Bump stops are springs, and they're doing more than you think

Most people treat the bump stop as a crash pad — the thing that stops metal hitting metal. On a modern car it's a second, progressive spring that's carrying load long before you'd call it 'bottoming out'.

Ask what a bump stop is for and you’ll usually be told: it stops the suspension smashing into the chassis.

That’s true, and it’s about a third of the story. On a modern car — and on essentially every race car — the bump stop is a second spring, deliberately engaged during normal driving, chosen and positioned as carefully as the main one.

Why you’d want a second spring

Chapter 12 left us with a conflict that a single linear spring cannot solve.

Soft springs give a good ride and a low ride frequency, but they use travel fast: more sag, less bump travel left, and the suspension runs out of room on big inputs. Preload can put the ride height back (chapter 12), but it buys that by spending droop travel — it doesn’t create bump travel out of nothing. Stiff springs keep the travel but ruin the ride.

What you actually want is soft near ride height, stiff near the end of travel — a rising rate. You can get that from a progressive main spring, from rising motion ratio (chapter 10), or, most simply and cheaply, from a bump stop that starts contributing partway through the stroke.

That’s why the stop isn’t just a lump of rubber at the very end. It’s sized and positioned so the car starts loading it during ordinary driving.

The shape of the curve

The app models the stop as a linear rate multiplied by an exponential — which is a good description of how a real microcellular urethane stop behaves:

F = k_0 \cdot p \cdot e^{\,\alpha \cdot (p / L)}

where $p$ is penetration into the stop, $L$ its free length, $k_0$ the rate on first touch, and $\alpha$ the progression.

Two things fall out of that shape, and both matter.

It starts gently. At first touch the force is near zero and rises linearly — so the transition into the stop isn’t a step. A well-chosen stop blends in, which is exactly what you want and exactly what a cheap short stop fails to do.

It ends violently. The exponential means the tangent stiffness climbs without limit as you crush it. Past a point the stop is no longer a spring but a wall, and the app has a specific notion of where that is: it finds the penetration at which the stop’s tangent rate reaches a multiple of the main spring rate — 25× by default. Beyond that, the ride is being set by rubber, not by suspension.

Engagement is the setting that matters

The two knobs are Engage @ — where in the stroke the stop first touches — and Progression — how fast it stiffens after that.

Engagement is the one to think about first, because it decides whether the stop is part of your suspension or an emergency device:

Late engagement (stop touches only in the last 10–15% of travel): the car rides on the main spring alone almost always, then hits the stop as a distinct event. Simple, and what most people assume they have. The penalty is that all the rate change arrives at once.
Early engagement (touching within normal driving): the stop acts as a rising-rate spring throughout. The car can run a softer main spring for ride and still resist big inputs. This is how most modern cars, and effectively all long-travel off-road cars, are set up.

Early engagement also changes the car’s static behavior, which surprises people: if the stop is touching at ride height it’s already carrying load, so it stiffens the corner and the car sits slightly higher. The app accounts for that — static sag is solved with spring and stop together, which is why changing Engage @ moves the sag figure even though you never touched the spring.

Go look at it

Open the strut layout on the Springs/Shocks tab.

Find Engage @ and Progression in the bump-stop section, and note the static sag figure in the setup panel.

Run ▶ Play over Bumpy Back Road and note Max Travel (mm) and Peak Accel (g). That’s your baseline.

Move Engage @ later — so the stop only touches near the end of travel — and run the identical road. Travel used goes up, and the peak acceleration spike gets sharper when it does arrive.

Now move it much earlier and run again. Travel used drops, the car rides higher, and the peak softens — but check RMS Accel (g): riding on the stop full-time costs you ride quality.

Then change Progression alone, leaving engagement fixed. A high value blends in gently and then walls up hard; a low value is more linear but reaches the end of its usefulness sooner.

Finally, the diagnostic worth learning: run Rally Gravel Stage with the original setup. If Max Travel is pinned at the limit, the answer is usually not a stiffer spring — it’s engagement and fast damping.

The bump stop is a second spring. Where it engages decides whether it is part of your suspension or an emergency device.

Move the engagement point →

What it costs you

The bump stop’s cost is paid in ride quality when it’s engaged and you didn’t want it to be. A stop that engages too early makes the car feel firm and jittery at exactly the small-input, high-frequency conditions where it should feel supple, because the effective rate at ride height is no longer the spring you chose.

The other cost is heat and durability, and it’s the one nobody models: a stop that’s carrying load continuously is doing real work and will take a set over time. A car that “sat down” over 100 000 km often has stops that have compressed permanently, which quietly changes the rate curve and the ride height together.

And the trap: because the stop is invisible and unlabelled, it’s frequently the actual cause of a complaint blamed on something else. Harsh over sharp bumps with good body control usually means you’re hitting the stop, not that the spring is too stiff. The check is the one in step 6 above — look at travel used before touching the spring.

Rules of thumb

Engagement, road car: the stop should start contributing in the last 25–40% of bump travel. Later than that and it’s an emergency device; much earlier and you’ve changed the car’s spring rate.
Stop length: 15–30% of total bump travel is typical. Very short stops give no blend and feel like an impact.
Progression: enough to roughly double the effective rate by mid-crush. The app’s harshness threshold — 25× the main spring rate — is a useful ceiling to design against.
Race car: frequently on the stops in normal running, using them as the primary high-load spring while the main spring handles ride. Packers (spacers) are the adjustment.
Rally-raid and desert: the stop stops being a component and becomes a system. Hydraulic bump stops — small, self-contained dampers that engage in the last portion of travel — do the work rubber cannot, because the energy of a landing has to be absorbed, not stored and returned. A rubber stop gives that energy back and launches the car; a hydraulic one turns it into heat. That distinction is why desert trucks can land from heights that would destroy a conventionally sprung car, and it’s the single biggest thing separating long-travel suspension from a scaled-up road car.

Try this

Find the engagement point that gives the lowest Peak Accel (g) on Bumpy Back Road without raising RMS Accel (g). That’s the compromise the factory is trying to hit.
Soften the main spring by 30% and use earlier engagement to keep the same travel. Is the ride better or worse? Why?
On Off-Road Trail, can you keep Max Travel off the limit using bump-stop settings alone, without touching the spring?

Next: the other mass in the system — the one below the spring, which has its own resonance and cares about entirely different frequencies.

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