Car Performance & Handling: Swaybars!

It’s been several years since I autocrossed or owned a high performance car. I still like fun to drive cars, I just don’t have the time for it anymore. I finally got around to doing the first performance upgrades that I’ve done in years. Back in my SCCA days, the first 2 mods anybody did was (1) tires, and (2) swaybar.

Swaybar 101

When a car has a swaybar, in order for the body to roll L or R it must twist the swaybar. Stiffer swaybars reduce L-R body roll without affecting the spring or shock rates. If both wheels hit a bump and move together, the swaybar does nothing. It only kicks in when the L and R sides try to move differently. When one side (L or R) tries to move up or down, the swaybar forces the other side to also move up or down too. How much, depends on the swaybar’s stiffness.

A stiffer swaybar reduces body roll, which reduces weight transfer, which reduces overall traction. Yet at the same time, it improves response and agility. So it’s a trade-off. Put differently, you need to allow some body roll to get good traction, but too much of it reduces response.

Adjustable Swaybars

When the car’s body roll twists the swaybar, it does so through connecting arms. Longer arms give the body roll forces a longer moment arm, making it easier to twist the bar, making a softer bar (lower rate) with less resistance to body roll. Adjustable swaybars usually have several points along their arms where you can connect the end links.

Must both of the swaybar arms have the same length? Imagine if one arm is longer than the other; that is, connecting the end links to different mounting points on each arm. Newton’s 3rd law says the torques exerted are always equal and opposite, which seems to suggest that an asymmetric setup could give symmetric roll response. In this case, an adjustable bar with 2 holes actually has 3 different rates, or with 3 holes has 5 different rates.

However, while the torques are always equal and opposite, even when connected asymmetrically, the moment arms are not. And the torque on the bar exerts a force on the opposite side through its moment arm. So connecting the swaybar asymmetrically would create asymmetric roll rates: stiffer to the L than to the R, or vice versa.

The conclusion: always connect the end links to the swaybar using equal length arms.

Tuning the Response

Most cars are designed to understeer: that is, under most conditions the front slides before the rear does. This is easy to control, especially for unskilled drivers. But skilled drivers find excessive understeer to be less fun and even annoying. Excessive understeer makes a car less responsive. As a general rule:

  • A stiffer rear swaybar reduces understeer, increases oversteer.
  • A stiffer front swaybar reduces oversteer, increases understeer.

This is all relative. Most factory cars are too soft overall and also understeer, so a stiffer rear swaybar is ideal. But if the car is too stiff overall and also understeers, you might use a softer front swaybar.

Mazda 3

My 2014 Mazda 3 is actually pretty fun to drive, for a FWD economy car. At 36000 miles I finally had to replace the OEM tires. While I was doing this I figured I’d also install a stiffer rear swaybar.

This is such a common car there are many options. I used a 22mm Progress bar. It was relatively inexpensive and came with new bushings and brackets to handle the larger forces. The OEM rear swaybar is 18mm diameter with a rate of 334 in-lbs. The Progress is 22mm diameter with rates of 772 in-lbs (about twice as stiff), and 1,015 in-lbs. (about 3 times as stiff). This bar has excellent build quality and perfect fit with the end links pointing straight up and down just like they do with the OEM bar.

The soft setting made a noticeable yet not a huge reduction in body roll. I quickly shifted to the stiff setting which was completely different. Less body roll, quicker turn-in and more precise handling. But, it got twitchy. The swaybar was too stiff for the rest of the suspension. So I kept it at the soft setting (still twice as stiff as stock).

A few months later, I replaced the shocks & springs all around. With the stiffer shocks (Koni yellows) and springs (+20% rates from Racing Beat), the swaybar’s stiff setting was better matched to the rest of the suspension. The twitchiness was gone, now it was just a precise, sharp, flat tracking, great handling car.

Subaru Forester

My wife’s 2018 Forester is a lot softer than our 2004 Forester. It drives more like a bus than a car. Over the years, Subaru softened the suspensions. Subaru has OEM swaybar replacements in 2 sizes: 19mm or 20mm. Stock is 16mm, so the 19mm is about twice the rate. I got this bar from Subaru Online Parts, cost about $100 and included new brackets and bushings.

This makes the new Forester handle more like the old one. It’s less like a bus, more like a car. It feels tighter and more controlled. But not too tight. If you go too stiff with an AWD vehicle it can impair traction off road. It’s perfect for my wife, who wanted less body roll but is not a performance car enthusiast.

NOTE: the end links on this Subaru were quite nearly frozen. The end link attachment bolt was corroded to the nut. And the car is only 2 years old, 4700 miles, and has not been driven on salted roads. I removed the end links from the car, so I could remove the entire bar with end links attached to it. Soaked the end link bolts in liquid wrench and moved the nut back and forth chasing the threads until it finally came free.

More generally, a stiffer swaybar applies greater forces to the end links. So if you more than double the rates, don’t be surprised if the end links eventually break. Keep an eye on them and be ready to replace them with more robust aftermarket end links.