1887 Mustang Rear Suspension Diagram
4 Link explained further: Your stock Mustang comes with a brilliant design called the 4 link. I explained its flaws above and oft referred to as the "quadra-bind" rear suspension. The real fix is a package kind of thing, as there are too many jobs that the 4 link is trying to do. If you're not going to modify your suspension all at once, the rear is much more important to modify than the front; the front may be bad, but the rear is horrible. Maximum Motorsports measured 2" of lateral axle travel with stock suspension. That's the wiggle you feel in the rear end under hard braking coming into a corner. There are other options than the torque arm mentioned above, including but not limited to the decoupled torque arm, tri-link, and 5-link. The torque arm is the most used by road racers, but it does have it's detractions. For one, it's heavier than some of the other options, and for two it sacrifices braking stability for stability under acceleration.
Lower Control Arms: Lower control arms (LCA's) are the heart of your rear suspension. They hold the axle in place and transfer the majority of the energy to your car's body. The stock ones work well, they are just weak. New ones will firm up the rear end, making for better transfer of energy to your chassis, less axle bind and less windup. Several designs of LCA's exist and there are advantages/disadvantages to each when talking about aftermarket ones. For the following LCA information, we are ONLY referring to the solid axle applications here and NOT the '99~'04 Cobra IRS:
solid non-adjustable poly/poly LCA's:
These are hard on torque-boxes and detrimental to performance cornering of your ride. Made of round or squared tubular chromoly or aluminum, they have a polyeurothane bushing at each end. Such an arm offers less "give" than OEM arms found on the Mustang. They lessen the likelyhood of wheel hop and can allow the removal of the quad-shock found on '79~'04 rides. The quad-shock is in small part a contributer to rear end suspension geometry issues on the Mustang 4 link, so its removal can be of some benefit. The above design is also available in an adjustable length, although not as common. Those can be used to "fine tune" the axle center to help make room for larger drag tires and possibly avoid some rubbing. There are also disadvantages to the above design when it comes to road noise. Noise Vibration Harshness (NVH) is a trade-off for suspension upgrades and the above arms will add some. The bushings and arm itself is much less compliant than the OEM one, so additional sound will be heard inside your car. I cannot tell you exactly how much NVH you'll get, but WILL get some. You will get a bit more with the following types of arms, but much improved oporation of your rear suspension.
Solid poly/spherical LCA:
Adjustable poly/spherical LCA:
With a spherical Heim joint, these relieve some of the bind found on the '79~'04 Mustang suspension. The spherical end will not only allow the LCA to rotate freely lengthwise with axle articulation, but also add a bit more NVH than poly/poly design. The spherical bearing can be placed on either end, but I prefer the torque-box or body end. There are several other designs that lean more toward racing and differences in strengths and such. For the purposes of this post, I will not get into all the differences as this is really geared to fundamental issues reguarding suspension operation. The poly/spherical LCA is a big step in relieving some 4 link bind, but is less than 1/2 way to ridding your Mustang of its rear-end woes (see 4 link / suspension bind above)
The Jazzer approved LCA list:
OK, due my pepes always asking which ones would be best for his/her application, I have decided to make a list which is by NO MEANS complete. I do not want to push any particular company, but in order of what I consider a good design, I will list some of the most easily attained LCA's of which I am aware today. I will try my best to keep the list updated, but time will go by and will undoubtedly fall behind the times at some point.
The following is a list of subjective preferences, so if you don't agree, just buy something else instead. Here are LCA's I like and listed in order of a general purpose one for a DD (lighter duty requirements) and nice geometry, to ones that will work for DD, but also very strong for 1/4 mile applications:
1. UPR Chrome moly: ('99~'04) Preferred* ~ adjustable for DD with cornering and not my first choice for 1/4 mile use ~ some NVH
2. MM HD's: ('99~'04) Less preferred* ~ stronger than UPR above and some 1/4 mile use without DR's ~ a bit less NVH
3. MM XD's: ('99~'04) Preferred* ~ stronger than the above listed HD's for DD and DR's on 1/4 mile ~ LOTS of NVH
4. TeamZ: ('99~'04) Preferred* ~ adjustable
VERY strong, DD and 1/4 mile with DR's ~ LOTS of NVH.
