Hi, I understand this is an older thread, but I wanted to drop in and give my two cents since someone referenced one of my articles above (sways and the spring article).

My Recommendations

The front springs are under-rate for what I perceive to be the motion ratio. (~78%)

Be aware that increasing the front rate (and ride height) will cause additional static and dynamic weight on the front. Adjust accordingly.

Running a fixed height shock for this application is a really mediocre solution. Adjusting the ride height with an adjustable base mount would give you full travel without having to do janky stuff to get the geometry right. This would let you reduce the spring rates considerably.

Tie a small piece of string to the base of the shock shaft, drive around, and then see where the string is now located. If it's at the top, you are beating the shock to death. Increase dampening considerably.

The golden ticket is adjusting the front and rear harmonics to be near each other. Milliken & Milliken found a 20% lower frequency in the front worked well.

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Shock travel is not always limited by the stroke length of the tube itself. There is barely 5" of compression travel in a 8" long spring, nor is there enough compressive force at that motion ratio to even remotely achieve that without trying your hand at the Duke's of Hazard driving school. You have to consider a few things:
  1. Spring Rate & Spring Compression Travel
  2. Shock Compression & Rebound Dampening
  3. Shock Gas Pressure & Design
  4. Force Input Speed & Duration
  5. Unsprung Mass
  6. Tire Dynamics

Tires

When you hit a bump in the road the tire is compressed. The amount the tire is compressed is due to the resistance (damper / spring) and the inertia (corner weight / unsprung mass) acting to deform the tire. How rigid the sidewall is and the load bearing capacity of the tire also have an effect, a rather large one. A taller sidewall, larger load rating, non-reinforced sidewall (or lower plys), and lower pressure all allow for greater tire deformation, which acts to dampen the blow. A stiffer tire will deliver more force and with a sharper duration ("impact" loading).

The force accelerates the unsprung mass (wheels, tires, brakes, etc.) upwards. There are a couple of very vital components at this point, the speed of the unsprung mass and the weight of the unsprung mass. That's because velocity x mass = inertia.

Suspension

So up comes the wheel into the wheel well. The second the force starts to move the wheel the suspension is acting to counter the force. This counter force comes from several sources:
  1. Shock Compression Dampening
  2. Heim Joint Bearing Friction
  3. Spring Force
  4. Sway Bar Spring Force
  5. Suspension Geometry

Initially the upward movement creates a very fast piston travel speed within the damper. This causes the compression dampening to be considerably higher (stiffer) which creates a large resistive force to the unsprung mass traveling upwards. This has a second effect of low shock compression and delivering the load to the chassis. This causes the front to pitch upwards, and ultimately downwards. It is this upward / downward sine curve that the body perceived as the harshness. (It should be noted that rear frequencies are perceived as harsher than frontal frequencies).

Concurrently the coil spring is compressing from the same movement. The linear Hyperco spring above will produce, say 350 lbs. / in. of travel. Meaning, the first inch absorbs 350 lbs of force, and two inches absorbs 700 lbs of force. This force is transmitted to the chassis, but also acts to accelerate the unsprung mass on it's downward rebound travel with the same force, minus losses. Hence why we run shocks. During the rebound moment the piston travels back through the fluid which causes different flows through the valve stack, creating rebound dampening. In this case, the high speed rebound dampening region, which is a high amount of force.

This force travels back and forth several times until the shock (and other losses) overcome the force. This creates a decaying sine wave of body movement (vibration). Both overdampening and underdampening can cause excessive body movement which will be perceived as harshness.

In NVH studies, harshness is measured in Hz and kept to a specific range. Some people may experience discomfort at different thresholds depending on their relative tolerance to it. This is why I don't care much about running 600# springs on my car while my dad enjoys spring rates in the 300# range.

It should be of note here that a "5-inch travel shock" is a very limiting descriptor. If Shock A has very low force dampening, it will cover that 5" very quickly. If Shock B has very high force dampening, then it may not ever use it's 5" of travel.

Furthermore, a 8.00" coil spring with 6 - 0.500" coils only has 5.00" of travel before it is completely closed. At that point, any additional compressive load is transferred to the mountings as an impact and the bending stresses on the suspension immediately ramp up. It should be noted that packers, bump stops, and spring solid height should ALWAYS be used to limit suspension travel to avoid damage to the shock and/or control arms.

Sway bars only really add forces when one side moves in a different direction. So if you hit a pothole, for example, on the left side, it will add spring rate to that corner. The amount of rate depends on the sway bar setup (thickness, material, lever arm length, etc.) It should be noted that offset hits to both axle wheels creates a lot of torsional / weird movements that are perceived as very rough (because they are annoying). A sway bar makes this even worse because rate is quickly shared.

Lastly, you need to heavily consider the angle of the shock as it has an effect on the motion ratio. The further away from 90 degs. it gets the higher the necessary rate.

Suspension geometry affects shock piston speeds and effective spring rates, as well as other dynamic concepts.