Diagnosing rear end problem

[benno505]

Hey guys, need a little help here diagnosing a problem with my rear end.
I have an unequeal length 4 link in my charger with viking double adjustable coilovers
boze alloys and 295 tires
the car drives fine doesnt crab walk etc. but when i am up around 60mph and hit a bump the rear end wobbles, sort of like speed wobbles on a skateboard.
i stiffened the rear springs, and it went away a little bit but now so stiff for normal driving.

I am 99% sure the rear end is square, before i set the correct pinion angle properly it did nto do this.

any suggestions?

[andrewb70]

Hey guys, need a little help here diagnosing a problem with my rear end.
I have an unequeal length 4 link in my charger with viking double adjustable coilovers
boze alloys and 295 tires
the car drives fine doesnt crab walk etc. but when i am up around 60mph and hit a bump the rear end wobbles, sort of like speed wobbles on a skateboard.
i stiffened the rear springs, and it went away a little bit but now so stiff for normal driving.

I am 99% sure the rear end is square, before i set the correct pinion angle properly it did nto do this.

any suggestions?

Do you have a device to control the lateral movement of the rear end, like a pan hard rod or watts?

Andrew

[benno505]

has a panhard bar which bolts to passenger side lower front conrtol arm bracket then across to driver side lower rear control arm bracket

[jaybee]

How are your arms oriented? Possible you have excessive bump steer on the rear?

[benno505]

bottom 2 are parallel and running horizontal with the frame (possibly sitting a tad up at the diff. top ones slightly off parallel towards each other to miss the frame and going up from chassis to diff at probably 20* angle, panhard bar is parallel with axle at ride height.

[astroracer]

Sounds like you need to look into a "real" panhard bar. One that runs from the frame to the rearend. The bar you have is so compliant it will cause the wobbling you describe is it flexes. This style bar is used on drag cars that go in straight lines but does not work that well on a street cat that turns corners and hits bumps.
Going to a long panhard bar, mounted solid to the frame, running straight behind the rearend is the only way to correct this. To keep roll center under control try to keep the bar parallel to and as to the center of the housing as possible. A watt's link is another alternative but you need to connect the rear suspension to the frame with a less compliant control device.
Mark

[benno505]

ok cool, i will design one up and go from there, thanks for the feedback.

[benno505]

wont a panhard bar going from chassis parallel along with back of diff to opposite side cause left and right travel under suspension travel?

whats the best way to design this?

[mitch_04]

I could be wrong, but isn't the name of the bar Benno has technically a tracking bar? Or is that only when it's solid mounted and not using bushings? Do you have bushings or is it bolted solid and not allowing independent lower link articulation?

A panhard bar that runs horizontally from the one side of the axle to the frame of the other size (preferably level at ride height) will cause slight side to side movement of the axle, but it will be much stronger since it is not at an angle. The Watts link would get rid of almost all the side to side movement, but is more complicated to design and build or more expensive to buy.

[benno505]

the bar i have has heim joints (spherical rod ends) that allows up and down movement with suspension travel

here is the only pic i have at the moment, disregard the bent shock, i found the QA1 had a bent upper mount and replaced them all with viking shocks.
100159

second pic is of the same unit but with a watts link
100160

[astroracer]

wont a panhard bar going from chassis parallel along with back of diff to opposite side cause left and right travel under suspension travel?

whats the best way to design this?

Yes, it does, that's why you want to design it as long as possible without interfering with other suspension parts or the frame. The longer the length the smaller the "arc of travel" will be moving it side to side.
The best way to design this is to make it as long as possible and keep it parallel at ride height and as close to the center of the axle as possible. Keeping it close to the centerline makes the "side to side" travel equal in jounce and rebound.
Mark

[benno505]

okay cool, but wouldnt the style i have virtually make no side movement as it moves with the travel

[astroracer]

okay cool, but wouldnt the style i have virtually make no side movement as it moves with the travel

No, your track bar cannot stabilize the cross car movement of your suspension. Look at it like this. The track bar is at an angle to your suspension and also at an angle to the force being applied to your suspension. Being that it is on an angle it is trying to inscribe an arc, the center of which is at the front mount. The radius of the arc is the bar length, center of link to center of link. The track bar is simply letting your suspension swing around the solid mounting point. This puts a LOT of stress on all of the other suspension components and is what bent your shock mounts.
Mark

[benno505]

okay i will make a bracket, that hangs low enough to give me a panhard bar that is parallel to axles, weld it to passenger side chassis. then mount the other end to the lower shock mount, this should work right?
just shorten the bar i have now to suit.

I also might take my top 4 link rods, set my squareness, set my pinion, put one of the rods back in to make sure everything is right, then adjust the other upper rod out to suit and bolt it up.

last time i set my squareness and pinion angle with all 4 rods still on, this might be causing some binding of the 4 brackets and might be twistin gthe diff a little.

what do you think?

[mitch_04]

Make sure that your brackets are well supported so they don't flex when cornering hard!

[benno505]

roger, will get some laser cut at work and weld em up

[David Pozzi]

Your shocks need to be more vertical. They are not very effective as mounted.