Opel GT Forum banner

1 - 20 of 31 Posts

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #1
Dear Experts:

I have a bent Panhard Bar on my 75 Manta. Its bent in 2 axis and have been unsuccessful in getting it straight. I discovered this while performing a complete overhaul of the front and rear suspension, new poly bushings through out etc, etc... I read a comment from RallyBob awhile back stating he makes a new Panhard bar from stratch using tubing and spherical rod end (s). Could some one provide a few more specifics about the materials including the exact finished length. Also of note, I am lowering the car 1-1/4 inches using custom springs (275#ft & 170# rr). How, if any, will this affect the desired overall Panhard Bar's length.

Thanks for everyone's help.
Paul Crane
 

·
Super Moderator
Joined
·
13,576 Posts
Lowering the car will require the panhard bar to be made shorter. Otherwise, the rear axle will be diagonal relative to the chassis, and it will tend to "rear-steer". Also, one tire (but not the other)may rub the outer fender because of this.

The exact length is not critical, as it is adjustable in length anyway. The range of length needs to encapsulate both the short and long limits of the bar's adjustment however. Also, I do not recommend that you utilize heim joints at both ends. I am not even doing this on my race car. I use polyurethane on one end, and a heim joint on the other. This absorbs side loads that would otherwise bend the panhard bar. I use one "eyelet" from the stock panhard bar, welded to a 5/8" RH thread bolt. This threads into a thick-wall aluminum tube, and at the other end the tube is threaded LH, and uses a LH chromoly 5/8" rod end. I've used a Chevy van front lower shock mount in place of the standard mount that is secured to the rear axle, although my ITB car has a multi-adjustable mount. This allows the fitment of the 5/8" rod end. Jam nuts are required, and loosening the jam nuts and rotating the panhard bar will either lengthen, or shorten, the panhard bar according to the direction of rotation.
Do not use cheap heim joints, they have a lot of slop (more than poly), and wear quickly. Use a teflon-lined chromoly rod end, not "commercial grade" or "precision grade" made from regular steel.

Bob
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #3
Good info Bob, thanks. Also.....How much adjustment will be needed accomodate the change in ride height (s)? An inch or so? How do you get the length set right? On an alignment machine? Which end gets the Poly bushing? The end mounted to the chassis or the end mounted to the axle?

Thanks
Paul
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #4
Well I guess I could just use the bent bar since it is already shorter :D . Now If I can just calculate exactly how much it needs to be Bent!!!
Paul
 

·
Super Moderator
Joined
·
13,576 Posts
Paul said:
Good info Bob, thanks. Also.....How much adjustment will be needed accomodate the change in ride height (s)? An inch or so? How do you get the length set right? On an alignment machine? Which end gets the Poly bushing? The end mounted to the chassis or the end mounted to the axle?

*****I've found that the simplest way to adjust this is with the driver in the car, and have another person adjust the panhard bar until a "neutral" area is found while turning the bar. This needs to be done on a flat, level surface, with the car on the ground (not jacked up). The rear axle is basically self-aligning, the panhard bar will feel tight while turning it either side of the "neutral" centered zone of adjustment, but will have little or no resistance when it is centered for about 1/2 turn or so of the threads.

Personally, I mount the polyurethane on the chassis side, and the heim joint on the axle.

Bob
 
  • Like
Reactions: Nurelic 17

·
Member
Joined
·
315 Posts
The Panhard bar determines the roll-center at the rear of the car... the point at which the bar crosses the center line of the rear-end is the roll center. As the car "squads down" from being lowered, the roll center is raised. This is usually not good, and handling will probably suffer.

So the really "trick" thing that we do is to reinforce the place where the bar mounts to the chassis, and then make multiple holes in the mount. We make the mount a bit "longer"... i.e holes can be drilled so that the end of the bar can be closer to the ground. The holes are so we can raise or lower the end of the bar. Raising the bar raises the roll center and vice-versa and you can adjust the understeer/oversteer characteristics of the car.

I'll put up some pictures when I get a chance.
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #7
Bob,

By 5/8 Heim joint, do you mean 5/8 thread or 5/8 bore? Some of the ones I've seen listed have had different bores than threads. For example Pegasus has some with 5/8 bore and 3/4 thread. Racer Parts Wholesale has some that are 5/8 bore and thread. Both places list high strength chromoly heim joints with teflon/kevlar lining. But the prices are very different. $25 vs $43.

