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1969 Firebird, panhard bar with leafsprings?

12K views 56 replies 12 participants last post by  kso 
#1 ·
Would a panhard bar work with a sway bar and leaf springs on a 1969 firebird? How hard with this be to build and weld in myself?
 
#2 ·
Yes, a panhard works on leaf spring suspensions. It does a nice job of keeping the rear axle from moving laterally due to deflection of the rubber bushings on the spring eye and shackles. They are not hard to build, but they are challenging to build correctly. My goals for panhard bar installations are...
1. Level at ride height
2. As long as possible
3. Beefy enough so the bar and the mounts don't flex
4. Utilizing end bushings that will last for a long time with highway use (no heim joints)
5. Make it adjustable for fine tuning
6. Usually mount it at the axle centerline to help lower the roll center a bit
7. As close to perpindicular to the longitudinal centerline of the vehicle as possible for strength and control

Regards,

Andy
 
#4 ·
I agree with JakeBrake. Do not use a panhard bar with leaf springs.

A panhard bar pushes the axle left and right, because it is fixed to the frame and rotates around that point. Panhard bars are typically used on coil spring suspensions, and tranverse leaf applications.

A watts link would be better, but still shouldn't be needed on a parallel leaf spring application.

Why are you wanting to install a panhard bar?
 
#5 ·
Yeah, the panhard rod wants to make an arc and the leaf-sprung rear wants to go straight up-and-down. You'd at least have to keep the bar at the same roll center, dead-flat, and disconnect it to do any service. I believe they used them on the old Trans-Am cars, but another mod, if you believe your springs are rolling around any, is to just clamp the spring under the housing with no rubber and let the bushings at the end do all the isolating.
 
#8 ·
I think the issue of a Pan Hard rod arcing the axle left and right is a mute point for a suspension with not a lot of travel (unlike say an off road vehicle). Obviously a watts linkage does not arc but I think the arc for most cars is so small it is not an issue.

I have an old MGB with an engine conversion. It is a pretty spunky car. The problem with MG's is the leaf springs are past their parabolic arch. When you hit a bump they do not do much at first then all of the sudden play catch up. Lousy lousy setup.

I removed a couple leaf springs, added a coil over and threw in a panhard rod. The panhard rod was also added because I went to slightly larger tires and the rubber is about 1/4" from the fenders on both sides.

If there was much arcing at all I would raze a fender but there just isn't. It does exactly as I ask:




 
#9 ·
The idea of making a panhard bar as long as possible is to minimize the side movement from the arc. The bar on my sedan mounts to the left frame rail to a bracket to the right side of the pumpkin, making it a little longer than half the distance between the frame rails. Since it is level at normal ride height, there is very little arc throughout the range of suspension travel.
 

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#10 ·
A panhard bar will fight the leaf springs as the rearend travels up and down. Even in a perfect scenario where the panhard is mounted dead level at the middle of it's travel, it will still try to push the leaf springs sideways as the rearend moves up and down. Leaf springs are not meant to bend sideways, so either they will need to flex in a direction they're not designed to flex, or the panhard bar will try to push the frame, or bend.
Stick with a swaybar for your leaf spring rear, and forget the panhard on a leaf spring rearend.
 
#11 ·
Adding a panhard bar to a parallel leaf spring set up is a waste of time if you ask me..:nono: They just don't go together.. But if you want to add it.. That's you.. If you have that much play in your leaf springs.. You need to start checking your bushings ...
 
#13 · (Edited)
I once saw a parallel panhard setup that eliminated the arc bind but can't find any pics now. It had a bar attached to both frame rails. The bars then attached to a vertical fulcrum lever that had a center pivot. The center pivot attached to a bung welded on the center of the rear axle cover. As the suspension moves thru it's travel the bars move the fulcrum lever instead of binding up.
Anyone remember seeing this setup and can scrounge up some pics ??
Found it, it's known as a Watt's Link. Here's an illustrative pic.

 
#15 · (Edited)
And it's way over complicated for 9 out of 10 applications it is used on. Putting all the lateral side loads through the bolts that hold the rear differentail cover on seems quite silly, IMO. Not to mention the example shown above uses heim joints, which transmit all the suspension road noise from the suspension right up into the frame and into the vehicle.

Unless your building a road race truck, keep it simple. And even if you are, NASCAR uses a copy of our truck arms, the same basic set-up used on '63 thru 72 trucks.
 
#19 ·
The beauty of rubber bushings in a leaf spring set-up is that they do distort and deflect. When you go over a one wheel bump or when you are cornering and have body roll, the rubber bushings allow the rear suspension to twist slightly and not bind. If you replace the rubber bushings with polyurethane or aluminum or delrin or any other stiff material you have compromised the ability of the axle to "twist" in one wheel bumps. Stiff bushings are not a problem when both wheels hit a bump at the same time (speed bump for example) or if the car is used mainly for drag racing, but for the street, keep good rubber bushings in there.

