Sway Bar Adjustment
Attempting to build a driver controled sway bar adjustment similar to the one described in Carroll Smiths books. My question is,does only one side of the sway bar need to be adjustable? or do I need to figure a way to move both sides simultaneously?
Thank you in advance
Jeff
Assuming you are referring to a rotating flat blade bar, then, no, both 'ends' do not need to rotate.
That said, if only one blade rotates, you will get a smaller range of stiffness settings.
A single rotating end is mechanically simpler to implement.
Ian Macpherson
Savannah, GA
Race prep, support, and engineering.
Not the rotating blade but the sliding suspension link. In the very back of Carroll Smiths first book he describes a driver controlled adjustment, I believe this is what was used before the rotating blade method .
Originally Posted by Lotus7
In that case you absolutely do need to move both sides at the same time. If one side is longer than the other, the car will jack turning one way, and squat turning the other. This is because the side with the longer arm will have a lower spring (wheel) rate than the shorter side.
Last edited by DaveW; 01.14.20 at 11:21 AM.
Dave Weitzenhof
That is kind of what I was afraid of. Thanks for confirming my suspicion. Sound like I need to find a new sway bar.
Jeff
Interesting mind puzzle, I think the anti roll bar does not recognize wich side of the car is stiffer or softer. It only knows how stiff it is .
If you stiffen only one side by sliding the link back the other side will be that much more loose. and in theory could lighten that corner. But could make for some interesting corning techniques.
Dave, I have to disagree. The ARB works as a complete unit and doesn't care which side has the higher or lower rate, it is total spring rate of the entire bar.
So yes, moving one side will work but you will get half the value of adjustment.
Crossle had an adjustable bar slider on the 40f or 45f before they went to blades for the 50/55f
My question is why do you need an cockpit adjustable bar?
Robby
That is true as long as the lever arm length from the pivot to the link on both sides is the same length.
In the extreme, visualize a U-type swaybar with an extremely short arm on one side, and a long arm on the other. The side with the very short arm would hardly be able to move at all due to the arm's short lever distance and high force needed to move it. OTOH, the side with the long arm would move easily.
To be clear, the 2 arms can be a lot different in stiffness, but they have to be equal length to have the same effect in LH & RH turns.
Last edited by DaveW; 01.11.20 at 1:57 PM.
Dave Weitzenhof
Simple enough to test.... set one side full soft and the other side full stiff and drive the car. See if the car behaves the same in right and left corners.
This sway bar assembly has a fixed length blade and an adjustable length arm on the opposing side.
Nope.
It works as a complete system. Each side is not a fixed lever arm and a bar. The whole bar twists not just from the middle out to each arm. ARB's are not solidly fixed in the center, at least on all the cars I have.
I disagree. It has nothing to do with being fixed in the middle or not. It has everything to do with the lever arm length. If you do a torque balance you will see what I mean. The torque is constant (barring friction, etc.) from one end of the straight part of the bar to the other. If one lever arm is shorter, the force on it will be need to be greater to balance the torque created by the longer lever arm. This will create an imbalance in the amount of vertical force provided by the spring/swaybar combination side-side. How much this changes the amount of vertical motion one side v the other depends on the wheel rate stiffness v the swaybar stiffness. So as I said, the reaction of the car will be different RH v LH turns because the suspension on the side with the shorter arm cannot develop enough force at the link attachment to the swaybar to make the suspension move equally side-side.
As I said before, the car will tend to pivot around the side with the shorter arm, since it is stiffer. In the limit, with a very short lever arm, the swaybar will restrict suspension travel on that side. An analogy is a beam pivoting on a fulcrum close to one end. The end closer to the pivot will move less, but take more force (inversely proportional to the length ratio) than on the long end to balance. And, since the loads are coming to the swaybar from the upright, there is no way that making the swaybar stiffer on one end can result in equal reactions L-R.
Now, I do agree that if the length difference side-side is small relative to the lever length, this effect will probably not be noticeable.
Last edited by DaveW; 01.11.20 at 4:57 PM.
