Spring rate, wheel rate, ride frequency: the only three numbers

A spring rate on its own tells you almost nothing — it depends on the car's mass and the linkage in between. Ride frequency is the number that lets you compare a Miata to a Dakar truck and have the comparison mean something.

Module 1 was about where the wheel goes. Module 2 is about what happens when it gets there, and how long it takes.

Start with the question that ends most forum threads badly: “what spring rate should I run?” It has no answer, because a spring rate is meaningless without two other numbers — how much mass it’s holding up, and the motion ratio between it and the wheel.

Combine all three and you get ride frequency: how fast the body bounces on its springs, in hertz. That single number does transfer between cars, and it’s how suspension engineers actually talk.

Getting there in two steps

You already have the first step from chapter 10. Convert spring rate to wheel rate:

k_{\text{wheel}} = k_{\text{spring}} \times \text{MR}^2

Then treat the corner as a mass on a spring, which is exactly what it is:

f = \frac{1}{2\pi}\sqrt{\frac{k_{\text{wheel}}}{m_{\text{sprung}}}}

$k_{\text{wheel}}$ in N/m, $m_{\text{sprung}}$ in kg — the sprung corner mass, so subtract the unsprung mass, because the wheel and hub ride below the spring and aren’t part of what’s bouncing.

Worked example, on the strut layout. The car is 1200 kg with 64% on the front, so a front corner carries 384 kg. Take off 35 kg of unsprung mass and the spring is holding 349 kg.

The spring is 2.7 kg/mm at a motion ratio of 0.936:

k_{\text{wheel}} = 2.7 \times 0.936^2 \times 9.81 = 23.2\ \text{N/mm} = 23\,210\ \text{N/m}

f = \frac{1}{2\pi}\sqrt{\frac{23\,210}{349}} = 1.30\ \text{Hz}

The app reports exactly that: 1.30 Hz at the front. Which tells you far more than “2.7 kg/mm” ever could — 1.30 Hz is a comfortable road car, and now you can say so without knowing anything else about it.

Reading the number

Ride frequency maps directly onto what the car feels like, and the bands are remarkably consistent across wildly different vehicles:

Ride frequency	What it is
0.5–1.0 Hz	Luxury sedan, soft SUV. Long travel, floats over crests.
1.0–1.5 Hz	Normal road car. The layout above sits at 1.30 Hz.
1.5–2.0 Hz	Sports car, hot hatch, firm road car.
2.0–2.5 Hz	Track-day car, race car without significant downforce.
3.0–5.0+ Hz	Downforce car. The aero load, not the spring, sets the ride height.
1.5–2.2 Hz	Rally-raid and desert — surprisingly firm, for a reason below.

That last row usually surprises people. A Dakar car has half a meter of travel and looks like it should be soft — but it’s carrying two tons and landing off jumps, so its frequency lands near a sports car’s even though its spring is enormously stiffer and its travel ten times longer. Frequency normalizes all of that away, which is exactly why it’s the number worth quoting.

Working backwards, which is what you’ll actually do

You don’t pick a spring and discover a frequency. You pick a frequency and go buy the spring:

k_{\text{spring}} = \frac{(2\pi f)^2 \times m_{\text{sprung}}}{\text{MR}^2}

On that same front corner — 349 kg sprung, MR 0.936 — here’s what the target frequency costs you:

Target	Wheel rate	Spring needed
1.0 Hz	13.8 N/mm	1.60 kg/mm
1.3 Hz (as built)	23.2 N/mm	2.70 kg/mm
1.5 Hz	31.0 N/mm	3.61 kg/mm
2.0 Hz	55.1 N/mm	6.41 kg/mm

Note how brutally non-linear that is. Going from 1.0 to 2.0 Hz — doubling the frequency — needs four times the spring, because frequency goes as the square root of rate. Every extra tenth of a hertz costs more than the last one.

The third number: preload

There’s a variable I’ve quietly left out, and it’s the one that makes the setup above possible at all.

Ask the obvious question: how far does a 2.7 kg/mm spring compress under 349 kg? At the wheel that spring is 2.365 kgf/mm, so:

\text{compression} = \frac{349}{2.365} = 147.5\ \text{mm}

The corner only has 142 mm of travel in total. The spring cannot hold this car up. Left to itself it would compress past the end of its stroke and sit on the bump stop before meeting a single bump.

So how does the car sit at a healthy 30% sag? Preload. The preset winds 2600 N into the spring — force it is already exerting at full droop. At MR 0.936 that arrives at the wheel as:

\frac{2600 \times 0.936}{9.81} = 248\ \text{kgf}

Preload is carrying 248 of the 349 kg — 71% of the corner. The spring only has to compress far enough to pick up the remaining 101 kg, and that’s what puts sag at 42.7 mm.

That 30% figure is not evidence of a well-chosen spring. It’s evidence of a lot of preload.

Preload sets ride height; rate sets frequency

These are separate controls, and confusing them is one of the most common setup errors.

Preload (spring held at 2.7 kg/mm)	Static sag
0 N	76.2 mm — 54% of travel
1000 N	69.0 mm (49%)
2000 N	57.5 mm (40%)
2600 N (as built)	42.7 mm (30%)
3000 N	26.5 mm (19%)

Every row has exactly the same ride frequency of 1.30 Hz, because the spring rate never changed. Preload moved the car up and down its travel without altering how fast the body bounces.

