Factory Installed Swaybars
 

I understand a car not coming with a performance exhaust from the factory. Too loud. Performance suspension (springs, shocks)? Too harsh a ride. But a lot of aftermarket sway bar kits are sold, and they don't affect driveability all that much. So why don't they come standard from the factory with bigger swaybars if it's such a great thing?

In my opinion, not everyone who buys an RX-7 buys it for the performance. I have seen a grandma driving a Type R Integra. Also, swaybars aren't always better for every driver, each driver has a different driving style. I think where Mazda left their sway bar thicknesses is where they thought it would SUIT most of their market. instead of being the in the black or white part of the spectrum, they've kept it in the grey area, so to speak.

You also have to remember changing swaybars does affect it's tendancy to oversteer or undetsteer.

It's just my opinion.

Swaybars DO increase ride harshness, over uneven bumps especially. Larger sways do limit body roll, but they cause increased weight transfer, which is ultimately bad for grip, provided the increased body roll from not running them throws the dynamic alignment off too much.

They do not increase the weight transfer, but they do make the weight transfer faster. They also reduce the independence of the suspension. I do not come from the school of thought that you run softer springs with stiff bars....

I believe you're wrong here, rynberg. Twist the bar, and it pushes down on the outside tire and lifts up on the inside, right? If not, how does it resist body roll? And why then would a stiffer bar give less grip?

Overall weight transfer is unaffected by sway bars, as it is purely a function of track width and CG height. However, stiffening the bar at one end of the car will increase the amount of weight transfer at that end of the car (and lessen the weight transfer at the other end of the car).

-Max

Everything about swaybars by Grassroots Motorsports

GRM article is wrong ...
 

"In other words, a 160mm center-to-center bar produces only 80-percent of the torque that would be produced by a 200mm center-to-center bar of the same diameter. Or simpler yet, by using the 160mm end-link attachment points, we increase the stiffness of the anti-roll bar by an extra 20 percent."

Assuming stiff levers are used, as was done by GRM, Fred Puhn's famous (and 99% correct) sway bar equation would say going to an 80% lever length increases the rate by 56%, not just 20%. This is based on the square of the lever length ratio.

Take from the GRM article that long arms cause softer bar rates, and ignore the rest of the misleading lever length discussion.

Think of the link force created by 1" vertical motion at the connection to the bar.

Assume at the 200mm lever length the bar rate at the link is 500 lb/in, as body roll occurs. One inch vertical motion at the lever's link hole creates 500 lbs of link force, to the control arm.

Installed with the 160mm length, it takes 25% more force ( 200/160 ) to apply the same torque to the twisting mid length of the bar, due to the shorter lever arm. That makes link force 625 lbs.

But, although you have created the same torque in the bar's mid length, the 160mm link hole only moves vertically .8" (the unused 200mm hole still moves vertically the original 1" ). So to get the full 1" vertical motion at the 160mm link hole, the link force must be kicked up another 25% ( 1"/.8" ), for 25% more twist in the bar than the twist for the 200mm lever position. Link force is now 781 lbs, 56% more than the original 500 lbs for 1" deflection at the 200mm link hole.

This means the bar rate has increased 56%, not 20% per GRM, going from 200mm to 160mm levers.

Both GRM and I ignored corrections for misalignment of the vertical link, but that's ok to simplify the discussion.

^ Kevin, you're probably the only guy here who has devoted enough thought and analysis to this to even bother checking the math :p:

For those who are not KevinK2:

Puhn's equations (very closely) compute the rate of the bar when installed on the car. The GRM article tries to compute the rate of a plain bar laying by itself on somebody's table and they then extrapolate that into arm length which is misleading. If they'd stuck with merely discussing bar diameter they'd have been ok.

GRM missed the point that in order to calculate the actual installed rate on the car you have to realize the bar is twisted by an input over a certain distance. This means you have to take into account not merely the lever length, but the angular distance over which that lever was forced to travel in order to arrive upon the correct installed spring rate of the bar. A shorter lever length not only results in less mechanical advantage against the bar itself (which adds rate), it also means that for a given input from the suspension the shorter lever will twist the bar more (which also adds rate). Puhn's equations take both of these into account and (correctly) arrive at a greater rate than you would get if you merely compared lever lengths and ignored how a shorter lever forces the bar to twist more for a given displacement of the suspension.

GRM screwed up in computing the actual rate differences in their article, but other than the oversight in their algebra they're right on.

Which is negligble for very small slices of Pi. I prefer apple.

Dave

I still have issues with his lever logic:

"Using a setting closer to the center of the bar reduces the length of the moment arm, resulting in less torque against the bar, allowing less twisting motion of the bar, creating less body roll."

This only occurs with a .8 lever ratio if the bar in question is responsible for about 45% or more of the total roll rate, considering springs and bars. Otherwise, the bar torque will increase a small amount at the stiffer setting, all other stuff not changed. So a stiffer rear bar setting means more bar torque, not less, for an FD.

The link force always increases at the stiffer setting, with no other changes.

KevinK2
sway bar tech patrol