Jump to content

Roadmaster 8 airbag suspension H frame movement and resulting wandering


ok-rver

Recommended Posts

Most of us with Roadmaster RR8R or RR8S chassises experience a bit of wandering of our MH direction from outside influences like wind, truck air buffeting and G-forces on long radius curves. There is a lot of verbal discussion on the forums about what is going on. Several gentlemen that have researched this and come up with physical devices to minimize this wandering have a very good understanding on what is going on with the relationship of the H-frame and the Roadmaster chassis. I understand their explanations but when I read thru many responses to their posts, I hear comments that show some are not understanding the engineering principles. I have created a few simple sketches that I hope will give some insight into what I interpret as happening.

A review of the equipment. Front and rear H frames are very similar but I will discuss mostly the front. The H-frames are actually a double crossmember “H”. The vertical center line on an “H” is doubled and these run front to back on the chassis and will be referred to as H frame rails, as they run parallel to the rails of the chassis frame. The up down on an “H” will be referred to as H frame crossmembers, as they run across/perpendicular to the chassis frame at the front and rear of the H frame. Each corner of the H frame has an airbag and shock absorber. These air bags have very little horizontal stability. They are a fiber reenforced rubber tube with closed ends that supports the chassis frame using air pressure above the H frame. The front axle is bolted to the bottom center of the H frame. Typical front axle support about 10,000 lbs. Each of the 4 front airbags therefore support around 2,500 lbs. If the motor home goes around a long curve at about 0.25 Gs, a pretty easy turn, the front wheels carry 1/3 of the total MH weight and would experience 1/3 of 0.25 G force acting on 30k lbs MH weight with about 2,500 lbs of side force. The 4 airbags with no resistance to sideways force would be all over the place.

The designers of the Roadmaster chassis provided two means to try and hold the H frame in place. The first are driver and passenger side pairs of trailing arms (5’ long square tubes with bushings/steel eyes at each end) that keep the H frame/axle pretty much perpendicular to the chassis. Each of the tubes runs parallel to the other, about 12” between them vertically on each side of the H frame. They are anchored to the chassis at the front and to a vertical post very close to the axle on the H frame. On the front axle when braking, a rotational force is transmitted into the axle, to the H frame and to the vertical post. One of the trailing arms will be in tension and the other in compression to resist the axle from winding up. The rear trailing arms are also responsible for the differential twisting during acceleration. Bushings will deform during these loads. The amount could be in the range of 1/64”.  The bushing are an outer and inner steel sleeve with a flexible material, maybe polyurethane, in between. I used a pry bar to put about a 100 lbs side load on a trailing arm and the bushing allowed the arm moved sideways a lot. It would not take very much force at the end of a 5’ long arm to get the urethane to deform and the arm to rotate. These trailing arms allow the H frame to swing side to side, but still be perpendicular to the chassis.

The designers also added a Panhard rod that connects, on my MH, from the top of the driver side rail of the H frame across the frame to the bottom of the passenger side frame rail of the chassis, perpendicular to the chassis, level to the ground when the chassis is at ride height. The same bushings used in the trailing arms are use in the ends of the Panhard rod to allow movement of the H frame vertically. The Panhard rod is about 4 ft long. With the Panhard rod mostly level to the ground at normal road height, the Panhard rod will cause the H frame to move towards the passenger chassis rail as the H frame moves up or down with respect to the chassis frame. With a 3” up or down movement of the H frame, the pivot point of the Panhard rod would be less than 1/8” closer to the passenger side chassis frame rail. My MH H-frame is about 60” long. The axle is in the center front to back and the Panhard rod is half way between the axle and the rear of the H frame.

Back to our side load created by the curve, 2,500 lbs is a a lot of force. The force acting of the axle and the force resisting thru the Panhard rod are about 15” apart.  Because there is the 15” between them, a circular moment is created and the H frame rotates until components in the suspension system can pick up these loads. The bushings in the trailing arms as they rotate to the side will eventually pick up some load.

It would have been great if there had been a Panhard rod mounted front and back of the axle to equally pick up load, no rotation of the H frame with respect to the chassis. 