The reverse is also true. Hold preload at 2600 N and change the rate instead:

Spring rate	Ride frequency	Static sag
1.6 kg/mm	1.00 Hz	57.1 mm (40%)
2.7 kg/mm	1.30 Hz	42.7 mm (30%)
3.6 kg/mm	1.50 Hz	32.0 mm (23%)
6.4 kg/mm	2.00 Hz	18.0 mm (13%)

Two knobs, two jobs: rate for frequency, preload for ride height. Set the frequency you want from the mass and motion ratio, then use preload to put the car where it should sit.

The myth, and the real limit

Preload does not make the spring stiffer. A 2.7 kg/mm spring is 2.7 kg/mm whether you’ve wound 0 N or 3000 N into it — which is why the frequency column above never moves. Anyone who tells you cranking the collars “stiffens it up” is describing ride height, not rate.

What preload does cost you is droop travel. Winding the perch down compresses the spring, which raises the car and leaves less room for the wheel to extend. Push it far enough and the suspension sits hard against its top-out stop with zero droop — at which point it’s a solid link, not a spring, and the ride frequency figure stops meaning anything. The app detects exactly this state and reports the corner as topped out; on this corner it happens somewhere above about 3600 N.

So the honest summary of soft springs: they don’t inherently cost you sag, because preload can put the car back where you want it. What they cost is droop travel and margin — you’re relying on preload to hold up a corner the spring can’t, and there’s a hard ceiling on how much of that you can do.

Going the other way, a stiffer spring keeps travel but transmits more of the road. Above about 2.5 Hz on a road car you’re no longer isolating anything — you’re just relaying the surface to the driver’s spine and, worse, making the tire skip.

And the trap that catches people who’ve read this far and no further: frequency alone doesn’t describe the ride. Two cars at 1.3 Hz can feel completely different depending on damping, which is chapter 14. Frequency tells you where the body wants to oscillate. Damping decides whether it’s allowed to.

Go look at it

Open the strut layout on the Springs/Shocks tab.

In the setup panel on the right, find the RIDE FREQ readout — about 1.30 Hz at the front.

Find Rate under FRONT SPRING: 2.7 kg/mm. Change it to 6.4 and watch the frequency climb toward 2.0 Hz. You just turned a road car into a track car with one number.

Put it back to 2.7, then hit ▶ Play on Urban Road and note RMS Accel (g) — that’s how much the body is being shaken.

Now run the same road at 6.4 kg/mm. RMS acceleration goes up: a stiffer spring passes more of the road through to the body. That’s the cost of frequency, measured.

Now the point most setup guides skip. Put the rate back to 2.7 and set Preload to 0. The ride frequency doesn’t move — still 1.30 Hz — but sag jumps from 42.7 mm to about 76 mm, over half the travel. That spring was never holding this car up on its own.

Walk Preload back up in steps and watch sag fall while the frequency stays fixed. Two independent knobs: rate sets frequency, preload sets ride height.

Push preload past about 3600 N and the corner reports topped out — zero droop, sitting on its top-out stop, no longer behaving like a spring at all. That’s the hard ceiling on using preload to rescue a soft spring.

The corner is a mass on a spring. Ride frequency is how fast that mass bounces — and unlike a spring rate, it means the same thing on any car.

Change the spring and watch the frequency →

Rules of thumb

Front vs rear: not equal. The rear normally runs 5–20% higher, for a reason that’s the whole of chapter 13.
Static sag: 25–35% of total travel for a road car; 20–30% for a race car; 30–35% for long-travel off-road, where droop matters as much as bump.
Always quote sag with the preload it was measured at. A sag figure on its own says nothing about whether the spring is right — the layout above hits a textbook 30% with a spring that cannot support the corner unaided.
Sanity check the spring alone: divide sprung corner mass by wheel rate. If that exceeds your available travel, preload is carrying the car, and you should know by how much before you go any further.
Preload is ride height, not rate. It never changes the frequency. If you need a different frequency, you need a different spring.
Rally-raid and desert: 1.5–2.2 Hz, and the reason is landings, not cornering. Sizing springs for a normal ride frequency on a car that will drop several feet leaves nothing to absorb the arrival; these cars are sprung for the worst event, not the average one, and then use bump stops (chapter 16) for the truly bad case.
Downforce cars break the model entirely. When aero load at speed exceeds the car’s weight, the “mass on a spring” picture stops describing anything, and ride height control takes over from frequency as the design target.

Try this

Compute the ride frequency of your own car. Take corner weight, subtract a guess at unsprung mass, apply your motion ratio. Does the number match how it feels?
Find the spring rate that puts the front at exactly 1.5 Hz, then set preload for 30% sag. How much preload did it need, and how much droop did that leave?
Set front and rear to the same frequency and run a road. Then read chapter 13 and see if you can predict what you just did wrong.

Next: why matching the front and rear frequencies — the obvious, symmetrical, tidy thing to do — makes the car pitch, and what to do instead.

Preset spring and damper values are illustrative starting points, not a measured setup for any particular car.