Lifting rear of coach

[Toy Box]

To Grantracy... when you get back home, stop by if I can show you what I have learned during the past 20 years of RV's , bikes and trailers.

[grantracy]

Thanks for the offer,will do. My cell is 305-799-4193.Will be in town most of next week. Thanks, Granvil Tracy. grantracy@aol.com

[Petervs]

This thread has been going on for quite a while now, and I think perhaps one item has been overlooked. Granvil said he dragged the back of the coach on a few dips in the road. The first thing to check is to see if the ride height is set correctly. There are small lever arms at each axle that provide a signal to the air system to add or lose air from the air bags. When the Level low is in the drive mode, these sensors constantly signal the suspension to remain level. But if they happen to me mis adjusted, well, then you can drag the back end, and perhaps you are driving off level all the time. They are easy to adjust too.

The service manual has a simple test method which involves measuring the height of the coach body with a ruler while the engine is running. Start with this before deciding all the rest.

As for the weight and load from the bike and rack on the back, even with it on there all the axle and wheel loads can easily still be within limits. Just weigh the coach axle by axle. Trust, but verify, remember?

[hhoppe]

Peter: I think they all were trying to say he is putting his coach in peril with the possibility of breaking the frame and pulling the back cap off. Our busses are not built with full frames front to rear. The have a short frame in front designed to support the front running gear only. They have a short frame in the rear designed to support the rear suspension, engine, transmission, and other associated mechanical parts. They have a defined weight amount allowed for a trailer. Hanging large cantlevered loads onto the hitch is detremental to the whole coach. Between the two subframes is a unibody skeletin small tube frame designed to support the center body sections only.
Adding height to the rear suspension via air bag levers does not eliminate the above described problem.

[Petervs]

I am not suggesting he should add height to the air bag system so he can carry the motorcycle, only that he check to see if it is set correctly. If it is misadjusted then the bus will drag even without the bike and rack on there.

As for the frame members being strong enough or not, well the frame truss between the front and rear is plenty strong even if it is not one continuous hunk of steel channel. I do not like the way the typical bike rack is mounted only to the very rear end on the bottom, that is a bad design. And they want you to weld it all together which prevents unbolting things in order to remove the engine for overhaul without cutting it all apart again.

Again, I am not in favor of those "Overbuilt" style bike carriers, I was only suggesting he check to see that the air bag system was operating according to spec.

[Jon Wehrenberg]

I think the issue of bike carriers on the rear of our coaches is one in which not only is the design apt to lead to structural issues because so much weight is cantilevered off the hitch, but it may also impact handling and safety.

The weight of a full size heavy motorcycle located around 24 to 30 inches beyond the rear bumper is a fairly long lever arm from the drive/tag axles. If the weight of the bike was directly over the drive/tag axles the bus would handle it nicely, but with it extended that far to the rear it becomes a multiple of that weight and the air bags, their air supply lines, the valve orfices, and the shocks in the rear are not sized to handle that additional weight. As a result several things happen even if the ride height is adjusted perfectly.

First, the jounce from going over a bump is exagerated. That increases the magnitude of suspension travel. If the bus is cornering while this happens it worsens the situation. The rear air bags on a normally loaded coach actually do not require a lot of air pressure relative to the front air bags to inflate, but since there are six of them they require a lot of air flow. Add weight such as the bike and that air just can't get to the air bags quick enough and the result is the tail of the bus drags.

Driving on moderately curvy roads will allow the air bags to deflate faster than they can be refilled. I think as long as time delay ride height valves are used the situation will be less severe for curves of short duration, but more severe for curves of long duration. If the instant response valves are used, such as in replacements or newer coaches the air will be going out much faster than it comes in so the airbags over time on a curvy road are going to deflate. The air compressor is going to be working hard.

Where safety is affected is in braking distance. I am sure that the fronts will be less effective and when the ABS kicks in there may not be any drama in braking, but the fronts will tend to slide faster than when fully loaded. Whether the rear axles will have sufficiently greater braking to offset that I don't know.

[Petervs]

Jon,
Usually we are on the same page engineering wise on most things. On this one I think you are overly concerned and your concerns are unfounded.

These coaches are reallly big and strong, and the engineering is not barely adequate and marginal. These are not flying machines where every last ounce must be removed to increase useful load, etc. The thing is built like a tank!

If you hang an 800 pound motorcycle and a 400 pound lift off the back, that is not much different than if 6 200 pound people on a transit bus walked to the very rear of the coach all at once to use the bathroom. OK, they cant walk 2 feet beyond the back wall but it is pretty close. Nobody would think doing that would overstress anything. And it would not affect braking either.

Do the math on the weights involved. That same 1200 pounds is the same weight as 150 gallons of water. Does your coach really brake or drive on curves different if the water tank is full or empty? The 1200 pounds is only3% of the 48,000 pound gross weight of the coach. It is trivial.


What is not trivial is the way these motorcycle lifts are welded to the lower rear frames. That part can cause flexing of the rear lower part, and since the skins are pop riveted to the frames, that can cause wrinkles and popped rivets.

When I hang my 1600 pound Smart car on the back, plus the 350 pounds worth of lift ; it reduces the weight on the front axle by just under 1000 pounds. The front axle gross is 14,000 I think, and mine goes from 13,000 to 12,000. I am sure that is less than what an empty transit bus would have on its front axle. The rear weights go up a little, but they both remain well below their rated limits. The real difference in my design is that I am not carrying all the load from the area of the trailer hitch, mine attaches also to the upper portion of the truss which distributes the loads much better.

[Jon Wehrenberg]

Your design Peter is unique in that the "cradle" shares the load with the upper structure.

Our hitches rely exclusively on the engine cradle which does not place any of the load above that point. There are two POG owners that have cracking along the lower portion of the fiberglass cap and around the fasteners that attach the forward perimeter, indicating there was some force pulling downward, and someone has posted I believe about stress cracks in the axle frame structure.

But apart from that the steering gets squirrelly when the coach is not at ride height. I know because I once had something in the level low system stop working and the tail was lower than the front and the coach darted side to side until I stopped and manually got the coach to the correct ride height.

I do not disagree about a bunch of heavy people standing in the rear of the coach, but they are above the floor, which is carried in part by the side trusses.

[lewpopp]

Try stepping out on a diving board and jumping up and down 3-4 feet from the end and see how much stress you put on the board. Practically nothing. Go to the end of the board and jump up and down and the stress is multiplied by many, many hundreds of times. This makes a lot of sense to me to not load the rear of the exterior of the coach which is the end of the diving board.

[Jim_Scoggins]

I digress a bit from the original topic:

I spent years trying to work out easy convenient ways to carry a motorcycle along on trips. A pickup seemed a good solution after I broke the code to easily load or unload the bike: All towns have a store of some kind with a ramped loading dock. In a press used a strap under the bike frame and had a garage lift it on and off with a car lift. Of course, with smaller trucks we were left in a lurch about carrying four people to go some where.

Did the enclosed trailer trick, then we had no car for bad weather, etc.

What I finally came down to was the realization that fooling with a bike was an injustice to motorcycling and coaching. So now when I go coaching I just pull a nice four passenger Toad. When my motorcycle blood gets going I just saddle it up and head out on the bike-usually spring and fall when the weather is the best!