How safe are split bones on rear?

I'm planning to use split bones on the front of my first rod but have heard negatives about using them on back.A number of folks have claimed they'll break due to binding since the rear axle has no twist to it like an I beam front axle.I'd like to go with the same look on the rear but not at the sake of safety.If it makes a difference it will be a lightweight T on model A rails.What do you think?Tried 'em?What was your experience with 'em?

 

If you're going to stick with the banjo and torque tube, you shouldn't have a problem. Check out all the old track roadster pics plus look at any modern sprint car.

 

Wishbones on the bottom, top link, and gussets.

 

Zombie, do you have any more pics of this setup? And, are you running closed driveshaft or open with a torque arm?

 

I'm running a quick change with an open drive. No need for a torque arm with the top links.

I'll look around, but I don't think I have any other pictures.

 

A lot of guys have done it a million different ways. Anecdotaly, they say it is no problem. But I believe that you are raising a valid concern.

The point is that you need to provide for articulation. In the worst case, this is one rear wheel all the way up and the other all the way down.

On a drag car, this type of movement is not a problem. On a street car, it is, for example when making a right into a steep driveway. In reality, however, even the body sway you experience driving around a corner makes the suspension articulate.

It is easy to see that the stock Ford banjo rear with a torque tube and bones connected at the front can handle articulation no problem. You are just tilting a big triangle, and there is no interfference movement at all.

Split the bones, and you are not simply tilting a triangle any more, plus you are introducing interference motion. By that I mean that the various parts are moving in along different arcs, and that is what the twist of an I beam front axle can accomodate.

IMHO the best setup is to run a closed driveshaft with unsplit rear bones. That is not always feasible, so whatever it is you end up with, the closer your setup is to that ideal 'tilting triangle', and the more you can reduce the amount of interference motion, the better off you will be.

So, for one, mount the ends of the arms as close to the front ujoint as you can, and at the right height relative to the tailshaft and angle. Also, use resilient bushings (soft rubber), not heim ends or stiff urethane bushings.

As soon as you introduce interference motion, something yields or twists. That is a physical fact. If the suspension setup has too much interference motions and/or is too stiff (resistant to twisting) then something is eventually going to yield.

It might be that many rods are not driven enough to get to the point where something bad happens, or maybe it does and something else gets blamed.

We really have two extremes when we are talking about a hot rod rear suspension with immunity to these binding suspension movements. One is the stock buggy spring and torque tube. The other is parallel semi eliptic springs. Anything in between has SOME measure of bind and so the design challenge is to minimize it from happening at all PLUS selecting components (like soft bushings) that accomodate the movement.

It's a long answer and perhaps not what many would like to hear, but I believe the facts I presented are correct.

Again, I realize that there have been many rods built with all manner of rear suspension setups, but the cold hard truth is that no one really knows how they behave over the long term because we don't test for that.

On my rod, I have the torque tube and unsplit bones and I would not change that configuration.

Lastly, those of us who prefer traditional rods have an easier time of it. For example we can more easily justify the stock setup. The billet boys with their drag-car four bar setups and panhard bars have interference motion up the yin yang.

You can measure the interference motion of a particular setup. It takes work as you have to set up dial indicators or protractors or other means to measure the twist that the various parts experience, plus you need a way to get one wheel up and the other down. PITA to set up but the results could amaze you in terms of what components are subject to what kind of stress, and what other parts are flexing or twisting.

 

Dale,I'm planning on running an open drive but don't really see what that matters.As the car leans in a turn the forward ends of the bones will still be getting bent in opposite directions(one up,one down)which is what I'm worried about.PLYM,you make some good points on keeping the ends toward the chassis center and using flexible bushings.Thanks for the run down on the situation.I was a machinist in the army so I'v got a pretty good handle on tangents,arcs,etc,but it's good you confirmed my thinking on this was correct.

 

Here is my set up on my T, I used rod ends for the split ends but plym49 got my rethinking them. Still need to figure out a torque arm though.

 

Do a search we have covered it many times.

The stock Torque tube carries the load (torque) the bones just locate. If you remove it, the bones will BREAK. You need a torque tube, torque arm, gusseted bones like those pictured above by Zombie or upper arms like in a 4 bar setup.

 

Here is a little info on the torque arm...http://www.jalopyjournal.com/forum/showthread.php?t=379082&highlight=torque+arm

 

Your picture helps you visualize the tilting triangle pretty well. Imagine one rear wheel going up and the other down. Those bones will want to twist. Probably, they can't or won't, because the front or rear mounts might be more easily twisted. Say it is the rear mounts that flex the most. Eventually those brackets can fatigue. Make them stronger, and you simply move the flex to somewhere else - it can't be eliminated, because the geometry has interference motion built in.

The longer the bones and the closer to the centerline of the car, of course, the less twist as it is spread over a larger area.

If your front ends can twist adequately you might be OK.

