Flat ride: why the rear should be stiffer than the front

Matching front and rear ride frequencies is the tidy, symmetrical, obvious thing to do — and it makes the car pitch. The fix is a 1920s idea that still decides how modern cars are sprung.

Here’s a design decision that looks like a mistake until you do the arithmetic.

Drive over a bump. The front wheels hit it, the front of the car rises and starts to oscillate. A moment later — how long depends on wheelbase and speed — the rear wheels hit the same bump and the back of the car starts oscillating too.

Two ends, bouncing, out of step. The body doesn’t move up and down. It pitches, nose-up then nose-down, which is the motion humans are worst at tolerating and the reason a badly sprung car makes passengers queasy rather than merely uncomfortable.

The timing problem

The gap between the front and rear hitting the bump is just distance over speed:

For the 2467 mm wheelbase in the app:

Now compare that with how long a bounce takes. At the front’s 1.30 Hz, one full oscillation is 769 ms. So at 100 km/h the rear starts its bounce about 12% of a cycle behind the front.

If both ends have the same frequency, that 12% phase lag never closes. The two ends stay permanently out of step, and every bump leaves the car pitching until the dampers kill it.

Maurice Olley’s fix, from the 1930s: make the rear a little faster, so it catches up.

The number

If the rear oscillates slightly quicker, it can complete its cycle at the same moment the front completes its own — arriving back in phase. The required ratio is roughly:

At 100 km/h with a 1.30 Hz front and an 89 ms delay, that’s $1 + 1.30 \times 0.089 = 1.12$ — the rear should be about 12% stiffer in frequency than the front.

Check it against the setup in the app:

Sixteen percent, against a theoretical 12% at 100 km/h — and the app’s advisor grades 1.05–1.20 as good. This is a car sprung deliberately for flat ride, and now you can see it in two numbers instead of taking it on faith.

Notice the ratio is achieved even though the rear spring (3.3 kg/mm) is stiffer and the rear motion ratio (0.700) is much worse than the front’s (0.936). Those two effects fight each other, and the rear corner is also far lighter — 178 kg sprung against 349. Frequency is the only number that resolves all three at once.

The uncomfortable part

The equation has $v$ in it. Which means flat ride is only exactly right at one speed.

Tune for 100 km/h and you get 1.12. Tune for 50 km/h and the delay doubles to 178 ms, wanting a ratio of 1.23. At 130 km/h it wants 1.09. There is no single split that’s correct everywhere, so manufacturers pick a speed the car will actually spend its life at — usually a cruising speed — and accept being slightly wrong elsewhere.

This is also why the effect is more visible on some cars than others. A long wheelbase gives a bigger delay and a stronger requirement; a short-wheelbase car at speed has so little delay that almost any split works.

Why race cars ignore it

Flat ride is a comfort criterion, and it costs something a race car won’t pay.

On a circuit, the front/rear stiffness split is one of the primary balance controls — it sets how load transfers and therefore whether the car understeers (chapter 21). A race engineer choosing that split for pitch behavior instead of balance has spent an expensive tuning knob on passenger comfort.

Downforce cars have a second reason: ride height controls aero platform, and the springs are there to hold a ride height, not to isolate anybody. Pitch behavior is managed by dampers and by geometry instead.

So the honest hierarchy: road car — flat ride matters, use Olley. Track car — balance wins, ignore Olley. Rally-raid — read on.

Go look at it

Open the strut layout on the Springs/Shocks tab.

1. In the setup panel, find Ride frequency F/R — it should read about 1.30 / 1.50 Hz, and the advisor should grade it green.
2. Now break it. Set the rear spring rate down until the rear frequency matches the front at roughly 1.30 Hz. The advisory turns amber and tells you the car will pitch.
3. Run ▶ Play over Urban Road in both states and compare the body-movement trace. Matched frequencies produce a longer, more obviously pitching settle.
4. Push it wrong the other way — make the rear softer than the front. This is the worst case, and the advisor says so: the rear falls further behind with every bump.
5. Then reason about speed. At 50 km/h the theory wants a 1.23 ratio, at 130 km/h it wants 1.09. Which does the car you’re building actually spend its time doing?

What it costs you

The rear is now stiffer than it “should” be for grip. On a rear-drive car that costs some traction out of slow corners, and on any car it costs rear ride quality — rear passengers get the firmer end, which is a choice manufacturers make knowingly and which anyone who has sat in the back of a sports sedan has felt.

It also interacts with everything in Module 3. Front/rear stiffness split is a balance lever, so tuning it for pitch means you’ve partly spent it, and the remaining balance adjustment has to come from anti-roll bars instead.

Rules of thumb

• Road car: rear 10–20% higher in frequency than the front. The app grades 1.05–1.20 as good.
• Never run the rear softer than the front on a road car. It’s the one clearly wrong answer — the rear falls further behind on every input, and the car porpoises.
• Race car: ignore this and set the split for balance. Expect some pitch and control it with dampers.
• Long wheelbase (vans, pickups, limousines) needs a bigger split, because the delay is longer at any given speed.
• Rally-raid and desert: flat ride barely applies. At speed on open desert the inputs aren’t discrete bumps arriving front-then-rear; they’re continuous and often large enough that the suspension is working near its stops. The split gets chosen for landing attitude — usually a stiffer rear so the car lands rear-first and doesn’t nose in, which is a survival criterion rather than a comfort one.

Try this

1. Compute the ideal ratio for your own car at the speed you actually drive it. How far is it from what the car came with?
2. Set the ratio for 50 km/h, then run a road at 100. Then reverse it. Can you see the difference in the body-movement trace?
3. If your car is rear-drive and you need rear grip out of slow corners, which end of the flat-ride compromise would you give up first?

Next: frequency says where the body wants to oscillate. Damping decides whether it’s allowed to — and the preset in this app is a good example of getting it wrong.

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