If you want to get rid of that flexing feeling while cornering, but still want compliance in the rear suspension, then install a panhard bar as I described in an earlier posting. It will help also if your tires rub while cornering.

Regards,

Andy
 
#18 ·
Yes, poly bushings will definitely stop deflection. Aluminum will also, but it will transfer too much road noise to the body, and you'll get tired of that quickly!
I put poly bushings in my front control arms, body bushings, and rear spring bushings, and it really stiffened the suspension up on my '71 Camaro. I also added HD swaybars front and rear, lowering springs all around, and Lakewood ladder bars in the rear. It handles like a slot car after that.
 
#20 ·
There will be no binding action w/ stiff bushings. In the event of body roll or hitting a bump w/ one wheel etc., leaf springs simply twist. It's part of what they are made to do, and the reason why a leaf-spring car doesn't need much rear sway bar...the leafs are already (by design) providing roll resistance by twisting. (In fact you might save the weight/cost of having to use a rr bar by using stiffer bushings, depending on your setup.)

Binding is what happens when you have two conflicting types of suspension links working against each other...which you may or may-not end up with here, depending on who's advice you are going with...;)

The downside of stiff bushings is increased transfer of vibration and spike loads into the mounts which may become more prone to cracking, and of-course noise...for that reason I would hold the line at plastic and not go any stiffer.
 
#21 ·
There is no binding with poly or aluminum bushings in your springs. As mentioned above, a leaf spring can twist in it's length, just from design. If you rely on the rubber bushing to allow flexing instead of the spring's length, then your rubber bushings will also cause sloppy handling, and eventual wear. A poly bushing removes the play but gives you more positive feel for the road.
Car makers use rubber for one reason, it deadens noise transfer to the car. It does so at the cost of handling.
Swaybars and leaf springs do not fight each other, they compliment each other. Almost every performance car ever made that has leaf springs, also has swaybars. If you want positive handling, you can't do much better than multileaf springs with a swaybar front and rear.
 
#25 ·
As mentioned above, a leaf spring can twist in it's length, just from design. If you rely on the rubber bushing to allow flexing instead of the spring's length, then your rubber bushings will also cause sloppy handling, and eventual wear. A poly bushing removes the play but gives you more positive feel for the road.
Car makers use rubber for one reason, it deadens noise transfer to the car. It does so at the cost of handling.
Swaybars and leaf springs do not fight each other, they compliment each other. Almost every performance car ever made that has leaf springs, also has swaybars. If you want positive handling, you can't do much better than multileaf springs with a swaybar front and rear.
First off, I appreciate your opinion and my goal is not to start an argument. I just want to put out some more food for thought...

I would be interested to know where you got the information that a leaf spring can twist longitudinally within its length. If it does, the amount would be so small as to be insignificant. Especially since the center of its length is solidly mounted to the axle housing (except for some factory axle mounts that have a rubber pad). The majority of the rotation of the leaf springs are due to the fact that they are arched. In a one wheel bump the wheel moves up as the arch flattens out and the small amount of rotation is taken up in the bushings. Herb Adams in his book Chassis Engineering points out that since leaf springs are so stiff as to not need a lateral locating device such as a panhard bar (for a race car), he also states that you should never put poly bushings in the front spring eye of a leaf spring "as they will bind up and prevent the body from rolling in relation to the axle". To further illustrate the motion of leaf springs, some Hotchkiss suspension systems even mount the front of the leafs closer together than the rears to cause the rear axle to steer the car toward understeer in order to make it more stable and predictable to drive.

The great thing about rubber bushings, other than noise isolation and low cost, is the fact that they can repeatedly distort and then return to their original shape. Poly will do that also to a very small degree, but they won't be able to do that near as long as rubber.

As to anti-roll bars causing bind, they most certainly do. With an anti-roll bar you are tying one side of the suspension to the other with a very stiff spring. In the relatively small amount of suspension travel in most street cars it really isn't an issue (and as you pointed out it can be used to your advantage) but if you go too big (or stiff if you like) you can bind up your suspension so much it will hardly move. Almost like a solid axle. Look for photos on the web of cars that have too stiff of anti-roll bars, in a corner you can actully lift the inside tire off the ground! That is also why off-road guys who drive their vehicles on the street have quick disconnects on their anti-roll bars, it frees up the suspension when they are off-road so it can rotate to the limits of their axle locating linkage.

Good discussion...
 
