Do sway bars prevent rolling over off road?

robcarync

Sliders
Location
Raleigh, NC
Simple question, until I started thinking about it.

We all know sway bars prevent BODY roll by bracing one side against the other. But do they actually make it more difficult to roll over or flip your Xterra off road...or do sway bars simply make you FEEL less off camber in the cab?

I have both sway bars removed, and the Xterra is definitely flexy and has quite a bit of body roll. But even if the body is rolling, the axle and suspension/frame are still firmly planted with the additional flex.

The flip side (ha.ha...flipping on the side...get it?!) , is if I had both sway bars installed, I would not feel the body roll because of the sway bars, but the suspension would flex less, so my tires would come off the ground sooner if I am of camber.

What do you guys think?
 

Prime

Shut up Baby, I know it!
Admin
Location
Denver Adjacent
In theory, removing the sway bars helps prevent a roll over because it allows the Off camber terrain to be absorbed by the suspension rather than pushing the body to one side. Given the design of our trucks, you still have a center of gravity issue when operating off camber, but tue more flex you have, the harder you can push.
 

robcarync

Sliders
Location
Raleigh, NC
If your X is gonna be sideways it's gonna be sideways. Sway bar or not. IMO at least

I agree with you there...I was more just thinking along the theory of operation. I was offroading some Sunday and got in some tippy situations where I literally felt like I was gonna tip over...but I was just riding along a trail that had a side to side slant. I thought I may have had brown stains in my car seats, but it got me thinking...was I really that close to physically tipping over, or was it just that I felt it more without a front sway bar.

My conclusion agreed with Prime's statement...I felt like removing sway bars just let the suspension flex more, which would keep your tires on the ground more. I definitely agree on the center of gravity issue, though. Lifted SUVs packed down with gear are going to be top heavy
 

granitex

Skid Plates
Founding Member
Location
Columbus OH
I took my front off after fixing the mounts way too many times. sure it is like driving boat at times, and bigish mt's don't help.
 

robcarync

Sliders
Location
Raleigh, NC
Yes, sway bars can help prevent a rollover.
No, sway bars will not help prevent a rollover.

Confused yet?
Just like everything else it all depends on the situation and circumstances. Just like a super flexy suspension can prevent you from rolling over, and a super flexy suspension can CAUSE you to rollover.

Clear as mud. To be honest, I never did understand why so many folks take the front swaybar off an IFS rig. Gains are roughy 3/4 of an inch of extra travel (I've measured this personally) and the downsides are numerous. Those that say it makes no difference for on road handling are either lying or have absolutely no clue how to drive and what a vehicle should feel like and how it should respond. But hey, what do I know anyway.

Cheers,
Josh

Sent from something, somewhere.

I agree that it depends on the situation, terrain, and how the truck is leaning. I guess I was thinking of it at the most basic level as a hypothetical lab experiment...say, on a tilt table. Sitting on level ground, and gradually tilting it until the truck flips. Same loading, cargo, etc.

As for why remove the front sway bar....

Bolt head sheared off and I didn't feel like drilling it out. I think most people end up doing it for reasons like this.

It drives like a boat. I still have the sway bar and will probably end up drilling the broken bolt out and rebuilding it with new bushings and end links...but for now I am sway bar-less.

For what it's worth, with it on, I was never even able to flex the front suspension enough to hit either bump stop at the control arms...it would just lean the entire truck over until a back tire lifted off the ground.
 

dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
Yes, sway bars can help prevent a rollover.
No, sway bars will not help prevent a rollover.

Confused yet?
Just like everything else it all depends on the situation and circumstances. Just like a super flexy suspension can prevent you from rolling over, and a super flexy suspension can CAUSE you to rollover.

Clear as mud. To be honest, I never did understand why so many folks take the front swaybar off an IFS rig. Gains are roughy 3/4 of an inch of extra travel (I've measured this personally) and the downsides are numerous. Those that say it makes no difference for on road handling are either lying or have absolutely no clue how to drive and what a vehicle should feel like and how it should respond. But hey, what do I know anyway.

