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Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
I've long been mystified by this aspect of how ARB's work.

Why doesn't transferring weight from inside to outside tires increase their lateral tractive capability?

Seems to me that's what would should happen when increasing normal force w/o increasing the lateral force.

I understand that increased slip angles may require steering angle correction, but that's a different issue.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

The total axle lateral force is the sum of the inside and outside tire forces at some slip angle. Because tires act as "softening" elements ( the more you add vertical load to them, the less incremental output force results and vice versa as a generalization. Remove load from them and they produce incrementally more force). So, fy(alpha,wlf + deltaw) + fy(alpha,wrf-deltaw) is reduced as deltaw is changed.

where fy = tire lateral force output function
alpha = slip angle
wlf = left wheel weight at 0 sideforce
wrf = right wheel weight at zero sideforce
deltaw = vertical load transferred side to side due to cornering, suspension spring and anti-roll bar forces.

To see this, just draw up a parabola of tire force with vertical load on the x axis and output force on the y axis. Slap a dot on the curve at some convenient point and go the same distance (deltaw) in each load direction. Then connect the dots with a straight line between the two new tire outputs. The line will lie below single point representing the tires with no load transfer.

Actually, tires are not "springs" as far as sideforce generating elements. They operate like dampers. You need forward velocity in order to generate lateral velocity. (no speed, no cornering).

And there are cases in which the tires on a vehicle having VERY large load reserve (They are way under their rated load capacity) do just the opposite. It these cases, you will sometimes see that adding an anti-rollbar WILL increase the grip of the pair of tires up to a point. Corvettes come to mind in this case. Their tires 'like' increased vertical load up to a limit. Pressure, rim width, and construction details influence this trait. And this phenomenon can easily be lost in some (most) forms of tire math models which can not accomodate a linear or increasing stiffness tire trait. It's there in the raw tire test data but not in the analysis. Oops. From there on out, it's lies, untruths, urban legends, dogma and blatant ignorance. In God we trust, all others bring data...

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
Thanks for the nice response.

Right now I'm too lazy to use your nice formulae, but is this it - in the case of the weight transfer being on the same axle, the nonlinearity of lateral tractive capability vs. normal load results in less being gained on the outside wheel than is lost on the inside?

Either way, what about the case where only the rear ARB is added or stiffened?

Then normal force is transferred from the inside front tire to outside rear tire.

As far as the rear is concerned, why doesn't this normal force "stolen" from the front increase the lateral adhesion of the outside rear tire (not to mention decreasing adhesion at the front)?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

A stiffer rear ARB increases normal force at the outside rear AND the inside front. The normal force is "stolen" from the outside front and inside rear.

1. The total normal force at the front stays the same regardless of ARBs etc. Same for the rear.
2. The maximum lateral force available at the front for the given normal force will occur when each front tyre carries the same load. Any deviation from this will reduce the total lateral force available. (as explained by Cibachrome). Same for the rear. (This is why vehicles with a wide track and low CG corner faster.)

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Correction: Use a function like log(x-1) to model a tire. I was thinking parabola on its side. Brain fart. A sine function is more famous (as in Pacejakk).

The rear bar operates the same way. Because it also contributes to the total roll resistance moment, there is reduced load transfer at the front. The fractions of load transferred are the main factors in "TLLTD" (Tire Lateral Load Transfer Distribution) which is a major tuning setting and is specific to a tire construction, pressure, rim, etc. recipe.

Roll bars also affect other tire and chassis properties, some by intent and others unintentional. There are tire properties other than lateral force to deal with. And there are several types of "roll bars" (direct acting, etc).

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
> 1. The total normal force at the front stays the same regardless of ARBs etc. Same for the rear.

Incorrect and counter to long-established suspension tuning practice.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote (Noah)

Incorrect and counter to long-established suspension tuning practice.

How so, when it's the transferred load rather than the static total Fz that results in changes to the total Y-axis grip and the slip angle(s) associated with generating it? For a quasi-static midcorner condition, anyway.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
> How so, when it's the transferred load rather than the static total Fz that results in changes to the total Y-axis grip and the slip angle(s) associated with generating it?

