Last reviewed: August 3, 2026 · By the RaceYear team
Short answer: Race car shock tuning controls how fast weight moves onto and off each tire, not how much the car moves — springs do that. A stamped valving number is that shock’s damping force at three inches per second. One digit is compression, the other rebound, and the scale is not standardized between brands.
Key takeaways
Setup-direction note: Adjustment effects described here are directions to test from a stated baseline, not universal guarantees. Response varies with chassis and suspension geometry, tire construction and pressure range, track surface, corner phase, driver input, and what else changed at the same time. Confirm against your chassis builder, the tire manufacturer, the current rulebook, and your own scales and lap times.
You bought a set of used shocks off a guy in the pits. They’re stamped 3-5, 3-5, 5-3, and one of them just says 6. He said they were “good ones.” Now they’re bolted to your car and you have no idea whether you just made the thing better or turned it into a pogo stick. This is the part nobody explains, so here it is: what the numbers describe, what each end of the shock actually does to a tire, and which of the four to touch when the car does one specific thing.
Shocks control the rate of load transfer. Springs control the amount. That single sentence sorts out most of the confusion, because a car that rolls too much is usually a spring or bar problem, while a car that rolls too fast is a shock problem.
Compression is what happens when the shock gets shorter. Penske Racing Shocks defines it plainly — compression “refers to when the shock length grows shorter,” and rebound “refers to when the shock lengthens again after compressing” (Penske Racing Shocks). QA1 frames the same two forces as resistance: compression is “the amount of force it takes to—you guessed it—compress a shock,” and on the way back out, “the force that keeps the shock from instantly pulling apart is determined by the rebound valving” (QA1).
Now connect that to a tire, because that’s the part that matters. NASA Speed News puts the rule in one line: “In general, rebound damping controls how fast weight leaves a tire while bump controls how fast weight goes onto a tire” (NASA Speed News). Hyper Racing says it as a load statement you can actually use at the pit table: add rebound damping to one corner and that corner will “have less weight on it when that shock is in the rebound state,” while more compression damping causes that corner to “have more weight on it while that corner is in the compression state” (Hyper Racing).
| Compression (bump) | Rebound | |
|---|---|---|
| What the shock is doing | Getting shorter | Getting longer |
| What it controls | How fast weight goes onto that tire | How fast weight leaves that tire |
| Add damping force and that corner | Carries more weight while it’s compressing | Carries less weight while it’s extending |
| Symptom of getting it wrong | Too much is “harsh or jarring”; too little is “spongy or unresponsive” | Too little and “your shocks won’t properly control the movement of your springs” |
The other half of the rule is the one that saves you money: a shock only does anything while weight is actually moving. NASA is blunt about it — “when going into a corner, as long as the driver is moving the steering wheel or the brake pedal, the shock has an influence on tire loading,” but “once all weight has been transferred, the shock no longer influences handling.” A car that’s evil in the middle of a long, steady corner is usually not a shock problem. A car that’s evil in the first half-second of turn-in almost always is.
A valving number is a force reading at one shaft speed. Hyper Racing states the industry convention directly: “Almost all shock companies use the force dampening at 3 inches/second velocity to rate their shocks and to apply a valving number to that shock.” A “5” means the shock made a certain force on the dyno at three inches per second. That’s it.
Which is also the number’s biggest weakness, and Hyper says so themselves: “The shock valving number only describes the 3 inches/second number and nothing else… it totally ignores everything else about the shock dampening curve.” Two shocks can both stamp a 5 and behave nothing alike at the shaft speeds a rough cushion actually generates.
Here’s where published sources genuinely disagree, and you need to know it before you shop:
| Source | How valving gets expressed | What that tells you |
|---|---|---|
| Hyper Racing shock theory | Force at 3 in/sec; their example “a 5/6.5-1, which is a 5 rebound with a 6.5 to 1 adjustable compression valved shock” | Their example lists rebound first |
| AFCO 2026 Oval Track catalog | Separate “COMP. VALVE OPTIONS” and “REB. VALVE OPTIONS” per part number | You order the halves independently; no plain-English key is printed |
| PRO Shocks Tech Central | Sixteen separate valving sheets by model (A1754-7, A2853-7, comp lin-lin, reb dig-dig) | The scale is per model, not universal |
| PRO Shocks new valve bodies (2016) | “The new rebound ranges include 3-8, 6-11, 7-12, and 5-14” | On an adjustable, the number is a range the knob sweeps |
So there is no cross-brand standard, and there is not even a reliable digit order. A 5 from one builder is not a 5 from another, and a “3-5” might be compression-then-rebound or the other way around depending on whose stamp it is. AFCO publishes its tuning literature free at Tech Central and PRO posts model-specific valving sheets at Tech Central — pull the sheet for your exact part number instead of trusting a number in isolation.