Cheers,
Josh

Sent from something, somewhere.

this ^^

he hits all the nails on the head.

while you're still able to flex, it'll help keep you from rolling, however, once you hit the point where you aren't flexing anymore, your downward side suspension is now even softer than before and you're more likely to oop on over.

once I get done installing front skid, mobile ham, and that stuff, I'll see if I can find some 2" longer swaybar links and I'll probably reinstall mine...the stiffer suspension I added can probably handle it just fine without re-adding it (and it is adding weight back that I'd rather not add) but, in the end, like Casper said, on road, it's much better to have it....
 

lizardking

Bought an X
Location
wilkes-barre, PA
If you are worried about rolling over off road, buy an inclinometer. https://www.youtube.com/watch?v=S9Usf9xh7Pg

Sway bars are not about preventing rollovers in the first place. They are about suspension tuning and preventing body roll. Body roll does not directly equate to roll over. If you have 700# radflos and still have your swaybar on you actually might cause too much understeer. Disable your VDC and we have something to talk about... but only if you are talking highway speeds. VDC will not stop anything off road.

edit: I meant understeer....

edit2: er... Sometime I forget the first gens. So 700# spring aren't an option for you... but I still stand by the inclinometer :D
 
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dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
If you are worried about rolling over off road, buy an inclinometer. https://www.youtube.com/watch?v=S9Usf9xh7Pg

Sway bars are not about preventing rollovers in the first place. They are about suspension tuning and preventing body roll. Body roll does not directly equate to roll over. If you have 700# radflos and still have your swaybar on you actually might cause too much understeer. Disable your VDC and we have something to talk about... but only if you are talking highway speeds. VDC will not stop anything off road.

edit: I meant understeer....

edit2: er... Sometime I forget the first gens. So 700# spring aren't an option for you... but I still stand by the inclinometer :D

I'm not sure what grade of spring I have now (whatever the OME HD lift springs are) but, the issue is this, the sway bar adds another point of resistance to keep the springs from compressing quite so quickly. that's why your X didn't feel like you were driving a jon-boat with it on. VDC doesn't change the ride of the rig, it changes what the computer does with the brakes once it senses slip or roll, the problem with that thought, is that VDC is set up to react and adjust based on the FACTORY suspension dynamics, i.e. sway bar and coils/leafs of a certain load and spring rate. once you add a leaf, change out shocks, or coils.

Switching to SAW 700lb coilovers aren't an option for some 2nd gen people either, given that those run $1200 with the PRG UCA's...just sayin...
 

robcarync

Sliders
Location
Raleigh, NC
I'm not actually concerned about rolling over...it was just an experiment in thought. As a mechanical engineer...I always enjoy thinking about the complicated physics of the situation.

The sway bar has two underlying factors that impact how it rolls over...changing the effective spring rate by making it more difficult to cycle the suspension...and changing how the center of gravity changes as the suspension cycles (increased body roll)

So...the question more so was a hypothetical thought...thinking aloud...random musings...that the sway bars prevent you from feeling body roll, but does it actually make the truck safer or less prone to rolling over?
 

lizardking

Bought an X
Location
wilkes-barre, PA
I'm not sure what grade of spring I have now (whatever the OME HD lift springs are) but, the issue is this, the sway bar adds another point of resistance to keep the springs from compressing quite so quickly. that's why your X didn't feel like you were driving a jon-boat with it on. VDC doesn't change the ride of the rig, it changes what the computer does with the brakes once it senses slip or roll, the problem with that thought, is that VDC is set up to react and adjust based on the FACTORY suspension dynamics, i.e. sway bar and coils/leafs of a certain load and spring rate. once you add a leaf, change out shocks, or coils.

Switching to SAW 700lb coilovers aren't an option for some 2nd gen people either, given that those run $1200 with the PRG UCA's...just sayin...

Just for perspective, I have 600# radflos, full armor, winch and bumper and have been daily driving without a swaybar for over 6 months. Maybe longer since my endlinks always broke when I did have the sway bar in.

VDC works regardless of "factory" suspension. Your "factory" suspension is not the same as it rolled off the line. In fact, depending on which model, year and/or state of your vehicle the suspension will be completely different. The VDC is the same regardless. The VDC will sense you going out of spec and attempt to use the engine and abs to fix it. It doesn't care if your sway bar on or off. VDC (or whatever the OEM calls it) is one of the main reasons that moderns SUVs have lower roll over rates then their ancestors. The ancestors had sway bars too. Sway bars are tuning devices. I am not suggesting that people w/ stock rate springs go out an take off their sway bars. There is a good chance that will create over steer which, in and of itself, could induce a rollover. I am saying that taking off your sway bars will always cause you to roll over is incorrect. Putting big springs and huge sway bar in the back might cause you to roll over too via to much oversteer. Putting big springs on front and not removing your sway bar may create too much understeer. Understeer (edit:I had oversteer here... it was late and I am dyslexic) is generally safer but it makes the vehicle turn at a slower rate. This all about high speeds though. The suspension does different things at 3 mph and a 30 degree tilt.