If the front has lower roll stiffness than the rear, then with latacc instead of normal force transferring proportionally from inside to outside wheel at both front and rear, weight will effectively transfer from inside front wheel to outside rear wheel.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"NoahLKatz (Mechanical)(OP)12 Sep 16 02:50
...Why doesn't transferring weight from inside to outside tires increase their lateral tractive capability?
Seems to me that's what would should happen when increasing normal force w/o increasing the lateral force.
I understand that increased slip angles may require steering angle correction, but that's a different issue."

Noah, your last statement is the most important issue for car handling and can limit what you would like to do with front/rear roll stiffness. The most noticed effect of roll bar selection is on handling. You could end up with a car that makes more Gs on a skid pad, but is undriveable on the track.
That last statement also partially answers your question. Note that tires make a side force when not pointed in the direction of travel. The angle they make is called the slip angle and the force they make is not perpendicular to the direction of travel, but points behind. If the car is cornering about a turn center, then the side force of the tire does not point toward the center of turn, it points behind the center. This resolves into two components, a force toward the center and a force opposite the direction of travel, a drag force that slows the car. Therefore, the "cornering" force toward the center of turn is less than the side force. Also keep in mind that though increasing the normal force increases the tire to track friction (you hope), the compliance of the tire is not changed. It distorts more and requires more slip angle to make more side force.

Now suppose you had suspension with zero roll stiffness (Z-bar springing or beam axle with a single center spring). You have two tires always sharing equally the weight of that end of the car. For a certain cornering speed they will corner with a certain slip angle. Now with the same tires add the maximum roll stiffness that just begins to lift the inner wheel in a corner. The outer wheel now carries twice the weight as before and requires approximately twice the slip angle. You see that the side force points farther behind the turn center, etc, etc, etc.

Another aspect involves what happens in the contact patch. You know that a tire rolling with only a vertical force has its contact patch in static contact with the road. You also know that for rubber, static friction is much greater than sliding friction. Well, when a tire is cornering, braking or accelerating there is both static and sliding friction. Suffice it to say that under side force with the carcass and tread under high distortion the proportion of sliding vs static goes unfavorable. The tire with the higher loading and greater tread distortion loses G force due to an increase in sliding friction with a loss of static friction.
The tread tends to break away at the rear of the contact patch where the distortion is greatest and the weight is being lifted. But there is also sliding and gripping and sliding and gripping going on throughout the patch.

There are also changes due to heating and additional effects that are mysterious to me and some more effects that seem to be mysterious to everybody.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
140A,

"... Therefore, the "cornering" force toward the center of turn is less than the side force."

Thanks, I hadn't read or thought of that aspect before.

I'm aware of the control and safety issues; I'm not proposing anything counter, just trying to understand what is still a conundrum for me.

I don't see how the case of zero roll stiffness at one end addresses my question; in that case there would be no lateral weight transfer at that end and all of the overturning moment reaction would appear as higher downward normal force on the outside tire of the other axle.

So I ask again, why doesn't that increase its lateral adhesion?

Or does it, but it's unusable because of the control issues?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

The illustration about the zero roll stiffness with both tires sharing the vertical and cornering loads equally vs total weight transfer onto one tire shows the lessening in the geometric cornering power due to higher slip angle, as you acknowledge, and the poorer performance of the contact patch due to more distortion and a greater proportion of sliding.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote (Noah)

I don't see how the case of zero roll stiffness at one end addresses my question; in that case there would be no lateral weight transfer at that end and all of the overturning moment reaction would appear as higher downward normal force on the outside tire of the other axle.

Zero roll stiffness does not mean that the geometric portion of LLT is zero. That would require locating the geo roll center at grade level - and even then, and together with a zero roll stiffness suspension, there'd still be a little LLT from the effective amounts of unsprung mass (wheels, tires, hub-mounted brake rotors, suspension uprights, etc.).


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote (Noah)

If the front has lower roll stiffness than the rear, then with latacc instead of normal force transferring proportionally from inside to outside wheel at both front and rear, weight will effectively transfer from inside front wheel to outside rear wheel.

Juggling roll stiffnesses alters the amounts of roll moment carried at the two ends of the car, and the load transfer remains proportional taking each axle and its roll moment loading by itself. In your case here, there would be less LLT up front covered by more LLT out back. What does change is the crossweight percentage (circle track terminology that I'm not at all sure how to work with).


