Anti Sway Bars 101 (LXi and LX)

[dentmac]

Back to the sway bars.... The CG is moved laterally with the roll. So keeping the roll to a minimal is paramount. With air suspension ,both IFS and solid axle, exhausting the inside and airing the outside has to help. Perhaps not so much in emergency maneuvers but most of us are not at the track.
With IFS ,especially multi link (over "king pin" style ), are the effects more pronounced? Possibly with the geometric differences -- ie Scrub steer?

I would think that IFS suspension requires something-- either solid metal sway bars or "Active Air".
Interesting NHTSA recall here, possibly giving validity.

Ross

Report Receipt Date: JAN 28, 2014
NHTSA Campaign Number: 14V020000
Component(s):
Potential Number of Units Affected: 452
All Products Associated with this Recall expand
Details close
2 Associated Documents expand
Manufacturer: Spartan Motors, Inc.
SUMMARY:
Spartan Motors Chassis, Inc. (Spartan Chassis) is recalling certain 2010-2014 model year MM motorhome chassis manufactured December 11, 2009, through January 30, 2013, and equipped with a sway bar and model 1253 front suspension. This front suspension design has increased rebound travel which may result in a binding condition of the sway bar end link which could lead to the breaking of the sway bar end links.
CONSEQUENCE:
Broken sway bar end links may result in poor vehicle handling, or sway, which increases the risk of a crash.
REMEDY:
Spartan Chassis will notify owners, and dealers will replace the old end links with new end links and an improved mounting, free of charge. The recall is expected to begin in February 2014. Customers may contact Spartan Chassis at 1-517-543-6400. Spartan Chassis's number for this recall is 12012.
NOTES:
Owners may also contact the National Highway Traffic Safety Administration Vehicle Safety Hotline at 1-888-327-4236 (TTY 1-800-424-9153), or go to http://www.safercar.gov.

(11-06-2013 16:18)davidbrady Wrote:  

(11-06-2013 04:36)mhughes01 Wrote:  Are the bars solid steel, David, or castings?

Mike, The roll bar is made by Addco and it's a solid mild steel, low carbon. It could be turned but it might be easier to simply bend a new one. IDB, Addco, or Roadmaster can make up any bar we'd like and we can use a high quality spring steel if we prefer.

I think the ride is intolerable with the bar that Blue Bird used, but that's my coach with my OEM 2.125" diameter bar. Some coaches have a 1.75" bar which is better but others have reported an improved ride by removing it too.

I also run my tag air pressure at 55 psi, my steer tires at 95 psi, and my Koni adjustable shocks set to full soft. The ride is fantastic - I search out bumps on the road just to marvel in the suspensions ability to soak them up! No more dentures jarring or heads banging the ceiling. Even the porpoising is vastly improved. The soft shock setting lets the steer suspension recover quickly morphing porpoising into a gentle heave. It's really very good but as Pete points out your coach and your preferences are probably different.

(11-06-2013 09:20)ernie ekberg Wrote:  haven't some folks taken those off with good results?

Bill Mattocks, principal engineer at Ridewell, and the man who designed our suspension told me that the bar isn't needed and the suspension works just fine without one. Wanderlodge has a long history of using his Ridewell suspension without a sway bar.

(11-06-2013 10:08)dentmac Wrote:  Interesting. Probably too sophisticated for a solid axle in an RV but for IFS the response/change speed would be far better then active air since RV's have high roll and understeer. The article refers to safety aspects of controlling roll and pitch.
Sadly , the Hadley modifications done to the 450 removed the "Sway bar" type function of the active air.