If you are at an angle of 35 degrees or more off road, you might roll. Gravity doesn't care about your sway bar.

I'm not actually concerned about rolling over...it was just an experiment in thought. As a mechanical engineer...I always enjoy thinking about the complicated physics of the situation.

The sway bar has two underlying factors that impact how it rolls over...changing the effective spring rate by making it more difficult to cycle the suspension...and changing how the center of gravity changes as the suspension cycles (increased body roll)

So...the question more so was a hypothetical thought...thinking aloud...random musings...that the sway bars prevent you from feeling body roll, but does it actually make the truck safer or less prone to rolling over?
There are so many variables. Speed. Road angle. Road surface. Tires. Vehicle load.....
 
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dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
Just for perspective, I have 600# radflos, full armor, winch and bumper and have been daily driving without a swaybar for over 6 months. Maybe longer since my endlinks always broke when I did have the sway bar in.

VDC works regardless of "factory" suspension. Your "factory" suspension is not the same as it rolled off the line. In fact, depending on which model, year and/or state of your vehicle the suspension will be completely different. The VDC is the same regardless. The VDC will sense you going out of spec and attempt to use the engine and abs to fix it. It doesn't care if your sway bar on or off. VDC (or whatever the OEM calls it) is one of the main reasons that moderns SUVs have lower roll over rates then their ancestors. The ancestors had sway bars too. Sway bars are tuning devices. I am not suggesting that people w/ stock rate springs go out an take off their sway bars. There is a good chance that will create over steer which, in and of itself, could induce a rollover. I am saying that taking off your sway bars will always cause you to roll over is incorrect. Putting big springs and huge sway bar in the back might cause you to roll over too via to much oversteer. Putting big springs on front and not removing your sway bar may create too much understeer. Oversteer is generally safer but it makes the vehicle turn at a slower rate. This all about high speeds though. The suspension does different things at 3 mph and a 30 degree tilt.

If you are at an angle of 35 degrees or more off road, you might roll. Gravity doesn't care about your sway bar.


There are so many variables. Speed. Road angle. Road surface. Tires. Vehicle load.....

ok....I'll bite.

1. VDC is a computer program.

2. Computer programs are set up with specific parameters

3. the program will attempt to function within the parameters that it's programmed to function within.

4. when the computer functions in a manner within its parameters, but the suspension doesn't react as is expected, the vehicle can react in a completely unintended manner.

5. you're right, when you get too far off camber, you can roll, weight shift is a major factor in that, if your weight shifts excessively as you transfer from 30-35deg, you're likely to roll. if you're gently arriving at 40deg, you might just stay wheels down instead of wheels upward (or at minimum sideways).

if we break it down to a simple answer, it's this. a swaybar will *not* keep you from rolling, however, it certainly *will* keep you from rolling as easily.
 

P&P

Need Bigger Tires
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Location
Middle TN
I agree that it depends on the situation, terrain, and how the truck is leaning. I guess I was thinking of it at the most basic level as a hypothetical lab experiment...say, on a tilt table. Sitting on level ground, and gradually tilting it until the truck flips. Same loading, cargo, etc.



In this situation, no the sway bar has no effect on tipping point. But in this situation where the vehicle is static the only real thing that has any effect will be the angle of inclination and locations of the center of gravity and the moments of inetria. If you're just tilting the room the suspension is doing no work, and once the CoG is outside the safety triangle it's all over, suspension or not.

Suspension comes into play when dynamic factors are introduced. Vehicle travel in 3 dimensions, spring rates, wheel base, tire width and offset, even just how gentle the driver is on the controls and what they ate for lunch could be the difference between a roll over and a cool youtube video.

I tend to think in *most* situation on and off road having the extra spring rate and in the orientation of a sway bar will keep you off your side vs. not having had it for this reason. The extra spring rate will control weight shift and transfer of the center of gravity. I think *typically* it's momentum generated by too much speed for conditions coupled with weight transfer which pushes a vehicle beyond what its suspension can cope with.

But then again, the bigger we make our trucks the easier it is to get the CoG outside what the wheelbase and gravity can help us out with.
 

lizardking

Bought an X
Location
wilkes-barre, PA
ok....I'll bite.

1. VDC is a computer program.

2. Computer programs are set up with specific parameters

3. the program will attempt to function within the parameters that it's programmed to function within.

4. when the computer functions in a manner within its parameters, but the suspension doesn't react as is expected, the vehicle can react in a completely unintended manner.

