Lowered panhard bar mount

[slowTA]

Evening all, this is my first post on this forum to help me figure out why my contraption is bending under load while another design is not. I'm going to try to describe the issue the best I can for those of you not familiar with the car.

I'm working on an autocross car (about 3,400 lbs. and competitors with similar setups and times are pulling about 1.5G, haven't run the app on my car yet) and lowered the panhard bar to get more rear stability. In the factory configuration they run a diagonal bar from the driver side frame down to a frame horn on the passenger side due to the distance from the frame down to the attaching point of the panhard bar. At the bottom of the frame horn the panhard bar goes back to the driver side and connects to the axle.

In my design I made a vertical square tube that attaches to the lower hole on the frame horn with a stubby horizontal bar that connects to the upper hole. The factory diagonal bar is ditched for one of my own that attaches to the same spot on the driver side and the vertical bar on the passenger side at a point just below the lower bolt hole. The three tubes are tied together with another short square tube. The tubes in question are 1.5" x .125 wall for the vertical and 1.25" x .095" wall for the diagonal. The height of the panhard bar is adjustable with clamp on mounts from a race car shop.

The kit most people use retains the factory diagonal bar, welds to the outside of the frame horn, and hangs down about the same length as mine. The material in this kit looks to be 3/16" thick and almost 2" wide.

Pictures and video of my insert are here (link) with the youtube link here (link)
Pictures of the kit everyone uses are here (link)

In the video you can see the right bolt head on the adjuster moves back and forth compared to the corner of my spare tire well. If you look carefully it appears that the lower portion of the frame horn is twisting about a point somewhere between the 2 bolts, it does not appear that the whole frame horn is twisting.

My 2 questions are, why is mine bending and how do I fix it. I've been getting suggestions that range from connecting the bottom of the vertical bar back to the driver side (making a large oddly shaped triangle) to adding metal over the weld between my diagonal and vertical tube to strengthen the joint.

Thanks for any help you can offer. This was my winter project and this past weekend was the first test... now I have to scramble to make it right!

Chris.

 

[cibachrome]

My first impression from the video and pics (nice diagnostic job BTW !) is that you now have a bolt clamping torque retention problem. Spray paint the connection from your square tube (both sides) to the old frame bracket and tighten it to your spec. Rerun your road tests and then disassemble (carefully your parts. Look for "itching" of the paint. That means you are asking a bolt clamping load to handle a very large moment (Not a good idea). Sometimes we use modeling clay to check the joint movement, too. So, bolt may be too small for the job, and larger may crush the tube. So, weld a slug into the tube to handle the bolt torque. Can you somehow retain the "former" bracket brace ?

I've seen it where you would buy another frame drop bracket and cross-link it to the running track bar bracket frame side. Then you would weld or bolt the two braces at the cross where they meet. That takes the 'match-boxing' movement out of your bracing linkage.

You really need a gusset to triangulate these parts, otherwise you essentially have a 3 bar linkage with body, brackets and tubes (plus weak clamping torque retention)

Take a "fish scale" and a come-a-long and apply enought force to move the bracket, say an eigth or a tenth of an inch deflection. Record the force on the scale before and after modifications and road testing. I'd recommend doing this with the OEM setup if its still possible. Your design may be worse than the OEM lateral stiffness.

 

[cibachrome]

Take a look at this for similarity. I believe that if it takes that much track bar drop to get the load transfer distribution you need, you actually have a FRONT weight transfer problem.

 

[MikeHalloran]

You sure went to a lot of trouble to provide infinite adjustability.
With all the tubes welded to tubes and such, the stress situation is complicated.
The tube walls are pretty thin for bending loads, but that's just me eyeballing.
I can't see what's flexing in the video.
Maybe coat everything with brittle varnish, run it a bit, and look for cracks in the varnish.


Mike Halloran
Pembroke Pines, FL, USA

 

[BUGGAR]

3400 lb divided by 2 (50:50 weight distribution, right?) = 1700 lb static on rear axle x 1.5g lateral = 2550 lb on your bracket without any impact factor.

 

[gruntguru]

Looking at your video, I would say the entire frame horn plus your additions is moving laterally. Hard to see what you have braced it back to with the long tube, but I think the whole lot is moving.

je suis charlie

 

[slowTA]

Thanks for all the replies, I'll try to cover all the topics in this post.

-I painted what I made before reassembly, and it is a bit of a tight fit, see the picture here. There does seem to be some 'itching' but I can't tell if it's just from installation and removal a few times. In this picture you can tell I welded a heavy wall round tube in the square tube, OD is 1" and the metric bolt size is about 5/8". Although there may be some movement between the frame horn and tube, it appears that both are moving in the video.
-My design contained a small triangle tying both bolts into the diagonal bar.
-I can only imagine how much a 2k lb. fish scale would cost, but I get the point you're making!
-I've seen a bunch of panhard bar kits out there and they tend to look skimpy, the one linked above might be aimed at the drag race crowd.

