Suspension Tuning

This is my section on suspension tuning, and even more specifically dampers. Please have a look and enjoy.

Springs

The spring rate (or spring constant) describes how much force is needed to compress a spring a given distance, usually expressed in lb/in or N/mm. A higher spring rate means a stiffer spring; a lower rate is softer. Springs may be linear (constant rate) or progressive (rate increases with compression).

The wheel-rate (Kw) is the effective rate at the wheel, accounting for suspension linkages. It’s calculated from the coil rate (Ks) and the motion ratio (MR):

Kw = Ks × MR²

  1. Wheel Rate Discussion
    • Definition: Wheel rate is the spring rate as experienced at the tire contact patch, factoring in any leverage created by suspension geometry.
    • Importance: It directly affects ride frequency, load transfer, and how the chassis responds to bumps and weight shifts.
    • Adjustment: Changing spring perch position or using helper springs can fine-tune wheel rate without altering the coil’s spring rate.
    • Impact on tuning: When swapping springs or changing ride height, always recalculate wheel rate to ensure your targeted natural frequency and handling characteristics remain consistent.
  2. Calculating Bounce (Natural) Frequency
    • Natural “bounce” frequency (fn) is how fast a corner oscillates when disturbed:
    • fn = (1 / 2π) × √(Kw / msprung), where msprung is the corner’s sprung mass.
    • To target a specific frequency: Kw = (2π fn)² × msprung.
  3. Typical Bounce Frequencies by Application
    • 0.5 – 1.0 Hz: Street / OEM comfort
    • 1.0 – 1.5 Hz: Lowered street / sports
    • 1.25 – 1.75 Hz: Aggressive back-road cars
    • 2.0 – 2.5 Hz: Autocross & low-downforce track
    • > 2.5 Hz: High-downforce race cars
  4. Choosing the Right Spring Rates
    • Weigh your corner at ride height to get msprung.
    • Pick a target fn based on usage (see above).
    • Compute Kw = (2π fₙ)² × msprung, then Ks = Kw / MR².
    • Use dampers set to ~60–70% of critical damping for balanced response.
  5. Practical Tips
    • For a dual-purpose street/autocross car, aim ~1.8–2.0 Hz with adjustable shocks.
    • Tire sidewalls add compliance—tall/soft tires can mask stiff springs on the street.
    • Verify suspension travel to avoid unwanted bottoming or topping out.

Starting with these guidelines and then testing actual bounce targets with data logging will get you to a spring/damper setup that balances comfort, responsiveness, and control for your driving goals.

Bumpstops

Bumpstops are a crucial component of a vehicle's suspension system, designed to limit the travel of the suspension and prevent damage to the vehicle's components. They are typically made of rubber or foam and are located at the end of the suspension travel, where they compress when the suspension reaches its maximum extension. Bumpstops help to absorb shock and provide a smoother ride by preventing metal-to-metal contact between suspension components.

In addition to protecting the suspension system, bumpstops also play a role in tuning the vehicle's handling characteristics. By adjusting the size and stiffness of the bumpstops, engineers can fine-tune the suspension's response to bumps and dips in the road, improving overall ride quality and handling performance.

With that out of the way, let's get into the nitty gritty of Bumpstops. If you have driving a car in the last 30 years, you have driven a car that not only uses the bumpstops to limit suspension travel, but also uses them as a tuning aid to affect the ride quality and handling of the car. When you are driving in stock-class Autocross where we are rule limited to shocks and bumpstop changes only, on a weak-spring car like a Mazda Miata, bumpstops are your friend as they can provide additional spring rate and affect the balance of the car in ways that are not possible without changing the springs.

Now when it comes to choosing the right bumpstops for your car, there are a few things to keep in mind. First, you need to consider the type of driving you will be doing. Are you going to be driving on the street, or are you going to be racing? If you're going to be racing, you'll want to choose a bumpstop that is designed for that type of driving. If you're going to be driving on the street, you'll want to choose a bumpstop that is designed for that type of driving.

There are two types of bumpstops: progressive and linear. Progressive bumpstops are designed to compress more as they are loaded, providing a softer initial feel and then becoming progressively stiffer as they compress further. Linear bumpstops, on the other hand, provide a consistent level of resistance throughout their travel.