Thanks
Paul
 

·
Super Moderator
Joined
·
13,576 Posts
I use 5/8" bore and thread, I think this is more than adequate on a 2200 lb. car. Price varys GREATLY with brands, so this is part of the discrepancy you've encountered. I've used Aurora pretty religiously, since they're reasonably priced (about $34) and readily available locally. NMB, Alinabal, Rose, SKF, and many others are available. Heck, you can even get titanium if you have the $$$$$.
 

·
Super Moderator
Joined
·
13,576 Posts
Here is the panhard bar for my ITB car, showing the end that attaches to the rear axle. The aluminum tube is "off the shelf" and is tapped RH on one end and LH on the other. This rod end is LH thread, 5/8 x 5/8.

Bob
 

Attachments

·
Super Moderator
Joined
·
13,576 Posts
This is the opposite end of the panhard bar which attaches to the chassis using a urethane bushing. The stock eyelet was cut off an old panhard bar, and rewelded at the tubing/eyelet junction. A 5/8" bolt was ground down and inserted into the old panhard tube, plug-welded, then welded at the insertion point. This end is RH thread. With this setup, the locknuts are loosened, and rotating the panhard bar will lengthen or shorten the panhard bar for side-to-side adjustment of the rear axle within the chassis. Vertical adjustment of the panhard bar (roll center height) is accomplished via an welded extension on the chassis of slotted steel square tubing, and on the axle a serrated "NASCAR-style" plate for 1/4" incremental adjustments has been welded on.

Bob
 

Attachments

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #11
Thanks Bob and everyone!

I've started ordering the stuff I need, based off your suggestions and great pictures! Now, a couple of pictures of the modifications to the chassis mount...... Well... I can always wish.....

Got my custom valved Bilstein shocks back and now I'm just waiting on the Custom Springs and the suspension will be done....except for the Panhard Bar... and Chassis Mount (Hint, hint).

Thanks Again
Paul
 

·
Super Moderator
Joined
·
13,576 Posts
This is the "serrated plate" I weld to the rear axle for vertical adjustment of the panhard bar. I'll take photos of the chassis mount this weekend.

Bob
 

Attachments

·
Senior Contributor
Joined
·
903 Posts
Bob,

Can you give a source for the serated plate? How about OD and wall thickness of the tube? 6061? No sense re-inventing the wheel...

Thanks,

-Travis
 

·
Super Moderator
Joined
·
13,576 Posts
The serrated plate is made by UB Machine, part number 46-1601-B for the small block (5/8 hole), and part number 46-1604 for the long slotted plate (again, 5/8). I got mine from Motor State Distributing (800-772-2678), but a lot of circle track shops can get them. This company also sells pre-tapped aluminum tubing for suspension links, but I got the tubing above from Behrents Performance Warehouse (www.behrents.com or 845-651-7389). The one for my Ascona is 30" long by the way. Polished aluminum, 5/8 tapped both ends (one left, one right), 1" OD, and it cost $13.85. Hardly worth making it yourself for that money, right?

Bob
 

·
Super Moderator
Joined
·
13,576 Posts
As promised, here is a shot of the chassis side of the panhard attachment. This is on my old man's GT, but the theory is the same for all models. Also, I cannot stress this enough, if you are modifying the panhard bracketry, it MUST be strong. Failure would likely cause a crash. If you don't trust your own welds, pay someone to do it!

Bob
 

Attachments

·
Super Moderator
Joined
·
13,576 Posts
Also, a photo of the axle-mounted serrated adjustment plate. This one has been slightly shortened. It is welded to the spring bucket, and to a triangulated bracket to the axle tube. No, this panhard bar is not for this car! It is merely there to keep the suspension from falling full droop, as the car is elevated on a chassis stand.....

Bob
 

Attachments

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #17
Thanks Bob for all of the info you (and Others) have supplied about how to make the Panhard bar fully adjustable. But with all the adjustability, now we'll need some more information about what we are trying to achieve with the adjustments. Are we looking to keep the same angle of the bar wrt axle as before the car is lowered? Are we trying to lower both ends of the bar's mounting locations? I see/understand how you are accomplishing the adjustability. What I don't understand is how this affects understeer/oversteer. Is the stock bar in a good location w/ proper geometry to begin with and you are just putting it back where it needs to be after the car has been lowered 2 or 3 inches? Or was the bar mounted in such a way that it compromised good handling and need to be put into proper location?

Sorry for all the questions. I guess I need a good education in rear suspension geometry.

Thanks
Paul
 

·
Super Moderator
Joined
·
13,576 Posts
I agree, sometimes adjustability is too much of a good thing, as you can easily chase down a problem that did not exist before by making an incorrect adjustment.

Essentially, the panhard bar needs to be adjusted for length so that the rear axle is centrally located within the chassis, to avoid the car "crabbing" or rear-steering.

