Don Keefhardt

Sigh.

I remember, probably a decade ago, a debate among the RennFrench related to whether "caliper on the back of the spindle" = "anti-dive" & "anti-squat". Someone tossed around free-body-drawings.

Sigh again. "Omniscient being on a biscuit".

The bar doesn't care which way the car is turning. It doesn't care which end is longer. All it cares about is the total resistive force that is then coupled to the chassis.

It does not know, nor does it care, left from right.

Quit with the attempts to anthropomorphize suspension systems. Don't be like Porsche guys.

SouthernSon

Solofast

Well at least someone here is agreeing that the moments in the bar are the same and the forces at the end links are different. That is progress and that picture does act as a geometric proof.

So we have established and can agree that the forces on the end links are different than they would be with equal length arms.

Now the question is, what does that difference in forces do on a system level what does that do to the loads on the tires. Your assumption that the system "does seek equilibrium" is also correct, but where is that state?

Solofast

Nope, you got that right...


Not quite....

Actually the reactions at the bushings are the story here. You sorta left out the important part, and that is how the chassis reacts to that pesky force difference.

Let’s go back to statics 101, and look at the sum of the forces on the chassis…

If you remember your statics class you will recall that any two forces can be resolved into a single force and a moment. But there is one special case. That is where the forces are equal and opposite, but offset. In that case the two forces are reduced to a single moment, and there is no resultant…. That’s exactly what you have when the arms are equal. There is a moment across the chassis, but there is no resultant force either up or down.

But if the forces are unequal there is a moment AND a resultant force…. Hummm,,,, this is what we’ve been talking about all along.

If you look at the forces on the spindles (assuming that the linkage ratio is one), the outside spindle sees a higher force due to the bar pushing down on the spindle and the inside wheel sees a lower force pushing up on the spindle. So the forces on the spindles due to the bar being offset are both increased by an equal amount and in the same direction…. As RBugBITme noted equilibrium has to be maintained… Since the total force on the axle is fixed by the CG location, the total force on the axles is the same, and if the forces are the same there is no change in the understeer/oversteer relationship.. Or so it would seem…

But what had to happen to maintain equilibrium? The short answer is that the spring forces are reduced on both sides by the same amount as the sway bar difference.

Humm… Spring forces reduced….. What happens if the spring forces are reduced? The answer is (when turning toward the short arm) the car rises up. And if you are turning the other way both spring forces increase and the car is lowered. What is happening is that the sway bar is now adding force or taking force off of the springs and the result is a change in ride height, even though the forces on the tires are the same.

Now I don’t have to remind you that raising the car up increases the height of the CG, raises the roll center so that there is an increase in the amount of roll stiffness contributed by the suspension arms, reduces negative camber, all of which would (when turning toward the short arm if it is on the rear) INCREASES OVERSTEER…..

And of course the opposite things are all happening when turning toward the long arm, the car is squatting, the rear roll center goes down, the negative camber increases… All of which tends to increase the amount of understeer…

It also makes sense if you take this to the limits as noted in one of the earlier posts, if you have a big offset and huge bars turning one way would result in the car lifting as though there was an extra spring being shoved under the car and when turning the other way the bar would act pretty much like it wasn’t there at all.

Now, if you think that I’m still not correct, go back to RBbugBITme’s free body diagram and put F1 and F2 at the bearings. Those forces can be resolved about the chassis centerline to a moment equal to F1 x half the bearing width, plus F2 x half the bearing width,,,, PLUS a force that is equal to the difference between F1 and F2….. That resultant force is equal to the force that is removed or added to/from the springs and the net effect is to either raise or lower the car on the springs to maintain equilibrium.

Now I've always said this isn't a big change, but it is a difference and that difference can be determined and quantified and it's real.

So this is why you don’t see different length arms on sway bars. It isn’t that there is a corner weight difference. There isn’t. And there isn’t a difference in basic cornering stiffness. It’s because the difference in forces causes squat or lift that is different depending on which way you are turning, and to keep the car from handling differently we would not want that force to get too large.

SouthernSon

You do realize that the bar mounting at the two points is only for the purpose of keeping the bar in plane with the cradle? One could actually place one mounting point in the middle and another on one end of the cradle; makes no difference as long as the bar remains in place allowing the springs to combine work for the outside wheel, notwithstanding torsional flex.

froggy47

Thread trending toward - don't try unequal length arms???

