ND Miata Sway Bars

A sway bar is a torsion spring that ties the left and right wheels of an axle together. In a corner, the loaded outside wheel tries to compress and the bar transfers some of that load across to the inside wheel, resisting body roll. Stiffen the bar at one end of the car and you make that axle work harder in roll — which loads its outside tire sooner and lets it give up grip first. That’s the whole tuning lever: a stiffer bar at one end shifts the handling balance toward losing grip at that end. Stiffer front bar pushes the car toward understeer; stiffer rear bar pushes it toward oversteer.

The ND is a good platform for this because the chassis is light, the roll centers are sane, and the factory bars are conservative. Mazda tunes the stock car to understeer — it pushes wide at the limit, because a car that runs out of front grip first is predictable and safe for someone who isn’t expecting the rear to step out. Most ND owners who upgrade are trying to dial some of that understeer back out and let the car rotate.

What the ND comes with

The factory front bar is a relatively thin solid bar (commonly cited in the ~19–22 mm range; Mazda doesn’t publish the figure and it isn’t worth chasing). The rear bar is much smaller (roughly half the front’s diameter), and on some markets and the cheapest trims a rear bar is omitted entirely. The takeaway matters more than the exact millimeter: the stock setup is front-biased and soft, which is why the car rolls noticeably and pushes at the limit.

That softness is deliberate and it’s fine for the street. The reason to change it is autocross, track, or a driver who simply wants flatter, more responsive turn-in. If the car only ever sees commuting, sway bars are not the upgrade you’re missing.

Front bar vs. rear bar: which one to touch

This is the question that decides everything, and the answer is unintuitive if you’ve never tuned a chassis:

The rear bar is the one most ND owners actually want. Stiffening the rear loads the rear outside tire sooner, which kills understeer and lets the back of the car rotate into the corner. On a car Mazda tuned to push, a stiffer (or simply present) adjustable rear bar is the single biggest balance change for the money. This is why the most common ND recommendation is a stock or mild front bar with an adjustable rear.
A stiffer front bar does the opposite — it adds understeer. People reach for a fat front bar to cut body roll, and it does flatten the car, but it also makes the front give up first. A big front bar is mostly for cars that have gone the other way (lots of rear grip, too much rotation) or for very high-grip tire setups where you need to manage front load. On a stock-ish ND it usually makes the push worse.
Don’t stiffen both ends hard at once. If you crank a fat front and a fat rear, you’ve stiffened the whole car in roll without changing the balance much — you just get a harsher ride and more snap at the limit. Pick the end you want to change and adjust the other only to fine-tune.

The practical mistake to avoid: adding a big rear bar with no front change can take a forgiving car and make the rear come around quickly, especially in the wet or on a trailing throttle. That’s why adjustable rear bars exist — start in the softest hole and work stiffer.

Adjustable bars and the holes

Most aftermarket ND bars are end-link adjustable: the bar arm has two or three holes where the end link attaches. Moving the link to the hole nearer the bar’s center end shortens the effective lever and makes the bar softer; the hole toward the tip makes it stiffer. Racing Beat’s guidance is the standard advice — start in the middle hole, drive it, then move one hole at a time once you know how the car behaves.

Adjustability is the reason to buy a real bar instead of guessing a fixed diameter. You’re not buying a stiffness number, you’re buying a range you can tune at the event.

Popular ND options

These are the bars ND owners actually run. All bolt to factory mounting points; the front bar shares its location and a couple are known to need the factory mount reinforced under hard use.

Racing Beat — the long-standing Miata sway bar maker. Front bar is a tubular 1.125” OD × 0.188” wall design with three adjustment holes; they sell it with center pivot bushings and brackets. The common track pairing is the Racing Beat front with a Cusco 14 mm rear bar — that combo is a well-known ND autocross/light-track setup that works on 245-section and smaller tires. Racing Beat also sells a matched front-and-rear package.
Flyin’ Miata — a tubular 1.125” × 0.188” front and a solid 0.625” rear, both adjustable. FM also sells a front sway bar mount reinforcement kit for the ND, which is worth knowing about: the factory front bar mount can flex or crack under a much stiffer bar on a hard-driven car, and the reinforcement plates address it.
Progress Technology — a front-and-rear combo developed on Progress’s own 2016 ND project car at SCCA events. Roughly a 28.5 mm hollow front and a 16 mm solid rear, with three holes front and two rear.
Eibach Anti-Roll Kit — a 27 mm front / 16 mm rear matched set, marketed at a large step over stock. A simple, well-supported bolt-in choice if you want a known matched pair rather than mixing brands.
Cusco, RoadsterSport, Ultra Racing, AutoExe also make ND bars; the Cusco rear in particular shows up paired with other makers’ fronts.

Specs above are as published by the makers and round numbers — diameters and wall thicknesses are the figures to confirm against the current product page before buying, since makers revise them.

A note on classed racing

If you autocross or race in a class, the rules decide your bars before your preferences do. SCCA Solo Street class allows one front or one rear bar to be changed but not necessarily a free choice of both, and end links must usually remain in a stock-style attachment — read the current Solo rules for your class. Spec Miata mandates a specific bar setup; you don’t get to tune it. Street Touring and the prepared classes open things up. Before you buy an adjustable kit to chase balance, confirm your class even lets you adjust it, or you’ll have a legal-but-fixed bar you bought for adjustability you can’t use.

Installation reality

A rear bar is a driveway job — drop the existing bar, swap bushings, bolt the new one in, a couple of hours with the car safely supported. The front bar is more involved because of its location behind the front subframe area and the end-link and bushing work; budget more time, and if you’re fitting a much stiffer front bar, plan for the mount reinforcement at the same time rather than after it cracks. Polyurethane or the supplied bushings should be greased on install or they’ll squeak; that squeak is the most common “did I do this wrong” question after a sway bar swap, and the answer is almost always dry bushings.

How sway bars fit with the rest of the suspension

Sway bars change balance cheaply without changing ride height or spring rate. They pair naturally with an alignment — more front camber does more for outright grip than any bar, and the two together are how you actually find lap time. They don’t replace dampers or springs; if the car is bouncing or the body control is poor, that’s a damper and spring conversation, not a bar one. And they interact with your tires: a bar setup that’s neutral on 200-treadwear track tires can feel different on a softer or wider tire because the grip balance underneath has moved.