Second, the panhard bar should be level with the ground when a normal load is placed in the car (driver). When lowering a car from standard ride height with a standard panhard bar mount, this will not be the case, one side will be higher than the other. Under certain circumstances, it is actually desireable to have one side higher than the other to plant one tire down to the ground (circle track for instance), but since most roads consist of left AND right turns, a level panhard bar is the best compromise.

Lastly, there's the overall height of the panhard bar. As TGSI mentioned, this is what establishes the car's rear roll center height. As to what is the "correct" roll center height, this cannot be easily established without testing. Ride height, spring rates, driving style, etc., they all affect this decision. Basically, a higher roll center will add roll stiffness to the rear axle, and induce oversteer. A lower roll center will reduce roll stiffness, and induce understeer. So for a given spring rate, you can fine-tune the car's handling with the panhard bar height. However, going too high with the roll center can make the suspension "jack up", and really screw up the handling. I prefer to make the rear roll center as low as possible and still retain the car's front/rear balance. So I prefer to do my primary tuning with spring rates, and then anti-roll bars, and then panhard bar height, and finally tire pressures. My preference only, not necessarily the "right" way, but it works for me.

Bob
 

·
Member 1000 Post Club
Joined
·
1,520 Posts
Discussion Starter #19
Sorry Again....But It seems the more information you folks give, the more questions I have. Some of the information I understand, but some of the things y'all write confuse me. Maybe I'm just reading some of the posts wrong or taking them out of context. Here goes... I try to show the bit and pieces that I can't add up.

TGSI Writes:
A) As the car "squads down" from being lowered, the roll center is raised
B) Raising the bar raises the roll center and vice-versa

***Doesn't the bar move lower toward thge ground, as the car is lowered? These two statements seem to contradict each other....

RallyBob posted Pictures of the Chassis mounting and the Axle Mounting brackets. The pictures seem to show the Chassis mount behind the RIGHT tire and the Axle mount behind the LEFT tire. On the MANTA these are reversed! Does this change how I apply any of the information supplied so far?

RallyBob Writes:
A) Second, the panhard bar should be level with the ground when a normal load is placed in the car (driver). **** This I understand......When lowering a car from standard ride height with a standard panhard bar mount, this will not be the case, one side will be higher than the other.
*** And THIS I understand

B) Basically, a higher roll center will add roll stiffness to the rear axle, and induce oversteer. A lower roll center will reduce roll stiffness, and induce understeer.
***This I kinda understand...

****Now If I lower the mounting points of the bar with respect to the axle center line(or road surface) and keep the bar parallel to the axle center line. Is the roll center raised or lowered?? Is the roll stiffness raised or lowered??? I assume that the ROLL CENTER of the rear suspension of the Manta is below the Center of Gravity. Is this correct?

Guess I need to buy a book (with Pictures) and study it. Y'all have a good suggestion?

Thanks
Paul
 

·
Super Moderator
Joined
·
13,576 Posts
I'll try to answer all your questions, although it's getting confusing to read these posts now!

Yes, as the car is lowered the panhard bar lowers as well. This lowers the roll center. Technically, only the end attached to the chassis gets lower (the other end is attached to the axle which only goes lower to the ground if the tire diameter is reduced or if an adjustable mount it used to lower it)

GT and Manta mounting points are reversed. Also, the GT has a shorter panhard bar, but on my father's GT I made the bar longer and relocated the axle mount further to the driver's side of the car. As it works out, the Manta's panhard setup tends to plant the right rear tire better than the GT's under right-hand cornering, while the GT's panhard arrangement is better for planting the left rear tire in left-hand turns. On my friend's circle- track Opel Manta, we switched the bar around to match the GT fitment, as we were only turning left.

In regards to your last questions, lowering the panhard bar relative to the ground will lower the roll center. I recommend keeping it level. If it is angled, it will tend to preload one side of the car more the other. Lowering the roll center has the same effect as softening the springs (less roll stiffness), so the car will have more body roll.
In most cases, the rear roll center will be WELL below the center of gravity, however, you could theoretically lower the car, lower the engine in the chassis, reduce weight of all components at the upper part of the car (glass, body panels, etc). If we then raise the rear roll center, we can certainly close the gap between the CG and the rear roll center. The smaller the distance between these two points, the less roll angle the chassis will exhibit.

As far as books on the subject, there are TONS of them. Some are simple, some read like an engineering course, but most are pretty helpful. Check out Classic Motorbooks, they offer a lot of books on the subject. I probably have about a dozen or so on suspensions.
 
1 - 20 of 31 Posts
Top