Rear roll center question

[Peter Mc Mahon]

I have been thinking about roll centers lately [mostly rear] and I have some thoughts










I would put rear suspensions into 2 different catagories, fixed and floating











An example of a fixed rear suspension would be a triangulated 4 link, I believe that the angles of the arms [due to triangulation] make a fixed roll center at a static position.




An example of a floating rear suspension would be a 3 link [no lateral locating device here], the roll center is floating around until it gets locked down with a locating device























So, going back to the triangulated 4 link, if I add a watts link to it and lower the pivot point of the adjuster to lower the roll center, I am conflicting with what is already there,
I am going to create bind because you can't have 2 roll centers on the same vertical plane. In a nut shell, the roll center is already defined and fixed so you can't [shouldn't] change it.






However on a floating suspension, think 3 link again, we have to add something for lateral location. When we do this we can choose the height of the roll center because there is not one there already.




















Now, this brings me to my main point, I have read that if we use say a panhard bar we should keep it equal from each end so the pivot points are the same so the car will act equal in left and right corners.




















So lets say we do that, and lets say that each end is adjustable in height so we can control our roll height.





























I think conventional wisdoms says that if one end is 11" off the ground and the other end is 12" off the ground the roll center is 11.5"


























I am having trouble wrapping my head around this as the vehicle does not pivot off the center, it pivots off the bolt at each end.



























I am thinking that there are 2 roll centers [they don't conflict as they are not on the same vertical plane] 2 distinct roll centers with corresponding moment arms measured from the
center of gravity to the pivot of the bolt on each side. NOT measured to the middle of the bar.














So, without going off in too much detail here, if the geometry of the locating points of your rear suspension lock in the position of it [think triangulation] there is no need for a lateral
device as it will only cause bind if the roll center is anything other than what is already there. However if your locating points do not lock it in position, you will require a locating device
and that device will dictate the roll center.


Now all of that is based upon a static height, if I am not too far off base we can get into when it is compressed etc.




























Am I out to lunch?

[Rod]

that's a lot of words, not really sure what the question was

a triangulated 4 link doesn't need a lateral location device, correct never needs one
a three link needs a lateral location device, correct always needs one

if you use a lateral location link on the triangulated 4 link I would use a watts link and you can override the built in RC to some extent and move it with the watts, I have done it and it works fine

[Peter Mc Mahon]

Ya, it is too wordy after I just re-read it. Not really so much a question as me thinking out loud.
I was looking at another thread for SSLance and watched a video about a triangulated 4 link binding with a watts added to it. I had been thinking this would happen due to a discussion with a guy I know who is a math whiz. I would think in a perfect world, but maybe not in reality due to weight and G forces etc, that adding a watts to a tri, 4 link will absolutely cause bind. I think the geometry would prove this. But I do think the weight of the vehicle and cornering dynamics would possibly overcome the battle of roll centers.

So what do you think about my thoughts that a panhard bar has 2 roll centers? Being each pivot point at the bolt?

[dontlifttoshift]

You are confusing the geometric roll center with a physical pivot point.

156525

In the case above, the roll center is in the middle of the bar at that height, the distance from that up to the CG determines the leverage and how much weight will transfer from the inside to the outside. Regardless of suspension type, roll center is roll center and for a static comparison, the math stays the same.

Dynamically, in this picture with the PHB chassis mounted on the passenger side, in a left turn the roll center gets lower as that side of the PHB travels downward. In a right turn the RC gets higher. The fast kids can tell the difference on track.

Your floating and fixed terms muddy the waters significantly.

[SSLance]

Here is what happens when you put a lateral locating device on a triangulated 4 link.


https://youtu.be/EDM9_-4kjHg

It binds and does not allow the suspension to work at all...

[Peter Mc Mahon]

Donny. Thanks for the reply and yes I am having issues wrapping my head around a difference between physical and geometric. In the illustration that you gave, where would the roll center be if the Panhard Bar was only 1/2 it’s length? Same mount on passenger side but mounted on the center of the differential? And, I have looked but can’t find a picture that demonstrates reference lines drawn to show why it is the roll center, any chance you can point me in the right direction? Thanks

[CSG]

Here is what happens when you put a lateral locating device on a triangulated 4 link.


https://youtu.be/EDM9_-4kjHg

It binds and does not allow the suspension to work at all...

