Front Coil Springs - selection, cutting, and installation

Coil springs, finally.

This isn't supposed to be hard, but I found a way to overcomplicate it anyway. My original intention was to remove the old front coil springs and put in new ones. There are many different springs available out there for the Mustang aftermarket, so many that it can get confusing pretty quick.

I'll warn you now, this one is a little long.

The old springs. I suspected all of this was original. 

Coil spring as it sits in the spring perch.

I've already posted on this when I pulled the springs out, and there's a lot of info on this on the interwebs already, so I'll just say be careful, as it's easily the scariest thing I've done yet on the car. 

When you go to order new springs for an old car like this, you need to know not just the year and model, but the engine, body style, and what accessories came on the car, such as A/C, power steering, or other heavy items, and which trim line it came with from the factory, such as the GT handling package. It matters because the springs carry the weight of the car and those things add significant weight, so they need to be accounted for in the springs to get the right height and spring rate. And that's only if you're trying to get exact replacements; if you want 'upgraded' springs, you have more work to do.

My car is supposed to have the competition handling package on it; as a rare option, it's harder to find replacements. I'm almost positive that it's the same package that goes on Shelby GT350's, and there's usually a substantial price premium for anything that says "Shelby" in the parts' application notes. Plus, I was worried that it might be stiffer than I wanted, so I decided a while ago the car was going to get GT springs all around, slightly softer than the competition handling package, but firmer than the standard factory springs. I've already put new GT leaf springs on (Eaton springs). I discussed this with John at Opentracker Racing and mentioned what I wanted, he suggested the GT springs from a big-block car to make it just a little firmer.

New GT (big block) springs on top, old springs on the bottom. The new springs are about 2" longer than the originals. 

The tell-tale yellow and pink paint daubs on the springs indicate the competition handling package.

I compressed the spring to put it in and found it was hard to get it compressed enough to fit easily. The spring pocket is about 11" high, and I struggled to get the spring down to 13"...too high still. I needed a spring that is about 2" shorter to fit in the pocket. I eventually had to disconnect the sway bar and the strut rod to get the upper control arm low enough just to fit the spring in. It has to have room to go in easy - no push/shove/rock-n-roll with a compressed spring. Eventually I got it in...

And it looked like this. Please notice the spring is actually bowing outward. This was uniquely scary,  as well as wrong, so I promptly pulled the spring back out again. I wonder if this is what working on a bomb squad feel like. I called up John at Opentracker again and told him my story, and he replied simply that , yes, you should expect to trim the tops a little for height - that's not surprising at all. So, now I know. Again, this is my first rodeo. 

So, how much should I cut off the spring to fit the car and still behave like it's supposed to? Turns out that's a hard question to answer simply.

Most folks I see cutting springs are talking about 1/3 or 1/2 coil, maybe 1 full coil, and they all say to do it in iterations - install, measure, remove, cut, repeat. No thanks - I want to minimize time with the coil spring compressor. I'm going to try to calculate an answer, cut it once, and get close enough for now. Needless to say, much research ensued. Please see the FAQ at Eaton springs. I used the same equation they list, and checked it several ways before committing to cutting.  

The springs have some fixed values like wire diameter and spring diameter, and some variables like number of springs and the height. I also spent a lot of time trying to figure out what factory spring rates were for original springs as a sanity test for my work. Then I wired up the spring equation in MS Excel and varied the number of coils (and thus the height) to see how much I needed to cut off the springs to get close to by desired spring rate (less than  the competition handling springs, close to GT rates). 

As it happens, cutting a spring raises its spring rate. Cut too much and you'll have a spring that's not only too short but too stiff as well. Both values change as a function of spring length, so I put in a range of values to see what would happen. I used the stock values, the rates I found for original competition handling springs, and some in-betweens.


At a glance, it looks like cutting one coil from my new coils will drop the height by 2 inches (which I want) and increase the rate from 284 lbs/in to 325 lbs/in (which I also want). It also shows that I'd have to cut these new coils down to 12.5" to match the old competition handling spring rates. Finally, I plotted the all these points as height versus rate to make sure no weirdness was afoot.

This also illustrates that when someone says they have "620" springs in their car, they're likely referring to the wire diameter, not the spring rate. I'd have to cut my big-block GT springs in half to get to a 620 lbs/in spring rate.

After all that, we're going to cut exactly one coil. Be patient, spring steel is harder than most. Be sure the cut end is as perpendicular as possible to properly seat against the spring seat tab.

And then one last time with the coil spring compressor. Sure enough, the trimmed springs go in a lot easier, and sit in the spring perch nicely. I won't know for sure until the car is pretty much fully assembled if I got it right or not. Let's pretend it's right.

And I never want to mess with them ever again.


Mark said…
Hello, First off, great blog and you're helping me out as I restore a similar car to yours - 67 GT 390 Fastback with Competition Handling Package. Secondly, using your numbers, I've confirmed I also have the 0.65 x15" springs, the 15/16 swaybar, along with some other CHP goodies. My questions for you; 1) Why did you decide not to try the stock front springs first before replacing them with new ones? 2) Where did you get all the CHP information from, like spring info, stripe colors, and so on? I can't seem to find much info on the interwebs...

I haven't updated it in a while but the URL I listed is where I started a build thread, if you'd like to take a peak! ;)