Can any rear radius rod handle SBC/open drive line combo?

This subject has been debated to death on threads prior to this one. With all due respect to other posters, plym49 has it correct (as I immodestly will claim for myself as well).

One need look no further than the GM pickup "truck arms" from the '60 to '72 period to see what and how the engineers at GM dealt with this issue. When rigidly mounted to the axle housing, even when triangulated the long arms WILL twist (or TRY to) when the suspension articulates with one wheel higher/lower than the other. If the arms can twist along their length all is well. If the material used resists twisting, which any tube shape will do, it sets up a twisting force at the axle housing. Now, I think we all will agree the axle housing won't twist, so the forces will be concentarted at the junction/joint of the long arms/axle housing.

GM avoided this conflict by using 'I" beam shaped arms that are quite strong as a beam but will twist along their length and absorb the forces without damage to any components. They also used fairly large and compliant rubber bushings at the forward end of the arms where they attached to the center crossmember.

The mere fact that several professional shops use a particular design, and do so with outstanding workmanship, doesn't in any way mean they are correctly taking into account the geometry and physics of a given setup. Some are and some aren't. Appearance often seems to carry more weight than science.

Ray

 

I see the science validity behind Ray's reckoning as well as the "all things considered" practical approach laid out by Dick Spadaro.

When I originally posted my query, I wanted to know if there was a setup that adequately mitigated the effects of dropping the torque tube for an open driveline that did *not* use ladder bars, but rather used some modified form of bones/radius rods (NOT the Rolling Bones style.....those are *still* ladder bars). Some guys will be engineering purists and say that looks don't matter, so just use the ladder bars...or that they LIKE the look of ladder bars, which is a completely acceptable answer. However, how many folks have been scared into liking ladder bars from claims that radius rods will shatter and you'll careen to your death off a cliff on some abandoned highway?

The nature of my paying work often balances mild engineering with a heavy dose of aesthetics. I have problems when I have to accept a purely engineered solution. I see what I like and want to figure out a way to make it happen while accepting that there might be *some* inherent, if only remote, risk. I think that this is where Dick's outlook comes in to play.

I have seen guys whose rigid mounts at the castings have cracked...but only a few. What's the percentage of these that DO crack? What were they doing when it failed? What mill were they running? How triangulated were their bones? Were they using bushings, Heims, or tie rod ends? Who here has actually ran, or know *personally* someone who ran a setup like this who did have it blow up in their face?

Engineering absolutes is great, and I think it should be undertaken whenever it's feasible both economically and time-wise. Are we engineering to a fault, only for a slim chance that something will break? Are those instances when it will break catastrophic only to the wallet, to your time, or to your life? Are you taking a bigger chance with tragedy by just getting out on the road or not perfectly triangulating your rear end?

I'm asking all this in practicality, not to be a smart ass. Theory is great, but it has to be backed up with all supporting information and circumstances in order to be able to make a fairly definitive call as to what "fact" really is, no?

 

I don't have any formal engineering education so I just went with what I had for parts, and copied Fords idea from front wishbones.

I used a torched out center part of a 35 Ford X member, turned it backwards so the ball mount was facing the rear, then used a 32 front wishbone yoke/ball with a rubber ball cover, and used 35/36 rear trailing arms. Note that the ball mount is located directly below the U-joint.

I still need a torque arm.

Just me, but I think this is a very simple way to cure any binding, and is really easy to build.

 

That rocks. I'm a little late for that train, with the SoCal K member already installed, but if I end up needing to do something different, I figure I could fabricate something like that to hang off the bottom to accommodate that.

 

Your design is about as good as it gets.........you have addressed the 'twist issue' very effectively and it looks great too. I do agree that a torque arm will be needed as well but you have addressed that as well.

Nice work.