Dave Weitzenhof
Does anyone disconnect their Anti-roll bar when they scale the corner weights? Then set the lengths of the links between the ARB and the suspension so there is no preload?
Always unhook the bars during any alignment activities. When done hook them up with zero preload.
OK just to throw a wrench in, FV ARBs are fixed and non adjustable. 1/2" --3/4"
The center boss in the beam limits how big you can go ,3/4"seems to be the limit.
I once had a bar that was 3/4" welded to 1" bar just past the middle.
It seemed to be a better set up but was damaged and have never made another one.
On this winter weekend maybe some thoughts on how a FV ARB even works!!
Yes. Agreed. Unless you are preloading them for cross-weighting. And I would do that after everything else was set.
Dave Weitzenhof
There are a few interesting things about a FV front ARB:
1. Because the FV front suspension is usually run with the trailing arms severely tilted up toward the rear, the front suspension is heavily "rising rate." That means that as the car rolls, the outside suspension wheel rate climbs rapidly, making the car understeer. So a stiffer front swaybar, which in most suspension setups would make more understeer, in a Vee, by limiting roll, can limit the amount of rate increase on the outside wheel, thus reducing understeer. At the same time, because it reduces the amount of wheel rate increase on the outside suspension, a stiffer swaybar reduces the tendency of the front to "jack up" in a corner.
2. Because the front suspension has little or no camber gain, limiting roll makes the outside front tire have less positive camber in a corner, improving grip.
There are probably more, but those are the two I thought of at the moment.
Dave Weitzenhof
To Mondial, I think your idea of making one side of the arb cockpit adjustable is a good idea. To reduce the minor jacking effects that may occur try to keep the bar sliders as close as possible right to left.
Again no.
Reynard built cars in the 80's with an ARB that was fixed on one side and only adjustable on the other. How did these cars function as they were very competitive. According to your interpretition they would have been unbalanced.
When one side of an ARB is put under stress the entire ARB is activated. They are not split into sides like springs are. Does not matter if one arm is shorter than the other. Sorry but your logic makes no sense. How is a shorter arm stiffer if the force is transfered to the other arm where it is attached to the upright? It is connected to the other longer arm in order to have any action on the uprights. The force on each upright is the average of the 2 arms.
Carroll said. I use the simple way and I adjust both sides of the bar.
I am a pretty good engineer. I use known principles and logic to reach my conclusions. I will say no more on this subject.
Dave Weitzenhof
Dave
Thanks for your comments on FV front ARBs
I will use it as a reference for our new drivers who have
no experience with set up on Vees
Dave, when I first read your original response, my initial reaction was like some others above. However, once I thought about your description, I understand how it can exert different forces between left and right corners. Very interesting. I'm glad to be always learning, and a little embarrassed that I might have argued pretty hard for the past 40 years that you only needed to adjust one side.
Racer Russ
Palm Coast, FL
May I suggest a compromise?
Dave's point is that with unequal lengths on the attachment point there will be unequal amounts of motion necessary to transmit a given torque, and hence downforce, from one side to the other. In practical terms, however, the amount of transfer is determined by the spring rate of the entire assembly, and we generally tune the bar to enough stiffness, and with enough similarity in arm lengths, that there really isn't very much motion difference. The few examples I've seen of the old Crossle' system had quite a limited range of adjustment compared to the overall length of the arms.
Caldwell D9B - Sold
Crossle' 30/32/45 Mongrel - Sold
RF94 Monoshock - here goes nothin'
Agreed. That's why I said in post #14 that if the length difference between sides was relatively small compared the the distance from the pivot, you'd probably not notice any adverse effects.
Dave Weitzenhof
If someone wants to try unequal arm lengths on a car with a bent tube arb, go ahead and tell us the results.