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

The "zero" roll stiffness in the example is used for illustration of why and how load transfer diminishes cornering power. I don't know of a suspension with actually zero roll stiffness.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Just about all farm tractors have zero front roll stiffness due to a center pivoting solid front axle. Rear suspension is the tires, which operate at low pressure. On ag tires, they roll quite a bit. Can-Am Spyder motorcycles with only 1 rear wheel also have, you guessed it.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
Norm,

>In your case here, there would be less LLT up front covered by more LLT out back.

Isn't that more or less what I said?

Does anyone not agree that higher roll stiffness at an axle causes its outside wheel to have increased normal force transferred from the inside wheel of the other axle?

I understand that the increase in lateral tractive capability is not commensurate with the increase in normal force, but it seems like there should be an increase nonetheless.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"I don't see how the case of zero roll stiffness at one end addresses my question; in that case there would be no lateral weight transfer at that end and all of the overturning moment reaction would appear as higher downward normal force on the outside tire of the other axle.

So I ask again, why doesn't that increase its lateral adhesion?"


Depends whether you want to define "lateral adhesion" as cornering force (Fy) at that tyre or Fy/Fz (coefficient of friction). Fy increases but Fy/Fz decreases and therein lies your problem.

Repeat:
1. Cornering forces do not alter the TOTAL normal force at each end - just the L/R distribution.
2. Moving the L/R distribution away from 50:50 at one end REDUCES the total adhesion available at that end.

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
> Fy increases but Fy/Fz decreases and therein lies your problem.

Finally, an answer, which is affirmative.

Why is it a problem?

I know coefficient of friction decreases, but I'm only asking about lateral adhesion.

> Cornering forces do not alter the TOTAL normal force at each end - just the L/R distribution

That contradicts your answer above.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Why is it a problem?
You have posed us a question ie you have a "problem".

I know coefficient of friction decreases, but I'm only asking about lateral adhesion.
But you didn't define "lateral adhesion". If you are trying to predict understeer/oversteer from "lateral adhesion" the definition needs to be "effective coefficient of friction for that axle".
So if "that axle" is experiencing more weight transfer than the other, it will tend to have a lower "effective coefficient of friction" than the other.

That contradicts your answer above.
My answer above: "1. The total normal force at the front stays the same regardless of ARBs etc. Same for the rear."
My latest answer: "1. Cornering forces do not alter the TOTAL normal force at each end - just the L/R distribution."

Where is the contradiction?

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
> But you didn't define "lateral adhesion"

I believe I did - lateral tractive capability.

I guess my question wasn't clear.

I am NOT interested in oversteer/understeer.

It doesn't make sense to me that increased normal force doesn't increase it.

> 1. Cornering forces do not alter the TOTAL normal force at each end - just the L/R distribution.

I said why I don't believe that's the case; rather than repeat your statement, can you say why or what part of mine you disagree with?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Understeer or oversteer is going to put a limit to how much lateral acceleration you can actually use before it reaches the theoretical maximum (the coefficient of friction) - sometimes well before.

And, the average racetrack corner doesn't look like a skid pad, and the surface of the average piece of public road doesn't look like the surface of a racetrack, and keeping the average joe safe when he has to do an emergency lane change on a rain-soaked motorway doesn't look like much of any of that.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote (Noah)

I understand that the increase in lateral tractive capability is not commensurate with the increase in normal force, but it seems like there should be an increase nonetheless.

For the outside tire on a given axle taken in isolation, that is, of course, true. But it seems sort of academic to focus on the loading (and adhesion) at only one of the wheels on an axle while ignoring what's going on over on the other.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Sta bars change the left to right distribution of Fz, but they do not change the fore aft distribution. if they did one end of the car would rise up . Draw a free body disgram in side view.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"Sta bars change the left to right distribution of Fz, but they do not change the fore aft distribution. if they did one end of the car would rise up . Draw a free body disgram in side view."

Greg, you haven't seen racing cars where in a corner one end rises up?

Seen on FWD, forward weight biased cars where the rear rises and RWD, rearward weight biased cars where the front rises. This is a consequence of putting almost all roll stiffness at the light, non-driven end. I've even seen it on RWD, forward weight biassed cars -the heavy end rises up.
One could say that the bars connect to both ends through the chassis.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"Just about all farm tractors have zero front roll stiffness due to a center pivoting solid front axle. Rear suspension is the tires, which operate at low pressure. On ag tires, they roll quite a bit. Can-Am Spyder motorcycles with only 1 rear wheel also have, you guessed it."

