panhard bar

[Nate K]

I have a 71 dart that I am thinking about installing a pahard bar on. It has leafs in the rear. I am looking for opinions on if it is worth doing. I also need to know how I figure out what height it should be set at.

[baz67]

Why do you think you need one?

[Nate K]

i am autoxing the car and some of the other cars are running them. I just wondered if it was worth doing.

[Mean 69]

Yes, it is worth doing. Do a search and you will likley find discussion on the topic. There's only a few ways to do it correctly, but a LOT of ways to do it wrong, so do your homework.

Mark

[wendell]

Please post pictures of your autoX 71 Dart. That is a COOL car. My grandma had one. If you want to use the springs you've got, set the pbar horizontal and the height at the leaf pads. That will locate the housing laterally with a minimum impact on the roll center.

For materials I'd use the serated housing bracket #26122 here: http://stockcarproducts.com/rsusp3a.htm Weld it low so that the leaf pad height is in the upper 1/3 of the potential adjustment.

For the chassis side I'd run a 1.75 tube down from just inboard of the frame rail (to clear the leafs) down to a inch or so above the side wall of your rear tire. From the bottom of the 1.75 tube, run 1.5 thin wall up and across to the opposite frame rail. Mount the chassis side of the pbar with one of these http://www.howeracing.com/Suspension...-CastClamp.htm
That's how I'd do it. When you built it you want to bias the potential adjustment so that you have room to move the bar down as you develop the car. It's un likely that you'd want to move the bar up much. Don't worry what side the housing mount is on either.

Lastly, search the site for leaf/pbar info. You'll notice an improvement from the increase in lateral location but the pbar has the potential to be a much more powerfull tool if you're leafs are set up for it.
POST PICS OF THAT DART!

[Nate K]

Sorry I dont have any pictures of the car. It is stock right now I am collecting parts for this winter. It is scaping the door handles in corners when I race it right now. I dont even have sway bars. I have been searching this site and I think I am going to make a watts link. It just seems like it is better but I cant decide if it is worth spending the time versus the improvement.

[cagedruss]

Be very careful in the design. Panhard bars have a habit of putting the leaf springs in a bind under cornering. I have installed quite a few back in my stock car days and we played with alot of different ways of mounting. Ended up using polyurathane bushings for the mounting hardware instead of rod ends. Really helps keep the springs from binding.

You won't have the same force excerted on your panhard bar as others do with 3-4 link suspension since the leafs are holding the rear end in the car.

Also, think about using a floating rear spring perch. If will let the panhard bar do its job better.

Good luck!

[Nate K]

When I set the bar up should I start with it level with the axle at ride height and adjust it from there? Should I also use the longest bar that I can? How can I tell if the suspension is binding?

[Matt@RFR]

Up near the top of the page, towards the right, you'll see a button titled "Search". Click it, please. It's free. All of your questions have been answered before.

[astroracer]

How "stock" is "stock"? Have you done ANY shock or spring tuning? This includes up front also... What tire/wheel are you running?
A panhard bar is the last thing I would do if you haven't done anything else. A moderate lowering, poly bushings, sway bars and good shocks should be your first thoughts and I bet, once you get the suspension tightened up, you won't think twice about a panhard bar or watts link...
Mark

[Norm Peterson]

For any car with auto-x intent, the PHB should be as long as is practical and horizontal with the car at rest and loaded as it will be driven in competition. The reasoning for the length is to minimize the lateral fight between the PHB and the leaf springs as the car rises and falls on its suspension (it's a (1 - cosine) sort of thing). The PHB forces the axle to move along an arc as seen in rear view, while the lateral bending rigidity of the springs wants the axle to move in a purely vertical line. The discrepancy generates forces that do nothing productive, and will generate an effective roll resistance that can get the rear end rather "loose". Especially in 2nd gear around the cones.

As noted by wendell, you want to mount the PHB approximately at axle height. Again, this is to minimize conflict between the two separate axle paths defined by the PHB and the leaves. The actual height of the roll center as defined by your specific leaf spring configuration might be anywhere from approximately the axle's spring seat height and a couple of inches above the axle tube. Figure 17.44 in the Millikens' book "Race Car Vehicle Dynamics" shows a construction for the RC height as defined by leaf springs. Depending on how laterally rigid your (rear) shackles are, the rear point on the dotted line in that figure could be as low as the center of the rear spring eye.