One more wrinkle worth knowing: the low-speed/high-speed split moves with the surface. Hyper Racing puts the boundary at 1.5 to 2 inches per second on dirt versus 0.5 to 1 inch per second on asphalt, with low speed being “what the driver feels” and high speed being what happens “when it hits a bump.” That’s why asphalt shock advice sometimes reads backwards to a dirt racer. You’re not even talking about the same part of the curve.
Neither one is softer. They’re split differently, and the split is the whole point.
Run it through Hyper Racing’s load rule. The shock with the bigger rebound number holds that corner down harder as it tries to extend, so that corner carries less weight while it’s coming back up. The shock with the bigger compression number resists that corner going down, so it carries more weight while it’s being squashed. Same total, different behavior, opposite ends of the corner.
That’s a real setup decision, not trivia. On a slick track where you’re trying to keep the left rear planted and generate forward bite, you care about what the shock does on extension. On a rough, rutted surface where the car is getting slapped, you care much more about the compression side.
Before you act on any of it, confirm the order with the builder or a dyno sheet. Given that AFCO orders compression and rebound as separate options and Hyper’s own example lists rebound first, guessing which digit is which has a coin-flip failure mode: you’ll make the exact change you were trying to avoid.
Name the phase first — entry, middle, or exit — then pick the shock that is actually moving during that phase. A complaint that lives in one phase is a shock candidate. A complaint that’s the same everywhere is usually springs, bar, or air pressure, and you should go read up on diagnosing loose vs. tight before you touch a knob.
Here’s what published guidance actually says, quoted rather than paraphrased, so you can see who is saying what:
| Symptom | Where in the corner | What published guidance says | Source |
|---|---|---|---|
| Entry push (won’t turn in) | Brake and turn-in | “A stiffer rebound shock on the left rear will help the entry push problem” | NASA Speed News |
| Loose off the corner | Throttle up | “we could decrease the rear bump rate or increase the front rebound” | NASA Speed News |
| Unsettled on corner entry | Entry, dirt oval | “Use a stiff compression shock ahead of the axle on LR to improve corner entry stability” | AFCO Modified sheet |
| No drive off the corner | Exit, dirt oval | “Reduce rebound to improve LR drive off the corner” | AFCO Modified sheet |
| Harsh, skating over bumps | Anywhere | Too much compression and “your ride might feel very harsh or jarring” | QA1 |
| Soft, floaty, won’t settle | Anywhere | Too little compression and the car “might feel too soft, making it spongy or unresponsive” | QA1 |
Two of those rows point at the left rear from opposite directions, which is not a contradiction — it’s the entry-versus-exit trade every dirt racer eventually meets. Stiffen the LR to settle the car going in and you’ll usually give something back coming off. AFCO’s Modified sheet says both things on the same page (AFCO Racing).
And before you order anything, check where the shock lives. AFCO’s sheet notes that “a shock mounted ahead of the axle will provide more dampening than the same shock mounted behind the axle.” Two identical shocks on two different cars are not two identical shocks. Copying a valving number off a fast car with a different mount location is how people end up chasing ghosts all season.
Find out what your single adjuster actually moves before you turn it, because the category name means two different things depending on who built the shock.
The general-market answer is that one knob moves both. QA1: “Single adjustable shocks change both compression and rebound with a single knob.” Penske agrees — “with a single adjustable shock, one knob typically changes both compression and rebound together” (Penske Racing Shocks). But not every one-knob shock moves both. PRO Shocks sells a rebound adjustable line alongside its double adjustables, announcing new valve bodies in 2016 “for its Rebound and Double Adjustable shock lines” and speccing them by rebound range: “The new rebound ranges include 3-8, 6-11, 7-12, and 5-14” (PRO Shocks). Same one knob, different job.
Once you know what it moves, singles are perfectly workable. You have four adjusters on the car, so treat which corner as your variable instead of which end of the shock. Penske’s rough test for when singles are enough: you “race the same type of track every week,” and your “adjustments always move the same direction.” Go looking at doubles when the car is “harsh over bumps but loose in corners” — two problems that need opposite answers on the same shock.
A few habits that make single adjusters honest:
Slide a zip tie onto the exposed shock shaft, snug enough that it holds position but loose enough to slide, and push it down against the body seal. Run a heat race. The tie gets shoved up the shaft by the shock’s deepest compression stroke and stays there. The gap between the tie and the seal is the travel you used.
Do all four, and do them the same night, because the useful information is in the comparison, not the absolute number. Left front used two inches and right front used four? That’s your car telling you how it’s loading, and it’s a lot more reliable than a driver trying to describe corner entry at 10:30 at night.