*I prefer that if an LCA have a non-spherical bearing, it be on the axle end. This will create more NVH than the reverse, but I happen to like the relationship between an OEM type rear sway-bar and plate in which it mounts.
UCA's = HORRIBLE LITTLE CREATURES & THE BANE OF A 4-LINK SUSPENSION *
(see PHB/WL & TA below)
Upper Control Arms: Upper control arms (UCA's) are the wonder twins of your rear suspension. They are simply a couple a steel pieces that connect at a funny angle to your upper differential casing. In normal driving conditions, they are fine, but when at the limits, they are plain dangerous. They bind under hard turning making your tire grip unpredictable, they help limit, but in no way prevent, axle wind up. They also contribute greatly to what is oft referred to as "snap-steer" (< bad thing!). With the installation of a WL or PHB and TA, these are COMPLETELY REMOVED to rid oneself of the horrible bind created by such a pair of hideous beasts. This binding is hard to explain, but take a look at the image below:
Imagine the movement of both sets of control arms, but focus specifically the UCA's. When the axle articulates (driving at an angle up a steep incline) imagine what position you will see the UCA's in such a case! There is NO WAY IN HELL they will move freely to a position which is asked of them. They will go a very short distance, until they must flex via poor structural strength and soft rubber bushings. When UCA's are pushed to the limit around a high-speed corner with lots of body roll, this binding can result in snap-steer and the possibility of losing control goes way up. There are a couple of solutions in which to address this binding issue and my particular preference is with the installation of two parts. These would be either a WL or PHB with TA and will do the entire job of the UCA's in a FAR, FAR, FAR better way. Now... imagine the scenario described above with a set of solid poly UCA's installed (seen directly below) in their place and the resulting effects seen in the following picture 
solid poly UCA's:
This design should NEVER be used in the desire to gain better handling of your ride! They are completely inflexable and greatly increase the suspension bind already inherant in the Mustang 4 link rear suspension. They are generally a solid arm and not even adjustable for pinion angle, so less than ideal for even a 1/4 mile ONLY application as well. There are adjustable poly UCA's if one does NOT DAILY DRIVE HIS/HER RIDE , but only wants the ability to address pinion angle. Short of going Panhard bar or Watts link (see below) this is another location I like to see a spherical end (see below for further details, if considering aftermarket UCA's).
Below is a damaged poly boxed UCA's with OEM rubber bushing on axle end:
This end has been "pryed" open via articulation of the rear axle on a DD ride. This is due to arm having NO GIVE with only a little between the poly and OEM rubber bushings. It was forced open and eventually destroyed them, due to the normal events of the DD car. If the arms do not break... the mounting connections of your ride will.
UNLESS YOU LIKE THE UCA ABOVE... DO NOT USE POLY BOXED ARMS ON YOUR RIDE! Adjustable spherical UCA:
Like the LCA's above, these UCA's have a spherical (Heim jointed) end. This "eyeball" socket allows the arm to rotate multiple directions simultaneously. Combined with the poly/spherical LCA's above, this is a major relief to the rear axle bind of the 4 link * . For the next step in solid rear axle (SRA) cornering performance, one must forgo the UCA's altogether and can go with a Panhard bar or Watts link and torque-arm. There are also some other designs, such as the tri-link or 5 link conversion, but will not get into those here.
* Even though this is a major improvement for freedom of rear axle, it is my understanding that such a UCA can allow undesired movement at extreme end of axle articulation. For this reason, I recommend that one invest the $$$ to go PHB or WL with TA for improved performance in ALL aspects of your ride. This will greatly improve the function of your rear suspension under hard cornering and MAJOR improvement in your roll-axis (RA) Oops..... I said I wasn't going to talk about roll-axis, my bad!
*WL or PHB with TA = THE 4-LINK... SAVED!