5. you're right, when you get too far off camber, you can roll, weight shift is a major factor in that, if your weight shifts excessively as you transfer from 30-35deg, you're likely to roll. if you're gently arriving at 40deg, you might just stay wheels down instead of wheels upward (or at minimum sideways).

if we break it down to a simple answer, it's this. a swaybar will *not* keep you from rolling, however, it certainly *will* keep you from rolling as easily.


VDC is a safety device and will function if you have a shredded tire, a blown shock, a blown endlink (which negates the say bar), on ice, snow, rain.... Yes it is a program with parameters but its goal is to keep the vehicle upright and pointed in the proper direction.

https://www.youtube.com/watch?v=RtCfHQxJFYw

Obviously you believe the swaybar is a safety device. It is not. It is a suspension tuning device. It knows nothing. It only increases your spring rate when it attempt to compress the other side. It is only a percentage though. You don't double your spring rate and it doesn't compress the other side in a 1 to 1 ratio. If you have 500# springs (stock second gen) and a thick sway bar, you might get 650# of spring in a hard turn. If you take a turn too fast with a blown sway bar endlink or a improperly tuned suspension, you do increase your chance of rolling. It is about the poorly tuned suspension. Not about the sway bar. Again, I am not saying that you should go out and take your sway bar off if you have stock springs. It can cause oversteer (did I get it right that time? coffee...) which is bad for inexperienced drivers. I am just saying that it is not instant doom. Most of the titan swap guys run no sway bar because it doesn't fit anymore. They do run higher spring rates. So their spring rate in a hard turn is already 650#. Is it just that simple? No. Road surface, weather condition, speed, body roll, wind, weight, the fact we drive a brick on wheels, and on and on... Don't turn your VDC off though.


VDC and sway don't help or in the OPs question. Your suspension setup may help you not get in a tippy situation but if you are titling above 35 degrees, you are in the danger zone. You might regret that tire on your roof or the burrito that you ate for lunch.


In this situation, no the sway bar has no effect on tipping point. But in this situation where the vehicle is static the only real thing that has any effect will be the angle of inclination and locations of the center of gravity and the moments of inetria. If you're just tilting the room the suspension is doing no work, and once the CoG is outside the safety triangle it's all over, suspension or not.

Suspension comes into play when dynamic factors are introduced. Vehicle travel in 3 dimensions, spring rates, wheel base, tire width and offset, even just how gentle the driver is on the controls and what they ate for lunch could be the difference between a roll over and a cool youtube video.

I tend to think in *most* situation on and off road having the extra spring rate and in the orientation of a sway bar will keep you off your side vs. not having had it for this reason. The extra spring rate will control weight shift and transfer of the center of gravity. I think *typically* it's momentum generated by too much speed for conditions coupled with weight transfer which pushes a vehicle beyond what its suspension can cope with.

But then again, the bigger we make our trucks the easier it is to get the CoG outside what the wheelbase and gravity can help us out with.

Off Road rocks: There is so much going on. Different angles different rocks. Uphill tippy I might want soft springs so that the suspension compresses more and bounces less. Downhill tippy I might want high spring rate so my motion is more limited. Real world I have seen our trucks with all different setups (stock to titan swap with #750 springs and full armor) go through very tippy spots. Experience, smooth control and low center of gravity would be what I would vote for.

High speed: A properly tuned suspension with all its bits working.
 
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robcarync

Sliders
Location
Raleigh, NC
In this situation, no the sway bar has no effect on tipping point. But in this situation where the vehicle is static the only real thing that has any effect will be the angle of inclination and locations of the center of gravity and the moments of inetria. If you're just tilting the room the suspension is doing no work, and once the CoG is outside the safety triangle it's all over, suspension or not.

Suspension comes into play when dynamic factors are introduced. Vehicle travel in 3 dimensions, spring rates, wheel base, tire width and offset, even just how gentle the driver is on the controls and what they ate for lunch could be the difference between a roll over and a cool youtube video.

I tend to think in *most* situation on and off road having the extra spring rate and in the orientation of a sway bar will keep you off your side vs. not having had it for this reason. The extra spring rate will control weight shift and transfer of the center of gravity. I think *typically* it's momentum generated by too much speed for conditions coupled with weight transfer which pushes a vehicle beyond what its suspension can cope with.

But then again, the bigger we make our trucks the easier it is to get the CoG outside what the wheelbase and gravity can help us out with.

