Lets talk rear suspension - technical

[1968SWBBigBlock]

Ok - I need a lesson in rear suspension geometry for my 68.
It has the stock trailing arm x pan hard bar suspension.
I was visiting a shop today that builds really nice street rods and was showing him some pictures of the truck - he noticed the pan hard bar travels up hill from the rear center section, he seemed to think this was a big issue and effects the way the rear suspension should work.
it seems to me that all of our trucks need the pan hard bar needs to travel up hill and not be level.
Have I gone goofy?
Can some one explain in technical detail how our rear suspension really works and why a pan hard bar needs to travel in some arc other that flat?


Many thanks

Bill

[Twisted Minis]

Your panhard bar should be level at your ride height on a sprung vehicle. On something with air ride, it should be level at the halfway point of the suspension travel. And ti should be as long as possible.

Also your panhard bar mounting points set your roll center height, so that is something to take into consideration.

[ENHANCED AIR RIDE]

Word!

[LONGHAIR]

Originally Posted by Twisted Minis

Your panhard bar should be level at your ride height on a sprung vehicle. On something with air ride, it should be level at the halfway point of the suspension travel. And ti should be as long as possible.

Also your panhard bar mounting points set your roll center height, so that is something to take into consideration.

While this is absolutely true....it doesn't explain why?

The reason for the level panhard bar (and for having it as long as possible) has to do with side to side travel of the entire axle.
If the panhard bar is level, the axle moves less than if it was at an angle.
Example:
When the panhard bar is mounted higher on the frame than it is on the axle at ride height, as the suspension compresses, the panhard bar forces the axle to move away from the frame mounting point. This pushes the tire closer to the wheelwell on the side opposite the frame mount.
When the panhard bar is mounted level, the arc of its movement is smaller. That way the axle moves less.

[XXL]

Originally Posted by 1968SWBBigBlock

...and why a pan hard bar needs to travel in some arc other that flat?

If you visualize the way that a Panhard bar laterally locates the suspension, you'll see that it CAN'T travel "flat." It must travel in an arc as a result of one end being at a fixed point.

Simple drawing...

[krue]

Pictures are always good for conveying info.

[XXL]

Originally Posted by krue

Pictures are always good for conveying info.

Or getting a timeout.

[cparman]

so is a watts link better or is it just another way to do the same thing, and if so , which would work better on a truck with air ride on all 4 corners?

[lofly'a]

a watts link correctly engineered, and installed will swing less "horizontally" than a panhard bar will. that horizontal swing is crucial for clearance if you are tucking big wheels/tires for fender to frame clearance and when using bags. the only downsides of the watts versus the panhard are the cost to build, and it takes up more sometimes needed space in the undercarriage.

[smashingchuck]

Originally Posted by Twisted Minis

Also your panhard bar mounting points set your roll center height, so that is something to take into consideration.

explain.

[leftcoast66]

Originally Posted by lofly'a

a watts link correctly engineered, and installed will swing less "horizontally" than a panhard bar will. that horizontal swing is crucial for clearance if you are tucking big wheels/tires for fender to frame clearance and when using bags. the only downsides of the watts versus the panhard are the cost to build, and it takes up more sometimes needed space in the undercarriage.

A properly engineered watts link shouldn't swing at all side to side through the normal range of motion.

[Tony@AirRideTech]

I would add that the panhard be needs to be level at ride height. On alot of vehicles ride height is not center travel but about 2/3 to 3/4 of the way up.

[lofly'a]

thats true leftcoast, "normal" but extreme travel will cause minimal side to side travel. once it swings outside it's normal arc travel it will exhibit a small horizontal swing. the
"totally polished" unit which is very well designed and built, did not swing at all thru it's 12 inch travel but past that did show small deviations from center. check out their video.

[Twisted Minis]

Originally Posted by smashingchuck

explain.

I feel like this could have been a question, rather than a demand.

But the height at which you set your panhard bar becomes the height of your roll center. If you put your panhard bar above the third member, you will have a relatively high roll center. This is not necessarily bad, but it can feel weird if your front roll center is low, or you are used to the stock roll center, which is the axle center line on leaf springs. If you place your pnahard below the third member you have a pretty low roll center. This creates a more balanced feel with the front suspension, while also making the vehicle feel more top heavy. The placement won't make a drastic change in your suspension, but it will be noticeable. Most race cars use a low roll center. A street driven vehicle I feel is best set up anywhere from the axle center line, to the bottom of the third member housing.

Your lateral roll center is still determined by the mounting point of your 2, 3 or 4 link.

[Shane]

Relax, I think it was meant as a request, not a demand.

[Twisted Minis]

I'm just expressing my opinion, I'm not upset.

[N2TRUX]

He just forgot to throw in the obligatory smileys. Here's a couple for y'all, and thanks for the detailed insight. Your knowledge is appreciated.....

[lofly'a]

just good dialogue, i sure learned a few things from this post.