Wheel Rate Calculator converts spring rate through the motion ratio squared and the spring angle, then adds the tire in series to find ride frequency and the spring a target needs.
Sprung weight is the corner weight on the scale minus the unsprung parts, such as wheel, tire, brakes and part of the arms.
Motion Ratio and Spring Angle
Motion ratio is spring travel divided by wheel travel. Measured from arm lengths, it is the pivot-to-spring distance over the pivot-to-ball-joint distance.
Tire and Target Frequency
The tire acts as a second spring in series, so the body sees a little less than the wheel rate.
Wheel Rate Calculator Basics for Spring Swaps
The Wheel Rate Calculator works out how stiff a spring really is at the tire once the suspension’s leverage and the spring’s lean are taken into account. It then adds the tire as a second spring to find the ride frequency, how far the corner sinks, and which spring rate would hit a target frequency. Coilover buyers, autocrossers, and track day drivers can use it to compare springs by what the car actually feels instead of by the number on the coil.
Spring, Corner Weight, and Units
The quick setup takes the suspension layout, the spring rate, and the sprung weight on one corner, each with a preset menu and a Custom option. Sprung weight is the scale reading minus the wheel, tire, brakes, and part of the arms, because only that weight sits on the spring. The presets marked “typ.” are starting values, so enter your own figures when you have them.
Spring rates come in three unit systems, and the Wheel Rate Calculator handles all of them. The conversions it uses are 1 N/mm = 5.710 lb/in and 1 kg/mm = 55.997 lb/in, with weights in pounds or kilograms and lengths in inches or millimeters. Every result is shown in the unit system you pick, and the hero line also lists the wheel rate in the other two.
Why the Motion Ratio Is Squared
Motion ratio is how far the spring moves for each inch of wheel travel. Penske Shocks gives wheel rate as the spring rate times the motion ratio squared, so a 400 lb/in spring at a 0.7 ratio works out to 196 lb/in at the wheel. The ratio is squared because the leverage cuts both the travel the spring sees and the force it passes back to the tire, as members of the Grassroots Motorsports forum work through step by step.
$$K_w = K_s \times MR^2 \times \cos^2\theta$$
Here Ks is the spring rate, MR is the motion ratio, and θ is the spring’s angle from vertical. With a 550 lb/in spring at a 0.65 ratio and a 10° lean, the Wheel Rate Calculator gives 225.4 lb/in. That is less than half the spring’s rating, even though nothing on the car has changed.
Measuring the Motion Ratio
The ratio can be typed in directly or worked out from two arm measurements. Hyperco measures from the control arm pivot to the point under the spring, then from the same pivot to the ball joint, and divides the first by the second. The defaults of 10.4 in and 16.0 in give the same 0.65 ratio as the wishbone preset.
A LocostUSA forum builder took the other route and measured travel directly, getting 2.6875 in at the shock for 4.1875 in at the hub, a ratio of 0.642. Speed Academy found that the ratio can also change through the stroke, reading 0.65 at ride height on one car and rising to 1.00 by 3 in of travel. Measure near ride height for the most useful number, and remember that the result is a snapshot of one position.
Three Ways to Correct for Spring Angle
A leaning spring pushes partly sideways, so only part of its force holds the corner up. A Honda-Tech forum thread quotes an Eibach formula that applies a single cosine of the angle, while Speed Academy reports that the consensus is to square the angle factor just like the motion ratio.
Hyperco builds the angle into the motion ratio before squaring it, which gives the same result as the squared method.
The Angle Correction menu lets you pick the squared method, the single cosine, or no correction at all. At 10° the three give 225.4, 228.8, and 232.4 lb/in, which is close enough that the choice rarely matters. At 20° the squared method gives 205.2 lb/in against 218.4 lb/in for the single cosine, which is a 6% gap worth checking before buying springs.
Ride Frequency and Spring Choice
The four result cards turn the wheel rate into numbers you can use when choosing springs. They show how much of the spring reaches the wheel, how fast the body bounces, how far the corner settles, and which spring would hit your target. The alert box under them says whether the current spring is above, below, or on the target.
Share of the Spring That Reaches the Wheel
The first card of the Wheel Rate Calculator shows that 41.0% of the spring rate reaches the wheel at the defaults. Leverage takes away 57.8%, and the 10° lean costs a further 1.3%. That split shows at a glance whether the motion ratio or the spring angle is doing most of the damage.
Ride Rate and Frequency
The tire flexes too, so the body rides on the wheel rate and the tire rate working in series. The ride rate is the product of the two divided by their sum. The body’s natural frequency then follows from that ride rate and the sprung weight.
$$f = \frac{1}{2\pi}\sqrt{\frac{K_{ride} \times g}{W}}$$
At the defaults the ride rate is 188.4 lb/in and the frequency is 1.57 Hz, with g taken as 386.1 in/s². Ignoring the tire would push that to 1.71 Hz, so the tire matters more than it seems. The Eibach version of the same formula quoted on Honda-Tech gives cycles per minute by multiplying 187.8 by the square root of wheel rate over sprung weight.
