Swaybars and what they do

[bluemax_1]

*Posted this in another thread but figured it belonged in the Suspension forum.

Well, most of us have some idea of what a swaybar is and what it's for right? It reduces chassis roll when the car is cornering. But how exactly does it do this and what does installing a stiffer swaybar REALLY do?

First of all, a swaybar is actually a torsion spring. Swaybars are made of spring steel and do their work due to their resistance to twisting.

The swaybar is connected to the suspension linkages between the suspension on a given axle. When cornering loads are imposed, the suspension at the different corners compresses different amounts due to weight transfer with the outside wheel's suspension experiencing greater loading and thus greater compression and the inside wheel having weight transferred off it thus allowing the spring to uncompress and push that corner up.

Well, that all means that the car rolls over to the outside while cornering. Is this necessarily a bad thing? No, and yes. If the car rolled all over the place but did it quickly and all the tires remained flat on the ground, aside from making the occupants seasick, performance wise, it wouldn't necessarily be that bad (an oversimplification since the speed a suspension takes a set and how weight is transferred among other things affects the handling but that's outside the scope of this at the moment). The problem is that suspension geometry affects the tire's camber when the car is rolling. Instead of cornering flat on the tires, the rolling car changes the camber to positive camber and we start riding on the outside edge of the outer tires = reduced traction = not good.

So, we install swaybars to reduce that rolling. Keep the tires contact patch flatter and give better traction right? Yep, but how does this actually work?

The swaybar as mentioned, is a torsion spring connected to the suspension on both sides of the car on a given axle. When one side tries to compress more than the other side, this twists the swaybar. Since it's a torsion spring, it resists the twisting and (here's the important part), effectively increases the spring rate of the outside wheel's suspension.

So if you're going straight, or hit a speed bump or brake or anything that causes the suspension on both sides to compress equally, the swaybar doesn't twist and does nothing. When something happens that compresses the suspension one side more than the other, the swaybar twists and in resisting that twisting, adds it's spring resistance to the coilspring's spring rate of the outside wheel based on how stiff the swaybar is and how much differential loading there is.

It has another effect however. When it resists twisting by increasing the effective spring rate of the outside wheel (in effect, pushing that wheel/tire harder into the ground), it also affects the inside wheel because that resistance to twisting is trying to reduce the difference in compression amplitudes between the 2 corners, so the swaybar reduces the load and pressure on the inside wheel/tire.



Now comes the fun part. Selecting the stiffness of the swaybar is a compromise (as with most things) and to figure it out, you need to consider a LOT of info, but we can narrow it down when we figure out what we really want from the car.

I want optimal handling and the greatest amount of traction possible, so I approach things from that perspective.

Why use swaybars at all? Why not just use stiffer springs in the suspension in the first place to have better resistance to compression caused by weight transfer from cornering, braking, accelerating etc? Comfort is the real reason. The human body has particular sensitivities to acceleration (and vibration) at particular rates and frequencies. The stiffer the suspension, the more jarring the ride. Above a frequency of about 2.7Hz, our internal organs aren't too happy, so most cars tune the stock suspensions for natural frequencies ranging from 1.5 Hz (grandma's rollinng Caddy) to 2.2 - 2.5 Hz, (stiff sportcar suspension).

Sportscar owners have come to expect the stiffer ride and better handling so they have stiffer suspensions. Joe average prefers a cushy ride so he gets the softer suspension that compromises performance for the sake of comfort. With the softer suspension, to reduce body roll, we install swaybars as a compromise. Over speedbumps, they do nothing, in the corners, they increase spring rate. OK.

So what does installing a stiffer swaybar do? Well, as already mentioned, it resists body roll by increasing the effective spring rate of the suspension corner being compressed while increasing the load on that tire and decreasing the load on the inside tire.

Well that's a good thing right? The outside tire has more grip than the inside tire in a turn anyway so it makes sense to put more load on it right? Not exactly.

See, tire grip and traction is dependent on many things but in this case, we'll simplify things by looking at a very specific scenario of a specific tire at specific temp on a specific road surface. In this case, the tire's grip increases as pressure pressing it to the road surface increases. All good so far,... so the swaybar increasing load/pressure on the outside tire which has more grip than the inside tire to start with must be a good thing right? As I said, not exactly.

See, the relationship between tire grip and load pressure is not linear. As you increase the load on the tire, grip DOES increase (why aerodynamic downforce is a good thing), but it's not linear. As load goes up, you get diminishing returns on the increase in tire grip.

That's where the stiffer swaybars come in. By stiffening the swaybar, you're increasing load on the outside tire, but at the same time reducing load on the inside tire, in simplest terms, you're transferring tire load from one side of the car to the other. This is crucial, because due to the diminishing returns from increasing load vs tire grip, when you increase this load transfer, you never return as much overall traction between the 2 tires combined, because the traction you gained on the outside tire from increasing the load on it is not as much as the traction you lost from transferring load off the inside tire. That's why you lose traction on the axle where you install a stiffer swaybar.

Does that mean swaybars are a bad thing? No, changing the stiffness of the swaybar helps in changing the cornering attitude of the car, so you can tune and balance the handling better (assuming you have bodyroll under some control with a stiffer suspension etc.). Car understeers like a pig? Means you have less traction on the front axle than on the rear. Ok, well you can install a stiffer rear swaybar, reduce the total available traction of the rear axle so it more closely matches the front and voila, you have a more neutral handling car. Oversteering? Ok, stiffen the front swaybar and you'll have less traction in front to match the minimal traction in the back so the car's more neutral through a turn.

Wait,... Why do we want to keep reducing the total amount of traction available to neutralize the car's handling? Well, that's the million dollar question isn't it? If you can tune the car's handling for neutrality by increasing traction at the axle lacking it, wouldn't that be better? You'll end up with greater ultimate traction that way. Bingo!!! Now how exactly we do that is for another day, but that in a (rather large, bulky, verbose) nutshell, is why I have coilovers and stock swaybars.


Max

[andyvr4]

You diffentely need to put this in the 3si wiki because it's by far the most comprehensive write-up I've ever seen regarding the way that the sway bars effect the overall handling attitude of the car. I'd love to see some other write-ups you could do about the suspension on these cars also.

Thanks for spending the time to do this to help out all of the people (including me) that only have a general knowledge of how parts effect our suspension, but not a complete knowledge of why they do certain things.

[sammage]

Very good info as always Max, definitely belongs in the wiki.

[MaG18]

but if u have coilovers, you dont need sway bars lol

[darkkiller]

good read. alot of interesting info.

[GTO Assassin]

Again Max, awesome post. Thankyou.

Scotty

[J-Groove]

Great read, thanks for posting that up!

I LOVE my suspention set up on my car, it took a while to try different mods while AutoXing. But man, to get the rear end to step out controlably on a FWD car is heaven.

-John

[91Stealthrt2]

Originally Posted by MaG18

but if u have coilovers, you dont need sway bars lol

UNLESS you have a FWD, that puny bar in the back is the best upgrade ever IMO.

But I don't recommend front bars for cars with coilovers.