It depends on bushing material. With rubber bushings they don't bind that much.

[dontlifttoshift]

Donny. Thanks for the reply and yes I am having issues wrapping my head around a difference between physical and geometric. In the illustration that you gave, where would the roll center be if the Panhard Bar was only 1/2 it’s length? Same mount on passenger side but mounted on the center of the differential? And, I have looked but can’t find a picture that demonstrates reference lines drawn to show why it is the roll center, any chance you can point me in the right direction? Thanks

Roll center is still in the same place, with the PHB at half length. When I wrote above about it being in the middle of the bar, that was just coincidence and poorly worded on my part, it's in the middle of the car in that picture.

[Peter Mc Mahon]

So I assume the rear roll center is always in the middle of the car? Are there points that someone can reference and measure to triangulate where the center is? ( assuming that it is a triangulation calculation). Thanks again

[RobNoLimit]

Not always. The roll center on a panhard bar car is at the center of the 'bar'. (I state that because of J bars and such, so the RC is real at the center of the two panhard bar mount pivots). Back when we raced Mods, we offset the parhard rod to move the RC to the left (inboard) side. But, on a PT or street car, I would say centered LH/RH is the goal. - Unless you are going to use an offset RC to balance the car due to a drastic weight imbalance....... another story.

[Peter Mc Mahon]

Not trying to start any arguments but I think Rob and Donny are saying very different things here. My way of thinking ( and I don’t get it, that’s what I’m trying to figure out here) is that the rear roll center must be plotable to make a geometric formula ( the front works that way )so, If the body is pivoting from a single point be it rolling left or right then that point must be triangulated from some given points? What are those points? Thanks in advance for any help ( diagrams would be best in helping me get it )

[dontlifttoshift]

Honestly, you may be above my pay grade at this point. Rob's an engineer, I'm a guy that answers the phone and sweeps up the shop........

Most of this stuff has been put into words far better than I could ever explain it but here is an excerpt from here. http://kb.fmiracing.com/caster-camber-setup-document/roll-center-explained


The roll center is an imaginary point around which the rear of the race car rolls.

Just like front roll center is not an actual attachment point, but a theoretical point in space.

[Peter Mc Mahon]

My head hurts too Donny! I do realize that the front roll center is an imaginary point, but it is a real point in a math equation, just not getting where the rear is provable with geometry being a bolt or theoretical point. I asked my friend who is in my opinion a trigonometry wiz, but not a car guy, and he is scratching his head where the center of the bar comes into play The search continues!

[RobNoLimit]

and I thought Donny and I were on the same page. ?

[Rod]

Rob and Donny are way above my pay grade....

[dontlifttoshift]

and I thought Donny and I were on the same page. ?

We are. I just don't know how to answer Peter's question......technically.......so I was deferring to you. A large downside to RSRTs involvement in the forum was a lot of the "answer" people got ran off with the unwillingness to debate theory. Guys like exwestracer, jason rhodes, and norm peterson typically would have all chimed in on this. Everyone takes in information differently and reading it from different sources helps it make sense sometimes.

Here is everything I know about it. Rear roll center, low is good. With a panhard bar, it is very hard to get it too low. A watts link is better in every way save for cost, moving parts, and packaging. I prefer panhard bars, they are easy to build and package and the difference in handling, ASSUMING THE SAME RC HEIGHT, is minimal.....like measured in thousandths......on a timer system where the error is measured in thousandths.

[Peter Mc Mahon]

Roll center is still in the same place, with the PHB at half length. When I wrote above about it being in the middle of the bar, that was just coincidence and poorly worded on my part, it's in the middle of the car in that picture.

