Where structural tortional rigidity is unavailable it makes sense to design with high roll centres and low roll stiffness, perhaps with some sort of front-rear interlinkage. Most solid axles, like beam axles, live axles, or DeDion axles, may be located by means that place the roll centre near to the
physical axle. A roll centre any distance above or below this would require an impractically bulky, and probably heavy, arrangement.
The following is hard to describe verbally. Consider the length of a vehicle to represent x-axes, the width y-axes, and the height z-axes.
A fairly rigid tube is mounted across the frame, on pivots that allow it to rotate or swing about a y-axis. To the ends of this are mounted adjustable hubs, with rotary Vernier plates, with their axes normally substantially on x-axes. Rotation of the tube will allow the angle of their axes to vary in the x-z plane.
Each adjustable hub incorporates a pivot shaft at a right angle to its own axis, so that different configurations of the Vernier plates allow different orientations of these pivot shafts in (normally) the y-z plane. The point where the pivot shaft axes intersect will define the roll centre.
Each pivot shaft serves as base to an A-arm similar to those found in conventional dual-A-arm systems. The apices of the A-arms are pivoted to the axle. The axle will require another longitudinal link, or two, to the frame; that is, if one considers a 4-bar-and-Panhard system, this proposal replaces two of the bars and the Panhard bar.
In bump, the linkage rotates about the lateral tube. In roll, only the A-arms pivot. Note that the roll centre does not migrate but remains constant in relation to the chassis and, therefore, the CG. It moves fore and aft slightly with bump motion, but that does not affect anything.
This will work best where the desired roll centre is fairly far from the linkage itself, especially if that position is occupied by something that cannot be relocated. Where the desired roll centre is near the axle a Panhard bar, Watt's linkage, etc. would be better, as this system runs out of A-arm travel over a small zone where the A-arm pivot axes near y-axes. One would need these pivots to be at least about 5 deg off y-axis.-- Ned_Ludd,
Jan 25 2007
Here's a drawing:
http://i88.photobuc...oetzee/3link-01.jpg [Ned_Ludd, Jan 29 2007]
http://i88.photobuc...oetzee/3link-02.jpg [Ned_Ludd, Jan 29 2007]
Sorry, you lost me at "Vernier plates." Maybe you can post a picture or something. Is this a different form of independent suspension, or is it just a method for locating a solid axle? I honestly know very little about solid axle suspension.-- discontinuuity,
Jan 25 2007
It's specifically for a solid-axle set-up. I'm rather fond of lightweight solid axles, and think they deserve a second look. Elements of it could be used on an independent set-up, losing the lateral tube, e.g. a dual-A-arm crossmember with a severe anti-roll-bar that can pivot as a whole on the linkage's A-arms, but it becomes cumbersome.
The Vernier plates I've got in mind are circular plates with a ring of holes around the perimeter, say, 48 holes in one and 47 holes in the other, held together by two bolts: a big one in the middle and a small one through one of the perimeter bolts of each plate. There are therefore 2256 different angles at which the A-arm pivot can be set. I've done some drawings I can e-mail, if you can give me an address.-- Ned_Ludd,
Jan 25 2007
//and think they deserve a second look.//
and this is where you lost me. Solid axles are not generally a good choice except where extremely high load carrying or strength are needed and even then they are limited even extreme duty vehicles such as Hummers have abandoned solid axles.
Independent systems almost universally can be designed with lower weight, reducing unsprung weight, increasing performance. They also by design provide better suspension geometry allowing better handling as well.
A big fishy solid axle for you.-- jhomrighaus,
Jan 25 2007
Yes, [jhomrighaus], everyone knows that, so it must be true.
Independent systems generally require a lot of torsional rigidity, and present a nasty choice between a roll centre pretty much on the ground and unwanted camber/track changes. That is a problem in the sort of application I have in mind.
Solid axles are not necessarily the crude things you're thinking about. Unsprung mass comparable to strut or modified-semi-trailing set-ups are quite possible without recourse to exotic materials. They do, however, make short packaging difficult, and preclude cobbling a good ride quality by shoving the engine between the front wheels to increase polar moment of inertia - which was the real reason ifs was adopted.-- Ned_Ludd,
Jan 25 2007
+ for drawing, even though I have no idea
what it does-- xenzag,
Jan 29 2007
Based on your drawing it appears that the axle will slide from side to side under a roll condition causing a shift in the center of gravity over the wheels, which could destablize the vehicle.
