Last reviewed: August 3, 2026 · By the RaceYear team
Short answer: A four-link setup locates the rear axle with two bars per side, running from the frame to a birdcage that rotates on the axle tube. On a dirt track, bar angle controls how much the rear end steers and squats under load, and bar length controls how hard the car drives off the corner. The left-rear bars own forward bite; the right-rear bars own entry.
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.
Nobody explains the bars. You buy the car, the guy selling it says "it's in the third hole," and that's the entire handoff. Then the track goes slick in the feature, you've got twenty minutes and a wrench, and you're staring at a bracket with six holes in it doing math you never learned. Here's what each bar is actually doing, one adjustment at a time, on any brand of car.
A four-link setup locates the rear axle with four trailing bars, an upper and a lower on each side, running from brackets on the frame back to a mount on the axle tube. Modified racers say four-bar, Late Model racers say four-link, and it's the same four bars doing the same job.
DIRTcar's Modified rulebook defines the class by it: "Rear suspension configuration used on current and new chassis(s) must be the design commonly known as four (4) link" (DIRTcar Modified rules). On the Late Model side, DIRTcar allows "a maximum of two (2) attaching (radius) rods per side" (DIRTcar Late Model rules). That's a ceiling, not a requirement, which is why cars in the same class can run different rod counts on the left and the right.
Every bar gives you three separate adjustments, and racers mix them up constantly:
Harris Auto Racing's Modified/SportMod manual gives a length baseline you can measure tonight with a tape: "Start by setting the 2 lower 4 link bars at 15" center to center of the rod end and the uppers at 18" center to center of rod ends" (Harris Auto Racing). That's one builder's car, but it tells you the scale of the parts you're arguing about: the uppers are about three inches longer than the lowers, and a couple of turns on a rod end is a real change.
A birdcage is a bearing housing that rides on the rear axle tube and is free to rotate around it, so the bars, shocks and springs bolted to it can move without being tied to the axle housing's own rotation.
The patent literature describes them about as plainly as anyone: "Birdcages are typically used in connection with the rear axles of high performance vehicles, such as race cars or sprint cars," with the bearing's inner race turning with the axle while the cage carries a torsion arm at the bottom and a pair of radius arms at the top, the radius arms running forward to the frame (US Patent 7,021,414).
The failure mode is in that same patent. As the axle moves out of parallel with the ground, side loads and axial forces get exerted on the bearing and the birdcage, and the patent says plainly that this can cause the bearing to bind. A bound cage isn't rotating, which means the geometry on your setup sheet is not the geometry the car has. Spin both cages by hand on the jack stands before you go chasing a handling change with a wrench.
Indexing means rotating the cage on the axle tube to a set angle before you tighten it, which decides where the bar holes sit relative to the ground. Harris's manual gives a number: "The birdcages need to be indexed at 2-3° on LR and 0° on RR."
Then it gives the warning almost nobody passes along in the pits: "Be sure that the ride heights are set prior to setting birdcage index and reset ride height, keep in mind ride height will change index." Read that twice. Change a spring, change a shock, put a round of wedge in, and you moved ride height — which moved your index, which moved your bar angles. If you changed ride height and didn't re-check index, you made two changes and only wrote down one.
A solid, welded bracket ties the bar straight to the axle housing, so anything that rotates the housing under power or braking moves that bar with it. A floating birdcage puts a bearing between the housing and the bar, so the housing can rotate while the bar holds the angle you set. That's why rulebooks treat them as two different animals, and why your sanctioning body has an opinion about which one you're allowed to bolt on.
IMCA's 2026 Modified rules put the choice in one sentence: "One bracket mounted solid to axle tube with lower link OR one floating birdcage with upper and lower links allowed per side" (IMCA 2026 Modified Rules). Read it carefully — under those rules a solid bracket gets one bar, and if you want two bars on that side, the side has to float. IMCA also requires that "all trailing arms/link bars must be solid tubing," allows "one mechanical traction pull bar," and flatly bans lift, brake and sway bars.