-As for the amount of work I went through, the kits available are either very expensive (for the Watt's link) or relatively difficult to weld to weld to the car due to differing metal thickness and tight confines under the car. The majority of the welding I did this way were on a bench, the exception to that is the weld to the axle which wasn't that difficult to get into position and would be needed for the kit I linked to above.
-Agreed, there is a lot going on under the car to figure out what is really going on. The thin wall tubes are up for debate, I sized the tube based on my 'worst case' calculations of the entire weight of the car x 1.5G and came up with 0.070" of movement if I put the adjuster all the way at the bottom. I'm obviously getting more than that and it looks like it's mostly in the frame horn somewhere. Going up in wall size reduced the movement by less than half and added more weight.
-I thought about spreading some bondo along the bar and seeing where it cracks... if I still have this problem then I'll look into that to be sure.

-See the figures I used above, and the car is a bit nose heavy.

-Agreed, everything is moving. The diagonal is bolted to the driver side frame, in the factory position.

My solution for now was to add a bar from the bottom of the bracket back to the driver side. So there are 2 bars heading off to the left side of the video now. I don't have any pictures of it installed, because it isn't yet. Here is a picture of it hanging for paint. The part you see in the video is hanging at the bottom, I have an extra long tube sticking through towards the front of the car in case I have to add yet another diagonal tube. I would have added it this time but things are getting heavy and it would be really close to the differential cover!

With all this said, I still can't figure out why my version is bending more than this kit like I mentioned above. Someone brought up the point that the factory bolt holes are offset and that the axle side of the panhard bar is usually higher than the passenger side (at ride height) and that it puts the frame horn in more of a tension/compression situation instead of a moment. I'm not sure that I buy this... any comments?

My next event is Sunday and I should have everything put back together buy then, but it's more expensive (fewer drivers allowed to get more runs in) but I'm a bit apprehensive about paying more just to find it's still bending... then going home early since I'll be watching the videos very carefully this time.

Chris.

 

[gruntguru]

"- Agreed, everything is moving. The diagonal is bolted to the driver side frame, in the factory position.

My solution for now was to add a bar from the bottom of the bracket back to the driver side. So there are 2 bars heading off to the left side of the video now. I don't have any pictures of it installed, because it isn't yet"

If "everything is moving" (translating laterally) your extra brace won't fix it because the problem is not the stiffness of your added structure but the rigidity of what its bolted to. (the driver side frame)

je suis charlie

 

[slowTA]

I meant everything on the passenger side in the video is moving, looks to be bending mostly.

I doubt the driver side is moving too. The flange that bolts to the frame doesn't show any scratches from the serated washers. Also, if both the driver side and passenger side of the frame are moving and the camera is located on the frame behind the rear, then the whole car should be either turning or folding in half!

Chris.

 

[gruntguru]

Perhaps the camera is moving. How rigid is its mount?

je suis charlie

 

[slowTA]

Lol, I asked myself the same question! It's a go pro stick on mount, if it was moving then it should look like the bolt is moving the wrong way based on the tension and compression of the panhard bar. I also looked at the details around the perimeter of the frame and it doesn't appear that the camera is moving.

Chris.

 

[NormPeterson]

PHB force should be in the direction opposite to the direction that your resiliently mounted exhaust is moving. Don't forget that you're looking at what the rear tires' lateral force is doing to your vertical post, not what the sprung mass inertial force would do.

How much clearance is there in your bolted joints? They look pretty close together, so a little clearance in each could result in significantly more motion down at the PHB pickup height. In this scenario, things are rotating rather than bending (and it might be difficult to determine which it really is even with a GoPro).

I don't recall offhand whether the F-body rear subframe was resiliently mounted or not, and if so whether yours still is. There will be relative movement from that source of compliance if it's present, and the subframe element that the passenger side OE bracket is welded to is still going to be subject to torsion.

Side question . . . is UE back in business?


Norm

 

[slowTA]

Norm!!! It's been a while since I've seen you on the F body forums.

The bolt clearances are pretty tight, I machine out the welded inserts to just clear the stepped shoulder on the bolts. Also, after 1 event the top bolt now has to be hammered in place. So something in there moved around a bit, permanently!

Third and fourth gen F bodies are unibody so there are no bushings between the body, suspension mounts, and camera mount.

Here is another video with my fix, something is still moving but it is a lot less.

https://youtu.be/WcCOKUiiSGo

UE is still around, never thought they closed up shop to begin with.

Chris.

 

[NormPeterson]

I think the entire bracket, OE portion and all, is moving/rotating. Watch in full screen with the exhaust pipe and its hanger covered up with a piece of paper.

Maybe it was Evolution Motorsport I was thinking of. Anyway, I've been tinkering with an '08 Mustang GT, also PHB-equipped as OE, so at least some of the F-body information carried straight over. And the autocrossing I used to do has been replaced with a big-track habit.


Norm