Ray

 

ha, here is an "old school" 36 modified radius rod mounted to a 57 chevy rear...using a stick welder and some big plate and angle iron. Time to re engineer
what you see is the bottom of my frame, someone in the old past welded the original spring hangers to the rear axle, which is pretty common, then they welded a plate to the rear axle and triangulated the radius rods, bolted to the plate, and then attached that to an angle iron welded to the bottom of the frame. again attached by a bolt. The holes and bolt were not exact, and my Dad had told me that he kept on snapping the bolts so he would file the hole and make the bolt bigger, so now I have my 350 with a manual shift PG and at each shift, I could hear that bolt move, and over time I guess it fatigued the frame where it was welded and pulled off. So I am going to make some new ladder bars this winter.

 

I have seen firsthand the damage that comes from setting up a suspension in a bind with incorrect geometry with split rear radius rods and wishbones. I JUST got finished replacing the entire front and rear suspension on a T touring hot rod with a whooped pinto 2.3 in it and it was able to crack the rear radius rods and destroy the front wishbone (it came apart at speed and flew through the splash apron). It's not a matter of how often does it happen, more like when... if you set it up with inherent problems it WILL break.

I do think that many folks try to overengineer the suspension on an early hot rod like it's going to be driven as a rock crawler or something. Most of these cars will never see more than 4" of suspension travel. There will always be some sort of a compromise, knowing which way to adjust for the desired outcome is crucial. If you must use radius rods, mount them under the driveshaft yoke with large tie rod ends and add a torque arm or use tham as the lower links in a four link arrangement. Any other way is just asking for trouble.

BTW, Frank that "front style" rear wishbone arrangement looks great!

 

Here's how I currently have my rear end set up:





If need be, I do have tie rod ends that I can swap out with the Heims, but it'd also necessitate getting rid of one of the tabs for each mount and replacing the other with one more beefy and suited for the tie rod. Anyway...there it is.

 

I'm one for using Ford or any make vintage parts in either their stock or modified form when ever i can, they just look good. Some parts are asked to do MUCH more than they were built to do, but hey, isn't that the hot rod way Some here have given the right answers, and depending on how you drive some of the others will work too...but for how long

To the OP you don't like the look of ladder bars, that's cool, it's all a matter preferance. I think that the bones mixed with the later rear is like reverse engineering. You have added a torque arm and that will greatly help with the stresses. The wishbone tubes although not as strong as the 35-36 ones will do the job, the factory bracket is the major weak link. How about using the tubes and fabbing some thicker brackets?

Make a three link using some later front split bones, they are much thicker and run them paralell to the frame to have the bone look. You could run the tie rod at the front and a bushing at the axle and the top of the diff for the torque/upper link so all can pivot and there is no bind, the transverse spring is your locator/panhard bar. There are alot of solutions posted here, some better than others, i still think the ends are your weak link though..jmo

I think F&j has it...nice work

 

Back to Poboyross's issue, From the picture you have posted your major issue will be the torque arm rod you have installed. This small diameter tubing is not strong enough to control any major braking nor acceleration forces. . The acceleration and braking of the vehicle will place torque forces down the lengths of the control arms, all will be fine until the axle torques exceed the wall strength of the tubing used and produce a bow in them that leads to part failure.

In addition to the strength of the tubing you will have to address the respective arc made by the rotation of the rear axle radius rods and the swing of the torque arm. These items must swing in radius congruent to each other. Because you have used a fixed mount on the torque arm, this means that to eliminate any strains on the control arms, all swing arcs must be parallel to each other. This means all parts must pivot from a central point, in this case they don't. If you will notice most torque arms use a rubber bushing mount to allow the center arm to deflect at a rate different than the control rods. This is another way to eliminate any bind and external forces that can cause the unit to fail.

The lower arms are just made from rolled sheet metal and being old are most likely rusted thin in spots so although they appear to be strong can easily fail. The center torque arm is also made from small diameter tubing, unless it is exceptionally heavy wall material this will not withstand much stress before it defects.


In regard to the design degrading due to torsional twisting, the installation of a heim style end limits the rotational stress in the tube. The problem with heim ends is that they continually require lubrication and that when they run dry ,wear easily and are such a sold mount, transmit road shock throughout the vehicle.

