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Roll Center

Roll Center

Roll Center

I wonder if someone can explain the attached suspension geometry to me. It's on my motorhome which is essentially a 3 axle bus. The attached drawings show the rear suspension which has a single hanger that supports both the drive and tag axles. The drive axle has 4 air springs, the tag axle 2. The drive axle has a panhard rod (item #34) that connects from the frame rail to the differential housing. I know that this is where the roll center is located for the drive axle. What I'm not sure of is the significance of the line of attack of the tag axle control arms. A line drawn thru the tag axles bushings 53 and 38 intersect the panhard bushing at 34. What was the engineer achieving by this arrangement?

RE: Roll Center

Thanks Greg,

The manufacturer says this: "Single central hanger unitizes drive and tag suspensions allowing suspensions to operate in correct phase with drive line angles for maximum life of components". The part about "correct phase" prompted me to dig a little deeper. This is a diesel pusher bus suspension. I understand the desire to have the driveshaft move in a similar arc as the final drive, and the 4 leading link drive axle provides that. Is there more to the "correct phase" statement than that? Is there anything to do with body roll of the bus and the effects of axles moving in phase with one another? BTW, the tag axle suspension looks like your typical high roll resistance class 8 trailer suspension but it actually has significant one wheel bump independent motion. It's mounted to the hanger with 6 bushings, 3 on each side, and these bushings are capable of 6 deg of cardanic movement. FWIW.

RE: Roll Center

I cannot see any particular motive and I doubt whether there is an engineering motive behind it. Any solid axle where the "axle beam" is in the center line of the wheels and is not meant to "flex" will have a roll center height of approximately at wheel center height (independent from where the lateral load is being put into the chassis, obviously without over constraining the system).


dynatune, www.dynatune-xl.com

RE: Roll Center

Thanks everyone, and thanks dynatune for anticipating my next question, which would have been "where's the roll center on the tag axle"? I now know it to be at the wheel center height. The steer axle suspension is a beam axle and looks like the drive axle but rotated 180 deg; that is, 4 trailing links instead of 4 leading. From what I can tell the links are essentially parallel to one another and to the ground, so it appears that the bus is anti-neutral (dive and squat). The steer axle has the same panhard rod as the drive so that's where the roll center would be. The roll axis is quite high then and is parallel to the ground from steer to drive but then the tag takes a nosedive to the center of the wheel. I don't know the impact of not having all the roll centers line up. The suspension is simple, it works, it's durable as all get out, and as Greg astutely noted: it fits and it allows the axles to move up and down! What more could I ask for in a bus??? smile

RE: Roll Center

Trucks and buses are not designed for handling prowess. If it rides okay and it goes straight when aimed straight down the road, and it can make it around a corner somehow when driven conservatively enough, that's about the extent of it. The wheelbase is usually so long that fore/aft weight transfer won't matter too much; hence lack of anti-dive or anti-squat in the geometry.

If you tried to corner hard, it would probably lift up the inside wheel of the tag axle, followed shortly thereafter by very alarming levels of body roll that hopefully scare the driver into slowing down before the bus rolls over. It might have antiroll bars built in somewhere; can't tell from the diagram. Air suspension, particularly with the air springs forced a good way in from the outside of the vehicle due to the significant width of the tires, is not good at resisting body roll.

RE: Roll Center

It rides good, goes straight, and negotiates a turn, but you're absolutely right about driving conservatively. The weight distribution is 30% steer and 70% drive/tag. It weighs 47500 lbs. I probably have 4300 lbs of unsprung weight: (1100#, 2000#, 1200#: steer, drive, tag). No anti-sway bars, but that might be something to play with sometime. It uses a tripod setup of height control valves - one on the steer axle and two on either side of the drive. The left and right steer axle air springs are plumbed together (air freely moves from one side to the other) which doesn't help roll resistance. Most of the roll resistance comes from the drive/tag axles where there's one height control valve on each side and the left and right air springs aren't plumbed together. The six bushed (3 on each side) tag axle also offers some roll resistance. I think the CG is fairly low and close to the roll axis. It's a point and shoot kind of thing - no moose maneuvers here! I'm getting the picture about axle roll centers and appreciate why the tag axle would lift first. Calculating a spring rate for a possible steer axle anti-sway bar would be a fun exercise. I figure the air spring wheel rate at the steer axle to be about 1200 lbs/in (one wheel bump). The chassis stiffness probably needs to be considered - I don't want to twist it like a bath towel. I figure a steer axle bar would help with windy conditions and help with steering response. Any ideas?

RE: Roll Center

Are you sure the front steer axle is plumbed to exchange air?

If you cross the inlet and outlet lines on one side, you will create an air spring roll stabilize bar equivalent. If you put an orifice restriction in the crossover line, you will create roll damping as well.

RE: Roll Center

Thanks cibachrome. There is a company that makes something like you describe. They're called motion control units (MCU). The steer suspension on my bus definitely has the left and right air springs plumbed together using a T - this is pretty standard on buses.

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