How do they get away with this?

This subject has been discussed many times here before....I have read most of the threads. The subject is locating the rear radius rods way outboard onto the outside of the chassis, parallel to the frame rails, rather than inboard nearer the center of the car. This pertains only to what appear to be extremely lightweight cars....say under 1800-2000lbs. Many knowlegable folks say don't do it....something will break etc.

But the jury still seems to be out, at least for me. I have a ton of pics that show this type setup on many, many light cars, so I'm guessing it works ok, I understand the physics of the issues (I think I do anyway) of lack of rear axle flexability, need for triangulation/movement etc. etc.. Maybe most of these cars don't get driven much, having traded form for function. But I do know that the Bo Jones modified (pictured) was driven and driven HARD.

I'm no genius. Just a backyarder getting started on a new project Really want to know "how do they get away with this". I have attached some pics for examples.

 

I have heard that hairpins if not extremely reinforced, can twist enough to work without breaking something else. But I can't speak from experiance. However, I would think if the out side bone was treated as part of a 4 link, and the other links tucked inside of the frame, they would work. But they would need to be attached on a single point on the axle end, and have a bushing of sorts to provide some give. Hopefully someone with more experiance will correct me if I am wrong.

 

This type of setup will last a lot longer on a light car with a stiff spring. A swivel where the radius arms attach to the frame also helps a lot. The more it freely turns the better but it must still securly locate the axle fore and aft.

 

Since I decided to build a 26 RPU with a 36" box rather than a modified and to go with coilovers instead of the transverse spring, I designed and fabricated my own rear bones to mount on the outside of the frame rails to match the front bones I built.

They are part of a triangulated 4 bar design that I came up with that articulates well and should ride well too.

There are pivot points at the bottom of the rear bones under the axle and at the top bars (urethane bushings) along with heim joints at the front ends of the bones where they mount to the frame.

 

Because someone did it wrong once...

The front mount and/or the bolt is the fail point because it's single-shear.

Triangulated arms(unsplit bones) don't take any lateral force because their mount lies on the roll axis of the chassis. Splitting the arms does make them take some lateral force in body roll because the mounting points are outside the roll axis.

Your panhard bar or transverse leaf is what ultimately locates the rearend side to side.

Look at a three-link circle track car.

 

I think the use of a heim joint at the pivoting end makes it possible. It's able to swivel, keeping the twisting motion from being transferred to the mounting bolt or stud, yet retain the for-aft control.

 

They "get away with it" because as usual theory and practice are two different things. If you slam too much meat to it and you get traction something is going to get stressed then you may have to beef the radius rods. Remember when they first started doing this they were using flatheads and skinny tires.

You can beef the rods and many have done that before. I have actually posted cheesy drawings in the past to show how they used to do it on cars with lots of zot. I won't do that today, maybe you'll get lucky and find something.

 

Gearheadbill,
I agree with you completely, doing the rear this way looks good but is just bad engineering. The reason it "works" is as Porknbeaner said, they flex. When I did my 28 modified I built an "A" frame and mounted to a 3/4 inch aircraft heim joint and that is located exactly below the front U joint of the drive shaft. I have a T rear spring and use it to locate the rear end side to side, no panard. I probably have 10-15000 miles on my modified and i beat it like a "rented mule"and it handles pretty well.

Rex

 

Mine is like that, was told it wouldn't work. I guess the front spring has enough movement for the whole car to get by on. I have had no signs of stress or cracks, check often. I have drainage ditches built into the back yard/woods and before the body was on I ran it on angles to see if it would move well or cause traction problems, could find none. Hillbilly country, all roads I travel are crap so it doesn't have an easy life.

 

Front I-beams can twist. This lets the front hairpins act slightly independent of each other and move.

Round tube alxes (front or rear) don't twist so easy. Stuff gets stressed.

 

Mine has quarter elliptic springs, forces are all on the same plane (I think)!

 

Learned something looking at another thread.

I was thinking in terms of fabricated trailing arms with heims or rubber bushings on the axle end.



A trailing arm parallel to the chassis, solidly mounted to the axle is a poor idea. The only way the suspension can move without binding is straight up and down, no roll, no one wheel bump.

Yes that has to put a lot of stress on the arms over time.

Thing is they're so massively overbuilt for a light car it probably will never matter.

 

Limited suspension travel along with some chassis flex allows these to work in many cars, esp cars like a T-bucket with a simple ladder frame.

Stick that design in a car with long travel and a stiff chassis and it will fail sooner or later.

 

The Ford original design has the rear axle located to the chassis in one place - the centre of the rear spring, the wishbones mount to the torque tube which in turn is mounted to the trans. In this setup there are two pivot points when the chassis rolls, the rear spring and the torque tube. These points are in the center line of the chassis so when the chassis rolls side to side there is little stress on the mounts. The 'bones simply triangulate the rear axle and are under very little stress.

When the wishbones are split and mounted to the chassis a triangle is created, the axle is mounted in three places, the ends of each wishbone and the spring. This sets up lots of stresses in the wishbones, the further the wishbones are spread apart the bigger this triangle becomes and the worse the stresses. The more the axle moves in relation to the chassis the worse the effect on the wishbones. It is bad engineering but people have got away with it because if the suspension is solid or close to it the wishbones are not under a great deal of stress.

