View Full Version : Suspension Q--why anti-sway on live-axle cars?
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
Never knew this was the case. I'm baffled as well. It makes no sense. :?
clyde
01-27-2005, 11:52 AM
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
What is the mechancial function of a sway bar? (Hint: Thickness is the commonly listed spec of a sway bar, but the important part is actually how s***f it is.)
What does a spring do that a live/solid axle does not (or least should not) do?
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
What is the mechancial function of a sway bar? (Hint: Thickness is the commonly listed spec of a sway bar, but the important part is actually how s***f it is.)
What does a spring do that a live/solid axle does not (or least should not) do?
Resist body roll. The implication of your post is that an anti-sway bar acts as a torsion bar, rather than simply mechanically tying one side of the suspension to the other (which is sort of the way that I always imagined that they work). Is that correct?
I suppose it must be, now that I think about it. Here is a pic of a Mustang GT's rear suspension:
http://bradbarnett.net/mustangs/timeline/05/05/64.jpg
You can see that the sway bar connects directly to the shock absorber bracket; the bracket, in turn, is welded to the axle. The function of the bar must be to resist upward movement by one wheel through torsional strength, rather than resisting it by simply pushing the other wheel down (because the axle itself would do that on its own).
But a sway bar serves a different purpose than a spring, because the sway bar only exerts torsional resistance if the wheels move at different rates. That's why you couldn't accomplish the same thing by simply jacking up the spring rates (or you could, but the side-effect would be the world's most punishing ride).
Am I barking up the right tree?
clyde
01-27-2005, 01:58 PM
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
What is the mechancial function of a sway bar? (Hint: Thickness is the commonly listed spec of a sway bar, but the important part is actually how s***f it is.)
What does a spring do that a live/solid axle does not (or least should not) do?
Resist body roll. The implication of your post is that an anti-sway bar acts as a torsion bar, rather than simply mechanically tying one side of the suspension to the other (which is sort of the way that I always imagined that they work). Is that correct?
I suppose it must be, now that I think about it. Here is a pic of a Mustang GT's rear suspension:
http://bradbarnett.net/mustangs/timeline/05/05/64.jpg
You can see that the sway bar connects directly to the shock absorber bracket; the bracket, in turn, is welded to the axle. The function of the bar must be to resist upward movement by one wheel through torsional strength, rather than resisting it by simply pushing the other wheel down (because the axle itself would do that on its own).
And what of those two links on the bar that attach to nothing in that image? :dunno:
A solid axle will tend to stay parallel to the surface the car is on (at least parallel to the line drawn between the two points the tires on). The main deviation from that parallel line will be due to the spring that acts on it first...the tires and their deformation due to various loads. The "regular" springs affect the relationship of the car's body relative to the line defined by the axle.
Now, go back and look at the image again. As one of those springs compresses or extends, does it necessarily mean that the other must compress or extend? No. On that basis, each side can move independently of the other. It just so happens that with a solid axle, movement of one spring affects how the tire on the other side interacts with the road surface. That's where the meaningful difference is between a solid and independent rear.
Now, look at the image one more time. Remember those links that I asked about a moment ago? Those connect to the body (term for argument) of the car. Unlike the regular springs which are meant to allow the body of the car move in relation to the axle, those links are meant to keep the sway bar at a fixed distance from the body...at the point where those links are fixed to the bar.
So, you've got an axle that remains stationary (using the road surface as the reference point since that's what matters), with a body, sitting on springs, on top of it that moves around as loads are applied. The body, in turn, is attached at a fixed distance to a bar at to points. As the body moves, the bar also wants to move along with it. However, the bar is also connected to the axle, which, as we know, does not move with the body.
So, something along the line has to give. What gives is the elasticity of the bar...which is what makes it a spring. Disconnect the bar from either set of pickup points and it ceases to do anything (other than add weight).
But a sway bar serves a different purpose than a spring, because the sway bar only exerts torsional resistance if the wheels move at different rates. That's why you couldn't accomplish the same thing by simply jacking up the spring rates (or you could, but the side-effect would be the world's most punishing ride).
Am I barking up the right tree?
The sway bar doesn't do anything relative to what the wheels are doing. It does everything relative to what the body is doing in relation to the bar's pickup points, whatever they may be.
I know that there is an obvious answer to this question that I am just missing, but why is an anti-sway bar necessary on a car with a live axle? Why doesn't the axle itself act as the world's biggest anti-sway bar?
What is the mechancial function of a sway bar? (Hint: Thickness is the commonly listed spec of a sway bar, but the important part is actually how s***f it is.)
What does a spring do that a live/solid axle does not (or least should not) do?
Resist body roll. The implication of your post is that an anti-sway bar acts as a torsion bar, rather than simply mechanically tying one side of the suspension to the other (which is sort of the way that I always imagined that they work). Is that correct?
