Last reviewed: August 3, 2026 · By the RaceYear team
Short answer: Here’s how to scale a race car at home: level your four pads on a checked-flat spot (shim if needed), set full race trim — fuel, tire pressure, ride height, and stagger — with the driver or matching ballast in the seat, disconnect the sway bar, bounce and roll the car to settle it, then read left side, rear, cross, and total weight. Do it the same way every time and the numbers compare.
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 just bought a used set of scales, hauled them into the garage, and rolled the car up onto the pads. Now what? This is the part almost nobody writes down clearly: the actual garage procedure that makes this week’s numbers mean the same thing as last week’s. Get the procedure wrong and you can own the best scales money can buy and still be chasing ghosts.
One thing up front, because it trips a lot of racers up. This post is about how to scale a race car — the mechanical steps that make the numbers repeatable. It is not about what the cross-weight number does to how your car handles once you have it. That’s a separate question with a separate answer, and we’ve written it up in Cross Weight (Wedge) Explained. Read this one first to get a number you can trust. Read that one next to decide what to do with it.
Scaling a car means putting it on four individual pad scales — one under each tire — and reading the weight each tire is carrying. Those four readings are your corner weights, and every percentage on your setup sheet (left side, rear, and cross weight) is calculated from them.
That’s the whole idea. It sounds simple because it is simple — which is exactly why it’s easy to do sloppily and still get a number. A number on the scale readout isn’t the same thing as a number you can trust six weeks from now.
Here’s the trap. You scale the car Tuesday night, write down a cross weight, and it looks fine. You scale it again in three weeks and the number’s moved — but nothing on the car changed. What happened is the procedure changed: different fuel level, tires at a different pressure, floor not quite level in the same spot, sway bar left connected one night and not the other.
Corner weight scales are precise, but precision doesn’t fix an inconsistent setup. A scale that reads to the pound will faithfully report a wrong number if the floor’s tilted or the tank’s half empty when it was full last time. Grassroots Motorsports’ scaling guide is blunt about the floor alone: “Make sure the floor is perfectly level; use shims under the scale pads if needed” (Grassroots Motorsports). Everything below is built around removing that kind of variable, one at a time, so the only thing that changes your numbers is an actual change to the car.
Most home garage floors look flat and aren’t. This is the step people skip, and it’s the one that quietly wrecks every number after it.
NASA’s Speed News found exactly that problem in their own shop: “not all shop floors are created equally level, and we found that ours was actually not as level as we were hoping for” (NASA Speed News). Their fix didn’t require jackhammering the slab — they checked levelness with “two equal-size sockets, a piece of aluminum square tubing and a level,” measuring “laterally, longitudinally, and across corners between each scale,” then shimmed the low spots with thin floor tiles because it was “much cheaper” than re-pouring concrete.
A few ways to get the same result at home:
Do this once, mark the spot, and you’ve solved the floor problem for every session after this one.
A cross weight taken with the wrong fuel, tires, or ride height isn’t wrong math — it’s the right math on the wrong car. “Race ready” has to mean it, not “close enough.”
Longacre’s scaling checklist runs through the fluids and settings first: full fuel, full radiator, oil circulated, gear oil and transmission full, stagger set, tire pressure set, camber set, rear end square (Longacre Racing). Speedway Motors’ checklist covers the same ground — fluid levels topped off, stagger and tire pressure set, ride heights adjusted, caster, camber, rear end square, and toe checked — before the car ever sees a pad (Speedway Motors).
| What to set | Why it has to be right first |
|---|---|
| Fuel load | A full tank versus a half tank moves real weight, mostly toward the rear — pick one convention and scale to it every time |
| Tire pressure | Set to your actual race pressure; a pound or two of air changes the tire’s footprint and the reading under it |
| Ride height | Set at all four corners before you scale — ride height and cross weight move together, so scaling at the wrong height gives you a number for a car you don’t race |
| Stagger | Set before scaling; it walks around as tires wear, so measure it fresh |
| Driver or ballast | Driver in the seat is the gold standard; if you can’t, use the same ballast in the same spot every single time |
On fuel, pick a convention and stick with it. Grassroots frames it as scaling with a load of fuel for where you want the car balanced — “either at the start of the race, the end of the race or an average between the two” (Grassroots Motorsports) — any of those works, but only if it’s the same one every session.
On the driver, Speedway Motors puts it plainly: “I like to see the driver in the car during the set up process as he is going to be in the car when it goes around the corner” (Speedway Motors). If the driver genuinely can’t be there, weigh in with ballast that matches his gear-on weight, bolted down in the seat — and use that same ballast every time, not a rough guess.
A hooked-up sway bar is one of the most common reasons a home-scaled number doesn’t match what the car does at the track. The bar ties the left and right suspension together, which means it’s quietly sharing load across corners while you’re trying to read them individually.