Many have contributed to the design of the Watts linkage and X-braces that are being sold to minimize the rotation of the H frame. Going to the rear suspension were the trailing arms are behind the differential with minimal equipment in-between the arms, the addition of X-braces does a very good job of minimizing the H frame rotation. Each member of the X-brace works mostly in tension to keep the trailing arm that it is attached to it at a right angle to the differential. This side load from driving around the curve (twice the front value) is transmitted along the trailing arm and is resisted by the bushing at each the end of the arm. The bushings give so not all the movement of the rear H frame is eliminated with the X-braces. It does minimize the swinging side to side of the differential housing/H-frame while under load. Not as good as a PanHard rod front and rear but probably reduces the rotational movement in the 80% range. The deflection in the bushings is what gives that 20% movement compared to the original with out X-braces movement. Many have stated that adding a watts linkage to the rear H frame helps stabilize the H frame a bit more than just the X-braces.

Back to the front H frame, with the designers using most of the available space, the generator is mounted in-between the front trailing arms. Maybe very short X-braces could be install and not hit the equipment but they would not be very efficient. On some of the watts linkage kits, the watts lever is mounted to brackets off the hydraulic jack(s) attached to the chassis frame and the rods connect to the rear crossmember of the H-frame. The distance from the Panhard rod to the rear of the H-frame is around 15”. The load the watts linkage would see in our turn is equal to the side load generated by the tires, 2,500 lbs. The Panhard rod load has twice the load (5,000 lbs) generated by the tires. Even though the bushings in the Panhard rod give as well as the watts linkage gives some, there is much less rotation than the original (no watts linkage) suspension had. Those that have install a watts linkage in this rear position are very pleased with the improvement in steering/tracking of their MH. The pivot point on the lever at the center of the watts linkage moves vertically by the design of the linkage. A small amount of rotation of the H frame would occur when large bumps cause the axle/H frame to move up, caused by the watts going up straight and side deflection from the panhard rod. Under normal running down the road situations, the very slight side motion would not affect the steering.

On my 2004 HR Scepter, the rear of the generator frame is a couple of inches in front of the front crossmember of the H frame. The forward air tank support frame is a couple of inches behind the front crossmember of the H frame. Around 6” of total clearance. A chassis frame crossmember is almost directly above the front H frame crossmember. A mock up has been with an 90% angle clamped to the bottom side of this chassis crossmember that allowed a piece of card board to be positioned vertically down from the angle to just in front to the H frame.  The vertical plate would need to be about 14” tall. Mounting at this forward position, the watts linkage would be 45” from the Panhard rod. The side load at the front mounted watts linkage would be 1/3 of the load generated by the tires. The Panhard rod would see 2/3 of the load generated by the tires. If the watts linage deformed under load the same amount as the rear mounted watts linkage, the rotation of the H frame would be 1/3 of the rear mount rod due to the longer lever arm. As the front mounted load is much smaller, the actual deflection of the watts linkage would be even less.  This is the direction I am moving to stabilize my front suspension.  The correct offsets for mounting the watts linkage lever and brackets are being researched.

So now to the sketches. Top view of H frame, axle, tires, chassis frame, front of MH to left. Solid line circle represent the bottom of the airbag mounted on the corners of the H frame. Dashed circles are the top of the airbag mounted to brackets on the chassis frame. Size difference is only so they can be seen when concentric in the non-rotated sketch. Chassis frame shown in blue.

The first of the sketches has just trailing arms shown in the straight position. Second sketch with a side load applied that has pushed the H frame down in the sketch. No rotation of the H frame as this is a parallel linkage. The two trailing arms rotate with the bushings distorting at each end.

The second pair of sketches has a Panhard rod across the H frame in orange. First sketch with everything aligned. Second sketch with a couple of degrees of rotation of the H frame due to a side load. The rotation center for the H frame was at the H frame mounted end of the Panhard rod. Note that trailing arms are not shown. This would have changed some of the rotation and the center point. Sketch is simple so the trailing arms were left out.  The effects of the trailing arms on keeping the axle perpendicular to the frame is not simple. Additionally, the steering gear (not shown) is affixed to the frame and the axle end of the drag link is moving with the H frame. As the center of gravity is very high on out MHs, this causes the body to roll and more movement between the H frame and chassis. This is all pretty complex.