A very important thing to keep in mind is that bumpstops are not just a "one size fits all" solution. Different vehicles and different driving styles require different bumpstop setups. For example, a car that is primarily used for street driving may benefit from a softer, more progressive bumpstop setup, while a car that is used for track racing may require a stiffer, more linear setup.

Dampers

Compression damping is the resistance a shock absorber provides when its piston is pushed inward—when the wheel moves upward relative to the car body (over a bump, dip, or during body roll). Inside the damper, hydraulic fluid must flow through tiny orifices or past adjustable shims/valves; the smaller the opening (or the stiffer the shim stack), the more resistance the fluid encounters, and the firmer the compression damping feels.

  1. Controls suspension travel rate
    • By resisting rapid compression, it prevents the suspension from collapsing too quickly over large bumps or under heavy loads (e.g., braking dive or rear squat under acceleration).
  2. Manages weight transfer speed
    • Stiffer compression damping slows how quickly weight shifts to the front or rear, sharpening turn-in response and limiting pitch motions; softer damping lets the car settle more gradually, improving comfort but at the cost of sharper handling.
  3. Influences ride comfort vs. grip trade-off
    • Higher compression damping → firmer ride, reduced body motion, less brake dive, but can feel harsh and may cause the tire to “skate” over small bumps if the suspension can’t react quickly enough.
    • Lower compression damping → smoother, more compliant ride, better small-bump absorption and mechanical grip, but increased body motions (roll/pitch) and a “wallowy” feeling under sudden maneuvers.
  4. Low-speed vs. high-speed damping
    • Many performance shock systems split bump damping into low-speed (motions under ~0.08 m/s (3 in/s) piston velocity—body roll, brake dive, pitch) and high-speed (quick hits like potholes or curbs). Tuning each separately lets you dial in a composed, responsive feel without making the ride punishing on broken pavement.

Rebound damping is the resistance a shock absorber provides when its piston is pulled outward—i.e., when the wheel moves downward relative to the car body (after compressing, as the suspension “re‐extends”). Inside the damper, hydraulic fluid must flow back through orifices or shim stacks; restricting that flow controls how quickly the suspension returns toward its ride height.

Key functions of rebound damping:
  1. Controls extension rate: By resisting rapid extension, it prevents the suspension from “popping” back too quickly after hitting a bump, which could unload the tire and reduce traction.
  2. Manages weight transfer recovery: Stiffer rebound damping slows the body’s return to equilibrium, smoothing out oscillations after braking, acceleration, or cornering; softer damping allows a quicker return but can lead to a bouncy, unsettled feel.
  3. Influences ride comfort vs. control trade-off:
    • Higher rebound damping → car settles more gradually, reducing successive oscillations and maintaining tire contact, but can feel like the chassis is being “tied down” and may lead to a harsher note as the damper drags.
    • Lower rebound damping → suspension feels more compliant on repeated or rhythm bumps, but can oscillate or “porpoise,” especially under heavy loads or aggressive inputs.

Most adjustable dampers break rebound into low-speed (body motions, pitch change, roll recovery) and high-speed (reacting to quick successive impacts). Tuning these independently lets you achieve smooth, controlled extension without sacrificing the ability to follow fine surface detail.

The magic of a well-tuned suspension lies in how bump (compression) and rebound damping work together to control both how quickly the wheel moves up and how quickly it comes back down. It’s not enough to dial each in isolation—what feels right on the bump stroke will only stay right if the rebound stroke is matched to it.

  1. Cycle control & tire contact
    • If compression is very firm but rebound is too soft, the damper will resist wheel travel into the bump but then snap back too quickly—unloading the tire and sacrificing grip on the next bump.
    • Conversely, very firm rebound with soft compression lets the wheel dive easily into a bump and then drags it back so slowly that the chassis “sits” on the bump mid-stroke, again upsetting traction.
  2. Energy balance & heat management
    • Every time the shock cycles it must dissipate kinetic energy as heat. Ideally you want roughly equal energy absorption on each stroke. Mismatched valving shifts heat into one half of the cycle, which can lead to fade or inconsistent feel when the damper heats up.
  3. Ride frequency & settling
    • Together, bump and rebound set the “natural” frequency of your suspension’s back-and-forth motion. If rebound is too light relative to compression, you’ll get a quick “chatter” after a bump (too much high-frequency motion). If rebound is too stiff, the chassis feels sluggish and won’t settle between inputs.
  4. Dynamic load transfer
    • Under braking, you compress the front struts. Compression damping resists dive; rebound damping then controls the pace at which the car comes back up when you release the brakes. Too much imbalance here makes the car pitch forward or rearward uncontrollably between braking zones.
  5. Mid-stroke “packing” vs. “porpoising”
    • If rebound > compression, the shock can “pack” (stay compressed) over a series of bumps because it won’t extend fast enough between impacts.
    • If compression > rebound, you risk “porpoising”—the wheel skips off the road because it rebounds too fast and then smacks back down.

