Last reviewed: August 3, 2026 · By the RaceYear team
Short answer: A smash number (or load number) is the amount of force a coil-over spring is holding when it’s compressed to a specific shock length — for example, 606 pounds at 18.250 inches center-to-center. It lets you swap springs at the track and keep the exact same corner load, so your setup stays repeatable no matter which spring rate you bolt on.
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.
Most racers set ride heights with a tape measure and change springs by feel. That works right up until it doesn’t — until you throw a stiffer right front for a black, heavy track and the car pushes off the corner and you have no idea why. Smash numbers fix that guessing game — but only if you write them down every night. Let’s break down what they are, how they’re measured, and why a logged history beats the best memory in the pits.
A smash number is a load reading — the weight a spring is carrying when it’s squeezed to a set shock height. According to BSB Manufacturing, smash numbers are interchangeable with “load numbers”: “a number that represents a weight for that corner of a race car, not to be mistaken with wheel load or a scale number.”
Here’s the plain-English version. Take a coil-over off the car and put it in a tool that compresses it to your target shock length — say, 18.250 inches from the center of one mount to the center of the other. Whatever load shows up at that height is your smash number. In BSB’s example, a Late Model spring reading “606 @ 18.250” means it’s compressed to hold 606 pounds at that length.
That single number does something a spring rate can’t: it tells you what that spring is actually carrying on the car, not just how stiff it is.
They’re not the same thing, and the difference is the whole point. These three get mixed up constantly.
| Term | What it measures | Units | What it tells you |
|---|---|---|---|
| Spring rate | Stiffness of the spring | Pounds per inch (lbs/in) | How much force it takes to compress the spring one inch |
| Ride height | Static height of the chassis | Inches | Where the car sits at rest |
| Smash / load number | Force at a set shock length | Pounds (lbs) | The load the spring is seeing at that height — not the weight on the tire |
Spring rate is stiffness. As Maximum Zone Systems puts it, it’s “the amount of weight (in pounds) it takes to compress that spring exactly one inch.” A 200 lb/in spring takes 200 pounds to move one inch, 400 to move two, and so on.
But here’s the trap: two totally different springs can hold the exact same load. Maximum Zone’s example nails it — an 800-pound corner can be carried by a 200 lb/in spring compressed four inches, or a 400 lb/in spring compressed two inches. Same load, same corner weight. One dives deep and feels soft; the other feels punchy. The smash number ties the spring rate back to what the car actually sees.
That’s why more teams now think in load, not just rate. Marshall Fegers of QA1, quoted in Performance Racing Industry, says smash machines led to “a major transition in how we talk about setups”: “we don’t really worry about our ride heights as much, and we care a lot more about the spring force at the height that the car is seeing at speed.”
You measure a smash number with a spring smasher — a bench tool that compresses a coil-over to a known height and reads the load off a gauge.
The good ones use a calibrated load cell. Ultra Force (a Wehrs Machine Racing Products brand) calls their smasher “a precision measurement and calibration tool” that “applies a known, repeatable force to a spring while measuring load versus deflection” — letting you “map load at specific installed heights, match springs side-to-side, and validate setup changes before they go on the car.”
The basic process:
Do all four corners and you’ve got a load map of your baseline. Now any spring you test on the bench can be matched to that number — or deliberately offset from it — before it ever touches the car.
You have to scale your car to your target percentages before smash numbers mean anything. Smash numbers are not a shortcut around good baseline work. They’re a shortcut after you’ve done it.
BSB is blunt about it: scale the car “to your desired percentage on a standard spring package” and confirm weight distribution first. Only then do you pull the coil-overs and record what each corner is loading.
Why? A smash number describes load at one corner — it says nothing about whether your cross weight, nose weight, or left-side percentage is right. If your baseline is crooked, every load number you record just preserves that crooked baseline. Get the car scaled and your percentages where the class and track want them, then capture your smash numbers as the fingerprint of that good setup. Skip the scale and you’re chasing load numbers while a real problem hides underneath.
Here’s the payoff. Once you know your right front holds, say, 610 pounds at 18.250 inches in a setup you liked, you own that target.
Say the track goes slick and you want a softer right-front spring for more forward bite. Instead of bolting on a new rate and re-scaling the whole car — burning a heat-race worth of time — you smash the new spring to the same 610-pound load and set the collar so it holds that load at your ride height. The corner weight stays put. You changed the spring’s behavior without blowing up your baseline. That’s repeatable speed — the difference between “I threw a spring at it and hoped” and “I made one controlled change and I know exactly what moved.”
A number you can’t find on race night is a number you didn’t record. Log it so future-you can use it:
Paper works. Grab a binder, print a sheet per corner, and log it in pen every time you touch the car. The only rule is that you actually do it — and that you keep it forever, because a smash number’s value is the history behind it. (More on building out the whole sheet in our guide to the race car setup sheet.)
The biggest smash-number trap is treating the load number as the goal instead of a description of a good setup.
You can match a spring to a target load perfectly and still have a car that’s junk — because you never re-checked cross weight or left-side percentage after the change. Load numbers describe one corner in isolation; they don’t tell you the car is balanced. Keep pushing loads around chasing a feel without putting the car back on the scales, and you can walk your weight distribution out of the window and never see it coming.
Rule of thumb: use smash numbers to make repeatable spring changes, then verify the big changes back on the scales. The two tools answer different questions. Tire temps answer a third — how the setup is actually working the tire on track. (Here’s how to read tire temps after a run.)
Yes. “Smash number” and “load number” are used interchangeably in racing. Both describe the amount of force a spring is holding when it’s compressed to a specific shock height. The name comes from the spring smasher tool used to measure them. Some racers say “load numbers,” some say “smash numbers” — same idea.
To capture accurate numbers, yes — a smasher with a calibrated load cell is how you read load at a set center-to-center height. Many racers share one within a team or shop. Without it you can still track spring rates and ride heights, but you won’t have true load numbers to match springs against.
Measure at the center-to-center length you actually run at that corner. There’s no universal number — it varies by class, chassis, and setup. Record the height alongside the load and stay consistent, so your numbers compare apples to apples night to night.
The concept works anywhere you run coil-overs — Limited Sportsman, Sportsman, NE Modified, Late Models, and up. The specific loads are class- and track-dependent, so build your own history rather than copying someone else’s numbers, and confirm any setup against your rulebook and chassis builder.
No. Scale the car to your target percentages first, then record smash numbers to capture that baseline. Load numbers describe individual corners; they don’t verify cross weight or weight distribution. Use them for repeatable spring swaps, but put the car back on the scales for big changes.
You can do every bit of this by hand — a smasher, a binder, a pen, and the discipline to log all four corners every night. Plenty of racers do, and it works. The part that breaks down isn’t the measuring. It’s the record-keeping: the sheet that got soaked, the setup from two seasons ago you can’t find. That’s the job RaceNight was built for. Here’s how the post’s tasks map to the app:
Start free. Download the printable Spring Smash setup sheets for Limited Sportsman, Sportsman, and NE Modified at /downloads — built to capture load numbers the right way. Then log it all automatically in RaceNight at /racenight.
Set the baseline. Log the loads. Swap with confidence. When you’re ready, dig into the full race car setup sheet and how to read your tire temps after a run. Both of those live next to the number that decides how the car turns — cross weight. Smash numbers are a shock’s report card, so read them next to race car shock tuning.
Not sure what a term above means? Every one of them is defined in the RaceYear racing glossary.
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