Without the rear MonacoWatts X-bars, I might have sold my MH as it required extreme concentration to drive. I have over 3,500 miles since the X-bar install and I am much more comfortable with the handling. I am sure with Mike’s help, I will make another major reduction in the wandering of my MH with a front watts linkage.

I am new to MHs but have 50 years of mechanical design experience. This explanation is simplified as a starting basis for a discussion. When I started this search for better handling, I read thru over 80 pages on one discussion on IRV2 about watts linkage and X-braces. About 60% of the posts were “Now I understand” or completely off topic.  I am hoping that comments like those “just to be comments” will not be included in this discussion so that those looking for technical details have a good resource. I would hope that things I have stated that are wrong, confusing or right will be discussed.

Just my thoughts on how all this works.

trailing arm straight.png

trailing arm rotated.png

panhard straight.png

panhard rotated.png

  • Like 3
Link to comment
Share on other sites

Thanks for your help. I look forward to seeming your front Watts placement. As you know its the direction I need to go with mine. 
 

I appreciate your insight. You may have followed my perils with the wandering. It improved greatly with the rear of the coach at a ride height closer to factory specs. 
 

How much do you think the bags play in the movement of the H frame? 

Link to comment
Share on other sites

The air bags on the rear of our MH, sleeve type, have the internal metal cylinder that causes the bag to roll as the bag collapses. The cylinder is small enough to allow  the bag to rolls inside. At ride height, maybe 1/3 of the bag is in contact with the sleeve. I do not believe it contributes any shear strength to resist movement until a lot of sideways movement. The front bags are the two chamber bellows type. No internal sleeve.  Maybe not an exact comparison, but think of 4 balloons at each corner supporting a board. Even if the balloons were connected (glued) to the support and board so they did not roll, it would simply float as a sideways load was applied to the board.

With regards to handling improving when at published road height, this positions the panhard rod horizontal. Think of a pendulum on a clock. It only goes up from the vertical position. If the panhard rod is not horizontal, it pushes the H frame away as it comes up to horizontal, pulls the H frame as it goes away from the current ride height position. When horizontal, there is very little initial movement. This might have been part of the wandering you experience.

Been thinking about how the H frame reacts to bumps as that effects the functioning of the panhard rod. The trailing arms keep the vertical bracket at the axle pretty much vertical (parallel linkage)  which means the H frame stays pretty much horizontal. Bushings distort so not a perfect parallel movement. Originally I though the front of the H frame would compress and then the rear as the tires hit a bump. Hitting a bump may cause a horizontal force from the wheel into the H frame pushing it backwards. This might cause the rear set of air bags to compress and fronts to extend.

I pulled an old GoPro camera off the shelf last week and charged the batteries. Looking at maybe mounting it under the coach and recording the H frame movement.

  • Like 1
Link to comment
Share on other sites

The airbags do not, and are not designed to, resist sideways motion at all.
The only thing they do is support the weight and allow for a spring effect.

All the axle location is done by the training arms and panhard rods.
I agree, if these had been designed with two parallel panhard rods, both in front of and behind each axle, there likely would not be any handling issues with these coaches.

  • Like 1
Link to comment
Share on other sites

Lot of good information but my 2003 Imperial is very easy driving no wandering will hold the road with no hands very stable. I am running just under 32000 lbs and towing A 2020 Ford Edge at 65 mph.

Link to comment
Share on other sites

Interesting discussion.

There are a lot of additional variables at work.  Wheelbase long vs short and with/without a tag axle make a big difference.  The other large variable is the type of steering box.  The TRW is much tighter than the Sheppard.  

One of the IRV2 members found his 2003 Dynasty had the rear axle installed with unequal length trailing arms.  This resulted in the rear axle being out of alignment with the front.  

Regarding the front Watts linkage: I saw some folks are moving the generator forward 2" when installing the front Watts linkage.  

Link to comment
Share on other sites

Vito - can you provide a link too were they are moving the generator forward.