I suppose it must be, now that I think about it. Here is a pic of a Mustang GT's rear suspension:
http://bradbarnett.net/mustangs/timeline/05/05/64.jpg
You can see that the sway bar connects directly to the shock absorber bracket; the bracket, in turn, is welded to the axle. The function of the bar must be to resist upward movement by one wheel through torsional strength, rather than resisting it by simply pushing the other wheel down (because the axle itself would do that on its own).
And what of those two links on the bar that attach to nothing in that image? :dunno:
A solid axle will tend to stay parallel to the surface the car is on (at least parallel to the line drawn between the two points the tires on). The main deviation from that parallel line will be due to the spring that acts on it first...the tires and their deformation due to various loads. The "regular" springs affect the relationship of the car's body relative to the line defined by the axle.
Now, go back and look at the image again. As one of those springs compresses or extends, does it necessarily mean that the other must compress or extend? No. On that basis, each side can move independently of the other. It just so happens that with a solid axle, movement of one spring affects how the tire on the other side interacts with the road surface. That's where the meaningful difference is between a solid and independent rear.
Now, look at the image one more time. Remember those links that I asked about a moment ago? Those connect to the body (term for argument) of the car. Unlike the regular springs which are meant to allow the body of the car move in relation to the axle, those links are meant to keep the sway bar at a fixed distance from the body...at the point where those links are fixed to the bar.
So, you've got an axle that remains stationary (using the road surface as the reference point since that's what matters), with a body, sitting on springs, on top of it that moves around as loads are applied. The body, in turn, is attached at a fixed distance to a bar at to points. As the body moves, the bar also wants to move along with it. However, the bar is also connected to the axle, which, as we know, does not move with the body.
So, something along the line has to give. What gives is the elasticity of the bar...which is what makes it a spring. Disconnect the bar from either set of pickup points and it ceases to do anything (other than add weight).
But a sway bar serves a different purpose than a spring, because the sway bar only exerts torsional resistance if the wheels move at different rates. That's why you couldn't accomplish the same thing by simply jacking up the spring rates (or you could, but the side-effect would be the world's most punishing ride).
Am I barking up the right tree?
The sway bar doesn't do anything relative to what the wheels are doing. It does everything relative to what the body is doing in relation to the bar's pickup points, whatever they may be.
Understood. And I had a feeling that this is where you were going all along. But that would mean that a sway bar does has entirely different role in an IRS application vs a solid axle application but with a smilar result... to associate the dynamics of the wheels with the body, minimizing roll.
But I can't help but think that the sway bar in a live axle undercuts the job of the springs. But clearly, I must not know what I am talking about since it must have a significant-enough effect for it to be there.
The sway bar doesn't do anything relative to what the wheels are doing. It does everything relative to what the body is doing in relation to the bar's pickup points, whatever they may be.
Then you wouldn't need a single sway bar, right? You could just have a torsion bar connected on one end to the body and on the other to the axle.
Put another way, if the bar has no effect on the wheels relative to one another, why does it connect the wheels to each other?
clyde
01-27-2005, 03:49 PM
Understood. And I had a feeling that this is where you were going all along. But that would mean that a sway bar does has entirely different role in an IRS application vs a solid axle application but with a smilar result... to associate the dynamics of the wheels with the body, minimizing roll.
But I can't help but think that the sway bar in a live axle undercuts the job of the springs. But clearly, I must not know what I am talking about since it must have a significant-enough effect for it to be there.
The role of a sway bar is the same whether the axle at that end is solid or not. An independent suspension allows each wheel (tire, actually) to maintain a better contact patch regardless of what happens to the other one. With a solid axle, when one tire hits a bump and rises (or a hole and falls), the contact patch of the other tire on that axle changes and that's what makes solid axles, as a general rule, worse from a handling perspective than an independent system. The weight of the car still moves around the same depending on load. The difference between an independent and solid system is that each tire on that axle is allowed to perform closer to it optimum level under all conditions.
As a practical matter, sway bars and springs (as well as shocks) are used together as a system to control the various aspects of weight transfer and tire contact patches.
The sway bar doesn't do anything relative to what the wheels are doing. It does everything relative to what the body is doing in relation to the bar's pickup points, whatever they may be.
Then you wouldn't need a single sway bar, right? You could just have a torsion bar connected on one end to the body and on the other to the axle.
And where would you connect the bar to the axle and body with just two total pickup points? You could mount it along the roll center, but if you did, it wouldn't have an effect since it would be acting on the roll center where there is no movement anyway. If you move it to one side or the other, you won't be shifting the car's natural roll center, but you'd essentially be forcing it to try to do so, leading to all kinds of nasty dynamics (which, of course, would be different for left and right turns), the further fromt he roll center you get, the more pronounced the effects.
Put another way, if the bar has no effect on the wheels relative to one another, why does it connect the wheels to each other?
Geometry.