An OptimumG tech tip by Matt Giaraffa puts passenger cars at 0.5 to 1.5 Hz, sedan race cars and moderate downforce formula cars at 1.5 to 2.0 Hz, and high downforce race cars at 3.0 to 5.0 Hz and up.
The second card labels your result with the matching range, so 1.57 Hz reads as the sedan race car range. An F1technical forum engineer notes these ranges describe where cars tend to end up rather than rules every car must follow.
Corner Sink Under Load
The third card shows how far the corner settles from unloaded to ride height, which is 3.98 in at the defaults. It also gives the spring’s own compression at rest, 2.13 in, and the extra sink for every 100 lb added to the corner, 0.53 in. Those numbers help when setting ride height on a coilover or checking how much a full load will drop the car.
Spring Rate for a Target Frequency
The fourth card works backward from the target frequency to the spring rate that would reach it. The default 1.5 Hz target needs a 495.4 lb/in spring, which is 10% softer than the current 550 and rounds to a common 500 lb/in step. The same corner would need about 298 lb/in for a 1.2 Hz comfort target and about 1,021 lb/in for a 2.0 Hz track target.
Hyperco advises choosing the lower rate when you are between two sizes, since dampers and anti-roll bars can add support later. The OptimumG tech tip also recommends running the rear slightly higher in frequency than the front so the car pitches less over bumps. Run the Wheel Rate Calculator once for each end of the car and compare the two frequencies before ordering.
Wheel Rate Calculator Limits
Spring rate and sprung weight must be above zero, the motion ratio must sit above 0 and no higher than 2, and the spring angle must be under 80°. The tire rate can be zero to ignore the tire, and the target frequency must be between 0 and 10 Hz. If the target needs a ride rate stiffer than the tire itself, the tool warns that no spring can reach it.
The math treats one corner on its own, with a single motion ratio and no bushing or anti-roll bar stiffness. Real suspensions change ratio through their travel, and roll stiffness comes largely from the bars rather than the springs. The damping side of the same corner is covered by the Shock Force Calculator, which uses the wheel rate to set a damping ratio.
Input Mistakes That Change the Rate
Using the full scale weight instead of the sprung weight lowers the frequency and makes the target spring too stiff. Start from a real scale reading in the Corner Weight Calculator, then subtract the parts below the spring with the Unsprung Weight Calculator. Hyperco puts typical unsprung corner weight at about 70 to 120 lb, which is a large share of a small car’s corner.
Applying an angle correction to a ratio measured from real spring and wheel travel counts the angle twice, since the measurement already includes it. Choose “None, Ratio From Travel” in that case. Entering the installation ratio, which Penske defines as the inverse of the motion ratio, turns a 0.65 into a 1.54 and inflates the wheel rate more than fivefold.
Mixing up kg/mm and N/mm is another easy slip, because spring makers label their rates in different units. One kg/mm is about 9.8 N/mm, so typing a kg/mm figure into the N/mm setting leaves the spring roughly ten times too soft. Check the unit on the spring label before entering it.
Wheel Rate Questions From Tuners
What is the difference between spring rate and wheel rate?
Spring rate is the coil’s own stiffness measured on a spring tester, while wheel rate is the stiffness felt at the tire after the suspension’s leverage and the spring angle are included. Because the motion ratio is usually below 1.0, wheel rate is almost always lower than spring rate. At the defaults a 550 lb/in spring gives only 225.4 lb/in at the wheel.
Why is the motion ratio squared in the wheel rate formula?
Leverage changes two things at once, the distance the spring moves and the force it sends back to the wheel. Each change scales with the motion ratio, so together they scale with the ratio squared. A spring halfway out on the arm, at a 0.5 ratio, delivers only a quarter of its rate to the wheel.
Should the spring angle correction be squared?
Forum sources disagree, with some quoting Eibach’s single-cosine formula and others arguing the angle should be squared like the motion ratio. Speed Academy reports the squared version as the consensus, and it matches the result you get from Hyperco’s method of folding the angle into the ratio. The Wheel Rate Calculator shows both, and the gap only becomes large once the spring leans more than about 15°.
What ride frequency should a street car use?
The OptimumG ranges put passenger cars between 0.5 and 1.5 Hz, and the tool’s presets use 1.2 Hz for comfort and 1.5 Hz for a firmer sport street setup. Track cars without downforce usually sit higher, and the track preset uses 2.0 Hz. Treat these as starting points, since tire grip, road surface, and driver preference all shift the best number.
How do you measure wheel rate on the car?
A LocostUSA forum member suggests sitting on one end of the car and measuring how far it drops, then dividing half your weight by that drop. A 200 lb person causing a 0.5 in drop points to a wheel rate of about 200 lb/in. It is a quick check on the calculated figure, though the tire flex is included in the result.