Panhard bar / Watts-Link:
The Panhard Bar (PHB) or Watts Link (WL) are two items designed to take over one job of the 4 link. They bolt to your frame and connect to your axle with either a single pivoting bar or 2 bars connected to your differential housing. They both align the axle with the middle of your car. This prevent the rear axle from ever moving out of alignment and eliminates side to side axle bind. This means you can drift or turn hard without having to worry about the rear end skipping off the road and ruining your traction. When both the PHB or WL along with torque-arm (TA) are installed, you will lower what is known as the roll center to improve roll axis and is a good thing. Again, as mentioned at top of this page, we will not be getting into this specifically, but can research the subject if you like Google roll center. The '05+ (S197) models have a factory Panhard bar and control arm installed. This control arm is an improvement over the UCA's in the previous 4 link design, but does not offer the geometry improvements of a good torque arm.
Maximum Motorsports Panhard Bar:
Watts Link of Fays2 (one of two different basic designs of the WL):
Torque Arm: A torque-arm (TA) is a piece that takes away the second job of the 4 link. It bolts to the housing of your differential preventing axle wind up entirely. It also runs parallel to your driveshaft and is welded to your subframe connectors via cross-member. If you install it and the panhard bar or Watts link, you can remove your upper control arms completely. This will save you a decent chunk of weight and give you a stiffer yet a more freely moving suspension along with improved roll axis. This leads to better wheel rates and solid transfer of energy to the wheels and chassis. Torque arms not only increase the ability to turn and accelerate, it also moves the connection point of the rear wheels to the body, making your car actually stop faster too. Instead of causing the car to dive as much, it keeps the car more level allowing your rear tires to do more of the work than stock.
Maximum Motorsports TA:
Griggs Racing severe-Duty TA:
Additional benefits of a torque arm in terms of braking
The installation of a TA changes the rear suspension geometry in such a way as to defer much of the braking load to the center of your car (simple explanation). This change, allows you to move from what is approx. 70/30 in front/rear braking bias, to somewhere around 60/40 (lots of variables here and depends on some other factors) To do this, one needs to replace the pre-set OEM brake-bias valve with a mechanical proportioning valve. You will also need to do a major upgrade of rear brakes, as the OEM ones cannot provide enough stopping power to make any significant difference. Will not get into any more detail here, but I have done such an upgrade and my brakes .
See: Upgrade Brakes ~ Speeding up slowing down
Bump steer: This is the tendency of a wheel to steer the car as it moves upwards over a bump and can be an issue when a car is lowered. This doesn't seem to come in to play until you have a 1.5"~2" drop. With a 1.5" drop, however, this is only noticeable in pretty extreme suspension travel. Something one would be close to upon bottoming out the front suspension.
Below is a drawing illustrating the use and purpose of the bump steer kit. The red lines represent the steering rack (center of each image) and tie-rod ends (connecting to each wheel). This assembly operates best when all three parts are in the same line and best to be in this position driving down the street. The first is the stock set up and the second one is lowered. As you can see, the "A" arms and end-links are already at an angle. When you go over a bump, you will force them even more verticle, this will max out their range, and the car's front tires will toe inwards. Meaning, they will both turn in towards the car, if you happen to be turning at this time, you will lose the ability to turn at least one of the tires.
The final picture is a lowered car with the bumpsteer kit. The kit as you can see is blue, and corrects the angle, putting the end links, back in parallel with the road, giving you desireable handling again.
Wheel Rate: The force it takes for your tire to move up and down. This means how quickly your tire will move into the bumps on the road. You calculate it by the spring rate multiplied by the leverage. In the case of your stock front suspension. Your spring is roughly 1/3 - 1/4 of the way from the pivot point to the wheel. This means you have a 1/3 to 1/4 mechanical advantage on your wheel. For example: With 800 lbs/in springs, the wheel rate is closer to 200-250 lbs/in. and my force on the bottom of my car is closer to 2/3 - 3/4 advantage. Meaning I feel 3/4 of the bump, and have 1/4 the wheel reaction time it could. This is the main reason coil-overs (CO's) are so much lower in spring rate. A 300 lbs spring will get 90% of its force on the tire and the same amount on the car. This means you will feel roughly half the force on the car and gain 50% more force on the wheel. This leads to better handling and more comfortable ride. There is no real downside to going CO's up front other than cost.
