Split (decoupled) anti-roll bar?
Split (decoupled) anti-roll bar?
(OP)
I just had this idea and wanted to bounce it off of you guys. Have any of you tried or had any experience with a split anti-roll bar. What I am thinking of is something like a standard anti-roll bar that has been split in 2 with a rotating coupling in the middle. The coupling would rotate a certain nuber of degrees before "locking" to two halves together. I know that some high end race equipment does this with a hydraulic chamber that links the shocks. I am trying to brainstorm a way to achieve the benefits of installing a larger anti-roll bar on a softly sprung "luxury" vehical with out adversly affecting the ride.
I'm thinking that the stops could be adjusted so that under normal one wheel bump conditions, the bar would be decoupled, and in a roll condition the bar would lock together and act like a normal anti-roll bar but with a higher rate than normal (due to larger size). This would eliminate the harsh one wheel bump ride that is normally associated with larger anti-roll bars.
Am I smoking something, or is this possible?
If it has been tried, were there issues with the sudden change in spring rate when the bar "locks-up"? Possibly there is a way to make the bar get progressivly stiffer as it twists.
Thanks
Ed
I'm thinking that the stops could be adjusted so that under normal one wheel bump conditions, the bar would be decoupled, and in a roll condition the bar would lock together and act like a normal anti-roll bar but with a higher rate than normal (due to larger size). This would eliminate the harsh one wheel bump ride that is normally associated with larger anti-roll bars.
Am I smoking something, or is this possible?
If it has been tried, were there issues with the sudden change in spring rate when the bar "locks-up"? Possibly there is a way to make the bar get progressivly stiffer as it twists.
Thanks
Ed





RE: Split (decoupled) anti-roll bar?
Kinetic also cross link their shocks, which can be a clever way of controlling the roll response. I am slightly dubious about this one - shock absorbers are the slowest way of reacting forces across the car in transients.
Cheers
Greg Locock
RE: Split (decoupled) anti-roll bar?
Just use sholder bolts in the attachment points for the drop links, so there is some slack in the rubber bushes.
Correctly designed bushes and bush bearing surface for the drop links could result in a progressive application of the anti roll bar.
I am sure this is a currently used technique.
Certainly in the past, I have used bigger dia harder rubber, with big thick steel washers, on the drop link attachment to the moveing suspension member, done up much tighter than normal, so as to increase roll stiffness.
Regards
pat
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RE: Split (decoupled) anti-roll bar?
I also did some work on this some years ago.
It does have advantages for off-road vehicles where you can maintain wheel contact through free articulation at low roll angles.
However, we found it a bit unnerving for high speed handling on the road. We applied the device to the front only; the front turned in nicely but as the lateral forces built up and the roll bar came into play, the vehicle understeered violently (as one would expect). Bumps in the pavement also made the handling very unpredictable.
A gradual transition from 'off' to 'on' would therefore be very desirable for road applications. In theory, you could match front and rear but I doubt you would ever keep them synchronised and the vehicle roll stiffness would go through a series of discontinuities just when you didn't want it to.
As you say, the gradual increase in roll stiffness would be a good objective. You could look at stranded torsion bars (say, 4 bars held in a square formation). These give a progressive spring rate for large angular displacements but there may be insufficient deflection in a roll bar.
Regards - John
RE: Split (decoupled) anti-roll bar?
RE: Split (decoupled) anti-roll bar?
Harrisj suggests that a gradual transition would be better, but the manufacturer has already done this. The rubber mounts on the linkages and mounts have strongly progressive rates, so the anti-roll bar is actually less effective at center than when rolled a bit. You could try adding rubber to the linkage to make it softer at the center, with a gradual, but you will have to play with it at each end to get good handling balance, since the springs have an affect on the total roll rate that is unaffected by what you do to the anti-roll bar. No mater what you do, transient response will suffer because weight transfer is being delayed.
The real solution is to make the car stiffer in roll while making it softer in warp. Anti-roll bars make the car stiffer in both roll and warp. It is the roll stiffness that has the greatest effect on handling and the warp stiffness that has the greatest impact on ride comfort. I have written an essay that goes into more detail at http://members.cox.net/rseike/modes/index.html. Citroen has a rally car that has split anti-roll bars that are interconnected so as to stiffen under true roll conditions (both axels tilting the same way), but not under warp conditions (tilting opposite ways). This isn’t too practical unless you have the means to build fairly complicated hydraulic systems, and is probably patented anyway.
RE: Split (decoupled) anti-roll bar?
Best regards,
Matthew Ian Loew
"Luck is the residue of design."
Branch Rickey
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Split (decoupled) anti-roll bar?
Regards
pat
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Split (decoupled) anti-roll bar?
I didn't realize that there was that much work being done with this idea. I like Pat's idea, or maybe a center joint with some rubber in it to make it more progressive, but I think (as John points out) it might be better suited to off road situations, where you would normally not have a anti-roll bar due to loss of wheel articulation. I may have to test this on some of my off road stuff.
For the road, I really like the Tenneco\Kinetic idea. What do you think the red object in the lines connecting front to back is??? I am guessing that it is some sort of pressure accumulator/regulator, that acutally allows the articulation, but can also be adjusted to fine tune roll stiffness.
Ed
RE: Split (decoupled) anti-roll bar?
Cheers
Greg Locock
RE: Split (decoupled) anti-roll bar?
The cylinders are not zero-displacement type, consequently the rod end chambers have lower displacement than the blind ends. This means that there is a net displacement of fluid during operation, which must be accommodated by the system. Secondly, all hydraulic cylinders leak a little bit, and must be replenished. Third, there is certain amount of fluid transfer past the pistons from one chamber to the other. This is not likely to be equitable over long periods of time and needs to be equalized to keep the system functioning properly. Finally, temperature changes need to be accommodated.
RE: Split (decoupled) anti-roll bar?
could variable coil springs be integrated into the drop links?
but to my point, is there a way to make a torsion bar variable rate?
RE: Split (decoupled) anti-roll bar?
RE: Split (decoupled) anti-roll bar?
That would make the bar variable dependant on the wheel travel/position, although how much you could vary is something else.
Wouldnt halving the rate of the bar on a champcar be a bit scary? Id say it was more of adjustablility thing with the springs in contact 100% of the time (or loaded 100%).
RE: Split (decoupled) anti-roll bar?
RE: Split (decoupled) anti-roll bar?
The bore/annular area is not an issue as the system connects bore/bore and annular to annular - hence there's no volume inbalance.
Second, the later systems do not link the shocks - those cylinders are not shocks and have no flow across the piston - and hence are a stiffness device as well as damping.
In general my preference is to stay away from non-linearities ( including switching systems ) as you lose center feel and the vehicle never feels "right". What you ideally need to do is separate the modes of the suspension -so that you can optimise more than one attribute at a time.
RE: Split (decoupled) anti-roll bar?