Ross, your coach with its independent front suspension is different from the Ridewell that Blue Bird used on their earlier Wanderlodges - big surprise :-). Our Ridewell equipped coaches have a horizontal roll axis. Your coach probably has an inclined roll axis. Your steer axle probably has a roll center close to ground level. The drive axle's roll center is above the differential. Your roll axis starts off low and inclines upwards as it moves to the rear. On my LXi the roll axis is at about frame rail height and is level from the steer to the drive axle. In a turn your seat-of-the-pants feel is much different from mine. My coach rolls. Your coach rolls and yaws. The front of your coach yaws out of the turn. Because of this you probably feel more movement in the driver's seat than I do. I can see how you might want more roll control.

The safety aspect is interesting. In a high G turn the limit that automobiles approach is loss of grip and a spin-out. The limit that buses approach is a tip over. We all know this is due to the higher center of gravity of our buses. It turns out that if the CG is higher than about half the track width then the limit is tip over; otherwise, it's spin-out. But, we hardly ever hear of buses tipping over. If they do it's usually due to hitting a curb or an embankment or running off the road. All of these things can tip a car too. I think this is due to a bus's CG not being quite as high as we think it is - it's probably closer to half the track width or around floor level. This is why I don't feel that much roll in a turn.

In a turn the CG rotates outward on the roll axis - there's a lateral shift of the CG. If the CG shifts out enough then the bus tips over. The trick with active sway bars is to keep the CG from shifting. This is something that a passive sway bar can't do. An active sway bar can actually roll the bus inward towards the center of the turn to displace the CG to the inside. This creates stability and further removes the risk of a tip over. I don't think HWH Active Air or Hadley SAMS are capable of this; a high G turn will simply pick the bus body up off the exhausted inside air springs. An active sway bar will force it back down.

Incidentally a big advantage that the wide bodies (102") have over the narrow body (96") birds is tip-over resistance. The wide bodies are more stable in a turn.

(11-06-2013 11:08)pgchin Wrote:  FWIW please keep in mind most of the info David posts about LXI suspension changes he's done applies to "his" generation of LXI's, one of the last years with the latest chassis upgrades.

Very true Pete. Each bus is different and needs to be custom tuned and every owner has his preference.

[davidbrady]

Hi Ross,

It could be that the Spartan recall is addressing the issue of a sway bar failure in the midst of a turn. Imagine vehicle suspension transients when the sway bar lets go during a high-g high-speed turn.

I'd say yes, the M380 and M450 would do well with a front anti-sway bar. What you have now is a coach where the lateral load transfer isn't balanced according to fore and aft weight distribution. You have a very biased situation where close to 100% of your lateral load transfer is taking place at the drive and tag axles. The front flops over but the air spring pressures equalize resulting in little to no roll resistance or weight transfer. The rear of the bus is holding up the front, the chassis is twisting, and the front tire contact patch orientation is probably less than ideal.

Your situation with IFS is markedly different from the 43'LXi with it's Ridewell stick axle. The 43'LXi has a level and high roll axis whereas yours is inclined - low at the front and high at the rear. In a turn the 43'LXi has instantaneous lateral load transfer thru it's panhard rod attachment points. The front and rear load transfers are relatively balanced with respect to front/rear weight distribution. There's very little roll because the lateral loads are applied geometrically thru the panhard rod; hence, no need for a steer axle anti-sway bar, no excessive roll, little chassis twist, and balanced front/rear lateral load transfer.

[dentmac]

NHTSA concerns of the sway bar failing during a turn is logical.
Sorry to keep this going with IFS but very interesting.

Does the M380 have a 4 point levelling system? I don't think it was an active Hadley like the original in the M450 and there is no anti-sway bar. So why is it that the 380 doesn't seem to have the sway issues of the 450. Shorter, lighter and therefore less "Flop"? Would the CG of the 450 may be higher due to the use of steel shell -- therefore more front roll/twist? After the Hadley change there were complaints front door issues.

Ross MacKillop
2006 M450

[davidbrady]

Hi Ross,

The M380 has the traditional tripod Height Control Valve setup with no Hadley active air. I think everything you said is true which would make the M380 less prone to sway, but it still could use a steer axle anti-sway bar. M380 owners have told me so.