Without developing a relatively complex mounting arrangement for the leaves or getting into a bit of experimentation with soft rear springs, the advice that regards leaving a little bit of compliance in the PHB bushings is well-given. With a leaf spring car, the purpose of a PHB should only be to limit gross lateral movement of the axle. That lets the springs mostly just support the car's sprung weight plus whatever amount of lateral load transfer occurs in the corners. Using bushings with a little "give" provides a place for the difference in the PHB arc and the springs' vertical path to be resolved, and allows for a somewhat greater vertical distance to exist between the two competing roll center geometries. Poly might actually be a little on the stiff side, actually, but there are ways of reducing that.


FWIW, Philly Region SCCA has an event this Saturday (Aug 12th) at Ripken Stadium in Aberdeen, Md. (just off I-95). At this point, I'm planning to run in one of the "morning session" classes. Any chance you can make it there?


Norm

[wendell]

Astro racer makes a good point but Matt made a better one. It seems that this site can go around in circles for months at a time. tubular a-arms, g-mod, pbar/watts, truck arm/3L, tubular a-arms....

[wendell]

Norm, could you post the picture from Milliken's? calculating RRC on leafs continues to allude me so I always assume it to be around the top of the axel with solid front eye bushings.

Norm, Chicane Tom et. al.
I've been able to get effects "symptomatic" of lowering the RRC by lowering the pbar up to 3.5" bellow the axel center line. "Symptomatic" is important because as you know the lower the pbar the more roll resistance from bind. The lower RC should have the oposite effect as the increase in bind but it's an important caviat. I've also tried to prove that the RRc is creating the changes by experimenting with a 3/4 solid rear sway bar.

The hypothesis that the pbar can be the dominant factor in determining RRC up to 3.5" from the spring's RRC height has been supported by lap times and driver feed back. Do you think this is likely the case? I'd be interested to hear your feed back.

[Nate K]

How "stock" is "stock"? Have you done ANY shock or spring tuning? This includes up front also... What tire/wheel are you running?
A panhard bar is the last thing I would do if you haven't done anything else. A moderate lowering, poly bushings, sway bars and good shocks should be your first thoughts and I bet, once you get the suspension tightened up, you won't think twice about a panhard bar or watts link...
Mark

The only thing that is changed from stock is the front brakes are disks from a 74. I have been collecting parts for this winter. Right now I have new adjustable upper control arms with heim joints instead of bushings, .990 torsion bars, strut rods with heim joins, nylon lower contol arm bushings, and braces that need welded to the lower control arms. I am going to make subframe connectors and torque boxes like the hemi cars came with. I am also going to install a roll bar. I am deciding what rear leaf springs I am goin to order and what bushings I am going touse in the rear. This is why I am asking about a panhard bar. If I am gonig to install one it will be easier while I have the car apart. The wheels I plan on using are 17" BBS RK 17x8 front 17x9.5 rear.

[Norm Peterson]

Norm, could you post the picture from Milliken's? calculating RRC on leafs continues to allude me so I always assume it to be around the top of the axel with solid front eye bushings.

Done.

[Norm Peterson]

I've been able to get effects "symptomatic" of lowering the RRC by lowering the pbar up to 3.5" bellow the axel center line. "Symptomatic" is important because as you know the lower the pbar the more roll resistance from bind. The lower RC should have the oposite effect as the increase in bind but it's an important caviat. I've also tried to prove that the RRc is creating the changes by experimenting with a 3/4 solid rear sway bar.

The hypothesis that the pbar can be the dominant factor in determining RRC up to 3.5" from the spring's RRC height has been supported by lap times and driver feed back. Do you think this is likely the case? I'd be interested to hear your feed back.

It all depends on the details (doesn't everything?); in this case involving a stiffness and deflection analysis if you want the numbers.

Basically, the greater the separation between the PHB and leaf spring RC's, the greater the "bind" becomes (I think more properly this is unwanted rear roll stiffness).

This additional roll stiffness is composed of some amount of lateral force from the springs working in opposition to the PHB lateral force over the RC separation distance. The overall lateral stiffnesses of the PHB assembly matters, which includes bushing stiffness and PHB bracket rigidity (these things are not infinitely rigid either) as the main factors. So does the overall lateral stiffness of the leaves (bushings again, brackets, shackles, and leaf width(s)/thickness(es)/arch).

What ends up happening overall depends on just how stiff the PHB and the springs really are. Think of them as horizontal springs with that vertical separation, and if their stiffness happened to be equal you could compute the amount of roll stiffness in inch-lbs/degree that they create from:

[Lateral Stiffness (lb/in)] * [RCH Separation (in)]^2 / 114.6

and you could then compute an sta-bar size with equivalent roll resistance. Or the equivalent uprate for a pair of rear springs, for that matter. And once you know the amount of roll, you can solve for the PHB and spring forces due to roll and add those numbers to the direct lateral forces in each that are holding the rear end centered under the car. Algebraically, as one of the "anti-roll" forces will oppose its direct lateral counterpart, while the other "anti-roll" force will add to its.