I think Donny is saying the roll center is always in the middle of the car in this statement, and I think Rob is saying it is always in the center of the 2 pivot points of a panhard? That is what I meant by saying Rob and Donny are saying different things. Donny, Rob? Am I misinterpreting? Thanks again guys

[SSLance]

Guys like exwestracer, jason rhodes, and norm peterson typically would have all chimed in on this. Everyone takes in information differently and reading it from different sources helps it make sense sometimes.



Man, I talked with all 3 of those guys quite a bit early on about my car...an how to make it work. All very helpful...

Did they just disappear or move on to other pastures?

[dontlifttoshift]

Exwest moved on to other stuff I think, but I see Norm and Jason on other forums all the time.

Peter, I honestly do not know if it is in the middle of the bar or the car. It is entirely possible that my statement above was incorrect. I never thought about and never needed to. It is very rare that rear roll center is adjustable and with a PHB even if it is adjustable its an inch or two. The lateral location of the roll center has never factored into anything I was trying to do and I always just figured it was in the middle of the car.

If the car is loose, lower the bar. If you are out of adjustment, do something else.

[RobNoLimit]

Maybe this will help. ** Disclaimer: This is mostly still up for debate, but you would need a pretty solid argument. ** (this is just my opinion)

To start with, triangulated 4 bars are tricky to understand. Also, whats most important to the car is the Roll Axis. This is an imaginary line that passes through the front and rear roll center. - Keep this in mind. If you look at a "top view" of a Tri-4 bar car, Pict-2, to plot the RC you would extend the centerlines of the angled bars out until they meet. This point would be the Roll Axis point. Not really the roll center we would want to chart, because it's not on the rear axle CL.

Tri-4 bar side view, Pict-1, shows the elevation of the roll axis point behind the rear axle.

In the third sketch, we draw the imaginary Roll Axis line from the front RC to the rear roll axis point. At the point where this line crosses the rear axle CL, would give you the rear RC height that you would want to chart - compare - tune from. IMHO, Tri-4 bar set ups are always a compromise. Things that you do to lower the RC have a negative effect on the Instant Center, and can hurt traction. The Big three did this to save $$. 4 bars are cheaper and easier to package than 5, - and, they're not really building HIPO cars.

Pict 4 is a set of "rear view" drawings of typical Panhard set ups. In each set up, the Roll Center is at a point half way between the two mounting points of the bar. And the last pict, 5, is a top down view of the effects of an offset RC in the back of a car as it relates to the cars Roll Axis.

Now for a discussion topic. IF yo were clean sheet designing, and at some point you knew the loaded sprung weight on each corner, could you use this to your advantage? I say yes. Lets say the LF = 880lbs and the RF = 800 lbs. A 10% difference. If you offset the front RC 5% of the track width, (take 5% from one side, give 5% to the other = 10% total) the RC would be in the center of the cars mass. And, if you applied the same concept to the rear, you would end up with a roll axis that was centered in the mass of the car. And you would have a car that loads the tire the same in both a LH and a RH turn. Your thoughts?

[SSLance]

Nice work Rob... **polite golf clap** :D

Appreciate the time you put in to make the illustrations, it really helps.

[Peter Mc Mahon]

I like where you’re going with this! How would you offset the front roll center? Just built right in from the original design?

[cornfedbill]

I like where you’re going with this! How would you offset the front roll center? Just built right in from the original design?

Yes, it would need to be built into the front suspension. Circle track chassis builders do it all the time. Often they use different height spindles right and left.

It seems to be interesting in theory for a road course, street or autocross car. I'm not sure I would be gutsy enough to try it except in a left-turn-only car.

[j-rho]

Wait, so it's safe to come back in the water? :)

An accessible entry-level text I really like is Herb Adams' Chassis Engineering- https://www.amazon.com/Chassis-Engineering-Building-Performance-Handling/dp/1557880557/ref=sr_1_1?ie=UTF8&qid=1537548882&sr=8-1

To me, the most important concept in understanding handling is tire load sensitivity, and Herb's book got it to "click" for me. Once you've grokked that, and have developed the driving and testing chops to know how tune a car to make it handle better, the rest kinda falls in to place. There are other entry-level books that explain these things (a lot like the Fred Puhn book), then the more advanced texts like Milliken. Even when everybody has the purest intentions, forums maybe aren't the best way to learn the basics.