Fun conversation guys...I enjoy the input.

now as a follow up PP: given the static test...no sway bar would put the COG outside of the safety triangle sooner, no? Without a sway bar, the suspension can't flex as easily. The less flex you have, the less your COG can tilt over, which keeps your COG in the safety triangle. Like you said, suspension is not doing any "work" on a tilt table...but the weight distribution has changed, making the down hill springs support more weight (compressed more), which changes the center of gravity location relative to the safety triangle. If the sway bar makes it more difficult to flex, you have effectively increased the spring rate, which would keep the center of gravity from moving as much.

I've always wanted to make this a fun dynamics problem to play with and see how much I remember from school...but as we all see, it quickly becomes very complicated

:)
 

lizardking

Bought an X
Location
wilkes-barre, PA
How much do springs really compress when you jack up one side of the vehicle on level ground? Might have to try this.

Everyone gets around 30 degrees on this trail. I will have to try and take more pictures. I am pretty sure both Xs have 600+ springs. They both look like they have sway bars. Not 100% sure though. The suspension doesn't look like it is compressing much. I will have to try to find a picture of one without a sway bar...


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dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
How much do springs really compress when you jack up one side of the vehicle on level ground? Might have to try this. Everyone gets around 30 degrees on this trail. I will have to try and take more pictures. I am pretty sure both Xs have 600+ springs. They both look like they have sway bars. Not 100% sure though. The suspension doesn't look like it is compressing much. I will have to try to find a picture of one without a sway bar...
pretty sure at least the 2nd does from the shadows on the uphill side inside the tire, could be wrong.


To clarify however, my worry isn't when you drive into a situation, what happens if you're running sidehilled like those pics and you oops and slip off of the rut ridge and drop the downhill side another 5+deg, that's where I see the sway bar coming into play, because it'd help manage the body's "whiplash" so to speak...
 

GhostX

Allergic to Pavement<br><img src="http://i164.phot
Location
SoCal
What Casper said.

My "understanding" behind the removal of the front sway bar was for more "IFS articulation," meaning tires in contact with the ground off road. Not sure if that equates to less risk of rollover, but removal combined with the Titan Swap certainly has increased traction/off road capability in my case.

With that said, and back to the OP's question...I know for sure "without" sway bars, that California Cloverleaf's are much more "adventurous," and if approached at the same speed "with" sway bars, things would get very interesting very quickly.

Great discussion points all.
 

lizardking

Bought an X
Location
wilkes-barre, PA
So I couldn't find many xterra shots in that position but this is the same trail. The Jeep looks like it is at the same(ish) 30 degrees. No idea of his setup but I do remember I thought he was oversprung and had some issues getting the flex he was looking for. I am not sure that helps or hurts this discussion but I did find it interesting that as he was at middle of the rut and at the highest angle, his suspension seems to be the least compressed. Is because some of the weight of the truck starts to shift to the side of the axle? If that is true, springs and sway bars stop helping at some point and it is mostly about COG.... hmm. In any case, they don't compress much. There are many other places on the trail that flex will help though. So I guess the question would be is a little more spring in this spot worth less flex in other spots?


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robcarync

Sliders
Location
Raleigh, NC
How much do springs really compress when you jack up one side of the vehicle on level ground? Might have to try this.

Everyone gets around 30 degrees on this trail. I will have to try and take more pictures. I am pretty sure both Xs have 600+ springs. They both look like they have sway bars. Not 100% sure though. The suspension doesn't look like it is compressing much. I will have to try to find a picture of one without a sway bar...


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Not entirely sure to be honest. I haven't had a dynamics/statics class in a while...but assuming the center of gravity is in the middle of the body...as it tilts over, the COG is applied closer to over the downward wheel than the upper wheel. So if you sum the moments, sum the forces and solve for the force on each wheel...I think the math would work out that way...Of course, remember 30 degrees is a small angle compared to a 90 degree rotation...so it may not be much in a practical offroading scenario. I haven't actually done any math to verify my thoughts...but I did hi-lift my truck on each side to get my front CV axles past the LCAs and did the same thing again to get my low profile bump stops in past the UCA...had to flex the suspension some to clear.

Just conceptually thinking about it, at some heavy degree of angle tilt...you could just tilt the truck over by hand from the top side...which is because that downward side/edge is where most of the weight is centered over, and less weight is supported on the upper edge.