[Arcticdude]

So what does Prevost do with their front ends? Or maybe rear ends? Or maybe both!!

[dentmac]

From a Prevost site:
Independent Suspension System:

Prevost’s independent suspension includes precisely sized and tuned stabilizer bars on the front and drive-axles. This design, engineered to complement the ZF Servocom® power steering, provides exceptional directional stability, confidence-inspiring road feel, and best-in-class handling precision.

and not really related to this discussion but nice :
TOTAL SAFETY AND SUPERB DRIVEABILITY
Boasting such advances as ITS High-Efficiency Brakes, the Volvo Electronic Stability Program (ESP), a rigorously safety-tested body structure, and Volvo’s unique driver impact protection system, the Volvo 9700 represents the culmination of nearly 80 years of Volvo safety know-how. Innovations include the industry’s first Front Impact Protection (FIP) and Knee Impact Protection for drivers, to absorb and divert impact forces in the event of a frontal collision. The Volvo 9700 is also the first coach to be equipped with a Front Under-run Protection System (FUPS) that helps protect automobile occupants in the event of a bus-car frontal collision.

Ross
2006 450

[davidbrady]

That's right Ross, the XLII, and H3 Prevosts with IFS have steer and drive axle sway bars.

The downside of the Ridewell panhard rod approach is a choppy ride. The vintage (non-IFS) birds have the majority of lateral loads transmitted thru their panhard rods; the IFS-birds have the majority transferred thru their springs (assuming a Hadley active air setup). You can imagine how the ride is improved in the IFS-birds - lateral loads transferred thru the springs naturally soften and dampen the loads. Lateral loads transferred directly from the bus body to the tires via panhard and suspension torque arms have very little if any softening or damping. Anyone who's driven a non-IFS bird can attest to the chatter and choppiness that's felt throughout the bus when driving on rough, cobblestone-like surfaces. We can make the suspension air springs as soft as we want in the vintage birds, but you'll always feel the direct load transfers thru the suspension torque arms and panhard rods.

[dentmac]

(03-05-2014 18:56)davidbrady Wrote:  That's right Ross, the XLII, and H3 Prevosts with IFS have steer and drive axle sway bars.

The downside of the Ridewell panhard rod approach is a choppy ride. The vintage (non-IFS) birds have the majority of lateral loads transmitted thru their panhard rods; the IFS-birds have the majority transferred thru their springs (assuming a Hadley active air setup). ........

There is the 450 problem. The 4 point active air was removed on all (without owners knowledge or permission).
Ross
2006 450LXI

[davidbrady]

Even without Hadley, you still get roll resistance thru the front air springs. I didn't mean to down play this effect. It takes time for the air pressure in the outside spring to bleed into the inside spring during a turn. In a steady state circular path at some point all the air bleeds and pressure is equalized, but for most every day driving conditions, moose maneuvers, clover leaves, the out side air spring is providing instantaneous roll resistance. But, unlike Hadley, roll deflection is required before resistance is generated; i.e., springs need to deflect before they can resist the roll of the bus. With an anti-sway bar you can generate more resistance with less lean. Unfortunately the inside air spring offers no resistance to roll. With leaf spring suspension, both springs offer roll resistance, both the outside spring as well as the inside in a turn. Some truck suspension manufacturers use a semi-elliptical leaf spring in conjunction with their air springs to generate spring force on rebound and to improve roll stability.

[dentmac]

Agreed- Not instantaneous because the cross feed is only 1/4 inch .(Cutting the wires to the cross feed solenoid does very little). I would think that, this cross feed and losing the original design of the 4 point system where air is immediately added to the outboard and released from the inboard. (3/8 lines) allows significantly more roll. (not like a 380) In a cloverleaf or curved ramp, air flow was immediately heard. I feel that the change caused more wander on the highway, especially crowned . Changing back to the 4 point system will be interesting.
Ross
2005 450 LXi