Also while articulated it would appear that bump motion will be restricted and cause additional stress on the mounting points of the crossbar(leading to increased strength and weight requirement)
Even having seen it on paper how does this reduce the inherent issues of weight in a Solid axle situation nor does it address the issue of tread orientation in a roll or articulation condition.
Also if memory serves systems utilizing a "leading" arm arrangement tend to ride very harshly compared to a trailing arm configuration.
What is the benefit of this system over a conventional 2 or 4 link suspension? The axle weight will be the same, the a arms and pivots would be similar if not identical in weight and the cros bar would presumably be heavier as it does not exisit in a 2 link/4 link arrangement.-- jhomrighaus,
Jan 29 2007
So this is a front axle? I don't think many cars have had solid front axles for fifty years or more, and for good reasons. I don't really understand your drawing just by looking at it, but it might work for a rear axle better.-- discontinuuity,
Jan 29 2007
Perhaps [Jhorm] would disagree with me, but in the 4wd scene, the toughest (not toughest looking... with chrome everything and big subwoofers...I mean the ones that are still alive after being used as bull catchers for a few years) axle seup is still solid axles... Front and back. This is perhaps not the issue being debated here, but I just don't like the comment
/even extreme duty vehicles such as Hummers have abandoned solid axles./
...I don't think Hummers are particularly tough, from what I've seen and heard. They're just marketed well.
...I think there are still plenty of uses for solid axle geometry in modern 4wd vehicles, and am increasingly frustrated by car companies that convert their so-called tough 4wd's to independent suspension, that has been proven time and again to be fragile and susceptible to fatigue. I chose to buy a 10 year old 4wd and renovate it, as opposed to purchasing a new vehicle, for this reason amongst others.
/end rant.///-- Custardguts,
Jan 29 2007
In that application a solid axle is an outstanding choice. As to Hummers I was speaking towards the real deal not the road queen poser H2 and H3 but the real Hummer which is arguably one of the toughest most capable off road vehicles out there. They are at least on par with their Jeep ancestors.
As I stated for extreme strength and durability nothings better than solid axles but when it comes to handling and performance as most vehicles are exposed to it IS is superior. Baja 1000 Trophy Trucks use IFS with a solid rear axle and IS systems have been a part of all major rally vehicles for decades.
For rally cars(WRX, Quattro etc.) you would be hard pressed to locate a single vehicle equipped with anything but full independent suspension systems and those cars can handle just about anything.
I believe that almost every exotic high performance car on the road today(Lambo, Porsche, Ferrari, Lotus, Jag, BMW, Mercedes etc.) uses some form of independent suspension system. And outside of heavy trucks no-one produces a solid front axle vehicle at all.(Im sure theres one somewhere so bring it on you naysayers)
That being said a properly designed heavy duty Independent Suspension System can out perform a solid axle but at considerable additional cost, hence its lack of prevalence in the regular 4x4 market.
I would never be so stupid as to imply that an H3 or a Dodge Ram of 2007 has anything in common to a Solid axle Bronco, CJ or Land Rover of yesteryear when it comes to strength and durability, but both of those vehicle will handle, corner and brake better than anything on the 4x4 circuit which is what they were engineered to do.-- jhomrighaus,
Jan 29 2007
Look, this idea is not about solid axles for Ferraris. It is not about front axles or rear axles as such. It is a locating linkage that happens to work specifically with a solid axle, wherever you might find one. For all I know it might find applications in sidecar outfits or skateboard trucks: I don't know. Its advantage is that you can peg the roll centre exactly where you want it.
Having said that, I believe that a reading of technological history in terms of simple linear progression is just plain wrong. There are lots of reasons why ideas are abandoned and other ideas adopted, and many of them have nothing to do with the performance of the specific device in question. It is often useful to take a look at old ideas when other parameters apply.
For instance, a lot of current suspension thinking is indeed about getting an independent suspension system to act like a solid axle, because tyres have changed. Newer tyres are much more sensitive to camber changes than the early radials around which early independent suspensions were designed.
[jhomrighaus]: the axle will not slide but rotate about the roll centre, which may be located some distance above or below the axle itself. The motion will have some lateral component, but this is not unusual. The same happens in any system. Indeed, with a relatively high roll centre, the lateral CG migration will be less than with a conventional low roll centre.-- Ned_Ludd,
Jan 30 2007