#23 ·
I never seen where these cars are bad handing cars like they are..
I have, but only when things are not like they were when new. Worn out parts, or the wrong wheel/tire combo will cause poor handling, but if all is stock and in good shape they handle well. They all had front swaybars, and most performance versions of the Firebird had a rear swaybar too.
 
#27 ·
If I can toss out there, I don't know the exact issues with the Firebird, with the springs being as flat as they are and with new bushings and such I can't imagine the need to a "locator" of any kind unless some VERY serious racing was being done. At the new Goodguys autocross track, you bet your butt there are plenty of leaf spring cars out there with some serious rear axle locators like watts and panhard setups. But with any "normal" hot rod I don't see it honestly. But we all have different images and expectations so ok.

I ran a panhard bar in my truck with leaf springs not for the handling but because I had so little space between the tires and the bed running a 67 Camaro 12 bolt with 8" wide Americans I had about 3/8th inch room on both the inside from the tire hitting the bed side and the outside with the tire hitting the fender. That being said, with the large arch of the stock springs (with lowerblocks too) and the worn out original bushings. :rolleyes: I was running, I wanted the rear end to not move at all. So I wasn't using it for handleing, I was using it to correct a poor design of something else I had done, namely too narrow a rear end for the truck! Now, honestly, I don't remember if I drove it around and then added the panhard to stop it, and it worked. I honestly don't remember how well it worked from without it if I even knew or if I put the panhard from the start of the build, I don't remember. But that was my intentions at the very least, was to limit it's movement as much as I could. I don't have any photos of it in the truck but in the link below I have a few photos showing how I had it mocked up out in my wrecking yard behind my house. :D

http://www.hotrodders.com/forum/1930u-panhard-bar-geometry-188175.html


Brian
 
#28 ·
Like I mentioned earlier, how much suspension travel is a big key to how good or bad a panhard can be. In the MGB I put one on, I cannot get a finger between the tire and fender. Without a panhard minor movement in the suspension with side loads would make contact with the fender.

The movement with the panhard rod from arcing is so minimal it has not only made zero contact with about a 1/4" clearance and has in fact kept it located and kept it from contacting.

Binding is ZERO issue with this car. It is common on these sorts of cars and makes it a blast on curvy roads. It has allowed me to autocross without screwing up the fenders.

BUT....I still say success is largely dependent on suspension travel. More suspension travel = more arcing. Admittedly suspension travel on an MGB is less then on many other cars.
 
#29 ·
The twisting of leaf springs is not rocket science. One of the reasons (beyond greater spring rate) for multi leaf springs sets is to eliminate spring twist. A greater arc in a spring will also eliminate spring twist somewhat, and that also is a reason for greater spring arc beyond just giving it more travel. Stack enough spring leafs and make them greater length leafs, and you will almost totally eliminate spring twist, but use a single leaf like some 70's F body cars had, and spring twist will be very noticeable.
Rubber bushings eliminate spring twist by allowing the bushing to do the work, rather than the spring. I've read what Herb Adams has written, and he's extremely knowledgeable on suspension, but I still disagree on use of poly bushings in the front eye, as I think it's just making the rubber bushing do the work, instead of the rest of the suspension.
The reason track/street cars use a disconnect on their anti roll systems is because they use such heavy systems that they wont work well on the street. Swaybars designed specifically for street use do not bind so much as to lift the rear wheel when cornering. A swaybar's whole design is based on binding, but the material used, and diameter will dictate how much it binds, and that's why they work well if sized correctly.
 
#30 ·
Now again, just throwing something out there I don't understand. Wouldn't more arch have more sideways movement? I would think the straight spring of a 69 F body would have less sideways movement than a large arched spring because that large arch can "swing" side to side at the bottom of the arch like a pendulum. Even if the spring is a mono leaf as I think it is on that 69 Firebird, wouldn't it being flat make all the difference as far as side movement of the rear axle?

I personally am not a fan of poly bushings because I like the cushy ride and lack of noise of the rubber. On my Gran Sport I have poly on one side of the sway bar links for instance so I get a little bit of stiff without making it totally stiff with the poly at both ends. :D

Brian
 
#32 ·
You are correct, as the spring moves through an arc the axle will move longitudinally and laterally . This effect can be utilized, as I stated in an earlier post, to your advantage. Primarily, the angle of the spring as it is mounted in the car (as viewed from the side) will dictate how the axle rotates in a bump. Most designs when the body rolls over in a corner will make the axle turn the rear of the car roll into the turn (roll understeer) rather than out of the turn (roll oversteer). That is why if you look at the mounting of the leaf spring from the side, the front eye is lower than the rear eye that is mounted in the shackle so the spring is angled down toward the front. You can also see this effect when you lower a leaf spring car, the wheel is frequently no longer centered in the wheel opening, it has moved forward.

