Shock Force Calculator figures for a car shock absorber combine spring load and valving force at a set shaft speed, then compare damping ratio with a comfort, sport, or track goal.
Pick your car and the road situation. Open a section below only if you want to fine-tune it.
Corner and Spring
Shaft Speed and Travel
Damper Setting
Use the low-speed compression coefficient from a dyno sheet, or let the tool match your damping goal.
Damper Curve Shape
Most textbooks start at 3:1 rebound to compression. The knee is where the valving opens and the slope changes.
Damper Force Curve
Car Shock Loads From the Shock Force Calculator
The Shock Force Calculator works out the load passing through a car’s shock absorber or coilover at a chosen shaft speed. It adds the spring force holding the car up, the extra spring force from a bump, and the hydraulic damping force from the valving. It then compares the damping with a comfort, sport, or track target, so suspension tuners and coilover buyers can see whether a shock is too soft or too firm before they touch an adjuster.
Quick Setup and Detailed Inputs
The quick setup in the Shock Force Calculator picks a vehicle type, a road situation, and whether the spring is mounted on the shock. Each preset fills in typical values marked “typ.” in the menus, and the four panels below let you replace them with your own corner weight, spring rate, motion ratio, shaft speed, and dyno sheet figures. Any number you type switches the matching menu to Custom, so the presets never overwrite real data.
The US setting works in pounds, inches, lbf, and lbf·s/in, while the metric setting uses kilograms, millimeters, newtons, and N·s/m. The conversions are 1 lbf = 4.448 N, 1 in/s = 0.0254 m/s, and 1 lbf·s/in = 175.13 N·s/m. Weight entered here is the sprung weight on one corner, not the full corner weight on a scale.
Coilover or Separate Spring
On a coilover or strut, the spring sits on the shock body, so the shock mount carries the spring load as well as the damping. Because the shock moves less than the wheel, the static spring force is the corner weight divided by the motion ratio. A bump then adds the spring rate times the extra spring travel, which is the wheel travel times the motion ratio.
$$F_{spring} = \frac{W}{MR} + k \times (x_{wheel} \times MR)$$
With 700 lb on a 0.65 motion ratio, the coil already pushes 1,077 lbf at ride height. One inch of wheel travel into a bump adds 358 lbf more with a 550 lb/in spring. Together with 120 lbf of damping, that puts about 1,554 lbf through the shock mount.
A HybridZ forum member sums up the other layout well, saying a strut bears load and a shock does not. Choosing “No, Separate Spring” drops the spring load from the hero figure, so the shock shows damping force only. It also opens a Spring Motion Ratio field, because a spring mounted elsewhere on the arm moves by a different amount than the shock.
With the spring at a 0.50 ratio, the same corner puts 1,400 lbf into the spring at ride height and 275 lbf more in the bump. That 1,675 lbf goes through the spring seats rather than the shock, which only carries the 120 lbf of compression damping. The spring’s own ratio also sets the wheel rate, so the damping ratio changes along with the layout.
Damping Force Below and Above the Knee
Below the knee, damping force rises in a straight line with shaft speed. Above it, the valving opens and the force keeps rising at a reduced slope, which is called a digressive curve. Jim Kasprzak of Kaz Technologies places this digression point at 2 in/s, which matches the default knee.
$$F_{damp} = c \times v_{knee} + c \times \frac{s}{100} \times (v – v_{knee})$$
Here c is the low-speed coefficient, v is shaft speed, and s is the high-speed slope as a percent of the low-speed one. At 6 in/s with 30 lbf·s/in and a 50% slope, compression makes 60 lbf up to the knee and 60 lbf more after it, for 120 lbf in total. A linear shock with the same coefficient would make 180 lbf, which shows how much the valving shape matters at bump speeds.
Rebound uses the same curve multiplied by the rebound-to-compression ratio, so the default 3:1 gives 360 lbf. Kasprzak notes that most textbooks start at three to one, while OptimumG engineers have published guidelines that work out closer to 2.3 to 1. Kaz Technologies itself prefers more compression-biased damping, so treat 3:1 as a starting point rather than a rule.
Shaft Speed Is Not Road Speed
Shaft speed is how fast the shock body compresses or extends, and it has nothing to do with how fast the car is traveling. Penske Racing Shocks puts body roll and pitch between 0 and 2 in/s. It places heavy braking and quick direction changes between 2 and 6 in/s, and curbs and potholes above 6 in/s.
The road situation presets follow that split, running from 1.5 in/s for body roll to 12 in/s for a curb or pothole. The default expansion joint at 6 in/s sits past the knee, which is why the alert says the high-speed slope is setting the force. Pick body roll instead and the whole result shifts to the low-speed circuit below the knee.
