - 2000 lb on the rear.

 

After installing I got 2.75" which was too tail high for my taste, so I cut another 3/4 coil out of them and reinstalled, resulting in about 1.75" of lift, just right for the usual torsion crank with UCA's in the front. The ride is stiffer, but I felt the stock rears bottomed way too easily, especially at higher speeds off road. I think the on road ride is actually much improved with the stiffer springs combined with the Trailmaster shocks. I'll try them in Moab in the next couple months and see how the rockcrawling goes compared with before.

 

My point in all this is that off road shops are a great source of spring options at cheap prices, I scored one pair of coils for free (they ended up being too weak) and another that I used for $40. Second, don't be afraid to simply measure and find some that will work regardless of the vehicle they came off of, cutting them if needed.

 

Using the formula for calculating coil spring rate:

SR=(G x W^4)/(8 x # x D^3)

SR=spring rate

G=11,250,000 lb/sq in (a torsional modulus constant for spring steel)

W=diameter of the wire size

#=active coils (not including the portion of coils that are touching the spring seats)

D=diameter of the coils (OD of coil - wire diameter = centerline diameter used for formula)

You can calculate how much lift any coil spring will give the back of the Pathy.

 

In my example, I assumed about 1600 lb on the rear springs, probably 1800 lb with trail gear and camping stuff. Divide in half for the load on each spring. Using the stock spring info of 146 lb/in and 16.4" length (out of the factory '87 service manual) and my new spring info of 220lb/in and 17.25" length, calculate how much each spring will compress under load. 800/146=5.5", 800/220=3.6" Subtract these numbers from the overall length of each spring 16.4-5.5=10.9", 17.25-3.6=13.7" The difference between the loaded length of each spring is the resulting lift 13.7-10.9=2.8" of lift @ 1600 lbs.

 

Note that cutting the coils not only decreases the length of the spring by that amount, but also increases the spring rate. Larger diameter wire size means a higher rate, larger coil diameters and more coils means a lower rate

[88pathoffroad]

Good research, man! Right on.