Widewing

Okay, odds are that you are in sales at Full Traction, rather than engineering.

I'll discuss this from an engineering perspective.

To begin, I've worked as the chassis Engineer for a first tier Formula Atlantic team working with the March and Reynard chassis. Currently I am a Senior Technical Fellow for an aerospace company. That's honorarium title for an Engineer with 30 or more years of experience. Right next to my office is that of another Engineer. He has his Masters in Mechanical Engineering and designed the entire suspension for SUNY Stony Brook's championship Baja Buggy. He is currently a consultant to the team.

We had a brief engineering based discussion and he agrees with my conclusions. The problem is discussing this without causing headaches to the readers...

Before I continue, this is not about the design or quality of Full Traction products. They have terrific reputation for high quality and value. This is about the physics related to the function of a sway bar (aka, anti-roll bar).

The quoted text is not accurate. This is simple physicals at work. Anti-sway bars are torsion springs. A straight torsion bar is a spring that twists. There are several variations of straight torsion bars, including torque tubes, there’s splined bars with levers than can be positioned to adjust total deflection (rotational twist) and the more common version with ends bent 90° to create a fixed lever.

Sway bars are part of the entire suspension spring system. A heavier sway bar will have a higher spring rate. That means greater load for a given deflection. Indeed, the sway bar also works as a bending beam at the same time it is twisting. Since both the sway bar and coil spring are linked to the axle, the total spring force must be considered. One is not exclusive from the other. Thus, any increase in the sway bar’s spring rate adds to the total. Therefore, the ride must become stiffer. This is indisputable.

There is a difference that has to be accounted for. Because a sway bar is a linked torsion bar spring, it has a neutral position where it is, essentially, unloaded. In most cases, the neutral position is close to normal ride height. Coil springs will only be unloaded if they are removed. Thus, they are always under compression/preload. If not, they would simply fall out. In an ideal world, the sway bar should not be loaded at normal ride height, and only applies resistance as the suspension deflects.

When one side of the axle deflects up, it is resisted by both the coil spring and the sway bar. Since the bar is attached to both sides of the axle, the bar is twisted in different directions at each end, because the other side is now loading in the other direction.

Okay, lets discuss flex…

Here’s a simple experiment:

With your stock suspension, place a floor jack under the frame even with the back edge of the pinch seam. Jack up the Jeep and measure how high you have jacked it for the rear wheel to come off of the ground. Lower the Jeep back down. Roll the Jeep forward to settle the springs and get any stiction out of the suspension.

Now, remove the hardware and disconnect the sway bar link from the axle. Jack up the Jeep and measure how high you have jacked it for the rear wheel to come off of the ground. You will see that you will have to jack it considerately higher. This is because the sway bar is no longer constraining axle motion.

At this point, if you have the Full Traction sway bar, install it and connect the links. Repeat the jacking process and note how high you have to jack the Jeep.

What you will discover the 19mm bar has a greater spring rate, both in twist and bending. Thus, the wheel will lift off sooner than with the stock sway bar, and much sooner that if the sway bar links are disconnected. The real advantage of the Full Traction bar is that they have 3 positions for link attachment. This changes the effective length of the lever. Therefore, you can, as you stated, tune the bar to provide a better compromise between roll resistance, ride quality and flex. You can set it for the road, and quickly change it for max flex for off road. Quick disco links (preferably adjustable) can facilitate that in a matter of just a few minutes.

As a final note, one common way to mitigate "bounciness", is to look for shocks that are adjustable for rebound damping. Increasing the total spring force may exacerbate this. However, careful tuning of rebound damping can control that bouncing more effectively. Rebound damping if often a problem due to having to control all the stored energy in the springs and sway bar and the mass of the axle and tires.

yellow08jk

I am an elementary school teacher. However, I DO know that the spring rate is not affected by the sway bar when going straight down the road. When hitting RR crossing type bumps, speed bumps, or dips in the road you would not notice the difference in a 60mm bar or none at all. When hitting a pot hole the difference you notice would be negligible. The sway bar's force ONLY tries to keep the body parallel to the axle which prevents sway (or "tipsiness") around corners. There is no preload on a sway bar unless you have unequal length links which would cause the jeep to sit high on one side.

SO...... the answer to the OP's question is the thicker sway bar will make the jeep less tipsy on road and more stable. It will NOT affect "bounciness" however. Good shocks with better compression and rebound control will cure that.