If the ME guys are talking about a infinitesimal "unintended consequence" I (the OP who is not a ME) would love it if you could make that clear.

MY interest is "for practical purposes, and for a good (not pro) driver with suspension that DOES move pretty well" does using the unequal method give me two more adjustments or does it screw the handling? That's really a pretty straightforward question.

A couple of guys, so far, have said they do it & it works. I am inclined to trust their opinions (no offense to the college course guys) over all the theory stuff.

RBbugBITme

I see my over simplification missed what hklvette said but the difference seen at the tiny little stamped steel sheet sway bar mounts is small enough to ignore unless you have spherical bearings in your control arms and a pro-driver.

Solofast

Yup, no difference. It just moves the location of the forces. The resultant and the moments are no different.

Solofast

Highly dependent on the car, the suspension, the spring rates, the sway bar rates, the amount of offset in the holes your roll center height, the change in roll center height with height changes and so on...

For a car with a beam axle and the roll center at the ground it probably isn't going to make much difference. On a C4, where you can change the rear ride height by a half of an inch an feel a difference, heck yea, you easily could feel it.....

It doesn't hurt to try it, and on a C5 if you keep it one hole off you probably won't notice it much if at all. If you do notice you can try jacking a bit of weight to counteract the effect. Also try it on the rear bar since it is softer and the amount of jacking will be lower and the springs are stiffer.

It's all second order stuff, it's not going to make the car undriveable, it just may feel a bit weird as the car rolls in and takes a set. Turning one way you may feel it lift up a bit and going the other way it may feel like it is hunkering down and hooking up (that's always a good feeling)... But if you keep it small it's probably not a big deal. You just don't want to assume that you can go to big changes in arm length because that's probably not a good thing.

SouthernSon

[QUOTE=froggy47;1589722009....... does using the unequal method give me two more adjustments or does it screw the handling? That's really a pretty straightforward question.

........:[/QUOTE]

I think that is the consensus and my belief, as well. You simply have a setting between medium/medium and firm/firm that is medium/firm (a half step, if you will)

Xian

And FWIW, Lugod is running his STU C5 with a split adjustment... and Sam made mention of it without it appearing it was an issue.

froggy47

Ok, just going to do it one hole at a time for this Sunday.

froggy47

Video:


Please comment.

jtmck

You are correct. Each single hole can be an adjustment and it will not have different handling side to side

Formula cars and Trans-Am had cockpit adjustable sway bars and the rotating blade was only on one end of the sway bar.

SouthernSon

And then comes Jim. After all attempts to say exactly that by so many wordy posts, you find the exact succinct way to phrase it!

Solofast

The rotating blade approach doesn't change the length of the arms, it only changes the stiffness of one arm by letting it bend more or less. For that reason the forces acting on the sway bar system are still the same on each side of the car. The rotating blade or other sway bar stiffness changing systems work as springs in series. To calculate the spring rate of springs in series is relatively straight forward, but they still result in a balanced load system where there is no resultant force on the chassis, only a simple couple of equal force on the bar mounts.

froggy47

Doesn't shortening the bar = change the stiffness of the bar? That was my assumption.

I drove it today & it seemed "slightly" better so far as being to rotate the car (less rear grip more front grip (mid corner) so far as I can judge). Entry & exit seemed fine.

Being able to discern the changes from one different autox course to the next will be a challenge as there as many other variables, however over time I believe a trend will emerge.

I think I will try 1 or 2 more holes for the next practice & see how it feels.

Solofast

Shortening the bar changes the leverage on the bar. The bar always has the same torsional stiffness. When you shorten the arms the force is less effective at twisting the bar. If you lengthen the arms the same amount of force goes through a longer moment arm and it twists the bar more, making it softer in resistance to roll.

If you are wanting to make an additional change then move the other side one hole, you don't want to be more than one hole off from one or another.

froggy47

Thanks, yes that was my intention to go to the pass side & get 1 more hole.

Good thread! Your posts are appreciated. I am going to try & make the event on 6/14 El Toro lot, quite different from the Q West lot.