The rhyme is catchy but just you try finding something useful on the internet using "kinking linkages" as search terms. The correct technical term is offset. Chances are that sooner or later you'll find yourself needing to introduce an offset into a tensile/compression element, to clear something else. Experience leads me to expect that there'll be two schools of thought around offsets on the HAMB. On the one side I'd expect those who believe that tensile/compressive elements absolutely need to be dead straight, regardless of their cross-sectional dimensions, or the part will immediately fail, causing your car to explode killing everyone in a three-block radius, and you wouldn't want that on your conscience, would you? On the other I'd expect those whose buddy's cousin had a linkage shaped like a Slinky and it worked perfectly for years, no matter what your highfalutin' laws of physics say. In between there will be a few who recognise that offsets can work provided you make some adjustments to details and section sizes, and that those adjustments may come at a cost which may or may not be worth it. Two inevitable consequences of introducing an offset are that the part will be heavier, not only because it is now longer along its path but also because dealing with a bending moment which hadn't been there before might require a heavier material section; and that the offset will want to rotate about the part's axis, because the part's centre of gravity is no longer on the axis, meaning that a part with spherical joints at both ends will now require a compliant hinge at at least one end, or some other kind of aligning device, to keep it from flopping about. I was hoping to find a handy rule-of-thumb, because I like the broad accessibility aspect of rules-of-thumb, but no such luck. I think there are too many variables, which moreover influence each other: cross-sectional area and section modulus, which are interdependent, and the perpendicular height of the offset you need. The math itself is simple enough, unless you want to get into stress distribution ****ysis within sections and that kind of thing. In a nutshell, the sum of tensile stress and bending stress needs to be within the capability of the material used. To give an idea: I need to introduce an offset of maximum 85mm (±3¼") into a Panhard bar to clear an exhaust system throughout suspension travel. A straight solid round bar of about Ø8mm would be sufficient to deal with the tensile loads but would obviously be way too slender to handle compressive loads of the same magnitude, and we need a Panhard bar to do both. A straight circular hollow section bar of Ø20mm x 2mm might be rigid enough for compression but it is good for almost twice the required tensile stress. Even so, the 2mm wall thickness might make it a bit delicate in practical use. Beefing it up enough to make it practically robust would mean a degree of overdesign, so much that it is possible to introduce a certain amount of offset without having to go to a heavier section. It turns out that I'd have to go way past the point of practical robustness to get an 85mm offset to work. It ended up as a Ø43mm x 4.5mm circular hollow section — and it weighs about 4.8 times as much as the dainty Ø20mm CHS I started with. It's a 3.5kg (±7.7lbs) part instead of a 750g (±1.6lbs) part. Whether that extra 2.75kg, about half of it unsprung m***, is worth it is a judgement call. But that gives you an idea or the orders of magnitude involved. To keep the Panhard bar from clunking over and resting against the fuel tank on acceleration and then clunking back and resting against the rearend on braking, the bar would need to have a compliant cylindrical bushing at least at one end.
A solid bar (not being hollow) in about the same outside diameter as the original bar would probably do the job (though that is a pretty big offset), but the bends would likely need to be heated to accomplish the job. If the bar is heated to bend, it will need to air cool to retain the strength. I don't know how much weight you would save over the larger outside diameter hollow bar, but a solid bar increases the strength substantially over a much larger outer diameter hollow bar. As the hollow bar increases the outer diameter, it increases the the amount of flex then empty center space will allow. Just hands on experience, there may be physics to back it up, or not...
I would normally use observation to come to a rule of thumb, erring on the side of overbuilt. Small loads light weight = throttle, kickdown, auto shifters. Harder loads = manual shifter rods full vehicle loads I'd look at other suspension parts. Most will be straight. I can't imagine ANY auto maker offsetting a suspension part 3 inches to clear exhaust. The priority of suspension in stiffness, cost and unsprung weight would require this.
This is a screenshot of a panhard bar that I sold many of when I sold circle track parts. The ones I sold were not from speedway, but very similar. If memory serves correctly it was 1/4" wall tubing. Needless to say, in dirt circle track racing these panhard bars which were mounted to the left side ch***is rail and then to the right side of the pinion on the rear end took a tremendous amount of load and I never knew of one to fail. Lynn
Well, my example was just that, an example by which to give an idea of what an offset of that kind of height would entail. Having said that, I'm seeing S-shaped, J-shaped, banana-shaped, wave shaped links all over modern cars in the quest for tight packaging and, doubtless, strategic Chinese-puzzling. Indeed, Citroën 2CV suspension arms of 1948 are J-shaped links, offset to clear steering front tyres, as to a lesser extent are cl***ic Mini rear arms. I'm sure there are others. Every semi-trailing arm from the '70s? Europe was full of them.
Since you bring up " packaging limitations" that came into play redoing's rear suspension on my T roadster. I switched it from wishbones to a modified triangulated four bar to allow for articulation of the suspension because it was trying to twist the whole car when going through a dip. For packaging reasons The bars that served as the angled bars for the the wishbone style triangulated 4 bar had to have about a 30° bend in them to clear the frame rails. In the case of that roadster, it was only handling the torque of about 160 or 180 horsepower engine. But, the tubing I used was only .134" or .146" Wall thickness 1" OD DOM tube and it served the purpose very well. Personally, I think a lot of things in the construction of hot rods are often overbuilt. Granted, I'm no engineer so take my comments with that in mind. Lynn
If I understand the premise, I ***ume you are talking about the moment introduced as a result of the offset. If so, wouldn’t that moment (magnitude) be the same in either direction? It’s been over 35 yrs since graduating from eng school, so brain fog is big. Nominally, I recall a hollow tube section being stronger than an equivalent solid section, but not sure of the conditions under the which that would be true.
The example is just off the top of my head; it's not a real situation. But that is the sort of question which may arise: wouldn't a Watts linkage or a Mumford linkage be better here? Though I can think that the Watts linkage would be no better, because the rocker would probably be exactly where we'd have wanted to offset the Panhard bar. Edit: a Watts linkage with the rocker pivot on the frame rather than the axle might work, come to think of it.
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