Here's where published rules genuinely disagree, and it isn't a small thing: DIRTcar's Late Model book permits axle-housing mounts in aluminum or magnetic mild steel, while DIRTcar's own Modified book states that "aluminum bird cages will not be permitted." Same sanctioning body, two divisions, opposite answers. Never carry a birdcage decision across classes on somebody's word.
| Rule area | IMCA Modified (2026) | DIRTcar Modified | DIRTcar Late Model |
|---|---|---|---|
| Mount per side | Solid bracket with lower link or floating birdcage with upper and lower links | Solid (welded) or floating (birdcage), one per side | One axle-housing mount per side, assembled as one piece |
| Radius rods per side | Two only if the side runs a floating birdcage | Two maximum | Two maximum |
| Aluminum birdcages | — | Not permitted | Aluminum or magnetic mild steel permitted |
| Rod spacing on the cage | — | — | Minimum 6 in., maximum 12 in. |
| Adjusting from the seat | No driver-adjustable devices except the brake adjuster | No hydraulic or pneumatic weight jacks or trackers | Cockpit adjusters not permitted except brake bias |
A dash means the rule wasn't in the rear-suspension text quoted here, not that anything goes. Read your own book before you cut a bracket, because the tech inspection line is an expensive place to learn this.
That six-to-twelve-inch spacing window in the Late Model rules is worth sitting with. Where you put two rods inside a six-inch range on the same cage is a real geometry change that the rulebook explicitly leaves up to you, and it's a number most racers never write down.
Two definitions first, because the rest of this depends on them. Rear steer is the rear axle moving fore and aft on one side as the suspension travels, which points the rear tires slightly left or right and changes where the car wants to go. Forward bite is how hard the rear tires plant when you get back on the throttle.
Now the bars. This table is Harris Auto Racing's language for their Modified and SportMod chassis, which is one builder's car — but it's a written, dated, attributable starting point, which is more than most racers get.
| Bar | Harris starting hole | What the manual says the adjustment does |
|---|---|---|
| Left rear, lower | #6 on the lower | "Raising the lower arm will increase rear steer on entry and exit" |
| Left rear, upper | #4 on the upper control arm | "Raising the upper link will increase drive in the LR tightening the car under loose conditions" |
| Right rear, lower | #3 on the lower | "Increasing angle on the arm will increase RR drive in the car on entry and exit" |
| Right rear, upper | #3 on the upper control arm | "As you lower the angle it will free car up on entry but will help car tighten up on exit" |
Read that right-rear upper row again, because it's the one that pays. Lowering the angle frees the car on entry and helps tighten it on exit — the exact trade a car that pushes in and spins off is asking for. That's a single bar doing two useful things at once, which almost nothing else on the car will do for you.
One hard warning about the hole numbers: they are builder-specific. Hole 4 on a Harris bracket is not hole 4 on anybody else's bracket, and some builders count from the top while others count from the bottom. Before you copy a number off a message board, find out which end of the bracket the other guy is counting from. Better yet, mark your own brackets with a paint pen and count them the same way forever.
Move the hole when you want to change how the rear reacts to load, and move the length when you want to change where the axle sits. They feel similar in the shop and they are not the same adjustment.
Bar length is the one with a published number attached. Dirt Track Supply's four-link quick reference says to tighten the car overall, "decrease right rear trail (shorten right side 4 link rods 2 turns at a time)," and to loosen the car overall, "increase right rear trail (lengthen right side 4 link rods 2 turns at a time)" (Dirt Track Supply). Two turns at a time. Not a hole, not a guess — a countable, repeatable, undoable change you can make in the dark with a wrench and put back if it's wrong.
That's why length is a smart place to experiment and holes are a place to be careful. You can count turns back. You cannot count back a hole you don't remember moving.