Tack some welding rods to the front and rear and middle of the bones so that they point straight up in the air, like an antenna, and all 3 in line with each other. Now move one rear wheel up and the other down with the car on jack stands. By how the wires move you will easily see what/where it is twisting. Once you know that you can figure out what if anything needs to be done about it.

 

I've done searches and they leave a lot to be desired.If you put in "model t" with a space between model & t you get results for just "model" for instance.Plus every result does'nt yield relevant info.I also thought this was a subject that could use some discussion as I've seen some set ups that looked down right scarey.

 

I'd argue one point. If your componants are stought enough it won't twist. What will happen is the ride quality suffers. When one rear wheel goes up, so will the other (picture a VERY heavy sway bar). The rear axle ends up with only one plain of movement, straight up and down. An advantage for a drag car, but ill handeling on the street. Years ago I had a pro street car with ladder bars. Taking a driveway at an angle would practicly lift the outside rear tire off the ground, the only reason it didn't was the front suspension took the angle change (front sway bar removed), the right front would lift instead. In my case the front suspension was the loosest part of the car, rear axle wouldn't flex nor the caged chassis. Drove it for years with no problems, just didn't try to road race it around turns. The issue I think would be having a weak link elsewhere like light weight bones or a flimsy unboxed A chassis, anything that will be metal twisting and fatiguing. A stiff front suspension like a staight axle with split bones I'm sure adds issues moving the flex elsewhere.

 

Try this then type split+wishbone OR "split wishbone" Torque+arm is another great seach phrase. The + and or quotes keeps your words tied together. I can go find DOZENS of threads on the various methods of replacing the torque tube.

 

split+wishbones got me 495 threads. Many on the first page looked VERY relevent to your question. Good luck in your quest.

 

Metalman,what you say is true but the components are still under the same stress and will fail eventually.Til then you're looking at a lousy ride from lack of articulation and excessive unsprung weight.T-man,thanks for the search tips.I'll give it a try.

 

An interesting rear suspension idea might be to pair the Ford buggy spring with a pair of quarter elliptics. You'd have the articulation of parallel leaf rear springs but in a chassis that still ended at the rear crossmember.

 

Yeah,but I'd want 'em mounted somewhat inboard so they wouldn't be an eyesore and that would limit or eliminate their effectiveness.

 

You'd have to have the ends of the quarter springs meet the shackles where the buggy spring attaches. They could be right flush to the outer frame rails.

 

What motor trans are you using?
What rear axle ?

 

Here's a shot of my old roadster with a four link using '36 bones as lower links.



I ran it for some time as a true split wishbone and broke things twice, not due to engine torque but because of the normal side to side stress of street driving.

 

I don't think so. Yes, something will fail eventually but what I'm saying is properly engineered the part will fail that was designed to fail. For example the consenses is stock bones on the rear are not strong enough to take the stress and will fail so we see examples of reinforced bones. Now the bones will take the stress forever but you have moved the forces somewhere else (to the mounts), more in likely to a rubber bushed front mount. Eventually the rubber bushing will fail, but then the factories even design that into cars. Unless damaged by accidents or rust you never see a factory control arm fail but the rubber bushings will and sooner or later need replacing.

 

331 Caddy/Hydramatic/'40 Banjo

 

Nimrod & Blasted,OUCH!This is the kinda carnage I'm trying to avoid.I'm hearing some pretty good ideas here.Thanx guys.

 

Like this. I'm building another almost like it right now.

 

Here's what happened to me.

We have a stock '48 ford rear end with stock unsplit wishbones. However, we converted to an open drive line. Twice this past summer we broke the wishbone mounting bolt. Once during hard acceleration and the other under not so hard. We have it mounted so the wish bone mount is above the cross member so it can't hit the pavement. VERY unnerving when it happened, but no damage.

It appears that when we removed the torque tube we lost some of the rigidness of the unit and the twisting effect from the rear axle torque snaps the bolt like a kung fu chop.

I need to recreate the rigidity of the tube....

 

Here's a shot of the rearend showing the upper and lower mounting brackets.



I wouldn't ever run split bones on the rear of any street driven car.

 

Not really what I meant. You've got quarter elliptics with radius rods. I meant quarter elliptics with a buggy spring.

I'm not sure how your setup will do with articulation. Looks like you might be stressing the bones when it articulates. Also, with your setup will you need a panhard bar? Not sure if the quarter elliptics will handle the rear side to side since they are floating.

With a quarter/buggy setup, you have three points of contact so you keep the triangle. The buggy spring locates the rear side to side. The leaves of the quarter elliptics can flex to handle articulation. Extra benefit: no bones under the car to interfere with the exhaust.

Another way of describing what I am talking about is if you keep your quarter ellitpics, lose the bones, and add a buggy spring and rear crossmember.

 

i broke the stock radius rod's twice after going with the open driveshaft.
once just making a U-turn.
rich