Will it work yes, but at the low end of the scale, you should look for ways to beef up the rigidity of the parts such as a larger diameter torque arm and a thicker wall radius rod material.

 

Building horribly unsafe contraptions is totally traditional!

Back in the day kids built with whatever junk they had and when their crap flew apart at 60 mph they died or were horribly mangled, but at least it was in a traditional fashion!

All you clever folks, with your "physics" and "geometry"... Phffff!!! You can have 'em.

I'm going build mine out of an old bed frame with a stick welder. Right after I get done welding the body to the frame....

 

Dude....this is the second time your troll has come out form under the bridge, one more time and I'm going to bring out the sledge.

 

Ross, your setup will work fine. How many of these naysayers even OWN a hot rod? The Hot Rod Works has had success selling one just like yours. GV now has my old Touring with the same setup and 10,000 miles on it. The only thing that ever broke on that car was a "show" weld on a P&J rod end. And that was a case of poor penetration of a cold weld.

 

Hey TMan....thanks for your constant tending of my threads, no kiddin. While I appreciate learning the physics behind things in thread like this, I also appreciate when Akun's razor is applied to various degrees. I'm undoubtedly going to run it as is and keep close eyes!

 

Whoa buddy, I'm just having fun with you! Is my sarcasm not showing? Jeez!

 

Dick, you are saying the same thing I said, a little differently, however I submit that most setups - Heim joint, ball joint or urethane joint - cannot accommodate the amount of articulation required.

Perhaps the best check for any individual situation is to cycle the suspension with full articulation each way, checking the radius arms for twisting since the mounts at the front will probably run out of movement before the suspension is fully articulated.

 

You mean like twisting the empty rear with no rear spring? I'd bet it feels like it's binding pretty good.

I can't describe it very good, but something like the imaginary point/or line, where the two trailing arms would intersect further forward. If you twist the rear, the bars would want to rotate as one, at that imaginary point. But they are mounted closer to the rear. If you could make a cad movie, it would show that one bar front mount is trying to be pushed up, and the other arm is trying to push the mount down.

 

Right, you described it perfectly.

So what Dick said is that if you lay it out properly, you really have the 'triangle' with the tip lopped off. To make this happen you need two things: get the top of the trapezoid as close as possible to where the tip of the triangle would be, and use joints at the top of the trapezoid that can accommodate the 'rolling you would see. Remember, as one joint tries to twist up the other tries to twist down the same amount, just like you said.

Dick, not to speak for you, but you seem to believe that most Heim/tie rod/bushings use can handle the amount of twist needed. I do not out of hand agree with that, especially since most installations have a shorter trapezoid with the top farther apart and not in alignment with the longitudinal axis of the tail shaft (this all accentuates the problem).

The only way to know for sure is to check it by cycling the suspension at full articulation both ways, up against the stops.

 

^ this is the way to go. Why doesn't everyone embrace this method? It's easy, cheap, and EFFECTIVELY eliminates all the torsional issues.

(By the way, if anyone has a spare ball/yoke like this that they'd like to sell, PM me. I need one.)

 

Quick answer to Voodootwin, Because of the control arm length, under moderate to severe bump the rear end skews right or left enough to promote the vehicle to rear steer. The faster you are going the bigger the surprise. Now you say "but the front end is similar design" and I say you are correct to a degree on that but the rear axle doesn't have a steering wheel connected to it.

 

I want to understand how this happens, and what can be done to prevent it.....a pan-hard bar? Short control arms? How did Henry prevent this?