Grassroots is direct about it: “Disconnect the shocks, when possible, and the anti-roll bars” (Grassroots Motorsports). Speedway Motors’ checklist adds a real-world check for a bar you can’t fully unbolt: “verify that the sway bar is disconnected or completely neutral with plenty of slop” (Speedway Motors).
Then free the suspension itself before you read anything, because a bound-up spring or bushing will hold a reading that doesn’t match reality. The routine most of these guides describe is nearly identical: bounce the rear of the car, bounce the front, and give the roof bar above the driver’s head a few good shakes, letting the car settle naturally each time rather than forcing it down (Longacre Racing).
Getting the car onto the pads matters as much as anything before it. Roll the car straight onto the scales rather than dropping it from a jack, and make sure the tires are centered on the pads.
Bounce each corner one more time once it’s sitting on the scales, then roll the car off and back on and check that you get the same numbers. The Grassroots forum thread on this exact topic puts a number on it: “always roll the car and bounce the car for at least 20 feet after any adjustment” before you trust a new reading (Grassroots Motorsports forum). If the second read doesn’t match the first, something’s binding — a tie rod, a shock, a shim under one pad — and that’s worth finding before you touch a jack bolt.
Once the car’s settled and level, you’re not just reading one number off the display. Four numbers make up a scaling session that actually compares week to week:
| Number | What it is | Why it matters |
|---|---|---|
| Total weight | All four corner weights added together | Confirms you’re at your class minimum and haven’t drifted from last time |
| Left side % | Left front plus left rear, divided by total | For road racing, Grassroots calls 50% “the ideal left weight percentage” (Grassroots Motorsports); a circle-track car deliberately runs off it, within rulebook caps |
| Rear % | Both rear corners, divided by total | Sets how much static weight sits over the drive tires for corner exit — Grassroots notes this number “is less definite” and matters more the more power the car has (Grassroots Motorsports) |
| Cross % (wedge) | Right front plus left rear, divided by total | The number covered in depth in Cross Weight (Wedge) Explained — this post gets you a trustworthy number to plug into that math |
Write all four down every time, along with the conditions you scaled in — fuel level, tire pressures, and whether the bar was disconnected or just slack. Longacre’s advice on this is short and correct: “record your wheel weights, partial weights and percentages,” and check that “the left side and total are where they need to be” against your class rules (Longacre Racing).
One more honest note on targets. Some guides publish starting-point ranges for specific classes — Dirt Track HQ, for example, lists a rough baseline for quarter-mile dirt street stock and sport mod cars in the neighborhood of 52–56% rear, 52–55% left side, and 49–54% cross (Dirt Track HQ). Treat any number like that as a rough starting compass, not a target — your class rulebook and your chassis builder’s own baseline for your specific car are the real answer, and they can land well outside somebody else’s range.
Put it all together and one scaling session at home looks like this:
Do this the same way every single time, and the number you write down tonight means exactly what it meant last time you wrote one down.
The driver in the seat is the gold standard, since that’s the weight the car actually carries around the track. If the driver can’t be there, use ballast that matches his gear-on weight, bolted in the same spot every time. What ruins the comparison isn’t using ballast — it’s switching between driver-in and ballast-in from one session to the next.
Pick one fuel convention and use it every time — full tank, empty, or a half-tank average — because fuel is real weight that shifts mostly toward the rear. It matters less which convention you pick than that you never switch conventions between scaling sessions, or your numbers stop comparing.
No, but you do need to know how unlevel it is and correct for it. Check with a straightedge and level or a simple water level, shim the low pads, and mark that exact footprint so you set up the same way every time. An uncorrected tilt of even a small amount can throw your readings off.
Before. Set tire pressures to your actual race numbers before the car goes on the pads, because a pound or two of air changes the tire’s footprint and the weight reading under it. Scaling with the wrong pressure gives you a real number for a car you’re not actually going to race.
Any time you change fuel load, tire pressure, ride height, springs, or ballast — and at minimum, before every race weekend as a baseline check. If a number moves and you can’t explain why from something you changed, that’s the signal to check the floor, the bar, and the suspension bind before you touch a jack bolt.
You can absolutely run this whole procedure with a pen and a binder — plenty of racers do, and the steps above work with nothing but scales, shims, and a level. The part that’s genuinely hard to keep straight by hand is remembering exactly how you scaled it last time: what fuel load, what bar state, what tire pressure — the conditions that make one night’s numbers actually comparable to another’s. That’s the job RaceNight was built for:
Scale your car in RaceNight and let it hold the numbers and the conditions together — it works offline, and every plan starts with a 14-day free trial. Building your first sheet on paper tonight? Grab the free printable Spring Smash setup sheets to get in the habit. Once you trust your numbers, learn what the cross-weight number actually does to the car in Cross Weight (Wedge) Explained, and see where all four numbers live long-term in How to Build a Race Car Setup Sheet.
Ran into a term here you’d have to look up? They’re all defined in the RaceYear racing glossary.
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