I know what a shock does. I know what a spring does. I want to understand Sway Bars (AKA Anti-Roll bars) a bit more.

I have a fleet of hooptie Mazdas.

One of my kids drives a 2007 Mazda 3 hatchback.

My wife drives a 2014 Mazda 5 minivan.

And I have 2009 Mazda RX-8 and have a couple RX-7s in the garage (1st gen and 2nd gen)

The suspension on the Mazda 3 is modified. I have "Racing Beat" springs, "Koni" Shocks, and "CorkSport" sway bars on this car. But the key point is that the sway bars are of a matched set. The car handles very predictably through turns with minor understeer when exceeding the limits of the tires.

The suspension on the Mazda 5 is mostly stock. In working under it, I found the suspension and other components of the car to be remarkably similar to the Mazda 3. The kiddos found that it's got quite a lot of body roll when on a curvy road and for ***** and giggles, I found a "TriPoint Engineering" (defunct) sway bar for cheap on Facebook Marketplace designed for a Mazda 3 and put it under the Mazda 5. This bar is SIGNIFICANTLY stiffer than stock, as the diameter is twice as large and the radius portion of it is not bent round stock like the original was, but a gusseted laser cut piece of flat stock steel. The stiffer bar results in a bit of oversteer when driving spiritedly. I really like this.

So looking at the sway bars on my RX-8, imagine my surprise when looking at the bars and finding out the rear bar is significantly smaller than the front. Not only that, but the rear bar appears to be tubular over the solid front. I thought this may be more of a FWD vs RWD thing and I may be comparing apples to oranges...

Comparing my RX-7s, the 1st gen is a solid rear axle vs the 2nd gen which is independent suspension. In their stock setup, I found the 1st gen loves to oversteer when getting on the throttle in a turn while the 2nd gen is not as much. Someone told me to remove the front sway bar and see what happens, which I did and the 2nd gen became more tail happy. This seems to support a stiffer rear being better than a stiffer front, however these are RWD cars as well.

Can someone educate me or direct me towards resources on sway bars and how they affect driving feel?

 

Honestly the easiest way to understand how swaybars change handling is to hop in a game like Forza or Assetto corsa or most racing sim games and try playing around with the front and rear stiffness. Although the Rx8 is not the best in forza because they had messed up the suspension geometry and the car doesn't drive right......The only assetto corsa mods for the Rx8 were gameports from the broken Forza models. Maybe at some point I will post my recoded files I did based on my IRL cars.

Maybe some of the Autocross guys have better explanations, but here is mine. Generally swaybars are used for adjusting how much body roll you get when a either end of your car is loaded up. For example the front swaybar is most active when the weight of your car is leaning forward like during hard braking and turning in. Stiffening the front creates more understeer during braking and turn in. The rear swaybar will be most active when the weight is shifted rearward like during acceleration at corner exit so stiffening will promote oversteer.

The really simplified tradeoff is that the looser you are generally the more grip you will have because you can get your weight shifted to the outside. The stiffer you are the easier the car is to drive because you wont be sloshing around with body roll. Too stiff and you will induce unwanted oversteer or understeer

 

Think about what oversteer and understeer are: (controlled) loss of traction at the front or at the rear. Lose traction at the front before the rear and you get understeer, the tires are not rolling along their track but slipping to the outside of the the turn. Traction is a function of weight on wheel: more weight, more lateral force required to slide the tire. The sway bar connects the 2 sides springs and effectively adds spring rate to the outside wheel, using the other side's spring and it's own torsion. In the extreme it acts as a solid axel. So now you have less weight on wheel and less resistance to slipping on the outside, and more weight on wheel on the inside, but the inside isn't really doing a ton of work anyway. You can lift the inside wheel entirely with too stiff a setup. Result = more slip at that end of the car.

So why use bars at all? Because being able to add spring rate to the outside means you can run softer springs for every other condition and let the suspension work through bumps, uneven roads and acceleration/deceleration. Hard springs require smooth roads, which is usually not the case.