Dave is right
Dave is much better then a "pretty good engineer". He really knows this stuff and works to teach the Apexspeed
readers what he knows. Reynard likely got it to work by having long arms on the U and the difference was acceptable. I remember a Lotus? design with pushrod and bellcrank suspension way before anyone else and they abandoned it for rocker suspension on the subsequent F1 cars. The car probably had other reasons it was not a winner. They had the physics right but didn't get the results for other reasons. Reynard is seems got it the other way round
Thought experiment:
LHS wheel moves up 1 inch on a 10 inch long arm. The twist in the spring (Cross bar) wants to be 1 part in 10. If the bar doesn't want to twist (Super stiff) then the arm on the other side wants to move 1 part in 10. Say the arm on the RHS is 20 inches long. It will need the unloaded wheel to move up 2 inches. More than the loaded wheel!
Same car but loading the RHS wheel to move up 1 inch. 1 part in 20. The unloaded wheel (remember it has a 10 inch arm) will want to respond 1/2 inch on the inner wheel
very different response depending on the corner direction
Last edited by Roux; 01.12.20 at 7:50 PM.
I never said Dave was not a good engineer. I respect him for his knowledge and accomplishments. But that does not make him perfect nor infallible. Lemmings follow each other believing the one in front is correct. They ALL fall off the cliff. I would hope we here on Apex are not lemmings but read, analyze, question and check for validity the suggestions and ideas put forth on this site. If you find it offensive for me to question Dave, I am sorry. Much of what Dave posts is good information. This material on ARB's I just do not see it the way he does.
First we need to decide if we are talking tubular bent ARB's, tubular ARB's with mounted arms and Blade ARB's. Each behaves in a slightly but importantly different manner.
Second, are we talking forces on the arms or the degrees of an arc they move? These are 2 different things. In the above example the movement may have differed but the forces were average between the arms.
Last edited by BorkRacing; 01.12.20 at 8:07 PM.
Bote
Jeff,
give us the geometry of your sway bar and lets do a BOTE (Back Of The Envelope) calculation to see if you are OK with a single sided adjuster. It's winter, we have some time to spend on the internet:
Need arm length (nominal) Anticipated length range of your adjuster, Diameter of the arm (assuming it is a U shaped sway bar made out of a single solid steel rod) and the length of the bit between the bearings.
If your bar is not a single rod bent into a U show some pictures and dimenions
cheers
Steve
But how many degrees in the arc of the main bar do each of the arms move. I suspect it is the same.
Ever play golf? 5* on a putt is barely anything, 5* on a drive is in the next fairway. Same for the arms on an ARB, 5* at 10 inches may be 1/2 " and at 20" it may be 2". Same rotation of the main bar though. Just some food for thought.
I also agree with Dave, am not a lemming, and am also a ME. What dave is talking about is called a summation of both forces and sum of moments. First, looking at sum of moments, the bar for a given amount of rotational deflection, generates a given torque. To counter-act that torque, the length of the arm times the force on that arm generates the resisting torque. Equal length arm, equal torque. different length arm, different force. The force of the arm, connected to the upright, results in a spring rate when moved over a distance. As the car rolls, this determines the distance moved, resulting in a force. In summing of forces, the up and down forces of the arm connection, not being the same, results in a force applied to the two mounting points of the bar to the frame. These forces are not the same side to side, and will switch sides when turning opposite. This is what causes the difference one way VS the other. Also, as Dave said, small percentages of change side to side will most likely not make much difference. Just my $0.02.
john f
Different torque on each arm but same torque thru the entire system. You are treating each arm as a separate system, they are not. Your example would be true if the bar was held solidly by the mounting points, but they are not.
We are talking 2 different concepts here. I am referring to the rate of the bar and you and Dave are talking the distance the arms move. The reality is none of us will be using ARB setups with massively different arm lengths. So let's just agree we can't agree and move onto other more important tasks.
Last edited by BorkRacing; 01.12.20 at 9:56 PM.
Steve
This is where it gets interesting. You have to understand I am making a formula ford. I came across a Legrand MK21 frame and found that there is no parts readily available, so it is a little bit Legrand little bit Merylin a touch of Brabham and a smige of Lotus and Lola and .a lot of me fabricating what I could not find or make work., the rear sway bar is just a simple 3/4 inch u-shaped bar with adjustable links connected to the lower a-arms. I was going to make a driver controlled rear sway bar adjustment and could not figure out if it should be one link control or two. For right now I am thinking the sway bar will be later on once I figure out if this thing will even make a lap around a track. But it has created a interesting conversation.
