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Coil Spring Tech
There is a wide variety of lowering springs out there to choose from so how do you know what is right for your application? Eibach, H&R, Ford Motorsport, Steeda, and a plethora of other companies offer springs with unique spring rates, spring heights, wire diameter, wire spacing, number of turns per coil, coil diameter, and coil material. All these factors are related by the following equation which is used to calculate the spring rate in units of force per unit length:
K = Spring rate (lbs/inch)
W = Diameter of the wire (inches)
G = constant 12,000,000 for steel springs (psi)
N = Number of active coils
D = Diameter of the coil spring measured from center of wire to center of wire (inches)
A spring will be compressed when a load is applied to it. The distance it is compressed (or deflected from equilibrium) for that given load is referred to as the spring rate. Spring rate is specified in terms of force applied per distance of deflection from equilibrium. Typically, the units of spring rate are pounds (force) per inch.
Here are the numbers for a stock 87-93 V8 Mustang:
*Note: Factory spring rate varied slightly depending on body style, transmission type, and whether or not the car came equipped with AC. The stock springs are progressive rate, so the following spring rates are an estimate based on taking measurements in multiple locations on the coil spring and obtaining an average.
Front Springs
Rear Springs
W
N
D
K
W
N
D
K
0.66
8.75
4.24
426 lbs/in
0.55
7.5
4.625
185 lbs/in
Progressive vs Specific Rate:
A linear or specific rate spring has a given rate of resistance that is constant. If a spring has a rate of 450 lbs per inch and 900 lbs is sitting on it, that spring is compressed 2.0 inches. A progressive spring, on the other hand, has more than one single rate on that spring. Many springs are progressive rate because it allows the suspension to be softer initially (making ride quality better) but stiffening up as the spring compression increases. This allows the spring to be fairly soft at smaller compression while making it stiff enough at bigger compression to accurately control wheel and suspension movement under extreme load. Progressive springs usually allow the car to ride & handle better if they are properly designed, but the added complexity to designing and matching them to proper dampers raises the cost.
Here the two springs on the left are progressive while the spring on the right is specific rate.
Cutting Springs:
Cutting off a portion of the coil will reduce the free height of the spring. The reduction in the unloaded height can sometimes pose a problem with the spring remaining in the spring perch under suspension rebound. This is usually not an issue with stock springs since they are tall (they have a relatively low spring rate and thus must be tall in order to support the vehicle). Cutting aftermarket springs is not recommended because most of them are already so short that having the spring dislodge itself from the spring perch becomes a risk.
Most aftermarket companies who make lowering springs which reduce the vehicle ride height normally couple the shorter springs with a proportionate increase in spring rate to offset the reduced bump travel. The spring compression due to a specific load needs to be decreased enough to make sure the suspension does not bottom out since it has a shorter travel distance before coming in contact with the fender or any other vehicle components. A properly designed lowering spring will have a rate increase sufficient enough to offset this bump travel distance reduction. When cutting stock springs, don't over do it! Stock springs, even when cut, do not come close to matching the spring rate of the average aftermarket lowering spring, as I will illustrate below. If you lower your car by cutting your stock springs, chances are that your bump travel will be greater than with a high rate lowering spring.
Cutting springs also generates heating of the material which may surpass beyond its temper and can sometimes affect the composition of that material. A hack saw or dremel with a cutting wheel may be used, but be sure to take breaks and not allow the metal to get too hot.
Also, when installing any type of coil springs that have a higher spring rate than stock, be sure to also include a strut/shock combination that can handle the increased rate of the springs. This is a major reason of why cutting springs has a bad reputation. I would venture to say that the majority of people who cut their coil springs (thus increasing their spring rate), try to get away with using their stock worn out dampers. These are the same people that complain of poor ride quality and you see them bouncing down the road on the freeway because their worn out dampers cannot compete with the increased rate springs.