"K" Member / "A" arms: Your "K" member is the most important part of your front suspension. It is to which your engine, struts, "A" arms and sub-frame all connect. Your only choice, asside from OEM, is a tubular set up and usually requires tubular "A" arms and upgrade to coil-overs as well. The stock "A" arms can be used, in some cases, but are flexable and introduce some torsion issues between the mounting point, spring itself and strut. The stock "A" arms are also much heavier than tube ones and offer limited access for working on your car. For this reason, I recommend one goes all out at this point to get the most of the tube "K" upgrade. This can be a large change and a nice chunk of cash all at once, but definitely worth it. A new "K" member will give you improved suspension geometry and leads to better corner loading. It also adds stiffness to the front end with improved clearance for engine maintenance and the installation of turbo's and upgrades to exhaust. It is a big mod and takes some time, but a worth while one in the end.
Maximum Motorsports tubular "K" member:
Maximum Motorsports "A" arms (non-adjustable and no spring perch):
Caster/camber/toe: Caster, simply put, identifies the forward or backward slope of the wheels if a line were to be drawn between the lower ball joint and the location of pivot point above (center of strut on your Mustang). Camber identifies the angle of a wheel relative to vertical and can accelerate wear on the inside or outside of tires edge depending on the degree of camber applied. Negative camber is a wheel that is tilted inward on the top toward the car. Positive camber is the top of a wheel tilted outward and away from the car:
Caster:
Both of these can be adjusted with caster/camber plates (see below), but OEM ones do not allow much adjustment with the Fox Body and SN95 Mustang. Some negative caster can be applied to allow an improved contact patch upon turning a corner with wider than OEM tires. Some negative camber can be applied to the front wheels to improve grip around a corner, but will wear the inside of tire at a faster rate due to the permanently fixed contact patch which is not flat to the road surface. How much wear is directly related to the degree at which negative camber is applied. Some of those more interested in carving a corner will apply .5�~1.5� (race cars will go higher) to the front wheels in order to improve grip around a turn. To learn in much better detail, head over ***HERE***
Some good info from ReverendDexter on this subject:
Camber: Camber is the angle of the tire leaning inward toward the chassis (negative) or outward away from the chassis (positive). To get the most grip, you want the tire as flat against the pavement as possible as this gives you the largest contact patch. However, most suspension geometry doesn't keep the tire flat at all points of suspension travel, so you have to pick where in that travel you want the tire to be flat. Your typical A -> B driver wants that at straightline, as this gives the best braking. A corner-carver needs that extra grip when there's a bit of body roll (i.e. mid-corner), so they will lean the tire in so that it becomes flat against the pavement when it's the outside tire in a corner. Camber is going to be the most variable setup, as there is no static ideal - the ideal is going to be different for every different car on every different corner.
Caster: Caster is the angle of the axis about which the tires steer. By increasing the amount of caster (leaning the top of that axis rearward), you increase straightline stability at the cost of steering angle and steering responsiveness. HOWEVER, with a McPherson strut setup you lose negative camber under compression (a very bad thing); a large amount of caster can be used to fake in dynamic camber gain (SLA suspensions naturally have dynamic camber gain, which is why Miatas and Civics hand a stock Mustang it's *** in the corners).
Toe: angle of steering already dialed into the tire. If you look at just one side, toe in has the tire angle slightly "behind" where the steering wheel is pointed when you're turning the steering wheel into a turn on that side, and and slightly "ahead" of the steering wheel when you're going back to center. With both tires this way, it causes the car to "want" to be going straight, and resist going into turns, which is why factory alignment settings have a little toe in. Toe out will give you the opposite effect; the car will "want" to turn, but will resist straightening out the wheel. That's why qualifying setups for road track cars and autocrossers will generally run a hair bit of toe out; it sacrafices some straight line stability for better turn-in.
Source: https://mustangforums.com/forum/suspension/553765-the-official-suspension-guide.html
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