Thank you everyone for your input on this matter it has made for a very good read
Jeff
Jeff,
give us the geometry of your sway bar and lets do a BOTE (Back Of The Envelope) calculation to see if you are OK with a single sided adjuster. It's winter, we have some time to spend on the internet:
Need arm length (nominal) Anticipated length range of your adjuster, Diameter of the arm (assuming it is a U shaped sway bar made out of a single solid steel rod) and the length of the bit between the bearings.
If your bar is not a single rod bent into a U show some pictures and dimenions
cheers
Steve
Coming into this one slightly late (can I blame the time difference across the pond? No...ok!) and I have to admit I had to think about the two ideas for a while.
Eventually I reconciled it like this:
Different lever lengths - if one lever is made infinitely short, that corner will become solid, essentially, ie no suspension movement. The other, with some lever length, will continue to move on bump. Hence adjusting one side will give different side-to-side responses.
Same lever lengths - so the 'blade-type' arb. going back to the only car I had with one of these (Reynard 85F) the arm lengths were the same but rotating the 'blade' through 90 degrees allowed soft-to-hard adjustment. This was due to varying the flex in the blade only so, at the extremes:
(a) on 'full hard' the roll stiffness was controlled predominantly by rotational flex in the transverse element of the bar, and
(b) on 'full soft' the roll stiffness was controlled by predominantly by flex in the blade element.
Anywhere in-between, the overall stiffness was a combination of the two elements, the proportions varying with the angle of the blade.
No, same torque on each arm, yes, total system, no, rotatable in the mounting points. Torque is defined as "force X distance". Units are lb-ft or lb-in in my world. As YOU stated the torque thru the system, here the sway bar, has the same torque from one end to the other. If the arm length is "X" and the torque is "T", and the force at the connecting point on the arm is "F", then T=F x X or F=T/X.
Therefore, if X is made 1/2 as long on one side, F will become twice as much on that side, to keep T the same through the bar. The rate of the bar in T/degrees rotation has NOT changed in either case. The resultant, which is the load at the attached end of the bar, WILL change. My final $0.02 on this topic.
john f
First time since the FFU that I've ever disagreed with DaveW on some tuning/set up advice. Einstein and Edison, I'm certain were wrong at least once. Then there's me; a broken clock--I'm right at least once per day even if wrong all the other times.
Taking an extreme example: One lever arm 1" long and the other 10" long. The resulting degree of twist with any given force left or right hand turn is the same as when the two arms are 5.5" long.
If there is an upward force on the 10" long arm, the arm has lots of leverage on a very stiff "mount" (the 1" arm).
If you turn the opposite way and put the same amount of upward force into that 1" arm, you don't have near the leverage but your "mount" is very soft (10" arm).
The net result in terms of roll will be the "same". Where it gets wonky is not the difference in lever arm length but the angle that your drop links are acting upon those lever arms. When those drop links are vertical vs. more towards the horizontal.
It's like a balance or a see saw. 10 fat kids on the short side, 1 fat kid on the side that's 10x the distance from the fulcrum. Same balance as if you put 5 1/2 fat kids on each side with the fulcrum in the center.
No need to make it more complicated than it is.
Last edited by Daryl DeArman; 01.13.20 at 1:04 PM.
Yes the load at the other end of the bar has changed, but the length of the lever arm that load is acting on is a different length.
Exactly what I have said, the torque will be the average of that applied by the 2 arms. The load in a closed system like a bent ARB will be shared by both sides. You have to realize that when you look at the force on one arm, the other is not fixed. The opposite arm will also add to the total rotation force in the closed system.
BTW in my world torque is defined as :
Like I said we are discussing 2 different ideas.
FYI I took Millikens' Race Vehicle Dynamics Program at UB School of Engineering, so my understanding of physics and engineering is pretty good. Good book too, have you read it?
If I have time this weekend I will set up an experiment in my shop to test both of our hypothesis. Should be interesting.
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