Once that COG is centered directly over the downward edge, you are the theoretical tipping point
 
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Trexterra

Wheeling
Founding Member
Location
Alabama
In helicopter world we have a problem called dynamic rollover. The three factors in a rollover are pivot point, rolling motion, and exceeding the critical angle. The critical angle is relative to how much rolling motion you have so the faster you are rocking side to side the less extreme the critical angle. Obviously the tires are already the pivot point so the only thing left is to have a rolling motion. The faster you bounce = the faster you roll. My logic without doing physics/math would be that at low speeds it will not make a damn bit of noticeable difference because you are going to be tilting slowly. At high speeds I think the sway bar being connected would result in a faster bounce because of less body roll resulting in a smaller critical angle. But that's just me trying to apply helicopter logic to an Xterra.
 

dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
In helicopter world we have a problem called dynamic rollover. The three factors in a rollover are pivot point, rolling motion, and exceeding the critical angle. The critical angle is relative to how much rolling motion you have so the faster you are rocking side to side the less extreme the critical angle. Obviously the tires are already the pivot point so the only thing left is to have a rolling motion. The faster you bounce = the faster you roll. My logic without doing physics/math would be that at low speeds it will not make a damn bit of noticeable difference because you are going to be tilting slowly. At high speeds I think the sway bar being connected would result in a faster bounce because of less body roll resulting in a smaller critical angle. But that's just me trying to apply helicopter logic to an Xterra.

You Sir, hit the nail on the head, with exactly my point. Motion plays into the equation. Rollover comes from vertical force, horizontal force, and angle. If there is no horizontal force, the sway bar is pointless, when you add horizontal force, that's what the sway bar(s) help to combat. If a vehicle is going around a corner fast, and has sway bars which tie some rigidity from the frame to the axles, it's harder to roll, if it doesn't have that rigidity, the body roll increases the horizontal acceleration. Remember the law of Intertia? A body at rest will stay at rest unless acted upon by another force. A body in motion will stay in motion unless acted upon by another force.

Momentum (inertia) wants an object to keep moving in a straight line on a 3 dimensional axis. Gravity wants to move it downward at all times, but, your inertial energy (momentum) wants you to keep moving in that line, when you turn, traction (friction) is guiding your momentum into a different direction, however, it wants to keep moving in it's original direction. That's your horizontal force.

Now apply that to a trail...you're driving along, and slip into a rut, your vehicle rocks to the side hard, what happens if you're too far off camber when that force hits? A roll over. Again, the sway bar can help reduce some of that force, but, if it's too much, you're going over, hard. It won't save you, no, but, it'll sure buy you a little more room.

Just tipping? Negligible. Tipping with sideways motion? It'll make a difference.
 

TJTJ

Skid Plates
Founding Member
Location
NJ
OK

The way a roll works is that your center of gravity protruded outside of the rectangle formed by your contact patch. If that happens, you roll.

If it doesn't happen, you don't roll.

That's the physics of it.


So, imagine that the rectangle formed by your 4 tires is a "straight down from the tire" shape...so if you are tipped up on one side, the rectangle is narrower, as, straight down from the tires is now closer together. (Think of a plumb bob hanging straight down from each tire, and, where on the ground those 4 points land)

Now imagine your center of gravity, lets call it your radio dial on the dash...

And imagine IT also has a plumb bob able to hang straight down...

If the radio dial's plumb bob is pointing at a spot OUTSIDE of the rectangle formed by the TIRE'S bob's, then, you roll over.

It doesn't matter if you HAVE a suspension, it could be welded, it could be on slinky springs, it doesn't matter....if bob's outside the box, over you go. If bob's in the box, you don't roll.

That's pretty much it.



Now, where some posts were explaining that a flexier suspension of a stiffer suspension can change this, what they are REALLY explaining is how you get INTO these situations....not if you'll roll once there.


For example, if you have the "perfect suspension" the TIRES go up/down with the terrain, and the TRUCK stays dead level...and if the TRUCK stays dead level, well, bob's going to stay in the box.

Of course, there's no such thing as a "perfect suspension", so, **** happens. :D


So, if you are hitting a rock and its making a tire rise, the truck is level until the tire can't rise w/o lifting that corner of the truck with it. Once out of up travel, the corner of the TRUCK rises, and, you are swinging bob over to the other side of the box....swing bob TOO far, outside the box, and you've tipped yourself over.

If the suspension on the other side (being compressed) can overcome that force, then the truck might struggle to stay more level, etc....but, too stiff, and the rising side will win once the two sides leverage's even out.

IE: If tipped to say the driver's side, the compression resistance on the driver's side is fighting the tilt, and the stiffness on the PASSENGER side is trying to flip you to the driver's side.

You WON'T flip though unless bob strays from the box.



:D

As for the FRONT sway bar, yeah, its a good idea to leave it on (Says the guy with it off). The rig will handle a LOT better with the front bar on (Its the rear that's rubbish).