In practice, tuners start by matching low-speed compression and rebound to control body motions (roll and brake dive) in a balanced way, then dial in high-speed settings so that the car both soaks up sharp hits and recovers smoothly without oscillation. The goal is a single, coherent damper response rather than two disparate halves.

In suspension tuning, the low-speed damping ratio (often expressed as a percentage of “critical damping,” ζ) governs how the car responds to slow body motions—think roll under cornering, dive under braking, and squat under acceleration.

  1. Street Cars (ζ ≈ 20–30 % critical)
    • Comfort priority: A lower ζ lets the chassis pitch and roll more freely, which softens the ride and isolates occupants from undulations in the pavement.
    • Forgiving handling: If the car leans or dives too abruptly, it can feel twitchy to everyday drivers. A moderate damping ratio smooths transitions and keeps the car feeling predictable at typical road speeds.
    • Broader operating window: Consumers drive on surfaces from satin-smooth highways to broken concrete. A lower ζ provides a wider “sweet spot” so that both small and mid-speed inputs aren’t overly punished.
  2. Race Cars (ζ ≈ 40–60 % critical)
    • Performance focus: Higher low-speed damping controls weight transfer more aggressively—minimizing body roll, brake dive, and squat—so tire contact patches remain as consistent as possible through high-G maneuvers.
    • Sharper response: Racing drivers demand instant turn-in and a neutral balance. The stiffer low-speed valving reduces chassis pitch and yaw lags, giving more precise feedback.
    • Narrow tuning window: Racetracks are relatively smooth and predictable, so engineers can dial in a higher ζ without worrying excessively about harshness over potholes.

High-Speed vs. Low-Speed Damping
Suspension dampers are effectively two dampers in one: low-speed valving for slow chassis motions, and high-speed valving for rapid wheel impacts.

AspectLow-Speed DampingHigh-Speed Damping
Piston Velocity Range<= 0.08 m/s (≤ 3 in/s)>= 0.08 m/s (≥ 3 in/s)
Primary ControlsRoll, dive, squat; driver-initiated weight transferRoad-surface irregularities; wheel hop; harshness
Valve DesignSmaller shim stacks or fixed orifices engineered for fine metering at low flowHeavier shims or bypass passages that only activate under high flow pressures
Tuning EffectsInfluences balance and transient response; governs pitch and roll settling time; strong effect on lap-to-lap consistencyProtects chassis from jarring impacts; prevents chassis “packing” or “porpoising” at speed; keeps tires in contact over ripples
Typical AdjustmentAdjustable clickers or shim stacks allowing ~10–60 clicks of tuningOften a separate knob or shim stack, with coarse adjustment for harshness control
Trade-OffsToo stiff → twitchy handling; Too soft → wallowy responseToo stiff → very harsh ride, can upset tire grip on repeat bumps; Too soft → risk of bottoming out, excessive packing of suspension

Putting It All Together
A well-tuned damper blends these two regimes so that:

  1. Low-speed valving maintains body control and driver confidence during corner entry, braking, and acceleration.
  2. High-speed valving soaks up sharp bumps and keeps the tire in touch with the road without transmitting brutal shock loads to the chassis.

On street cars, engineers skew the balance toward low-speed compliance for comfort, with just enough high-speed damping to prevent bottoming or feeling floaty. Racers invert that emphasis—maximizing low-speed firmness for razor-sharp handling while relying on high-speed damping primarily as protection against chassis damage and tire unloading at speed.

