rear panhard bar tech

[Matt49]

I’m going to provide “my” answer and then get back out of the way.

There are things that need to be understood about rear roll center that are not very well known which is one reason there is a lot of conflicting information on this topic. There is a VERY common (and often published) misconception that the roll-center on a car with a j-bar is a point right between the mounting locations on each end. It simply isn’t true. This comes from an extension of OLD logic from panhard bar cars with VERY long bars and cars with springs matched on either side. A lot of stuff found in college level vehicle dynamics text books where the real world is the last thing considered. It’s “road car” vehicle dynamics and it doesn’t completely apply to late model or modified j-bar setups. This is reason number one why there isn’t a straight answer to the OP’s question.
The actual roll center location on a j-bar car is dictated (somewhat) by the j-bar mounting locations BUT it is also dictated by the upper spring mounting locations and the effective spring rates. That’s as much as I’m going to say about that because I could write for hours trying to explain it completely.

Reason number two that there isn’t a straight answer is that modern dirt cars on j-bars get a TON of motion which means that the roll center migrates quite a bit as the j-bar mounting positions move relative to the ground. And yes, BOTH mounting points move relative to the ground. Couple this with the face that both upper shock mounts move and the spring angles change which changes the effective rate and you’ve got a math problem that I have not the time or patience to go through right now. This is why we have computers.

Reason number three also has to do with the motion but more so because the vertical center of gravity is moving up at the same time the axle is moving left.

Bottom line is there is a lot going here and there are no “straight” answers but there are some “in general” answers that work for most people and those have already been provided.

[7uptruckracer]

Well said the bad thing is there are published diagrams that give people these ideas, and they don't hold water the best thing to do is to know how it works and adjust accordingly, take good notes, so you know how it effected the car and make your own adjustment guide. I have one on a asphalt mod that runs from the left side of the pinion diagonal to the RR trailing arm mount.....try explaining how that works to someone.....unless you know your spring tables your rear roll rates, you can have springs mounted toward the center or way out by the calipers all of it effects how it works and rolls

[dukeofpaducha]

Well said the bad thing is there are published diagrams that give people these ideas, and they don't hold water the best thing to do is to know how it works and adjust accordingly, take good notes, so you know how it effected the car and make your own adjustment guide. I have one on a asphalt mod that runs from the left side of the pinion diagonal to the RR trailing arm mount.....try explaining how that works to someone.....unless you know your spring tables your rear roll rates, you can have springs mounted toward the center or way out by the calipers all of it effects how it works and rolls

Why did a left side frame mounted panhard rod become the convention in dirt cars? Would a panhard rod mounted to the frame on the right lower the roll center as the car rolled over on the right rear?

[MasterSbilt_Racer]

Why did a left side frame mounted panhard rod become the convention in dirt cars? Would a panhard rod mounted to the frame on the right lower the roll center as the car rolled over on the right rear?

It may. But what does the front roll center do? Especially with the dynamic camber some are running today. I have had luck with a panhard like you describe, but I don't think it is a magic bullet.

[dukeofpaducha]

Thanks for the reply MasterSbilt_Racer. All else being equal would the right side frame mount panhard bar shift the dynamic roll couple resistance toward the front allowing the rear to roll over easier ?

[MasterSbilt_Racer]

Thanks for the reply MasterSbilt_Racer. All else being equal would the right side frame mount panhard bar shift the dynamic roll couple resistance toward the front allowing the rear to roll over easier ?

That has been my experience.

[Dirtrunner35]

Ever look at a data acquisition on a car, great stuff.