Very rarly is a leaf spring designed to be flat when installed. If the spring is flat when installed, it has likely lost some of its ability to support the load and needs to be rearched or replaced. If it is flat at ride height, it will reverse arch in bump and can tend to make the rear axle steer outward on a turn (roll oversteer). I am making several assumptions in saying this, but it is generally true.

Another intersting feature of many Hotchkiss suspensions is that the axle is mounted forward of the center of the spring. That helps in roll understeer, and I also have to believe that it helps with wheel hop under braking and acceleration. The traction bar effect.

Lastly, it was discovered by GM back in the 60's that having one shock mounted forward of the axle and one rearward helped control wheel hop too!

Just like everything else in the car, there is usually more to it than meets the eye.

Regards,

Andy
 
#31 ·
Spring twist is more about length, and height of the vehicle, as the height gives the vehicle more leverage on the spring. In some cases the greater arc is used to raise the vehicle's height, so it will then put more leverage on the spring, but not if the spring is mounted in such a way as to keep the weight lower. That's why many makers mount springs alongside the frame when large stacks of higher arc springs are used. It lowers the center of gravity, and keeps the vehicle's weight from working against the spring stack.
Some arc will give the spring more lateral support, but too much will indeed cause more twist, as there's just no way to keep the force against it low enough. Plus the spring needs to get longer as it's arc'd if it still has the same mounting points. Keep the length the same, and put some curve in it, and the spring will have a bit more resistance to twisting due to the curved shape.
You can test this with small spring material and a tension gauge. A flat piece of spring stock is easily twisted when gripped at the ends, but bend it and then try to twist it, and the tension will be greater. I've seen test sheets from the local spring shop where they use something similar to a big torque wrench to do these tests, and accurately measure spring twist.
 
#35 ·
I am not saying that the spring can't be twisted within its length when not installed in a vehicle. If you anchor it solidly at one end and twist the opposite, it most certainly will twist. And what you say about the arc increasing that effect is also very true.

What I am saying is that when installed in a vehicle and the spring is mounted at the frame on one end (in a rubber bushing), solidly tied to the axle near the center, and then tied to a shackle (in rubber or poly bushings) at the opposite end, you would have to rotate quite a distance before any spring twist would come into effect. All bushings would have to be fully compressed before the spring itself would begin to twist.

Then tie that same axle with one leaf spring mounted to another leaf spring on the other side of the vehicle, there is no amount of twist between those two springs (which are now tied together) that will be a factor in allowing the body to roll about the suspension. The rate of twist in that configuration would be massive and the shocks would not be able to control it. That is why I say that the ability for the body to roll about the suspension of a leaf spring suspended car is due largely to the arc of the spring and the bushings. The angle of the springs while mounted in the car (in side view) is the last major determining factor in allowing the body to roll.

An interesting way to test this would be to set the car on jackstands with the suspension loaded on three corners. Remove the leaf spring on one side of the rear axle, and with a floor jack under the end of the axle with no spring and move it up and down. That way you could easily see where the deflection is and also determine how much travel you have before the axle binds in rotation. If you were real ambitious, you could test with rubber bushings, then poly and then aluminum or brass, and see the results. My guess is that it would be a significant difference in axle travel between compliant and non-compliant bushings. If you did all aluminum bushings in both the front eye and the shackle, I would bet that you could hang the rear end nearly level with no support on the opposite side. I might try that the next time I have the opportunity.

(Warning!! I am now going a little off topic) To take this to the next level, try the same test with a parallel four link suspension with a track locator vs a longer and more lateral Panhard bar and see what happens. Or even a parallel 4 link with no lateral locator attached and see how much travel you have. After doing that you will likely see why I and many other folks that design and build suspensions prefer at least a triangulated 4 link for a car that has to turn, or my personal favorite, a 3 link rear suspension with a panhard for a live axle car. (Sorry, no more drifting off topic, I promise)

Regards,

Andy
 
#34 ·
Interesting stuff, as noted, gets more complicated than one thinks!

On the flat spring, how about a Ford 4x4 which has a reverse arch spring on the front? How the heck does that work?



Here's one out of the truck, it's almost flat with no weight on it! What is the thinking engineering wise with this?

Brian

 
#36 ·
I would guess with the reverse arch on the front spring, it goes along with what I was saying about roll understeer in turns.

With the other example, it is "nearly" flat, but not quite. You would have to see it installed to find out the whole story. Another explanation would be that is simply a very stiff spring for the application and it doesn't deflect much.
 
#38 ·
I'm sort of confused too. If you say a spring can only twist slightly, and is held ridgid by the spring perch and the mounts at each eye, then it seems your panhard bar would be of little affect.
And then the quote from Herb Adams about not needing a panhard bar confuses even further?
 
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