Sway bar size?

[Quiny]

My 2002 WRX wagon donor came with a 22mm rear bar. I believe it was aftermarket since I am having a hard time locating stock bushings for it. My question is should I use it? It seems most builders are using 17mm or smaller. Will I create a poor handling issue with using a stiffer swaybar?

[wleehendrick]

In general, a larger front sway bar will increase oversteer, making it more 'tail-happy'. How much so, probably only Wayne can answer since he's driven the 818 with multiple suspension set-ups. FFR chose the spring rates and shock damping assuming a stock bar, so personally I would swap it out. You should be able to get a stock bar for next to nothing since lots of WRXs upgrade them, so sell it and you should come out ahead.

[wleehendrick]

I'll add that Prothane universal 'A style' bushings fit perfect to mount the bar.

[Rasmus]

In general, a larger front sway bar will increase oversteer, making it more 'tail-happy'.

Wleehendrick, I have to completely disagree. The larger the front bar the more the car will under-steer/push. The Larger the rear bar the more the car will over-steer/loose.

Swaybars are touted as a way to control vehicle roll and keep the tire contact patch in it's sweet spot while keeping softer springs. That's great if you want a comfortable riding car. But the more bar you run, the less independent the corner suspension becomes. What that means: swaybars remove traction while turning. The more bar you've got the less traction that end of the car will have while turning. This is assuming you've got the tire contact patch dialed in.

Generally the less bar you run the more spring you run. You can even get away with running no bar up front and stiffer springs. Not only will the stiffer springs control sway and keep your tire on it's ideal contact patch, you'll also dive less under braking.

Grip > Lack of body roll. It's all about the tire contact patch.

[STiPWRD]

Right, a larger sway bar up front will increase the roll stiffness of the front and produce more under-steer. Stiffening the front springs or lowering the front will also produce more under-steer. I'm using a Cusco 22mm sway bar that I had on my wrx. The stock bushings worked fine with the 22mm diameter.

[wleehendrick]

Whoops, I was thinking backwards and responded too quickly. Thanks for correcting.

[Santiago]

Swaybars are touted as a way to control vehicle roll and keep the tire contact patch in it's sweet spot while keeping softer springs. That's great if you want a comfortable riding car. But the more bar you run, the less independent the corner suspension becomes. What that means: swaybars remove traction while turning. The more bar you've got the less traction that end of the car will have while turning. This is assuming you've got the tire contact patch dialed in.

Generally the less bar you run the more spring you run. You can even get away with running no bar up front and stiffer springs. Not only will the stiffer springs control sway and keep your tire on it's ideal contact patch, you'll also dive less under braking.

Grip > Lack of body roll. It's all about the tire contact patch.

Wish more folks would get this one so clearly. Nicely put.

Body roll can induce changes in the suspension geometry, but in and of itself it isn't such a terrible thing if the contact patches are not adversely impacted. Sway bars really come into their own when you want to split things like wheel rate for cornering from ride quality on the street...and when you want the ability to adjust wheel rates over the course of a significant change in running conditions (low fuel, rain, etc.) - but it takes in-car adjustability to gain that perk. Nothing to scoff at, btw; I know of a few enduro racers who are chomping at the bit to get this in their cars for the changing fuel load concern. And in the rain plenty of folks will disconnect their sway bars, but it might be nice to be able to go full-soft if it rains in the middle of a stint.

Anyway...

STiPWRD, I'm not sure I see the connection between "lowering the front" and "more under-steer." In relation to other things, I can see lowering the front having an adverse effect on front grip (like playing havoc with your roll center height) or if you lowered by cutting the springs, but in itself a lower front should contribute to front grip. Lower should be better up until it starts adversely impacting the existing geometry (and that's going to be application specific). What am I missing?

Best,
-j

[Rasmus]

Whoops, I was thinking backwards and responded too quickly.

I've done that myself. I've ninja edited a few posts when I've mixed up under-steer/over-steer.

[RM1SepEx]

Or you could take my approach and wait for others to post!

I figured that it was easiest to replicate the car that they did the development and tuning on... I did have to order the generic prothane mount as the stock ones were never going to fit without redrilling my powder coated frame. Do you use the FFR lower or upper hole to mount it, position does matter... never could get an FFR answer... The generic prothane is less than $20 and lines up perfectly!

[STiPWRD]

STiPWRD, I'm not sure I see the connection between "lowering the front" and "more under-steer." In relation to other things, I can see lowering the front having an adverse effect on front grip (like playing havoc with your roll center height) or if you lowered by cutting the springs, but in itself a lower front should contribute to front grip. Lower should be better up until it starts adversely impacting the existing geometry (and that's going to be application specific). What am I missing?
-j

Now I need to ninja edit my post lol. After skimming through my old vehicle dynamics book and double checking some equations, it looks like lowering the height of the CG or lowering the height of the roll axis will reduce the weight transfer on that respective set of front or rear wheels. In general, more weight transfer means less grip because the outside tires will slip more under higher load. So, lowering the front suspension will produce more over-steer, not more under-steer. This is a useful equation from Miliken:

1.jpg

W= total vehicle weight (lb)
H= height of CG above roll axis (ft)
Zrf= front roll center height (ft)
Zrr= rear roll center height (ft)
a= longitudinal distance from front axle to cg (ft)
b= longitudinal distance from rear axle to cg (ft)
l= wheelbase (ft)
tf= front track (ft)
tr= rear track (ft)
Kf = front roll stiffness (lb-ft/deg)
Kr= rear roll stiffness (lb-ft/deg)
Ay = lateral acceleration (g’s)
ΔWf = Front weight transfer(lb) [Solve for this!]
ΔWr = Rear weight transfer(lb) [Solve for this!]

[Wayne Presley]

Lowering the front suspension lowers the roll center, increases the roll moment and leans the roll axis toward the front. All of which increases front grip in turn in and steady state cornering. All of the suspension setups are a compromise of grip, turn in, drive off so trying to put them in an equation with not nearly enough data is pointless.

[RM1SepEx]

Nothing is free, everything involves compromise...

I'm relying on some skilled development work based on data and calibrated butts!

seat time is key...

[STiPWRD]

The equation merely shows the relationship between the variables and gives some guidance on the effects of adjusting suspension parameters. For example, lowering Zrf will reduce ΔWf and increase grip on the front wheels (assuming ΔWr remains constant). I wouldn't say its pointless.

[Quiny]

Lowering the front suspension lowers the roll center, increases the roll moment and leans the roll axis toward the front. All of which increases front grip in turn in and steady state cornering. All of the suspension setups are a compromise of grip, turn in, drive off so trying to put them in an equation with not nearly enough data is pointless.

Wayne what size sway bar would you run on an 818s intended only for moderate to aggressive street driving? The 22mm diameter bar I have is set up with 3 link anchors at each end. I figured I would use the outmost mounts to start with and if that was too stiff than swap out the bar or do you think I'm just wasting my time with it.

[Wayne Presley]

The 16mm was more than adequate to have mild understeer in the 818S, any larger is not needed. I would think that if the longest arm anchor points on the 22mm bar are at the stock 9" arm length, then it will be way too stiff. The roll moment on the 818 is not large because it has a very low CG and tends not to roll aggressively. The bar is on there to induce understeer and make it very friendly to drive. The 818S is very fun and easy to drive at he limits of the tires, I ran around one of the corners with the tail out 6" for 90% of the 270° corner. All smiles!