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    1. #1
      Join Date
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      Rear roll center question

      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?

    2. #2
      Join Date
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      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

    3. #3
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      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?
      Last edited by Peter Mc Mahon; 09-13-2018 at 08:26 PM. Reason: trying to not be too wordy!

    4. #4
      Join Date
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      You are confusing the geometric roll center with a physical pivot point.

      Name:  figure-5-panhard-bar-roll-center-rear-view.jpg
Views: 5623
Size:  20.1 KB

      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.
      Donny

      Support your local hot rod shop!

    5. #5
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      Aug 2012
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      Here is what happens when you put a lateral locating device on a triangulated 4 link.



      It binds and does not allow the suspension to work at all...
      Lance
      1985 Monte Carlo SS Street Car

    6. #6
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      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

    7. #7
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      Nov 2016
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      Quote Originally Posted by SSLance View Post
      Here is what happens when you put a lateral locating device on a triangulated 4 link.



      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.

    8. #8
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      Quote Originally Posted by Peter Mc Mahon View Post
      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.
      Donny

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    9. #9
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      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

    10. #10
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      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.

    11. #11
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      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 )

    12. #12
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      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-cambe...nter-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.
      Donny

      Support your local hot rod shop!

    13. #13
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      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!

    14. #14
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      and I thought Donny and I were on the same page. ?

    15. #15
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      Rob and Donny are way above my pay grade....

    16. #16
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      Quote Originally Posted by RobNoLimit View Post
      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.
      Donny

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    17. #17
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      Quote Originally Posted by dontlifttoshift View Post
      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

    18. #18
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      Quote Originally Posted by dontlifttoshift View Post
      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?
      Lance
      1985 Monte Carlo SS Street Car

    19. #19
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      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.

      Donny

      Support your local hot rod shop!


    20. #20
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      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?
      Attached Images Attached Images          

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