****OF COURSE When offloading we don't want the axle to be parallel to the body so that we can flex. That is why many of us disconnect the front when we go wheeling. Theoretically, the adjustable full traction units could offer the best of both worlds allowing you to flex well on the longest setting and then stiffen up on the shortest setting for drifting around curves when you drop the hammer on your 3.8

I hope my explanation didn't cause headaches for any pompous asses

mkjeep

FYI, Chris replaced that bar before he worked for FT and was stating at how it helped his rearend out on the long drive back from Moab. Soooo, he's speakin from personal(seat of the pants/jeep owner style) rather than just another salesman thank you very much.

Widewing

You are incorrect. There is always some preload. Put two passengers in the rear seat... The Jeep settles on its suspension and the bar is loaded on both sides. Weight transfer when driving will load the bar. Virtually any motion will load the bar. Albeit, only very slightly. However, variations in ride height will occur and that will load the bar.

Technically, a sway bar provides no force, it provides resistance, which is returned as a force on rebound. It twists and stores energy, and then releases that energy with some small loss. Increased roll stiffness means reduced body roll relative to the axle. That we can agree on. Excessive roll resistance can lead to lifting a wheel under hard cornering. That's why you lose flex. Large diameter sway bars will limit wheel travel and tend to unload the inside wheel while cornering.

A sway bar effectively increases the spring rate on whichever side is compressed more. Cross a speed bump perfectly square and both sides are compressed equally. You won't notice a thing. Run across it diagonally and you will notice the difference between a small diameter bar and a large diameter bar (or none at all).

Going to a larger rear bar can have unwanted side effects. Typically, if the bar is bigger than needed to control body roll, you can induce oversteer. That's due to unloading the inside wheel too much. For the average driver, oversteer is like the plague; you don't want it.

In a race car, with extreme spring rates and damn little suspension travel, you tune the total spring rate by adjusting the sway bars. If you've watched racing on television, you may have heard a driver or announcer talk about adjusting handling. The driver does that by adjusting the sway bar. Early adjusting methods were as simple as repositioning the links to increase or decrease stiffness for a given deflection while actually racing. Therefore, you can change from understeer to oversteer and back again. In the case of the Jeep, you can reduce understeer (that's "tight" for you NASCAR guys) with a bigger rear bar. Go too big, and you will have oversteer (loose). The trick is knowing what is too thick. That falls under the control of the Engineer who designs the bar. It should be designed not only to reduce body roll (which is not the real issue. The real issue is what happens when the body stops rolling and sets), it needs to be tailored to decrease understeer without being so stiff as to unload the inside tire. I'll assume the sway bar in question is well designed. Just keep in mind that no mechanical change exists in a vacuum.

I mentioned body roll. Why is body roll bad? Inertia. Typically, when driven close to the limit of a tire's adhesion, body roll can induce bad behavior. Again, it's not the body motion, it's what happens a second later. Let's say the tires are at 95% of their lateral adhesion. Commonly, the tires reach their limit before body motion stops. When the body motion ceases, that sudden transfer of inertia goes directly into the tires. Very quickly, you exceed the mechanical adhesion limit of the tires, and (if you are lucky) you spin the vehicle or understeer off of the road. Thus, vehicles with lots of body roll can't be driven to the tire's limits without great risk of adventure. Decrease body roll and you can drive closer to the tire's limits. That however, isn't what hellhound13 is worried about. I think he finds the "tippsy" body roll to be very uncomfortable.

You mentioned "stable". Define this.

To the point... Sway bars are absolutely part of the total spring rate. Relative to vehicles with comparatively low spring rates and lots of suspension travel, the resulting increase in rate due to minor deflections is small. Nonetheless, it is there. Will the driver notice? That depends on whether the deflection is limited to one side and how great the deflection is.

Feel free to argue that sway bars do not increase total spring rate. You'll still be incorrect, but you may avoid the headache.

falconfixer

I love listening to engineers talk, I listen to it all day. You hear words like "technically, "and according to the computer program" but in the real world it is a whole 'nother ball game. In reality I would put money on real world experience versus some geek behind a simulation.

Widewing

I truly appreciate "seat of the pants" analysis. The problem is that you can't quantify it. Moreover, I have no doubts that his Jeep handled better on the road. Especially if he's running large tires and a big lift. My point is that a bigger bar may not produce the same results on a stock suspension.

Chris mentioned that he couldn't see why one could not install a larger bar on a stock suspension. I offer three reasons to ponder it further.

mkjeep

Wtf quantify mean?
Saw a movie show long time ago called Quantify Leap or sumpin, made about as much sense to me as that long ass post I just skimmed over.

mkjeep

:b eer:

mkjeep

.........

Kauaian

i feel blood pressures rising