 

Answer to Plym49 post 78, Nothing wrong with with heim or spherical bearing, while you are concerned about the bearing not being able to rotate enough for various conditions is more based upon selection of the proper size bearing for application. In choice of bearing you measure angle of misalignment which is the maximum angle of axial rotation of the ball in the body of the rod end before it bottoms on the bolt or shaft. This is usually spec'ed by the part Mfg 10*, 8*, 12* or more, a common rod end used for hot rod application are for 1/2" shaft bolts and usually have a misalignment factor of around 20*, however increasing the bore diameter to say 5/8" may increse the misalignment to 26*+-. Now for most street applications an ablity to rotate 20* is plenty. This would mean that a rear radius rod end set at 0* could roll 20* before it bottomed out, that is quite a bit. Now substituting a ball end joint like a tie rod end will allow even more radial movement, the least movement is in a composition bearing like a rubber or urethane bushing. With this type of bushing to get higher angles of misalignment the composition becomes softer, the down side is the locating alignment point tends to wander because the material is easier to compress.

Now to Voodoo, you can't easily fix this, remember everything is in motion and because it is in correct static alignment doesn't mean its in correct alignment in motion. Remember your radius rod is what it says it is a "radius" meaning that it is the measurement point of an arc. Using the simple example of the lower radius rod being parallel to the ground and on center with the rear axle, if you view the axle from a side view the radius of the control arm swings thru the vertical axle center line. Now if the axle was bumped up ward it would be controlled by the radian distance of the control arm and its frame mounting point. If you plotted it, you would see that as the control arm moved upward an arc would be formed between the imaginary horizontal center line, the vertical center line and the fixed radius distance of the control arm frame mount. The swing of the arc would shorten the horizontal distance and pull the axle forward on that side and skew the axle to point away from straight creating a rear steer.

 

The wider the front of the trapezoid is, the more rotation you need. This is accentuated by the fact that with something like a spherical rod end, the axis of rotation of the joint is only an inch or so away from the ball. So even if the joint seemingly has enough of a potential for rotation, this can be diminished depending on how things are built. It is really important to get the front of that trapezoid as narrow as possible, and you should always test cycle the suspension to check for binding or twisting.

 

I'm going to stir this pot AGAIN.

How much more strength would I have gotten from using 36 Ford radius rods over the ones I used?



Of course this set pops up on the HAMB *RIGHT AFTER* I get this phase welded up :/ Gawdam Murphy......

 

3 times stronger or more.

 

Not a damn thing, send them to me!

 

LOL...you're more than welcome to them. I don't think I've got the money at this exact moment anyhow...not moving cross country in a month and all :/

Since I've already got rear spring hangers welded onto the axle housing, I may just figure out how to fab a thicker rear mount to replace the castings. My new job has all these tools in-house...and hopefully at my disposal. A lot of places don't let the "computer guys" touch the shop equipment.

 

this is what i will use to tie my rear bones together in a triangle manner.simmilar to how henry did it minus the ball.the ball inside the joint is greasable and the unit is forged.

 

Currie's Johnny Joint® offer 30 degrees of unrestricted movement in any direction as compared to 22 degrees on a common heim joint. The bushing in the Johnny Joint® rod ends are made with a high density, "tough 88" urethane that encases the inner ball. Its a very durable material and it is impervious to weather. The special ball is heat treated steel for extra strength. and on the 2" and 2 1/2" models the ball is cross drilled for thru-bolt lubrication. The 3" joint is lubricated via zerk fitting on the outer shell. Outer shells on the 2" and 2 1/2" are heat treated steel. The retaining washers on the 3" are machined out of 4340 chrome moly and feature a snap ring safety machined into the face of them to protect the snap ring safety machined into the face of them to protect the snap ring from coming out. 2" joints include a 7/16" greaseable thru-bolt, and the 3" joint accepts but does not include a 3/4" bolt. 2" joints are also available with a 3/4" RH threaded stud welded to them. The 2 1/2" joints are available in a forged version with either 1" RH or LH threads or 1 1/4" RH or LH threads, and the 2" joints are available in a forged version with 3/4" RH or LH threads.

 

add a simple torque arm and there you go .you can limit rotation and still have a modern but traditional looking rear wishbone setup.i will be using 40 front wishbones welded to 36 spring hangars, the torque arm will be 1 inch dom tubing with jonny joints on both ends