The Mark Ortiz Automotive
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Mark Ortiz Automotive is a chassis consulting service primarily serving oval track and road racers. This newsletter is a free service intended to benefit racers and enthusiasts by offering useful insights into chassis engineering and answers to questions. Readers may mail questions to: 155 Wankel Dr., Kannapolis, NC 28083-8200; submit questions by phone at 704-933-8876; or submit questions by e-mail to: firstname.lastname@example.org. Readers are invited to subscribe to this newsletter by e-mail. Just e-mail me and request to be added to the list.
THIRD SPRING CONFIGURATIONS
What are the differences in the characteristics of a 3rd spring that is actuated by swaybar movement, one that is actuated by a pivoting T bar type swaybar, and one that is hydraulically actuated? I have given it a bit of thought, but don't have it fully defined. I think hydraulic actuation with linked hydraulics would fight a swaybar... I would like the thoughts of a real expert on the subject.
I always remind people that the tires have no eyes, and only respond to the forces they are subjected to. They don’t know or care what the suspension system looks like. Often, the specifics of a particular design or setup count for more than the general conceptual nature of the layout.
That said, there are certain possibilities and limitations governing the forces we can generate with different types of layout.
Some readers may not know what a third spring is. It is a spring that acts on both wheels of a front or rear pair together. It acts only in ride, and not in roll. It is generally only found in cars with ample downforce. However, it has some other possible uses as well.
It might be a good idea to review the modes of suspension movement.
A front or rear suspension system, or half-car, has two modes:
1. Ride: synchronous motion of the two wheels; both moving together
2. Roll: oppositional motion of the two wheels; wheels moving opposite directions
Any possible displacement of the wheel pair can be expressed as a ride displacement and a roll displacement.
A four-wheel system, or full car, has four modes:
1. Heave: synchronous motion of all four wheels; synchronous ride at both ends
2. Pitch: oppositional motion of front and rear; oppositional ride at both ends
3. Roll: oppositional motion of right and left; synchronous roll at both ends
4. Warp: oppositional motion of diagonal wheel pairs; oppositional roll at both ends
Any possible displacement of the full car system can be resolved into some amounts of heave, pitch, roll, and warp.
Back in 1997, I had a set of two articles in Racecar Engineering which explained this, and more, with illustrations. I have for years been offering photocopies of these articles at $4 for the pair. I now have .pdf’s of these (from the black and white photocopies) for the same price.
The car will have a definable pound per inch (or N/mm) rate at the wheel (wheel rate) in each of the four modes. These rates will vary somewhat as the suspension moves. For some purposes, it may be desirable to have some of them vary dramatically.
Any springing device that acts on two or more wheels can be considered an interconnective spring. Interconnective springs can be arranged to elastically resist synchronous motion of the wheels affected (to be an anti-synchronous springing device) or to elastically resist oppositional motion (to be an anti-oppositional springing device).
Springs that act on individual wheels elastically resist all four modes of movement.
Any interconnective device that affects a wheel pair resists two modes, or in some cases just primarily two modes. Anti-roll bars (swaybars) resist roll and warp. Third springs resist pitch and heave.
It is possible to interconnect interconnective devices. By doing this, it is possible to get elastic resistance to only one mode, or primarily only one mode.
Third springs normally take the form of a coilover acting on the top end of a T-bar style anti-roll bar in a pushrod and rocker suspension. The T-bar is a vertically mounted torsion bar with a transverse member across its top end. At its lower end, it is held so it can rock fore and aft, but not rotate or rock side to side. The T-bar is operated by a secondary set of pushrods from the rockers that operate the main coilovers. The bar rocks freely in ride, but is twisted in roll. The third coilover is attached in the middle, at the top of the torsion bar. It is displaced when the T-bar rocks, but not when it twists. It often has a rubber snubber, to give a steeply rising rate. Sometimes the snubber is the only spring. Sometimes dual-rate or rising-rate coil springs are used.
A hydraulic third spring generally acts on shaft-displaced fluid from the main shocks. It is essentially a common remote reservoir, with a stiff spring of some kind. This works fine. It is a bit more difficult to provide a motion ratio for the third spring different than for the individual wheel springs. If large forces are required, the pressures can get quite high, as a typical 5/8” shock shaft only provides about ¼ square inch of piston area.
I don’t recall ever seeing a third spring on a passenger car style anti-roll bar, but I can imagine ways to do that. One possibility is to add a third arm in the middle of the bar, and arrange a snubber or coilover to act on that. This puts the third spring in series with the two halves of the anti-roll bar, unlike more normal third spring setups, where the third spring and the a/r bar are in parallel.
