I was at the pick a part and grabbed a set of rear springs for my T Bird IRS. The springs that came with the car had been shortened before and one was broke. I thought I read a thread on here someplace that said you will need to cut 1 - 1 1/2 coils off the oringinal spring to get the proper length for this install. Does this sound right. I just want to make sure before I cut anything.

 

Cutting coils is the cooterbob way of lowering. The right way is to measure the diameter, free and assembled spring length and weighing the truck's rear axle weight. You then go to a spring catalog like Moog or to a spring specialist like Eaton Spring (a site sponsor that sometimes posts here) and get a spring that has a rate that the rear weight will bring from the free length down to the compressed length. For example (numbers selected to make the numbers easy to understand) The uncompressed length is 16", the compressed length is 12", that's a difference of 4". The rear axle weight is 1600# (that's 800#/ wheel) 800/4 = 200, so a spring with a diameter as stock, with a free length of 16" and spring rate of 200#/inch would ideally be what you need. Cutting the coils doesn't change the spring rate just the uncompressed length, which means the springs will likely be too stiff for comfortable ride. If you cut enough length out, the spring can possibly come off the seat when the wheel drops and hammer back down slamming the passengers or could potentially jump out of the seat or even fall right out.

 

True, and once the truck is complete which is still always off, I can get a hold of a suspension specialist and buy a proper set of springs. Until then though I need a spring length so I can complete the install and boxing of the frame. First mock up and then rebuilding and finishing. Anyone know off hand the rear axle weight of a 49 f1. With the weight I could go ahead and order the proper spring now instead of having to wait.

 

I don't see where you have anything to lose with the stock springs unless you want the truck lowered. I used stock springs and it turned out ok for me. I did put a 1/2 inch spacer under my top springmount to raise it just a tad because I don't like the lowered look. It would also depend on where you have the top spring mount compared to the bottom at ride height. Like Ax mentioned there is a way to get it right without a lot of guessing.

 

Dang, to late. I cut 1 coil off so I could mock everything up. If the stock springs will work without cutting anything that would be perfect. No problem though, the wreckers has a 93 T Bird there and it doesn't take long to grab them. They call me a frequent flyer there and hardly charge me anything anyways. I was sure that I read on here where you had to cut a coil though and that was the reason for the question. It pays to have a little patience doesn't it? I am learning that ( sometimes the hard way). Thanks for the info. On a side note: With my 230 lbs sitting on the back, the back bottom of the frame rail is setting about 16 1/2" off the ground. With stock springs this will raise me to 17 - 17 1/2". If its to high then I will look at lowering it but this is pretty close to stock which is ok for now. And now it's time for some shock mounts. Have a good Easter everyone.