Sway-bars

Swaybars in general are the icing on the cake of suspension tuning. Unless you are stuck in an autocross class where they are the primary source of increasing the roll-resistance much like bumpstops in the previous section. For the rest of the world as a rule, the swaybar should be considered one of the final peices to your suspension puzzle and selected after everything else has been accounted for.

Swaybars (anti-roll bars, ARBs) are torsion springs that link opposite wheels across an axle. By resisting roll (body tilt) during cornering, they help keep tires flat on the road, sharpen turn-in, and tune under-/over-steer balance. Unlike springs, swaybars only engage when one wheel moves relative to the other, so they stiffen roll without affecting straight-line ride compliance.

How Swaybar “Rate” Works

A swaybar’s rate is its torsional stiffness, often expressed as lb/in (force to twist the bar 1 inch) or ft-lb/deg (torque per degree of twist). Rate scales roughly with the fourth power of diameter (d⁴), so small diameter changes yield big stiffness gains. Adjustable bars provide multiple end-link positions to fine-tune effective leverage and balance.

Miata Swaybar Specs by Generation

GenerationYearsStock Front (dia)Stock Rear (dia)Stock Rate FrontStock Rate Rear
NA/NB1990–200519 mm hollow; R-Package: 20 mm12 mm solid~120 lb/in~40 lb/in
NC2006–201521.1 mm hollow11.2 mm solid (base) / 12.1 mm (sport)≈160 lb/in*≈50 lb/in*
ND2016+22.9 mm hollow11.2 mm solid≈180 lb/in*≈30 lb/in*

* Estimated from diameter scaling; actual rates vary by arm length and mounting geometry.

Aftermarket & Application Recommendations

NA/NB (1990–2005)

Street:

  • Racing Beat hollow tubular front bars (15/16″ OD) and solid rears offer a balance of roll control and compliance; popular at ~$180 front, $90 rear.
  • Flying Miata solid bars (1.0″ front, 0.625″ rear) with multiple adjustment holes for fine-tuning.

Autocross:

  • Maximum front: Racing Beat 1.125″ tubular yields ~857 ft-lb/deg of roll torque—among the stiffest readily available.
  • Rear bar often removed for pure understeer bias, or replaced with an adjustable 11 mm bar to dial in oversteer.

NC (2006–2015)

Street:

  • Base OEM: 21 mm front/11 mm rear provides mild roll control.
  • Sport OEM: same 21 mm front with 12 mm rear adds a bit of rear stiffness.
  • Flyin’ Miata hollow 1″ front, 0.75″ rear adjustable; keeps compliance while flattening roll.
  • Whiteline/Eibach/SuperPro offer 24 mm front and 16 mm rear bars with multi-position adjustability.

Autocross:

  • Step up to 25.4–28.6 mm front (RX-8 bars or Progress: 25.4 mm/3.3 mm and 26.8 mm/3.5 mm options) paired with a minimal 16 mm rear or removal.
  • Adjust leverage to dial in transitional response and rotation.

ND (2016+)

Street:

  • ND OEM: 22.9 mm hollow front, 11.2 mm solid rear.
  • Flyin’ Miata 1.125″ OD front × 0.188″ wall, 0.625″ solid rear, three-position adjustable.
  • Cusco offers 24 mm front and 14 mm solid rear; RoadsterSport combos also popular.

Autocross:

  • Use FM’s 1.125″ front for maximum roll control; pair with a lightweight 14 mm rear or disconnect to tune bias.

Street vs. Autocross Setups

  1. Street: aim for balanced mid-corner grip with moderate bars (front: ~1.0″/25 mm; rear: 0.625″–0.75″/16–19 mm). Adjustable bars allow dialing for highway comfort and canyon carving.
  2. Autocross: maximize front roll stiffness (≥1.125″/28 mm), minimize rear (remove or use ≤11 mm) for a safe understeer bias and quick transitions and getting power down early with a Torsen LSD or viscous LSD.

Conclusion

By generation, Miata stock swaybars advance modestly from NA’s 19 mm to NC’s 21 mm and ND’s 22.9 mm fronts, with rear bars hovering around 11–12 mm. Aftermarket offerings span 19 mm–28 mm, with adjustability catering to everything from daily-driven grip to razor-sharp autocross performance. Matching bar diameter, wall thickness, and leverage ensures the perfect balance of roll control, compliance, and steering feel.