It would also be possible to add a Z-bar (like an anti-roll bar but with one arm pointing forward and one pointing back), acting either on the anti-roll bar arms or directly on the control arms or axle. It would be possible to use a “swing spring”: a transverse leaf spring mounted so it can pivot in the middle. This could also act either on the a/r bar arms or directly on the control arms or axle.
We can get any instantaneous wheel rates in the four modes that we want, using only individual wheel springs and either anti-roll bars or third springs. Why use both a/r bars and third springs at once? Why use both a device that acts in roll and warp, and another that acts in pitch and heave, when we already have individual wheel springs that act in all four modes? The answer is that we may want different rates of rate change in the various modes. Spring rate, or wheel rate, is the first derivative of force with respect to displacement. The rate of change of that is the second derivative of force with respect to displacement. We may want to tailor that differently for the different modes.
When winged race cars first appeared, it became desirable to arrange for the suspension to stiffen as it compressed. Designers first tried arranging for increasing wheel rate from the individual wheel springs. This had the disadvantage that the car’s elastic roll resistance distribution became highly pitch-sensitive. Third springs afford a way to get a steeply rising wheel rate in pitch and heave, without having the same in roll.
Does a hydraulic third spring fight an anti-roll bar? That is, does it not only not resist roll but actually create a pro-roll effect? Generally, no. It would actually be more likely for such an effect to occur with a coilover acting on a T-bar. If the coilover attaches at a significant distance from the center of the torsion bar, it could have an over-center spring effect that would actually try to add roll displacement once some roll displacement occurred. There would also be bending loads on the torsion bar. There would have to be some means of laterally constraining it at its top end, while still letting it move fore and aft. I’m not suggesting there would be any reason to want to do this.
It is very common for people to imagine that anti-synchronous spring devices promote roll, rather than merely not resisting it. They imagine that the device pushes the inside wheel down when the outside one comes up, so it must actually add roll. It doesn’t, though. It just freely permits roll, while holding the car up.
For passenger cars and trucks, and even for race cars that carry large fuel loads, it is desirable to have overload springs, usually at the rear, that act only in ride. If the overload springs act also in roll, rear roll resistance increases as the rear gets heavier and the springing gets stiffer. This aggravates the tendency for the added rear weight to add oversteer.
The most common way to add ride rate without adding roll rate is a set of air shocks that fill from a common valve. However, metal or composite springs that act only in ride and have a rising rate offer considerable potential for rear suspensions of trucks and cars – especially front-drive sedans, which today commonly use rising-rate individual wheel springs.
Hydraulic interconnection of the shocks at one end of the car is okay, but the biggest benefit of hydraulic interconnection is the ease of interconnecting wheels that are far apart, particularly front and rear wheels. It is possible to interconnect front and rear third shocks hydraulically. If both the shocks compress when the suspension compresses, such interconnection affords greater stiffness in heave than in pitch. If one of the two shocks is a “puller”, the interconnection can provide greater stiffness in pitch than in heave.
If we connect a pusher shock at one corner of the car to a puller shock at the diagonally opposite corner, we resist roll and pitch without resisting heave and warp.
If we hydraulically interconnect anti-roll bars, we can resist roll while achieving soft suspension in warp.
If we pursue the various possibilities, it will become apparent that we could just have hydraulic cylinders or rams at each wheel, and connect all of these to a central module that contains various springing and damping devices.
Kinetic, in Australia, have worked extensively on warp-soft interconnected suspension, and have obtained many patents in this field. They have worked with the University of Western Australia to produce a series of very successful, albeit complex and expensive, Formula SAE cars.
It is possible to use just “third springs” and anti-roll bars, and omit the individual wheel springs. This has in fact been done recently. What’s more, the system only has one anti-roll bar!
The two most recent UWA FSAE cars have used cost-reduced warp-soft suspension, partly in response to changes in the rules intended to more heavily reward cost-effectiveness. The simplified system uses beam axles front and rear, connected by an unsprung composite undertray that is stiff in bending but torsionally flexible. The undertray incorporates tunnels for aerodynamic downforce. The resulting channel sections provide the desired combination of beam stiffness and torsional flexibility.
The front and rear ends of the car are held up by swinging composite leaf springs. There is a damper at each corner, with no spring. An anti-roll bar behind the driver attaches to the undertray in the
middle of the car. This provides all the elastic roll resistance. A limited, but sufficient, adjustment of elastic roll resistance distribution is achieved by moving the attachment points for the anti-roll bar links on the undertray fore and aft.