To the OP's thoughts - the way I think about it, is each wheel can move in six possible directions (some call these "degrees of freedom") - up/down, forwards/backwards, in/out. We want to allow for the up/down but not the other four. If you had a live rear with nothing to constrain it laterally, when you turned into a corner, the front of the car would turn but the rear axle would keep going straight, which is an undesired outcome.

In suspension geometry, most of the stuff that constrains wheels can be looked at as circles, where the center of the circle is where the thing mounts to the chassis, and the point on the circle, where the arm/link connects to the axle/knuckle/whatever. In allowing for the up/down motion of the wheel/tire, since you're dealing with circles, it's tough to make the path perfectly up/down, and it's not always what you want (e.g. camber gain).

It can be neat to examine the forces going through all these links under various dynamic conditions (trail braking-on entry, powering at corner exit, etc.) to see how changing the geometric relationships would affect how loads/forces get distributed and moved around. It all comes back to tire load sensitivity. In general nobody's paper design is perfect in practice without real-world testing and iteration. So it's best to design in adjustability so you can tune things like anti-squat later, just as you'd tune spring rates, sway bars, shock settings, tire pressures, etc.

p.s. I'm flattered to be thought of as in the same league as Norm Peterson - he knows way more than I do!

[cornfedbill]

Wait, so it's safe to come back in the water? :)

An accessible entry-level text I really like is Herb Adams' Chassis Engineering- https://www.amazon.com/Chassis-Engineering-Building-Performance-Handling/dp/1557880557/ref=sr_1_1?ie=UTF8&qid=1537548882&sr=8-1


Yes, a very good book.

[dontlifttoshift]

Wait, so it's safe to come back in the water? :)
......bunch of stuff.....!

Good to see you back posting.

[raustinss]

I'd say this is a Ron Sutton question.....but anyone with half a clue knows what hes up to right now lmfao in a sad way

[Peter Mc Mahon]

Wait, so it's safe to come back in the water? :)

An accessible entry-level text I really like is Herb Adams' Chassis Engineering- https://www.amazon.com/Chassis-Engineering-Building-Performance-Handling/dp/1557880557/ref=sr_1_1?ie=UTF8&qid=1537548882&sr=8-1

To me, the most important concept in understanding handling is tire load sensitivity, and Herb's book got it to "click" for me. Once you've grokked that, and have developed the driving and testing chops to know how tune a car to make it handle better, the rest kinda falls in to place. There are other entry-level books that explain these things (a lot like the Fred Puhn book), then the more advanced texts like Milliken. Even when everybody has the purest intentions, forums maybe aren't the best way to learn the basics.

To the OP's thoughts - the way I think about it, is each wheel can move in six possible directions (some call these "degrees of freedom") - up/down, forwards/backwards, in/out. We want to allow for the up/down but not the other four. If you had a live rear with nothing to constrain it laterally, when you turned into a corner, the front of the car would turn but the rear axle would keep going straight, which is an undesired outcome.

In suspension geometry, most of the stuff that constrains wheels can be looked at as circles, where the center of the circle is where the thing mounts to the chassis, and the point on the circle, where the arm/link connects to the axle/knuckle/whatever. In allowing for the up/down motion of the wheel/tire, since you're dealing with circles, it's tough to make the path perfectly up/down, and it's not always what you want (e.g. camber gain).

It can be neat to examine the forces going through all these links under various dynamic conditions (trail braking-on entry, powering at corner exit, etc.) to see how changing the geometric relationships would affect how loads/forces get distributed and moved around. It all comes back to tire load sensitivity. In general nobody's paper design is perfect in practice without real-world testing and iteration. So it's best to design in adjustability so you can tune things like anti-squat later, just as you'd tune spring rates, sway bars, shock settings, tire pressures, etc.

p.s. I'm flattered to be thought of as in the same league as Norm Peterson - he knows way more than I do!

Does that book have a picture of the way to measure the rear roll center showing the geometry behind it?
Jaws is my favourite movie!