If you are against cutting the stock coil springs and would like to purchase aftermarket lowering springs, there are a few things to consider. First, if you want something that will give you a lower stance, improve handling, but you don't want a significant increase in spring rate, I recommend the: Eibach Prokit or Ford Motorsport "B" springs (both having a 425-525lbs/inch progressive rate). Both of these springs will drop you about 1-1.5" depending on your vehicle options and the type of spring isolators you are using (Poly has been known to raise a vehicle compared to the stock rubber isolators, and using no isolators can drop you as much as 1/4 inch compared to the stock rubber isolators, but I don't recommend it due to vibration and noise).If you are looking for something stiffer, there are a lot of guys using H&R Sports (490-575lbs/inch progressive), H&R Supersports (600-720lbs/inch progressive), Steeda Sports (650lbs/inch linear), or Ford Motorsport "C" (650lbs/inch linear).
If you are looking for something even stiffer for road racing, contact H&R or Steeda who both manufacture competition springs that range from 700lbs/inch all the way to over 1000lbs/inch.
For the rear springs, in my opinion almost all the spring manufacturers make rates that are too high for a solid live axle foxbody rear suspension. I know more than one autocross racer that uses stock cut rear coil springs with success, and this preserves ride quality compared to installing 250-300+ lbs/inch rate coil springs.
I should also note that shock/strut selection plays as big a roll in ride quality and handling as the springs themselves. I have ridden in Mustangs that have 700-800 lbs/inch springs with Bilstein shocks and struts that actually rode surprisingly well.
Brand new coil springs will also settle a tiny bit, but don't expect to ever install springs and have them drop by more than 1/16inch or so... In my experience brand new springs don't settle very much at all after use.
Make sure that if you install new springs and disconnect the A-arm from the K-member that you torque those bolts with the front suspension loaded, or you can create a condition where the rotation of the bushings will actually raise the vehicle.
One other thing I should mention before I get to the tech write-up is that the popular 01-02 Bullitt springs, which have a 600lbs/inch front rate and a 250lbs/inch rear rate, will only lower a foxbody Mustang about 3/4" all the way around, so be aware of that if you are particular about ride height.
Here is the tech article for installing springs:
This is the procedure I've used to change front springs in a Fox-body Mustang without the use of a spring compressor, which due to packaging of the front end components is often a source of frustration. And unlike other methods that require disconnecting the struts and actually prying the springs in and out, this one lets the spring completely decompress so that no unsafe prying is required.
Bear in mind that compressed springs contain a lot of stored energy. I am documenting the steps I took to perform this operation and although I am comfortable performing this task you should know that as with any job if you do it you do so at your own risk.
Required tools:
- Floor jack
- a 2nd jack, not necessary but helps
- 2 Jackstands
- Crowbar
- A good socket wrench is preferred, with the following sockets mandatory, box wrenches a less attractive alternative:
- 21 mm deep
- 24 mm deep
- 15 mm deep
- penetrating oil
- a "persuader" made of a roughly 12-inch length of roughly 2-inch pipe. Slipped over a wrench and used to effectively lengthen it, it is invaluable when it comes to applying real torque to really tight bolts.
- a small hammer
- a medium sized flat blade screwdriver
Now on to how I've done the install:
Block the rear wheels, raise the front, and place the car on jackstands. Keep in mind you'll want all available floor space to place the jacks under the inner side of the A-arm so the stands will need to be pretty close together and you don't want the car close to any walls or other cars.
Remove the front tires. Undo the lower swaybar end links (15 mm deep).
Spray some penetrating oil on the nuts and bolts that attach the inner end of the A-arm to the K-member.
Loosen the inner a-arm bolts but do not remove the bolts yet. You won't be able to, but don't even try. Breaking these loose will require a lot of torque on the 24mm. The bolt on the other end is 21 mm, put a wrench on that for leverage.
With the nuts loose, place a floor jack at the inside lip of the A-arm, between the mounting ears.
Raise the jack until you can see the tension taken off one of the bolts (probably the rearward one first). Punch the bolt out with a small hammer and jockey with the jack until you can remove the bolt. You may want to insert the screwdriver to keep things roughly centered while you remove the other bolt, just don't forget to remove the screwdriver before lowering the assembly.