Name the corner phase before you touch a bar. A car that's tight on entry and a car that's loose off the corner are two different problems, and the same bar move can help one and hurt the other.
| Complaint | Four-link move to consider | The source language |
|---|---|---|
| Pushing on entry | Lower the right-rear upper bar angle | "As you lower the angle it will free car up on entry but will help car tighten up on exit" |
| Spinning the left rear off the corner | Raise the left-rear upper link | "Raising the upper link will increase drive in the LR tightening the car under loose conditions" |
| Car won't rotate, needs more rear steer | Raise the left-rear lower arm | "Raising the lower arm will increase rear steer on entry and exit" |
| Tight everywhere, all night | Lengthen the right-side rods two turns | "Increase right rear trail (lengthen right side 4 link rods 2 turns at a time)" |
| Loose everywhere, all night | Shorten the right-side rods two turns | "Decrease right rear trail (shorten right side 4 link rods 2 turns at a time)" |
Now the part the charts bury. Dirt Track Supply's own quick reference lists "increase left rear bite" as a way to loosen the car on corner entry, and lists "increase left rear bite" again as a way to tighten the car on corner exit. Same change, opposite results, depending on where in the corner you're asking. Anybody who tells you more left-rear bite simply equals tighter has never read the chart they're quoting.
Second thing that chart tells you, if you count: most of its entries are panhard bar, spring and ride-height moves, and the four-link bars themselves come up mainly in the trail items. That's an honest signal — most nights, the bars are not your first move. Diagnose the phase, check the easy stuff, and get your loose vs. tight read straight before you start pulling bolts out of brackets.
Write down enough that a stranger could put the car back exactly how it was. Speedway Motors' Modified setup guide comes down to two habits: make one adjustment at a time, and record how the car performed into, through and off the corner (Speedway Motors). Every field below exists to serve those two habits.
The failure isn't measuring. It's that the sheet lives in three places: a note on your phone, a number in your head, and a chalk mark on the trailer wall. By the next race those three disagree, and the car is a mystery again.
Nothing mechanical. Four-bar is what Modified racers call it, four-link is what Late Model racers call it, and both mean two trailing bars per side running from the frame to the axle. DIRTcar's Modified rulebook uses the four-link name for the design it requires, while the same sanction's Late Model book talks about radius rods attaching to axle-housing mounts. Same hardware, different regional vocabulary.
Start at the right-rear top bar. Harris Auto Racing's manual says of the right-rear upper control arm that "as you lower the angle it will free car up on entry but will help car tighten up on exit," which is exactly the trade a car that pushes in and spins off is asking for. Move one hole, drive it, and write down where it was before you touched it.
Yes, and it's the most commonly skipped step in the pits. Harris Auto Racing's manual is explicit: set ride heights before you set birdcage index, then reset ride height, because "ride height will change index." A spring change, a shock change or a wedge change all move ride height, and every one of them quietly rotates your cages and moves your bar angles.
Depends on your rulebook and how many bars you want on that side. IMCA's 2026 Modified rules allow "one bracket mounted solid to axle tube with lower link OR one floating birdcage with upper and lower links" per side, so a solid bracket gets one bar and a floating cage gets two. DIRTcar's Modified book allows solid or floating mounts but bans aluminum birdcages outright.
One. Speedway Motors' Modified setup guide comes down to making one adjustment at a time and recording how the car performed into, through and off the corner. Two changes at once means you learn nothing, because you can't tell which one did what. If the car needs two things, make the bar change tonight and the spring change next week.
You can absolutely do all of this with a paint pen, a tape measure and a notebook in the toolbox. Plenty of track championships have been won exactly that way, and the measuring was never the hard part. The hard part is that hole numbers, bar lengths and cage index are the fields a racer swears he'll remember and cannot recall eight days later. That's the exact problem RaceNight was built for:
Log every hole, every turn and every degree of index in RaceNight — it works offline, and every plan starts with a 14-day free trial. Starting on paper this week? Grab the free printable Spring Smash setup sheets before you load the trailer. Then build out the rest of the sheet with our guide to the race car setup sheet, and get your wedge sorted with cross weight explained before you start moving bars. Geometry decides where the car sits; shocks decide how it gets there — race car shock tuning. The bar that sets the rear roll center gets its own page — panhard bar adjustment.
Hit a term in here you'd have to ask somebody about? They're all defined in the RaceYear racing glossary.
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