Back to the PHB/leaf spring roll stiffness - it's that squaring of the separation distance that makes things go sour in a hurry once it starts becoming significant. Four times as great at 2" separation vs 1" becomes over 12 times as great at 3.5" separation (than at 1"), and goes to 16 times as great in the next half inch (to 4"). So you can expect things to improve overall up to a point, in diminishing-returns fashion.

The math is similar for the general case where the PHB and the leaves are of different stiffnesses, but it's more complex.


BTW, Ripken is SATURDAY the 12th. My error.


Norm

[Norm Peterson]

Just to put a (simplified) ballpark number on that added roll stiffness, a low-ish value for bushing stiffness is 10,000 lb/in. One on each end of the PHB makes its overall rate drop to 5000 lb/in. Given an equal overall lateral stiffness for the leaves 3.5" away and you've pretty much matched the roll resistance of a typical 18mm rear sta-bar (0.71").


Norm

[Mean 69]

The hypothesis that the pbar can be the dominant factor in determining RRC up to 3.5" from the spring's RRC height has been supported by lap times and driver feed back. Do you think this is likely the case? I'd be interested to hear your feed back.

I'd say this is interesting to put it mildly. Lap times usually don't lie, but what does the driver say he feels in the difference of the car? Overall balance, more rear exit grip, etc? All things being equal, which they never are, it would seem to me that you might be able to better drill into this by tire temps? If the overall effect is a lessening of front tire temp, you could guess that the effect is more of a rear roll stiffness deal. If it's a roll center dominiated deal, the opposite would hold true, but in the case of the two "fighting" each other, there WILL always be a rear roll stiffening effect. Whatever makes the car faster and the driver happier in the end, but one very cool little exercise.

I know that on a bud's car (leafs, Watt's) we moved the bellcrank pivot higher than the setting that had been done by the builder of the car (which, based upon a lot of the silly things that had been done to the car, I'd really question what the real motivation of putting it there in the first place was). Too many other changes at the same time to see if this alone had a good or bad effect, but it was clear that the setup was in bind at the lower setup. How? When we had the car on jackstands, you couldn't rotate the links by hand, too much force on the heims. When we moved it up, you could. Not very scientific, but...

Mark

[wendell]

That certain car builder known of the manufacture of vintage cars who's funny-ness are only eclipsed by their price tags...

Work is destroying me, when i get a chance I'll come back to this. Generally, lowering the pbar was increasing corner exit bite, increasing rear tire temps and increasing left rear temps @ LRP up untill a level 3.5 inches bellow the axel centerline. LRP is a glorified clockwise oval.

This effect is probably attributed to clapped out bushings in the springs vs. well maintained hiems in the pbar. Either way, the car is fast.

[Brad74]

So is the official word on how to design a PHB for a leaf sprung rear is to:

1. Mount the bar @ axel height
2. Use soft bushings, poly perhaps rubber?? No heims.
3. Don't lower PHB height to try and lower RRC

The PHB forces the axle to move along an arc as seen in rear view, while the lateral bending rigidity of the springs wants the axle to move in a purely vertical line. The discrepancy generates forces that do nothing productive, and will generate an effective roll resistance that can get the rear end rather "loose". Especially in 2nd gear around the cones.

As noted by wendell, you want to mount the PHB approximately at axle height. Again, this is to minimize conflict between the two separate axle paths defined by the PHB and the leaves. The actual height of the roll center as defined by your specific leaf spring configuration might be anywhere from approximately the axle's spring seat height and a couple of inches above the axle tube. Figure 17.44 in the Millikens' book "Race Car Vehicle Dynamics" shows a construction for the RC height as defined by leaf springs. Depending on how laterally rigid your (rear) shackles are, the rear point on the dotted line in that figure could be as low as the center of the rear spring eye.

Without developing a relatively complex mounting arrangement for the leaves or getting into a bit of experimentation with soft rear springs, the advice that regards leaving a little bit of compliance in the PHB bushings is well-given. With a leaf spring car, the purpose of a PHB should only be to limit gross lateral movement of the axle. That lets the springs mostly just support the car's sprung weight plus whatever amount of lateral load transfer occurs in the corners. Using bushings with a little "give" provides a place for the difference in the PHB arc and the springs' vertical path to be resolved, and allows for a somewhat greater vertical distance to exist between the two competing roll center geometries. Poly might actually be a little on the stiff side, actually, but there are ways of reducing that.



Norm