True!

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

I'd be careful about interpreting visual-only observations. Rising-rate springs could cause an overall increase in ride height at that end of the car (the outboard side drops less than the inboard side rises).

That said, I think there are a couple of mechanisms by which there could be some coupling between ride height and the roll stiffness provided by a sta-bar. Unequal arm lengths, maybe. Asymmetric geometry changes in roll putting the endlink on the outboard side at a slightly larger motion ratio than the one on the inboard side.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

RE FWD lifting inside rear wheels. You have drawn an incorrect conclusion from a correct observation. Draw the FBD. Unless you were being ironic.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
My apologies for spinning everyone's wheels.

As I lay in bed not falling asleep last night, I saw my error.

I realized that if zero roll stiffness at an axle means that latacc can't xfer weight onto that axle's outside wheel, it also means it can't remove it from its inside wheel.

That put the lie to my belief that a rear ARB xfers weight from the inside front to the outside rear wheel (which I had read in a car magazine when I was young and never questioned) is not true.

So now I get it; the axle with greater roll stiffness loses total adhesion because it gets a poorer wt xfer distribution, and adhesion is gained at the other because it gets a better wt xfer distribution.

Which is what everyone has been saying.





RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Pheeeew!!!thumbsup2

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"RE FWD lifting inside rear wheels. You have drawn an incorrect conclusion from a correct observation."

Greg, the observation is that ALL normal car suspensions raise the CG of the car upon cornering. The ones that have one end with roll stiffness disproportionately high compared to the weight at that end will raise that end the most.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

140A.
That is not a correct assessment either. If the roll moment is purely moment, the chassis will rotate about the roll axis so no "jacking" will occur - unless a wheel lifts off the ground. OTOH "jacking" can also be caused by suspension design (eg swing-axle) where forces other than a "pure" roll moment come into play.

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Whatever happens, in side view, in steady state cornering, there is no front to rear weight transfer. Draw the FBD. Again. Which you obviously haven't. Again. JFDI.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Take pictures and measure what happens!

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Norm mentioned rising rate springs which I agree may be a frequent cause on track cars. A common source of rising rate is the trusty bump-stop. If the rate was symmetric about the unloaded ride height (ie a droop limiter) you would see a wheel lifting.

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

It's a common tuning condition on FWD. It is not ideal. The reason it happens is that to maximise traction you want the front roll rate to be soft, so to limit roll you have to use the back axle. But the rear axle is only lightly laden so the stiff rate makes it relatively easy to lift the inside rear wheel. The stiff rear roll rate also helps to reduce the understeer on the car, which is often helpful, it is otherwise quite easy to end up with a car that understeers if you accelerate, understeers if you brake, and understeers when you turn the wheel. Drivers whinge about that.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"Ah, not ironic then. Incapable."

Greg, I'm surprised you seem to believe "The" or "A" free body diagram can decide this question.
Of all the attitudes the suspension of a car on road or track can take for any condition of loading, side force and instantaneous ride height, the car would seek the lowest energy configuration at any instant, but this would be described by a family of free body diagrams. Vehicle dynamics software can provide this. If you have access to that I can be easily convinced if you would kindly post the curves for ride height and other interesting parameters.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Yes i spend every day running adams, or measuring cars on the track, or working things out with a pencil and paper. You are the one making the ludicrous proposition, it's up to you to show how it could work. A side view fbd of a car with weight and 2 axles shown is sufficient to demonstrate that in the absence of longitudinal acceleration there is no weight transfer.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

It just so happens that today in qualifying for Sunday’s F1 race in Singapore Sebastian Vettel’s car experienced some kind of suspension failure that had his front end lifting up in a corner with the inside wheel high off the ground. It was absolutely spectacular and it was a perfect example of what I had said. The failure was at the rear. A loss of roll stiffness at the rear made the front carry too much of the car’s roll stiffness, much beyond its share compared to the amount of weight it carries. The result was the front had extra lift. The rear then must have had less lift than normal.
Since the rear did not seem to drag, the failure most likely was only with the roll stiffness function of the rear suspension system. It was described as a broken rear sway bar. A perfect real world example of a sway bar at the rear affecting roll behavior at the front.