Of course there are wishbones and there are wishbones. the bulkier wishbones are stronger to begin with. I have mounted axles in this way using Pilot wishbones (British Ford) These things are super strong!. Hairpin wishbones have a lot more flex in them so absorb the stresses better by flexing.


There are other issues to consider like the relationship between the ends of the bones and the end of the drive shaft (whether open drive or torque tube) because further stresses can be set up if this is not right.

David

 

Early to Mid 1950's Indy cars with cross springs run split bones or hair pins. I am not a suspension expert and not trying to argue about right or wrong, just trying to understand. My experience with hair pins on a modified is they work. My original Model A spring would bottom out, spring eye hitting perch casting on 1934 rear end. Changed to a different spring, hitting or bottoming out eliminated. Point being that with the suspension was working, indicated by the bottoming out and then not bottoming out. Urethane bushings on rear axle and tie rod ends at the frame mount. Comments?
Duane

 

This makes sense to me....using of a flex mount on BOTH ends of the hairpin or wishbone. Allows for more movement. I'm not going to build a rock crawler that need a lot of suspension travel or articulation.

Anyone have any experiences with damage or breakage using this type setup?

 

I built my '27 Track T with the bones on the outside of the frame because I saw everybody else doing it, and I can honestly tell you that it sucked badly!!! I drove the car a few times and changed it over to a 3 link. I think that people get away with it because they probably don't drive the cars very much at all. It is like the ridiculously chopped and channeled Model A and T coupes that I see at car shows all the time, everything works great when it gets there on a trailer.

 

And yet,.. they have used this system for decades,.....

To bad somebody didn't tell these guys it shouldn't work.......




















Frankie's old Roadster,... I rode around a bit in that car,... wish I would have known the rear suspension shouldn't work,.... But, the car rode so good !


Between Hairpin and Wishbone pictures,.. I could fill a lot of pages with pictures,... Including a Lot of oval dirt track roadsters,.... If memory serves me right,.. they spanked this things like a rent-a-car.

 

i did not see it mention, lets not forget our bones may not be as strong as there. ie.. our steel is going on 60+ years.
chris.

 

Harm,
Ypu know what they say, ignorance is bliss. Sometimes things work real well because we never read a paper or attended a class.

I look at it this way. Back in the '70s I built floating boat docks from 1/8-3/16" angle iron. Some of them are still in use today with several changes of foam floatation. If I knew then what I know than I never would have participated in the practice. But back then I had never read a paper or attended a class.

 

double bones

 

I'm no engineer but when there's only one pivot per control arm and the left and right arm are mounted pretty much parallel to each other, your suspension is just a big hinge. So, on cars that have the control arms anchored outboard (front AND rear suspension, like those pictured in the initial post), the only reason they work is that one or more of the components are flexing and/or twisting.
When one end has more of a triangulated setup, it's still essentially a big ol' hinge but with a lot more leverage along with the resultant increase in suspension articulation. I think even tube axles that are not supposed to twist, comply at least a little.

Come to think of it...I'm guessing the only truly, full articulation hot rod setup is a stock Ford ball in socket, center mount wishbone. A close second would be a 4 bar setup. A long time ago on here, there was a guy who split a tube axle and joined the halves with a bearing. Anyone remember that? Pretty neat but probably not too necessary.

 

My old 31 Roadster was built that way with solid mounted radius rods [on axle end] and heim style ends at the front of each radius rod-[each being attached] to a point at the side of deuce frame rails with a 3/4" pin thru the frame in a weldin tube mount..........it had a model A rear Buggy-spring....
everyone said I would die...
It has since went thru 3 new owners in California over the last 6-7 years,seems to still be working fine.
I beleive how it worked on the Roadster was that the suspension was limited to about 2-1/2" total up and down travel by the HD GAS-shocks....an important factor in limiting those stresses.
Now thinking back if I had uses lever shocks or friction shocks instead it may have had more chance failing as a result of more up and down travel in the setup....

 

I was just thinking of something like that yesterday, while doodle-ing chassis diagrams. As long as the halves couldn't EVER come appart! Gary

 

Lever shocks and hair pins! Urethane bushings on the rear end and tie rod ends at the front. Building a car with a four bar front end with urethane bushings on the axle and rod ends on the frame to eliminate any binding. Comments?

 

"And yet,.. they have used this system for decades,.....

To bad somebody didn't tell these guys it shouldn't work......."



Kinda what I'm thinking right about now. Limited suspension travel, flexible couplings at both ends, light car, hairpin flex......seems like it should work.

 

To get away with this for any length of time, do two things: retain the torque tube, and use what racers call the "fifth spring", a frame so flexxy it will bend before the suspension breaks.

Sprint cars still use the torque tube, but the torque is not engine torque, but the attempt of the axle to rotate in reaction to braking or acceleration. The arms, which appear to be hairpins, on the later race cars, are attached to sleeves or "birdcages" which allow axle rotation without applying bending force. The tube controls axle rotation, the hairpins, or radius rods, locate the axle fore and aft, and a dead shackle on the spring provides side to side location.

 

My Track T has '36 bones mounted solid to the axle and tie rod ends on the chassis. Running a '40 front spring on the rear. Rides rough but I'm tough. Broke one once where the forged end attatches to the bone itself, but blamed that on rough-ass Nebraska roads. Local blacksmith welded it up, still going strong. Car has around 40k miles with this set-up.