  • Like 1
Link to comment
Share on other sites

On 11/16/2021 at 9:50 AM, dl_racing427 said:

The airbags do not, and are not designed to, resist sideways motion at all.
The only thing they do is support the weight and allow for a spring effect.

All the axle location is done by the training arms and panhard rods.
I agree, if these had been designed with two parallel panhard rods, both in front of and behind each axle, there likely would not be any handling issues with these coaches.

David is correct.  The air bags have NOTHING to do with the wandering.  It would wander just the same with coil springs.

And as once noted by Bob Nodine (with supporting trigonometry calculations) the "twist" of the H-frame is so small as to be difficult to measure.  I doubt you will be able to see it with a mounted camera.

Link to comment
Share on other sites

Van,

Most of what I have gleamed from discussions here and on IRV2 have been from your posts. I go back to one of the posts were u talked about sliding a 2" tube into the receiver hitch and setting the MH to swaying by push/pulling on the bar. My recollection of the post was that there was visible movement of the H frame. Have I misunderstood how the H frames were reacting to this side to side movement? You used "twist" in the H frame in your above comment. My thoughts have been that there was a combination of rotation and side to side movement that was affecting the alignment of the H frames and the MH chassis. I am very interested in your comments.

Link to comment
Share on other sites

3 minutes ago, ok-rver said:

Van,

Most of what I have gleamed from discussions here and on IRV2 have been from your posts. I go back to one of the posts were u talked about sliding a 2" tube into the receiver hitch and setting the MH to swaying by push/pulling on the bar. My recollection of the post was that there was visible movement of the H frame. Have I misunderstood how the H frames were reacting to this side to side movement? You used "twist" in the H frame in your above comment. My thoughts have been that there was a combination of rotation and side to side movement that was affecting the alignment of the H frames and the MH chassis. I am very interested in your comments.

That 2" square tube procedure was to demonstrate how "loose" the suspension is.  It is a very extreme procedure that will jostle things in the coach. If your coach ever experienced on the road what I caused with that 2" square tubing, you could not hold in in three lanes and it would be rocking violently side-to-side.  Again, the amount of H-frame twist required to cause incessant wandering is extremely small, and doesn't cause the coach to rock side-to-side.

Link to comment
Share on other sites

While I don’t want to get away from the topic of the H frame, I’d throw out there that under inflated air bags would contribute to wandering. In my other thread Bob had a link to an article that talked about car builders creating a frame in a jig for a “flat” or level frame. But when completed, they added rake to the finished car and thus it messed with the caster, thus causing wandering.

That really hit me as to why Monaco set specific ride height for the front and rear bags as ones had pointed out caster is set and cannot be alter without bending the axle. 
 

Am I correct in my thinking about the ride height and caster (thus wandering) correlation? 

Link to comment
Share on other sites

18 minutes ago, JDCrow said:

While I don’t want to get away from the topic of the H frame, I’d throw out there that under inflated air bags would contribute to wandering. In my other thread Bob had a link to an article that talked about car builders creating a frame in a jig for a “flat” or level frame. But when completed, they added rake to the finished car and thus it messed with the caster, thus causing wandering.

That really hit me as to why Monaco set specific ride height for the front and rear bags as ones had pointed out caster is set and cannot be alter without bending the axle. 
 

Am I correct in my thinking about the ride height and caster (thus wandering) correlation? 

While ride height can make a noticeable difference on caster on normal cars, it will have very little effect on our coaches, due to the much longer wheelbase.

A couple inches high or low on either end would only cause a small fraction of a degree in caster change.
That said, other aspects of suspension geometry could be affected much more by ride height errors.

Link to comment
Share on other sites

9 hours ago, JDCrow said:

Am I correct in my thinking about the ride height and caster (thus wandering) correlation? 

No.  Stop the H-frame from squirming under the coach and wandering is cured.

  • Like 1
Link to comment
Share on other sites

16 hours ago, vanwill52 said:

No.  Stop the H-frame from squirming under the coach and wandering is cured.

Van is absolutely right.  Until you keep all 4 wheels pointing in the correct direction, NOTHING else you do will ever cure wandering in a vehicle.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...