BTW "3-wheel racing" a term describing setting roll resistance front and rear to deliberately raise a wheel in cornering is well known and can be the fastest way around. It was used in F1 during the 2000s and on 911s in the 1980s.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"gruntguru (Mechanical)140A.
That is not a correct assessment either. If the roll moment is purely moment, the chassis will rotate about the roll axis so no "jacking" will occur - unless a wheel lifts off the ground. OTOH "jacking" can also be caused by suspension design (eg swing-axle) where forces other than a "pure" roll moment come into play."

What ultimately controls chassis lift is not the spring rates nor the suspension geometry. It is the fact that the tires exert centripetal force at ground level and the "centrifugal force" acts through the CG of the vehicle at some distance above the ground. As you well know, forces acting in opposite directions along vectors offset by a length produce a torque.
That torque can be shown, by drawing a line from the CG to the outside tire contact patch, to result in a lifting force at the CG. Ultimately, if the tire grip is sufficient combined with a CG high enough, it will actually lift both inside tires off the ground and tip the vehicle. ALL vehicles can be tipped with sufficient grip at the wheels and high enough cornering force.
I used to try to design (non-active) suspensions that would not lift the chassis in cornering. It can be done, but only up to a point and it requires compromising other characteristics like showing a kink in the curves of suspension behavior and may not be self restoring, especially not along the same characteristic curve.

With conventional suspension cornering force ALWAYS produces this lifting force at the CG.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

140A. Your "lifting force at the CG" does not exist. The couple due to centripetal and centrifugal forces is reacted by unequal normal forces at the inside vs outside tyres producing an equal and opposite couple. Yes, once the inside tyre normal force goes to zero, the wheel (and the CG) starts to lift. Until that point, there is no "jacking".

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"GregLocock (Automotive)
18 Sep 16 05:53
blaa blaa blaa. Look at the FBD I posted. Criticise it. "

Greg, your diagram is irrelevant to what I'm talking about. Since you seem not to know that, we must be talking past each other.

gruntguru, You imply that the inside wheels exert an equal force to the outside wheels in a corner. But, of course you know that is not true as you state in "Yes, once the inside tyre normal force goes to zero, the wheel (and the CG) starts to lift. Until that point, there is no "jacking"".

The inside tire normal force begins to go to zero (diminishes) just as soon as cornering begins. The suspension is usually designed to be compliant which means that as soon as the torque described exists, the CG begins to react to it by rising.

Originally I was going to give an example of a solid suspension vehicle to illustrate what it takes to avoid CG rising where what you describe can actually happen. And I was going to show what roll stiffness means with solid suspension and how you can get different roll stiffness front and rear with solid suspension and how this would also react in the manner I talked about with the end that has higher roll stiffness rising higher in roll (this time regardless of weight distribution), but that requires even a lot more mental flexibility and realizing that tipping involves roll.

Try to see if you can see what Sebastian Vettel experienced when his rear sway bar broke. I'd be glad to read your explanation of it that is different from mine.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

When his rear sta-bar broke, the boundary conditions for chassis attitude changed. Which in turn resulted in the inside front tire lifting (yet another change in boundary conditions - any/all further LLT had be taken at the rear for obvious reasons). IOW, it wasn't the same car any more, so an approach that implies linear behavior cannot be used to explain what was going on.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Greg: "Sta bars change the left to right distribution of Fz, but they do not change the fore aft distribution. if they did one end of the car would rise up . Draw a free body disgram in side view."

Airpower140: Greg, you haven't seen racing cars where in a corner one end rises up?

Ah I get it, you meant pitching, I meant literally rising up and up and up until the car was standing on end. Sorry.


Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"IOW, it wasn't the same car any more, so an approach that implies linear behavior cannot be used to explain what was going on."

Norm, the crew knew what was going on because the reaction of the car was explained by a loss of rear roll stiffness. Note that the exaggerated lifting only occurred in cornering. They therefore knew the vertical stiffness was ok. They did not look at the front where the strange behavior was occurring. They went straight to the problem. That was also my guess.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"Greg: "Sta bars change the left to right distribution of Fz, but they do not change the fore aft distribution. if they did one end of the car would rise up . Draw a free body disgram in side view."