Place the second jack so it will raise the other ear of the arm while you position the rest of the arm with the first jack. The arm will bend a bit if unsupported so you need the second jack for the second bolt.
Notice that the spring is in a perch and can't squirt out. Notice it's in a perch at the top too. Notice that when you lower the jacks, the spring will decompress vertically and that thanks to the strut and the tie rod still being attached it is very unlikely the spring will ever squirt out. But don't have your head in the wheel-well nonetheless and if you're the real nervous type tie it to the K-member with a piece of rope or chain. Lower it until the spring is fully decompressed, which will happen well before the jack gets to the bottom of its travel.
With the jack lowered and out of the way, swing the A-arm to the rear and the spring will practically fall out.
Cut the insulator off the bottom lengthwise so it will come off, then wind it onto the bottom of your new spring.
Clean off the crud out of the lower spring perch.
Position the spring so the bottom end (the one that isn't flattened) ends up between the two holes in the spring pocket. Position it in its pocket at the top, which may require raising the jack a bit just to hold it in place, and then get the jack positioned so the A-arm is roughly lined up.
Raise the jack and guide the A-arms into place. A little WD-40 type stuff on the pockets in the K-member eases the positioning process.
When you get at least one of the ears situated roughly (probably the rearward one), you'll probably have to pry on the ear out a bit to get the bolt holes to line up. Insert the prybar from the bottom between the K-member pocket and the ear on the A-arm and you can move the arm in and out pretty easily. By being precise with the jack and prying a bit if necessary you'll be able to slip the bolt right through and seat it. Place the nut on it for safekeeping but don't tighten it down yet.
Place the second jack under the other ear just like you did to remove its bolt, and positioning the jacks and wiggling things around as necessary, pop the second bolt through and install the nut but don't tighten it.
DOUBLE CHECK that you haven't bumped a jackstand out of the way in the process. It's easy to do because raising the A-arm will probably lift that side off the stand and it's easy to bump it out of the way when you move the jack around. So easy you might not notice you moved it so trust me on this, before you lower the jacks, double check the placement of the jackstands.
Lower the jacks and put one under the balljoint and raise it until the assembly simulates ride height position. This is so the bushings get torqued down in their natural position (If you don't do this you'll be "preloading" the bushings when at ride height). This will probably raise that side off the jackstand a bit again but that's OK, leave it right where it is for safety. Tighten the nuts down now. I don't have a torque reading but suffice to say it's tight as hell.
You're about 1.5 hours into the job and you're done with that side.
Don't replace the swaybar end link nuts until you finish with the other side.
Now do the other side as outlined above. Reattach the swaybar endlinks when you're done. You may have to pry on them a bit to give yourself enough clearance. When all is said and done you should be about 3 hours down with the whole front done.
NOW FOR THE REARS
Block the front wheels and raise the rear end. Leave the floorjack under the diff, and place the rear end on jackstands just ahead of where the lower control arms connect.
Remove the rear wheels.
Undo and remove the swaybar (15mm Deep)
Undo the lower shock mount, fasteners vary but it may be a Torx and it may be a 15mm 18mm combo.
Remove the quad shocks.
Lower the jack so the axle drops as far as you can without pulling the rear brake flex line.
Now put a second jack under one end of the axle and raise that end as high as you can.
You can now probably just pluck the low-side's spring out. If not, have a helper stand on the low end of the axle to help it the last little bit and pull the spring out, it's not under any compression at this point.
Swap isolators to the new spring, orient it so the flat side is up and the bottom pigtail points to the drivers side (for BOTH springs).
Lower the high-side jack, make sure the new spring is seated properly, and repeat the process for the other side.
When you're done, raise the diff back up, reconnect the quad shocks and shocks. Re-attach the swaybar, reinstall the wheels, lower the car and you're done in way under an hour.*Installation procedure courtesy of Frasier Elliott (MFE)
If you have any questions, please feel free to contact me at:
