Understanding Shock Absorbers for Autocross

Shock absorbers (dampers) are one of the most impactful upgrades you can make for autocross performance. Their primary job is to control spring motion and keep the tire in contact with the pavement. On an autocross course, where transitions are rapid and surfaces often uneven, a properly tuned shock absorber can mean the difference between a clean run and a spin.

How Shock Absorbers Work

Shock absorbers convert kinetic energy from suspension movement into heat through hydraulic resistance. The damping force is generated as oil passes through orifices (valves) inside the shock tube. Two critical phases exist:

  • Compression (bump) damping: Controls how the suspension compresses when the wheel hits a bump or when weight transfers forward under braking.
  • Rebound (extension) damping: Controls how quickly the suspension extends after being compressed – critical for maintaining tire contact coming out of a corner.

For autocross, you generally want stiffer rebound damping than compression, especially on the front axle, to prevent the car from rolling excessively and to help the car rotate on turn-in. However, if rebound is too stiff, the suspension may “pack down” over successive bumps, losing grip.

Types of Shock Absorbers

The article correctly lists gas, oil, and adjustable shocks. Here is expanded detail:

  • Gas shocks (monotube or twin-tube with gas charge): The gas (usually nitrogen) minimizes oil aeration and cavitation under hard use. Most autocrossers favor monotube gas shocks (e.g., Bilstein, Koni Sport) because they dissipate heat efficiently and offer consistent damping. They also tend to have a higher initial cost but deliver superior performance over a session.
  • Oil shocks (hydraulic only): Older or budget designs. They can fade quickly in hot conditions and are rarely used in competitive autocross unless running in a stock class that restricts modifications.
  • Adjustable shocks: This is where you can really fine-tune. Look for “single adjustable” (usually adjusts rebound only, or both rebound and compression together) or “double adjustable” (separate rebound and compression adjustments). Double adjustable shocks give you the most control – you can stiffen compression for slaloms and soften rebound for bumpy lots, but they cost significantly more. For most amateur autocrossers, a quality single-adjustable shock is sufficient.

Choosing Shock Valving and Spring Rates

Shocks must be matched to your spring rates. If you run very stiff springs (e.g., 800 lb/in front on a Miata), you need a shock with heavy-duty valving to control that spring. Many manufacturers offer “autocross” or “race” valving options. A common mistake is installing adjustable shocks but leaving them on a “comfort” setting – on course, that will make the car wallow. Start with the manufacturer’s recommended setting for your spring rate, then fine-tune based on feel and tire temperatures.

Pro tip: On the morning of an event, set your shocks based on the course surface. Smooth asphalt allows stiffer settings for quicker response; rough concrete or bumpy lots require softer settings to maintain contact. Always test a couple of runs before an event to dial in.

Mastering Camber for Autocross

Camber is the vertical angle of the tire relative to the road. Negative camber (top of the tire tilted inward) improves lateral grip during cornering because the tire’s contact patch remains flatter under body roll. In autocross, where you spend a lot of time turning, proper camber is essential for fast lap times.

Static vs. Dynamic Camber

Static camber is what you measure on a flat alignment rack. Dynamic camber changes as the suspension compresses and extends. Most cars gain negative camber when the suspension compresses (due to suspension geometry). So even a car with zero static camber might have 2–3 degrees of negative camber mid-corner. However, modern strut-type suspensions often lose negative camber as they compress (McPherson strut “camber loss”), which is why those cars frequently need more static negative camber to compensate.

How Much Camber Do You Need?

There is no universal number. It depends on tire compound, suspension design, driving style, and course layout. Ballpark chassis-specific recommendations:

  • McPherson strut cars (e.g., Honda Civic, Ford Focus, Mazda Miata NA/NB): Aim for -2.5 to -3.5 degrees of front camber. These cars need more because they lose camber in compression.
  • Double wishbone cars (e.g., older Hondas, many modern sports cars): Start around -2.0 to -2.8 degrees front. Rear can be -1.5 to -2.0 degrees for stability.
  • Solid axle or twist-beam rear: Often limited to factory adjustments, but you can use shims or offset bushings. Usually -1.0 to -1.5 degrees in the rear helps rotation without being too loose.

Too much negative camber reduces straight-line braking grip and can cause excessive inner tire wear. Look at tire temperatures across the tread after a run. The outer shoulder should be at or slightly below the inner shoulder temperature. If the outer is much cooler, you need more negative camber; if the outer is hotter (or you see shoulder wear), you need less.

Adjusting Camber

The original article mentions camber plates or adjustable control arms. Here is more detail:

  • Camber plates are common on McPherson strut cars. They replace the top mount and allow you to move the top of the strut inward. They also usually provide caster adjustment, which improves steering feel and camber gain on the outside wheel.
  • Adjustable control arms (A-arms) are needed on double-wishbone or multi-link rear suspensions. Some cars can use eccentric bolts in the lower control arm to adjust camber within a small range.
  • Shim kits are used on solid axle rear or on some front upper control arms. Adding or removing shims changes the arm’s pivot point.

Alignment tip: Always get a proper alignment after camber adjustments. Set camber first, then caster, then toe. For autocross, aim for 0 to 1/16-inch total toe-out at the front (improves turn-in) and 1/16 to 1/8-inch total toe-in at the rear (for stability under power).

Tire Choice: The Ultimate Differentiator

No component affects autocross performance as dramatically as tires. They are the only interface between the car and the road. The original article lists street, performance, and racing slicks – but the current state of the sport has evolved significantly, especially with the introduction of “200 treadwear” (UTQG) tires for Street classes.

Understanding Tire Categories for Autocross

  • Street -200 TW (e.g., Bridgestone RE-71RS, Yokohama Advan A052, Falken RT660): These are the top choices for Street classes (e.g., SCCA Street, CAM). They have a 200+ treadwear rating and are street legal. They offer phenomenal dry grip (often close to old R-compounds) and are predictable at the limit. They require ideal temperatures (typically 120-160°F surface temp) and will overheat if pushed too hard on a hot day. They also need a few runs to come up to temperature.
  • Extreme Performance Summer (e.g., Michelin Pilot Sport Cup 2, Toyo R888R): Often above 200 TW but still DOT legal. Used in some clubs for “Street Modified” or “Super Street” categories. They offer more outright grip than 200 TW tires but are more sensitive to temperature and may have shorter life.
  • Race Slicks (e.g., Hoosier A7/R7, Goodyear RS): The fastest tires available. They have no tread pattern and must be used in Prepared, Modified, or specific class categories (SCCA CAM-C, XS-B, etc.). Slicks require tire warmers to get them to optimum temperature; otherwise, they are dangerously slippery. They also wear very quickly – often lasting only 40-80 runs.
  • All-season tires: Avoid for competitive autocross. They will roll over severely, overheat, and have poor lateral grip. Only use if you are just trying to have fun without any expectation of winning.

Selecting Tire Size and Compound

Larger contact patch provides more grip, but only if the tire fits inside the wheel well and doesn’t rub. Most classes have rules on maximum tire width relative to wheel width. A common upgrade is to run a wider wheel with a square tire setup (same width front and rear) for better balance and rotation.

Compound choice depends on ambient temperature and course length. Softer compounds (like the A052 or RE-71RS) work best in temps below 85°F. On a 95°F day on concrete, they may overheat and fall off after 45 seconds. The Falken RT660 is slightly harder and handles heat better, but initial grip is a tad lower.

Tire Pressures: The Critical Variable

Proper tire pressure ensures the tire works in its optimal temperature range and that the tread wears evenly. Autocross typically requires lower pressures than the street because you want a larger contact patch and better grip. However, too low a pressure will cause the tire to roll over on the sidewall (especially on 200 TW tires), leading to sloppy steering and edge wear.

Start pressures for 200 TW tires (common example for a 2400 lb car on 225/45R15):

  • Cold: 28-30 psi front, 26-28 psi rear
  • Hot target (after a run): 34-36 psi front, 32-34 psi rear – adjust based on chalk marks on the sidewall. Use tire chalking: apply a stripe of chalk from shoulder to shoulder across the tread. After a run, check where the chalk has been worn off. Ideally it should wear to the edge of the tread block; if the chalk is gone on the sidewall (past the tread), the tire is rolling over too much – add pressure. If the chalk remains thick on the outer edge, reduce pressure.

Pro tip: On a hot day, you may need to bleed pressure after every run. On a cool day, you might want to add 1-2 psi to help bring the tires up to temperature. Never drive on tires below 25 psi hot – you risk damaging the sidewall structure.

Suspension Geometry Beyond Camber: Caster and Toe

While camber is crucial, caster and toe also heavily influence autocross handling. Caster is the steering axis tilt (like on a shopping cart caster). More caster increases dynamic negative camber on the outside front wheel when steering, improves straight-line stability, and adds steering weight. In an autocross car, aim for as much positive caster as factory arms allow (often around 5-7 degrees).

Toe settings affect how the car turns and its stability. As noted, front toe-out helps turn-in but can make the car darty under braking. Rear toe-in helps the car track straight but can reduce rotation. Many advanced autocrossers adjust toe between events: more toe-out for tight, slow courses, and less (or even zero toe) for fast sweepers. However, excessive toe dramatically increases tire wear, especially on the front tires, so adjust only if you are willing to replace tires more often.

Putting It All Together: A Systematic Tuning Process

Do not change everything at once. Follow a structured approach:

  1. Start with tires and pressures. Get a baseline tire temperature and pressure setup that works for the venue surface. Run a few laps or a test course to feel the grip level.
  2. Set alignment. Establish a target camber, caster, and toe based on your car’s common setups (check reputable forum threads for your chassis). Get it aligned at a shop that understands race alignment and is willing to accommodate autocross settings.
  3. Choose shock settings. With the alignment and tires dialed, set shocks to a recommended baseline (often the manufacturer’s performance setting or from known setups). Make small adjustments: for example, if the car understeers on entry, increase front rebound or decrease front compression. If the car oversteers on exit, increase rear rebound or reduce rear compression. Keep a log book.
  4. Fine-tune pressures. After alignment and shocks are decent, tweak tire pressures in 1 psi increments. Do not chase pressure to fix a handling imbalance that really needs alignment or damping adjustment.
  5. Data logging or video review. If possible, use a GPS-based data logger (like SoloStorm or RaceCapture) to analyze corner entry and exit speeds. Compare runs before and after changes.

Common Mistakes to Avoid

  • Setting camber too aggressive for the street (cause premature inner tire wear) – remember that autocross cars often drive to and from events, so a compromise is necessary unless you trailer.
  • Running extremely stiff shocks thinking it will reduce body roll – actually, it will make the car skittish over bumps and reduce tire contact. Shocks should be stiff enough to control spring oscillations, not to replace springs. If you have massive body roll, first get stiffer springs or sway bars.
  • Using tire shine or products that make tires slippery – avoid any dressing that could reduce grip.
  • Neglecting to check tire pressures before every session – ambient temperature changes of 10°F can change cold pressures by 1-2 psi. Always adjust cold pressure before your first run of the day.

External Resources for Further Reading

Final Thoughts on Tuning for Autocross

Tuning your autocross car is a continuous process of measurement, adjustment, and driving feedback. Shock absorbers, camber, and tire choice form the foundation, but they interact with spring rates, sway bars, ride height, and alignment settings. Do not underestimate the value of seat time – a well-tuned car driven by a novice will still be slower than a mediocre setup driven by an expert. Use each event as a test session, collect data (times, tire temps, driver notes), and gradually evolve your setup. The goal is not just to win trophies but to deepen your understanding of vehicle dynamics – and that understanding will make you faster no matter what car you drive.

Remember: the best tuning tool is an open mind and a systematic approach. Change one variable at a time, document everything, and never stop learning.