Airpower140: Greg, you haven't seen racing cars where in a corner one end rises up?

Ah I get it, you meant pitching, I meant literally rising up and up and up until the car was standing on end. Sorry..."

If you saw Sebastian Vettel's car and thought that was pitching then no, you didn't get it. But, I think you actually do get it, Greg, and are only being sarcastic as shown in your continuation.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

No i don't watch F1, and i was serious, a longitudinal weight transfer in the absence of longitudinal acceleration s would result in the absurd posture I proposed, eventually.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Noah,
I was going to add more to the discussion on the effects of slip angle, but the post would have been excessively long. However, I think you might find this interesting.
Remember that the slip angle points the side force generated by a tire in cornering actually behind the center of the turn which can be represented by a reduced force pointed toward the center along with a drag force pointed opposite the direction of travel. This describes a coasting wheel.

A driven wheel can generate a thrust force in the direction of travel that neutralizes the drag force resulting in the full side force being directed toward the turn center. So, a driven wheel can have better cornering power than a coasting wheel. However, for a 2-wheel drive car, one end is coasting and the driven end has to make extra thrust to make up for the drag at the coasting end. This kills the advantage the driven end has in cornering... UNLESS, you minimized it by concentrating as much weight as possible on the driven end. Then, there is always 4-wheel drive.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
Very interesting, 140AP, thanks.

Greg, regarding your FBD, I'm a bit puzzled as to your choice of view; in side elevation the lateral force vector is invisible so deriving its force resultants and effects is problematic, no?

Front or rear elevation makes more sense to me.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Greg, if we are talking about weight, (measured on a force scale), and not mass then sway bars can transfer weight front to rear and vice-versa diagonally which in a corner means that the most critical outside pair of wheels can experience a direct dynamic transfer of weight from front to rear or the reverse. You can crank a twist into the bars to create a static stagger of weight. They do the equivalent of this in NASCAR. And yes, when you increase the weight on a tire without increasing the mass on it (in effect, the crank of the sway bar adds to the compression of the spring), the reaction force is an increase in lift that raises the ride height for that wheel.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Every single post of Greg's as far as I can tell either uses the term 'steady state', or is explaining the logic of a post that used the term 'steady state'.

By definition, 'steady state' means that there is no weight transfer occurring- any and all weight transfer that ever will occur has already occurred and the system is in equilibrium.

The point (I believe) Greg is trying to make is that once the system has reached equilibrium (in which case all forces are constant, are in equilibrium, and any transfer of forces laterally, longitudinally, or diagonally have already occurred and settled to a constant) the only way to transfer weight front or rear is longitudinal acceleration, which is either not present in steady state, or is present but constant so that there is no change in weight distribution occurring through any axis.

He's talking about steady state and the rest of you are making arguments about dynamic situations.

Greg, I apologize for putting words in your mouth, that isn't really my intent here, but watching you all talk circles around each other is exhausting.

Quote (140airpower)

The inside tire normal force begins to go to zero (diminishes) just as soon as cornering begins. The suspension is usually designed to be compliant which means that as soon as the torque described exists, the CG begins to react to it by rising.

It is true that cornering causes the normal force of the inside wheel to diminish- but that does not mean the CG is rising. Think about where the roll center is.

Until the inside rear wheel is no longer contacting the ground, it is still contributing normal force to the total for the axle it is attached to- this means two things: 1) the axle in question is still contributing to the roll resistance of the entire vehicle and 2) the spring of the outside wheel is not yet supporting 100% of the axle normal force, so as roll continues to accumulate that spring will continue to compress, and in almost all cases that means the CG is moving down, not up.

At the exact instant the inside wheel comes off of the ground, the axle ceases to contribute to the roll resistance of the vehicle as a whole, so the vehicle-level roll resistance plummets; the wheel also ceases to contribute any of the total axle normal force- this means that the outside wheel bears all of it, and will not compress further. This behavior means that the roll center of this axle ceases to be the geometric roll center of the suspension, and becomes a point somewhere in the contact patch of the outside tire. Any further added cornering force will cause this end of the car to try and rotate around this new roll center, which will cause the CG to move up away from the ground- but unless the suspension geometry (and thus roll center location) are very strange, the CG will not move up until this happens.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote (140Air)

the crew knew what was going on because the reaction of the car was explained by a loss of rear roll stiffness.
Sure - that explains the visible effect. But not whether load was being transferred diagonally across the chassis by the sta-bar, which is nominally what this thread is about.