I put larger sways on my Grand Cherokee (solid axles) and I can tell you that they definitely make a huge difference.
Understood. And I had a feeling that this is where you were going all along. But that would mean that a sway bar does has entirely different role in an IRS application vs a solid axle application but with a smilar result... to associate the dynamics of the wheels with the body, minimizing roll.
But I can't help but think that the sway bar in a live axle undercuts the job of the springs. But clearly, I must not know what I am talking about since it must have a significant-enough effect for it to be there.
The role of a sway bar is the same whether the axle at that end is solid or not. An independent suspension allows each wheel (tire, actually) to maintain a better contact patch regardless of what happens to the other one. With a solid axle, when one tire hits a bump and rises (or a hole and falls), the contact patch of the other tire on that axle changes and that's what makes solid axles, as a general rule, worse from a handling perspective than an independent system. The weight of the car still moves around the same depending on load. The difference between an independent and solid system is that each tire on that axle is allowed to perform closer to it optimum level under all conditions.
As a practical matter, sway bars and springs (as well as shocks) are used together as a system to control the various aspects of weight transfer and tire contact patches.
The sway bar doesn't do anything relative to what the wheels are doing. It does everything relative to what the body is doing in relation to the bar's pickup points, whatever they may be.
Then you wouldn't need a single sway bar, right? You could just have a torsion bar connected on one end to the body and on the other to the axle.
And where would you connect the bar to the axle and body with just two total pickup points? You could mount it along the roll center, but if you did, it wouldn't have an effect since it would be acting on the roll center where there is no movement anyway. If you move it to one side or the other, you won't be shifting the car's natural roll center, but you'd essentially be forcing it to try to do so, leading to all kinds of nasty dynamics (which, of course, would be different for left and right turns), the further fromt he roll center you get, the more pronounced the effects.
So use three pick up points. I'm talking about two different bars, one on one side and one on the other, not connected to each other. You can mount them in whatever fashion you want--imagine adding two links right next to each other in the middle of the single bar, and then sawing the bar in half in between those two new links, such that each half of the bar has three links.
The only thing I'm trying to get at is whether there is any reason to have the same bar connecting both wheels, or whether the effects of the sway bar are wholly unrelated to the fact that the same bar is attached to both wheels, and the fact that one bar is used is out of convenience.
The sway on a solid axle exerts "upward" pressure on the opposite wheel just like it would on an ind. susp. Your hybrid multi-bar setup only controls the body.
The sway on a solid axle exerts "upward" pressure on the opposite wheel just like it would on an ind. susp. Your hybrid multi-bar setup only controls the body.
Ah. That is the gravamen of my question, so to speak.
Why do you want to exert upward pressure on the opposite wheel?
Think of the wheels in ralation to the body.....when you go into a turn the car leans and one wheel is stuffed into the wheelwell and the other moves down away from the body. The bar pulls "up" on the opposite wheel while pulling "down" on the body keeping everything parallel.
Think of the wheels in ralation to the body.....when you go into a turn the car leans and one wheel is stuffed into the wheelwell and the other moves down away from the body. The bar pulls "up" on the opposite wheel while pulling "down" on the body keeping everything parallel.
I see.
Pinecone
02-01-2005, 09:28 AM
Weight transfer.
Sways change body roll and weight transfer as you corner. Type of suspension doesn't matter.
When teh car corners, the body rolls since the CG is higher than teh roll center (and yes, you could make a setup that had the roll center higher than teh CG causing negative roll). As has been said, when the body rolls versus the suspension the sways resist that movement. The upgoing wheel exerts an upgoing force, through the sway to the down going wheel and vice versa. Taken to the extreme it wil actually lift the downgoing (in relation to the body) inside wheel.
A live axle doesn't resist such torsional forces since it has a spring at each end. So each end moves independantly, but movement of one end does create tilt changes in teh contact patch at the other end. But on a relatively smooth road a live axle will keep the tires the same relationship to the road at all times, even with body roll. The downside to a live axle is the extremely high unspring weight, which compromises the ability of the springs and shocks to control its overall movement. An in betwen suspension design that has the benefits of a live axle in tire location versus lighter unspring weight is the DeDion suspension.
Sways also modify weight transfer and this is where in many cars the handling improvement comes into play. Transfering the weight to the outside tire reduces overall grip on that end. And this is why, in most cases, increasing the bar size, decreases grip at that end of the car. The old big bar in front means more understeer. WHere this falls down is for independent suspensions with poor camber control. With an independent suspension the body roll translates into tire tilting and camber change WRT the road. This is not good. A GOOD suspension will have the camber change properly to maintain the tire/road angle as the body rolls. Most McPherson strut suspensions do NOT do this. So ANY reduction in body rol actually helps the strut end of the car. This is why in otherwise stock BMWs a massive front bar actually helps the car handle and reduces understeer (brought on by the outside wheel tilting as the body rolls).
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