What the readings tell you:
Ridetech gives a useful ballpark for how much room a shock should have to work with: “ideally you want 40-60% of your stroke length available when sitting at ride height,” and “generally, if the shock has 40-60% of its travel available at ride height then it should be able to travel over the worst bumps” (Ridetech). That’s written for a street rod, so treat it as a sanity check rather than a class spec — but a shock sitting well under that 40-to-60 band on the trailer is a shock that was never going to work.
The honest limitation: a zip tie records the maximum and nothing else. It cannot tell you when it happened, so it only means something next to a driver note. “Tie moved 3.5 inches on the RF, driver says it stepped over the cushion in turn three” is data. The tie alone is a number.
Assume the stamp describes a shock that no longer exists. Valving numbers are set at build, and Penske’s rebuild article describes exactly how they quietly stop being true: “performance doesn’t fall off all at once. The shock gradually changes as oil breaks down,” and racers end up building “a chassis tune around a shock that’s no longer performing to spec” (Penske Racing Shocks). Penske offers a 100-hour inspection as a sports-car baseline and says dirt rebuild windows come “significantly sooner” than that, without publishing a dirt number — so nobody can tell you how many nights are on the used set in the back of somebody’s hauler.
That’s the whole risk with a swap-meet shock: not that it’s broken, but that it’s a 5 that now behaves like a 3 and you’ll spend the summer tuning around it.
One change, driven and verified, beats four changes and a guess. Penske’s sprint car tuning guide reduces the whole discipline to six words: “One change. One direction. One verification.” They follow it with the warning every racer has earned the hard way — “don’t stack changes. If you change shocks, bars, and tires at once, you might get lucky, but you won’t learn anything” (Penske Racing Shocks).
Penske’s list of what to write down before the change is a good minimum: “track condition, tire pressures and stagger, any bar or indexing changes, exact shock settings and what you changed, driver feedback tied to entry/middle/exit.”
Then calibrate what you’re expecting. NASA Speed News attaches conditions to its number — “the handling problem must be small. The car should already be fast. Don’t expect more than .05- to .10-second improvement in lap times.” The same author documented a session that finished “.4 seconds faster than the morning baseline run,” then immediately added that “the day warmed up and the track surface was hotter, so we gained even more.” Both numbers are in the same article, and the gap between them is the reason you log conditions alongside settings. A shock change that looks like four tenths is usually a shock change plus a track that came to you.
It’s the damping force the shock produced on a dyno at three inches per second of shaft speed. Hyper Racing calls that the industry convention and also its limitation: the number “only describes the 3 inches/second number and nothing else,” ignoring the rest of the damping curve. Two shocks stamped the same can behave differently everywhere except that one point.
No, and treating them that way causes real damage. AFCO’s 2026 Oval Track catalog lists compression and rebound valve options separately without printing a plain-English key. PRO Shocks publishes sixteen model-specific valving sheets rather than one universal scale. Pull the valving sheet for your exact part number, and confirm which digit is compression before you order anything.
Change the end that matches the complaint’s timing. Shocks only influence loading while weight is transferring, so an entry problem points at whatever is moving under braking and turn-in, and an exit problem points at whatever is moving under throttle. If the car does the same thing everywhere in the corner, look at springs, bar, and air pressure before you touch a shock.
Check your specific model, because both answers exist. QA1 and Penske both describe a single adjuster as moving compression and rebound together with one knob. Meanwhile PRO sells a rebound adjustable line alongside its double adjustables and specs it by rebound range, so that one knob is working the rebound side. The category name won’t tell you; the builder’s valving sheet will.
Less than most racers hope. NASA Speed News caps the expectation at 0.05 to 0.10 second, and only when the car is already fast and the handling problem is small, though the same article documents a four-tenths gain over a morning baseline on a day the author says the track surface also got hotter. Shocks make the car easier to drive consistently, and consistency is usually where the real time hides.
You can absolutely do every bit of this with a notebook, a set of zip ties, and a good memory. Racers have been decoding shock stamps at the pit table since long before anybody had a phone. The measuring isn’t the hard part. The hard part is that a shock change only teaches you something if you can find the sheet from the night it worked, and remember what was stamped on the shock that was in the car at the time. That’s the job RaceNight was built for:
Log your valving numbers and your click counts every single night in RaceNight — it works offline, and every plan starts with a 14-day free trial. Starting on paper this weekend? Grab the free printable Spring Smash setup sheets before you unload. Tracking how many nights are on a set of shocks so you know when they’re due for a rebuild is a job for RacePrep. Then build out the rest of the sheet with our guide to the race car setup sheet, get your corner weights right with cross weight explained, and read up on slick track setup before the next dry-slick Saturday.
Hit a term in here you’d have to go ask somebody about? They’re all defined in the RaceYear racing glossary.
Setup notes, maintenance logs, race-day plans, and tax tracking — all in your pocket.
Get the apps