It's been common knowledge for quite some time that lifting a front tire on a rear-heavy RWD car can happen either during cornering or more likely/noticeably when on the throttle exiting said corner when you have a combination of pitch and roll. I'm sure I can find decades-old head-on pictures of a Porsche 911 doing precisely this. courtesy of a front sta-bar chosen overly stiff to reduce oversteer tendencies - this being essentially similar to it having a normal front bar combined with a broken rear bar.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

gruntguru said "The couple due to centripetal and centrifugal forces is reacted by unequal normal forces at the inside vs outside tyres producing an equal and opposite couple."

140A said "gruntguru, You imply that the inside wheels exert an equal force to the outside wheels in a corner."

gruntguru says "huh???"

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

(OP)
> if we are talking about weight, (measured on a force scale), and not mass then sway bars can transfer weight front to rear and vice-versa diagonally

Yes, upon further thought that seems right:

If both axles have roll stiffness, both will xfer wt w/lat acc.

Roll moment must be reacted, so taking the extreme w/zero roll stiffness at one axle, all the lat wt xfer must happen at the other.

IOW, wt will effectively xfer from inside wheel of one axle to outside wheel of the other.

So why the heck didn't anyone tell me I was wrong about being wrong?

Now we're full circle, and unless I'm missing something, my original question remains unanswered.

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

"Roll moment must be reacted, so taking the extreme w/zero roll stiffness at one axle, all the lat wt xfer must happen at the other.

IOW, wt will effectively xfer from inside wheel of one axle to outside wheel of the other."

No. The axle with zero stiffness will not change weight at either wheel (they remain equal). The other axle will see all the transfer ie from inside wheel to outside wheel - same axle - there is no diagonal transfer.

je suis charlie

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Quote:

if we are talking about weight, (measured on a force scale), and not mass
So let's use the correct term here to avoid any such confusion - it's load transfer, not weight transfer. Regardless of how common the latter term seems to be in nontechnical conversation.


Quote:

then sway bars can transfer weight front to rear and vice-versa diagonally
Unless what you're mean by "transfer weight" is really something like "transferring load transfer", no they can't. The resolution of front or rear roll moment resisted by each individual sta-bar into load transfers is statically determinate without looking at the other end of the car at all. Looking at both ends together was already done when the front and rear shares of the total roll moment picked up by each bar were determined, no need (not to mention misleading) to refer to both ends again.


Quote:

You can crank a twist into the bars to create a static stagger of weight. They do the equivalent of this in NASCAR.
Not the same thing at all. Here, you're changing a force boundary condition (maybe calling it an applied displacement boundary condition would be more accurate) rather than a stiffness. I get that you'd do this to shift the resulting wheel loadings around a little to benefit behavior at some point during cornering, but it's a separate effect from anything you could do with sta-bar stiffnesses alone.


Norm

RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Seems odd that all the handwavers can't put a diagram together to present their case analytically..

Noah wrote "Greg, regarding your FBD, I'm a bit puzzled as to your choice of view; in side elevation the lateral force vector is invisible so deriving its force resultants and effects is problematic, no?

Front or rear elevation makes more sense to me."

Well, that's the power of using the side view. It demonstrates that in the absence of external longitudinal or vertical forces, or accelerations, no internal reactions to lateral loads can change the vertical force on each axle. You can get absolutely tiny changes by jacking one axle or the other, and hence moving the cg fore-aft, but if you do the calculation it is a small effect even with a production car, with a relatively high cg. You certainly can get jacking just from rolling the car, it is not likely to be more than 50mm at the axle centreline. So it'll tilt the sprung mass by 1 degree (late edit), oh dear I just lost interest entirely.





Cheers

Greg Locock


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RE: Why doesn't increased normal force from ARB (antiroll bar) increase adhesion?

Don't lose interest. I think it was you that reminded me of the overloaded minipickup affect. Observed directly when following one of those little Toyota pickups loaded with three families worth of furniture piled 3.9m high and swaying side to side (and end to end) down the road. An effective exaggeration.

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