OFF road, the teeny gain in front articulation is somewhat negated by the LOSS in opposite side leverage. (Why a live axle can work better on uneven terrain). IE: When one tire is pushed UP, the other is pushed DOWN...increasing ground pressure for the drooped tire.

With IFS, there is no leverage pushing the drooped tire down, just the suspension's supported weight (Which works, but, not as well as leveraged force ADDED to the suspension's supported weight...)

Hope that helps!
 

lizardking

Bought an X
Location
wilkes-barre, PA
Ok. In a few very specific situations 100-200# of spring rate may be the deciding factor on flop or not. Personally, I don't want to be in that spot or at that angle anyway, so I tend to avoid those trials.

I hate to keep arguing with you... but the sway bar is not the only way to tune out body roll. You can put in stronger springs. If you have adjustable linear coilovers, you can increase the preload. Spring rate gives the rigidity. The sway bar is flexible. The bushings that attach to the frame are flexible. The purpose of the bar is to increase the spring rate without putting in bigger springs. It can also bring down the COG in a fast turn by lowering the other side but this is a secondary function. There are negatives to sway bars too. They can make the truck want to keep level to the ground (rather then the horizion) because of the limited flex and the one side pulling on the other. So in other off camber situations, they may make you more tippy to start. They also make the ride much harsher on bump roads and in the rocks. Less flex also means less traction which tends to mean more skinny pedal...

Body roll isn't the major factor in rolling over either. If it was, I would have rolled both my VW GTIs a thousand times. As soon as you started to turn those things, they would tip over and pick up a rear wheel. Sloppy and Floppy. There is a point in a fast turn that the body roll of the suspension stops and settles. We are roll prone because we are high off the ground and have a high COG. There is only one way to fix that.

x5.jpg
 

robcarync

Sliders
Location
Raleigh, NC
I don't believe anyone is really arguing here. If anything we are all saying the same thing in ten different ways. No one is debating where the rolling point is per se. It is and always will be when the center of gravity falls outside of the outer edge.


My original post...was rhetorical wondering of what angle of the ground gets you to that tipping point, and does a sway bar change that angle (by flexing springs less, keeping COG inside the box at a steeper angle)? Not saying it changes it by much or that it is significant.

As stated, this was just a theoretical pondering...like a static tilt table type of deal. Obviously, when you get into actual terrain conditions, speed, acceleration, which way you steer, etc...you have MANY more factors more important than a sway bar.

So the question really kinda boils down to whether or not the springs flex differently at different angles of slopes. It would be easy if the Xterra could be modeled as a rigid box with a CONSTANT COG. But the center of gravity can shift relative to the tire square because the BODY/FRAME is on springs, which allow the TOP half to sway and shift relative to the suspension and tires. That leaves multiple degrees of freedom in the system.



Part of me thinks it makes sense that the down hill tires will support more weight and flex more as a tilt table is lifted...

part of me thinks that all tires on the same angled table will flex the same at any angle because the added weight supported by the down hill tires are in lateral friction forces, which act perpendicular to the spring.

Not trying to cause a debate over here...just random musings. Maybe I will actually draw up the free body diagrams and do some math to figure it out for myself.
 

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dhyde79

Titan Swapped / SAS'd
Founding Member
Location
Amarillo, TX
Now, where some posts were explaining that a flexier suspension of a stiffer suspension can change this, what they are REALLY explaining is how you get INTO these situations....not if you'll roll once there.

agreed fully, but, momentum does come into play, as it can throw your "plumb bob" out of the safety zone, albeit temporarily.
 

Trexterra

Wheeling
Founding Member
Location
Alabama
My point was that the kind of situations we get ourselves in to (normal trails not baja) will have relatively 0 measure able difference with or without the front bar.


Sent from CoSprings
 

lizardking

Bought an X
Location
wilkes-barre, PA
I don't believe anyone is really arguing here. If anything we are all saying the same thing in ten different ways. No one is debating where the rolling point is per se. It is and always will be when the center of gravity falls outside of the outer edge.


My original post...was rhetorical wondering of what angle of the ground gets you to that tipping point, and does a sway bar change that angle (by flexing springs less, keeping COG inside the box at a steeper angle)? Not saying it changes it by much or that it is significant.

As stated, this was just a theoretical pondering...like a static tilt table type of deal. Obviously, when you get into actual terrain conditions, speed, acceleration, which way you steer, etc...you have MANY more factors more important than a sway bar.