Damping Ratio in the Shock Force Calculator
The damping ratio compares the damping at the wheel with critical damping, the amount that returns the car to rest without any overshoot. The damper acts at the wheel through its motion ratio squared, and the spring acts through its own ratio squared. The Shock Force Calculator scales each one that way before comparing them with the sprung mass.
$$\zeta = \frac{c \times MR^2}{2\sqrt{k \times MR_{spring}^2 \times m}}$$
On the default coilover, critical damping works out to 97.2 lbf·s/in at the shock, so the 30 lbf·s/in compression setting is a 0.31 ratio and rebound is 0.93. An OptimumG tech tip gives about 0.25 for passenger cars aiming at comfort and 0.65 to 0.70 as a race car baseline. With a separate spring at a 0.50 ratio, the softer wheel rate lowers critical damping to 74.7 lbf·s/in, and the same shock reads 0.40.
Valving Needed for Your Goal
The fourth card shows the low-speed compression coefficient that would hit the chosen goal, along with the matching rebound coefficient. For the sport street target of 0.40, the default coilover needs 38.9 lbf·s/in in compression and 116.6 lbf·s/in in rebound at 3:1. That is 30% more compression damping than the current 30 lbf·s/in.
The same corner would need about 24.3 lbf·s/in for comfort at 0.25 and 63.2 lbf·s/in for a track car at 0.65. Switching the Compression Setting to “Match My Damping Goal” lets the Shock Force Calculator pick the coefficient and list the high-speed target as well. The alert box reports whether the current damping is softer or firmer than the goal and what that does to ride and body control.
Reading the Damper Force Curve
The force curve plots compression in blue and rebound in red from zero to well past your shaft speed. A dashed line marks the knee, and a green line with two dots marks the forces at your chosen speed. It reads the same way as a shock dyno sheet, so you can hold it next to a real dyno plot and compare the shape.
Shock Force Calculator Limits and Mistakes
Weight, spring rate, both motion ratios, shaft speed, knee speed, and damping must all be above zero. The motion ratios can go up to 2, the rebound ratio up to 20, and the high-speed slope from 0% to 500%. Wheel travel can be zero, which leaves only the static spring load and the damping force.
The model uses a single corner with a two-slope damper, so it leaves out gas pressure, friction, and heat fade. Real shocks also change with temperature and wear, and the handling result depends on tires, anti-roll bars, and the road. Use the numbers as a setup baseline and confirm them on track or on a dyno.
Input Errors That Skew the Result
Entering the motion ratio upside down is the most common error, since the tool wants shock or spring travel divided by wheel travel. A wishbone at 0.65 entered as 1.54 makes the wheel rate more than five times too high and throws off every damping figure. Check it against the layout presets before trusting the ratio.
Comparing shocks by their valving code numbers is another trap. Roehrig Engineering dyno-tested “six compression, three rebound” shocks from five brands for Performance Trends and found one had over four times the force of another at 2 in/s. Use the coefficient from a real dyno sheet instead of a stamped number.
Using the full scale weight in place of sprung weight overstates the static spring load and the critical damping. Subtract the wheel, tire, brakes, and part of the links first with the Unsprung Weight Calculator. Take the starting corner figure from a real scale reading, such as one entered in the Corner Weight Calculator.
Shock Absorber Questions From Tuners
How much force does a car shock absorber produce?
Damping force depends on shaft speed and valving, so there is no single number. At the defaults, a sport coilover makes 120 lbf in compression and 360 lbf in rebound at 6 in/s. Add the spring it carries and the total through the shock mount reaches about 1,554 lbf in a bump, or about 1,010 lbf once it is scaled back to the wheel.
What damping ratio should a street or track car use?
OptimumG gives about 0.25 for passenger cars tuned for comfort and 0.65 to 0.70 as a race car baseline. The tool’s sport street goal of 0.40 sits between the two for firm road cars. A ratio of 1.0 is critical damping, which settles without overshoot but feels heavy and can make the tire skip over sharp bumps.
What rebound to compression ratio should I start with?
Most suspension textbooks start at 3:1 rebound to compression, as Kaz Technologies points out. Published OptimumG guidelines land nearer 2.3:1, and Kaz itself leans toward more compression-biased damping in its own work. Start at 3:1 in the tool, then compare the rebound damping ratio in card 3 with your target before settling on a number.
Does the shock absorber hold up the car?
Only when the spring is mounted on it, as with a coilover or a MacPherson strut. A separate shock with its own spring elsewhere carries only damping force, and the car stays up on its springs if you remove it. Set the spring mount menu to “No, Separate Spring” and the calculator moves the spring load to card 2 and shows damping alone as the force through the shock.
What is the difference between low-speed and high-speed damping?
Low-speed damping covers shaft speeds up to about 2 in/s, where body roll, pitch, and weight transfer happen. High-speed damping covers fast shaft movement from curbs, potholes, and sharp bumps above roughly 6 in/s. The knee setting in the Shock Force Calculator marks where one circuit hands over to the other, and the valving shape decides how steeply force keeps rising after it.