In the course of figuring out the suspension on my '31 project (http://www.jalopyjournal.com/forum/showthread.php?t=374184) it struck me that a tube axle with a central articulation that allows the two ends to rotate relative to one another about an axis parallel to the axle would be able to work with split wishbones without any unwanted effects. I didn't pursue that line for the '31, because I couldn't think of a way to fabricate such an axle without it being somewhat heavier than I'd have liked. It would have a straight middle section about 30" wide, comprising a smaller-diameter tube attached to one axle end running on needle bearings inside a larger-diameter tube attached to the other axle end. The smaller tube would have a sturdy threaded stub welded to its free end, that engages with a threaded bung welded into the larger tube, or some other measure to prevent free axial movement, and there would be a grease seal at the appropriate place. The whole ***embly takes batwings like any conventional axle, and can be sprung and located like any other axle, only it would allow a bit of free rotation between its two end and therefore render split wishbones, non-parallel four-links, or even ladder bars geometrically pure and finely tunable. Pros and cons? Worth the effort? If anyone wants to pursue the idea, there it is.
I'd be a little nervous about the beating the threads in the axle retainer would be taking. Interesting idea. Like to see some real-world statistics on tube axle failure in the split bones configuration other than engineering specs and theory.
If I understand what you are trying to do this might be and easier and well proven way. CAE (Culbert Auto Engineering ) sold many tube axles in the 60's for Sprint Car and Modified racing . This axle incorperated a a captured sleeve / batwing which allowed the LH hairpin to rotate freely on the axle tube.
I'd rather like to see the state of split-wishbone clevises that have been used on tube axles. My su****ion is that the holes would go oval and the bolts wear loose before the axle fails. I reckon the threads in my Morris Minor's front suspension trunnions take much more of a beating: they take vertical impact loads from the entire front half of the car. They're reliable as long as they're kept greased. The Aero-Willys used a similar set-up, with success. That's what got me thinking about a threaded coupling. It'd be a lot simpler than thrust bearings and long bolts, for instance.
That's the approach I ended up pursuing for the '31, except with a shorter batwing and a shackle rather than a sleeve. The twistable-axle idea arose out of the process: it's a bit of a tangent but perhaps worth a look.
Needle bearings in a two piece axle would be needlessly complex, although it would get you the degree of freedom you are looking for. For 3 inches of travel with a short 36" bone, the axle will twist about 10.8° from normal. Two rubber bushings in the batwings on each side could take up this travel. If you are really worried about it, you might just use an I beam. Or have an I beam section in the center of the tube axle something like an Anglia.
Simple solution if you make your split wishbones radius rods and use your center link as the third link of a three link.
Bugatti type 57's used a threaded coupling in the center of the axle (similar to a pipe union) Each tube bung was threaded with RH threads. A internel threaded coupler held both sides together then safety wired the outer sleeve to prevent it from spinning off. Simple but very effective. I am going to do this with a V8 tube axle and '36 split wishbones on my '34 speedster.
VW front beams use something along the same lines where the trailing arms enter the tubes. A simple Grub screw grabbing the torsion leaves is all that keeps the trailing arms from pulling out of the housing! Consider this: You fabricate a tube axle with an internally sleeved joint in the middle...including a thrust surface to limit how far the axle halfs can slip together. It's stable and rotates freely...but has no simple way to be held together. I would handle it this way... Inside the axle tube you insert a splined sway bar and Grub screw the two ends to Splined internal receivers on either side, but nothing in the middle to limit rotation. Result: You get a tube axle without terminal bind but with adjustable roll resistance...AND the axle halfs are held firmly together by the solid end mounting of the internal swaybar ends. It would look like a regular tube but would give no extra stress to the welded bat wings etc. Various recievers could be used to allow larger or smaller sway bars, depending on the required roll resistance...AND the internal swaybar is totally invisible. If thats too complicated a simple connector link, almost like a tierod, could be used to hold the two halfs externally. Mounted behind, but close to the axle, it would almost be invisible to the casual looker. No internal thrust bearing surface would be needed inside the axle either...and by not having one the minor contractions of the working axle would result in no binding from the connector link. Personally, I'd rather the adjustable swaybar, but no swaybar would be slightly easier, IF thats the driving force. (shouldn't be!) I can think of a dozen other ways to do this too...all very workable and simple. With regards to a rotating "cuff" on the axle...SUPPOSEDLY it can cause the car to pull one way or the other in hard braking due to rotational flex and "weight jacking"...for want of a better term. Also...the solidly fastened radius arm would be controlling ALL rotational forces from the brakes, so would need to be upsized to handle the strain. An interesting idea Ned! Gets the juices flowing!!! LOL
Fatman's Vintage IFS you mean? What Ned is considering isn't IFS. The axle isn't BENDING in the middle like the Fatman's IFS. It's rotating around itself to prevent binding. There's no change in Camber as there would be with the Vintage IFS.
Thanks, I'll do a search for Bugatti T57 axle info. Great minds, eh?! (or fools ...) Thanks, Bill. Getting the juices flowing is what I started this thread for. The more I think about it the more I'm sure I can get the weight penalty to go away by looking carefully at wall thicknesses. It shouldn't be that hard to get the combined section modulus of the concentric bit to equal the original tube. It can even be reduced a bit because the bending moment is going to peak at the wishbone/perch bosses and be less everywhere else. There will be an optimum length for the concentric bit: my guess is that it's probably nearer 15" than 30". The whole might even end up being lighter than a solid tube. Edit: I'm not thinking. The bending moment will be constant between the wishbone/perch bosses, and that supports my initial intuition of having the concentric portion run pretty much all the way between them. What I said about the combined section modulus still holds, though: the weight penalty should be reduceable to a negligible amount.
I'm not knocking you, buy why over complicate something that is already clean and simple? I beams look better too IMHO. Doc.
Doc, variety is the ****e of life. This is another way of doing things, and not a replacement for anything that's there already. The more, the merrier, I say. There are cases where an I-beam isn't an option anyway, like where clearance or dimensional issues mean the axle has to have a different shape. Then the options are fabricating an ifs or fabricating a tube axle. I don't know about you but I don't have steel-forging or even steel-casting facilities at home. And if I'm going to be fabricating anyway I am free to incorporate all kinds of stuff.
Fair enough, I didn't think about the limitations of space on a particular build. Would like to see what you end up with. Doc.
G'Day Ned , I have built a couple of tube axles in the way you describe for the owners of vintage race cars here in Australia one being an Allard and the other a Packard racer using quarter elliptics and wishbones . The way I went about it was ....the axle tube is 2" OD x 1/4" wall cold drawn seamless tubing , one side axle has a machined pin insert that fits into a machined bronze bushing on the other axle , using a sprung wave washer in between , the whole lot is held together with 6 - 1/4" roll pins , the unit is greaseable , I'm going back 25 years I forget the finer details , they worked very good and the customers were very extremely happy another set up that I had made quite a few of , one being Fred Cav***ini's '34 coupe.....was the opposed mounting twin axle .....ie the left axle pivots from under the right ch***is rail , the right axle pivots from under the left ch***is rail , which also works extremely good if set up correctly cheers Steve in Oz
The Jalopy Journal