So the question really kinda boils down to whether or not the springs flex differently at different angles of slopes. It would be easy if the Xterra could be modeled as a rigid box with a CONSTANT COG. But the center of gravity can shift relative to the tire square because the BODY/FRAME is on springs, which allow the TOP half to sway and shift relative to the suspension and tires. That leaves multiple degrees of freedom in the system.



Part of me thinks it makes sense that the down hill tires will support more weight and flex more as a tilt table is lifted...

part of me thinks that all tires on the same angled table will flex the same at any angle because the added weight supported by the down hill tires are in lateral friction forces, which act perpendicular to the spring.

Not trying to cause a debate over here...just random musings. Maybe I will actually draw up the free body diagrams and do some math to figure it out for myself.

I agree with the theory here. If you have some soft springs you could be at more of an angle then a truck w/ the same spot on the trail, line, setup and load. The question then becomes how much spring do you need and what is soft? Too much spring can cause issues of its own.

I am really not against sway bars. I am against constantly replacing my end-links. From experience of running the same trails in different configurations I can say that the truck feels better without both sway bars since I believe the traction off road is better (could just be my butt dyno). Norheast PA pot holes feel better with it off. Turns and abrupt maneuvers are definitely more interesting. All this is Second Gen of course....



And on the idea of swinging: a good setup should not bounce much. So, you should compress a then return. Too little dampening or too much spring might cause bounce....
 

GhostX

Allergic to Pavement<br><img src="http://i164.phot
Location
SoCal
In removing my front and rear sway bars, the purpose was explained as an effort to gain tire contact to the ground through increased articulation, improving control (braking, speed, etc.) over obstacles, and reducing, while not eliminating (speed, angle, etc.) the chance of an "off road" mishap (rollover, body damage, etc.).

Post sway bar removal, driving "on road" required an "adaptive" driving style for about 30 days, learning to co-exist with the softer suspension characteristics on tarmac.
 

robcarync

Sliders
Location
Raleigh, NC
From experience of running the same trails in different configurations I can say that the truck feels better without both sway bars since I believe the traction off road is better (could just be my butt dyno). Norheast PA pot holes feel better with it off. Turns and abrupt maneuvers are definitely more interesting. All this is Second Gen of course....

And on the idea of swinging: a good setup should not bounce much. So, you should compress a then return. Too little dampening or too much spring might cause bounce....

Agree with you on that. The front is much flexier on my first gen without it. Speed bumps and pot holes are hardly noticeable...very soft and nice. On/off ramps and 90 degree turns are...much more interesting :)



You are right on that...my thought was whether or not the "static" position changes with different angles with/without sway bar. Say comparing left/right tire stuffage on flat ground (equal)...and then checking it on a 35* tilt. If you did the test without
sway bars, and then with sway bars...would the right to left stuffage difference be less with the sway bar?

Well...I am a nerd and on my lunch break did a sample engineering problem of a basic model...I won't turn this into a lab report, don't worry
Smile(1).gif



In a normal static position, it doesn't matter one way or the other. Both sides should *theoretically* flex the same. Depending on slight differences in suspension from left to right, etc...you may see a slight difference in flex from left to right.

What you see is that as you tilt the truck to the side, there is less forces acting straight up and down on the tire, and an increase in lateral friction forces, which keep the truck from sliding down the hill. These lateral forces are acting *mostly* perpendicular to the spring motion, which is why you wouldn't get additional flex in a static position.

Looks like, as had been said, the sway bar may help keep you from getting into a tippy situation by controlling body roll during motion, but won't have any impact at all while sitting still.

I have a Lokka in the front, so traction isn't nearly as much of an issue as with a stock open diff :)

Man...I am glad my brain can rest now :)
 
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metzican

Suspension Lift
Location
Lafaytte, la
does the down hill spring compress more as angle increases. My guess (this is without a FBD) is yes as you increase from zero degrees it will compress more then at a certain value depending on vehicle and angle of the A arms and suspension setup it starts to decompress. for our vehicle I bet this is at 5-15 degrees. Does a sway bar help prevent roll over more then no sway bar in a static situation. No. I think this is due to the down hill spring decompressing at a certain angle in a static situation. Now in a dynamic side load situation I think a sway bar can help a little to a lot depending on speed and acceleration and direction of these vectors. Now in a dual washboard situation where opposite corners or sides of the vehicle would be flexing up then down repeatably in a rapid motion I think a sway bar would hurt you and cause you roll over more easily. There are a million different situation and in the end it is knowing your vehicle and how to handle each situation or which one are out of your vehicles limits.
 
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