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Old 05-25-2005, 10:40 AM   #43
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A crow bar is an example of "moment arm".

Flat spots on tires highly suggest "lack of proper running gear balance".

All too many times the brand tire is blamed for a failure, when it's really the owners fault for not having the running gear properly balanced.

There have been miles of conversation on this subject in this forum.

Lack of ptoper balancing does cause hot spots. That will cause a tire failure, guaranteed.

Andy
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Old 05-25-2005, 10:53 AM   #44
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Most radial "ST" tires, no matter what the "load range" have only 2 belts in the side wall. Bias belted tires, or commercial truck/trailer tires have thicker (more belts) in their sidewalls. Friends of ours with a 30ft A/S slide unit blow out tire sidewalls on everytrip. The trailer is just tooooo heavy for the Marathons Load Range "D". They are going to Carlisle Load range E and putting Centramatic wheel balancers on.
We have expereince with wheel balancers on our SOB motorhome, and are do pleased we put them on our Globetrotter. Very few places we have found are able to balance the running gear in totality. We are also considering replacing our tires with a truck type tire as well, since the "ST" tires aren't filling our bill either.
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Old 05-25-2005, 11:03 AM   #45
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Actually, a crowbar is an example of a lever. The example of the bike rack is a pitching moment causing ocillations up and down and sideways. It could contribute to exaggerated sway or bouncyness, but not to a factor of 4 or 5. The weight of the bikes and supports does contribute some moment to the frame, but this is fairly negligable. This moment is counteracted by by the front half of the frame, actually decreasing the deflection in the front half between the supporting axles and the hitch. This counteraction reduces the deflection in the front half of the frame because of course, the frame is a continuous and a cantilevered beam.

Would the weight of the bikes and supports cause problems? Maybe, if the frame is compromised already. However, if the supports are deep "C" sections backspanned and properly attached, the supports could contribute to a substantial strengthening of the frame. Also, It isn't just the frame which supports the shell, but the interior walls also. They act as full height box beams. Proper placement and attachment of the interior walls and you'd barely notice the weight back there. Success depends on the design.

Now for flat spots, occasionally when I'm crossing a remote area of Texas, I will have my wife ride in the trailer for 5 or 10 miles to check noises, vibrations, etc. The old tires had been balanced 8 months ago and the wife says she could feel absolutely no serious vibrations at towing speeds. Her comment was more about how serene it was in the trailer, save a few air whistles which have hopefully been corrected by new weatherstripping. We chech these things because our cats stay in the trailer when traveling and we wanted to make sure it wasn't exhaust fumey, shakey or otherwise uncomfortable. I stick by my assessment we were about to have problems and it wasn't because of things we hadn't done properly.
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Old 05-25-2005, 11:25 AM   #46
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The frame does not support the shell. The shell supports the frame.

Therefore when you add weight to the frame, especially at the rear, you will cause the shell to hold up even more weight.

The weight of adding bikes, indeed does multiply when bouncing. But the weight also multiplies when parked. Added weight to the rear also to some degree reduces the tongue weight.

Should any appreciable weight be added to the rear, say 50 pounds or so, in time, the rear shell to frame attachment will suffer.

This is not theory, this is fact.

If we disregarded moment arm, then there would by an awful lot of nose gear aircraft with the tail on the ground. When this is applied to a travel trailer that is being towed, several things happen, depending on the added weight.

The tongue weight is reduced, a permanent sway condition can result, rear end separation is almost a guarantee, and finally if the running gear is not properly balanced, amazing unbelievable things will happen to an Airstream or Argosy trailer, that at best becomes very expensive to repair.

Ask someone that's been there.

We see them all to frequently, in our service department.

Andy

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Old 05-25-2005, 11:58 AM   #47
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Aside from reducing the tongue weight, does weight in the cargo compartment under the rear bed have the same effect as a spare tire carrier (---or bike rack) mounted on the rear bumper? That may be a stupid question to those more familiar with the construction of the AS chassis and shell but, personally, I'd like to hear a simple explanation.
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Old 05-25-2005, 12:51 PM   #48
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Cracker.

Any weight placed rearward of the axle or axles, reduces tongue weight.

Any weight placed forward of the axle or axles increase tongue weight.

How much weight it adds or subtracts from the tongue weight depends on the weight and "how far" it's from the axle or axles.

Moment arm is the weight in pounds times the distance from the fulcrum.

Therefore a 10 pound weight at 1 foot weighs 1 pound. But that same one pound weight, 10 feet from the fulcrum now weighs 10 pounds. In the case of a crow bar, that same 10 feet would mutiply the force you have applied, times 10. Additionally, since the other end of the crow bar is less than one foot, the force continues to multiply. If the small end of the ten foot crowbar
was 3 inches, then a 100 pound force at one end now becomes 1000 pounds times 4 or 4000 pounds. (12 inches divided by 3).

No wonder we can all do magical things with a crow bar.

However, a mathimatician may not agree with that formula.

But, it works for me.

Andy
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Old 05-25-2005, 01:18 PM   #49
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Lets evaluate parts of the claim: "parked the forces are multiplied" using the engineering principle of "for an action there is an equal and opposite reaction". The moment of the bikes has to equal the moment of the reaction at the hitch ball.
Using my trailer as an example:
WDrear=WDfront W=weight D=distance
Weight of bikes: 60 lbs. assumed Equivalent weight of rack-supports: 15 lbs. assumed
Distance from center of supports (centered between wheels) to bikes at rear: 9.66 ft.
Distance from center of supports (centered between wheels) to hitch ball: 16 ft.
Weight of load: 75 lbs. Weight of Reaction: "Y"
(60 lbs.+15 lbs)x9.66 ft = 16 ft. x Y lbs
724.5 ft lbs= 16 ft. x Y lbs "Y" = 45lbs meaning 45 lbs less pressure is put on the ball
Decreased load on hitch: 45 lbs.
Increased load on tires is sum of bikes, supports and weight off hitch ball
Increased load on tires: 75+45: 120 lbs. Or 30 lbs per tire.

For a 190 lb man, (75x9.66)= 190 "Y" Y=3.8 feet.
so….. A man weighing 190 lbs standing 3.8 feet rearward of the center of axles is the equivalent to the bikes hanging off the back.

For the example "parked the forces are multiplied" that would be wrong, they are summed. The result is more downward force at the wheels, the sum being the weight off the ball plus the weight of the bikes and rack. Math works for me.

What does all this really mean? It means if you're going to hang the bikes off the back, it will increase tendancy to sway and to pitch, not severely, but some. If the total weight of 120 pounds causes more than that, your trailer strength is already compromised and needs immediate attention.
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Old 05-25-2005, 08:20 PM   #50
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Thanks Bob,

We added a steel bar the full length of the airstream's frame to support a bike rack on the rear of the Globetrotter. When we re-constructed the A/S' rear end from the separation, we added new woodflooring plus a steel beam bent to fit the curvature of the "C" channel all the way around the rear of the A/S and place new carriage bolts 6 inches apart. Between the frame ends at the back in the bumper compartment, we added a "like" beam under the "C" channel, running it under the new flooring, opposite the one we placed in the "C" channel. Those same carriage bolts go thru it to sinch it up.
With all this added metal, our hitch weight is still 600 lbs. So, if a couple of bikes reduces our hitch weight by say 120 lbs, we are still over what the original spec's say our hitch should be, 391 lbs. OUr Globetrotter is still front heavy with no sway. Additionally, with our new 6,000 lb axle, the fully loaded Globetrotter will weigh in at almost 5,000 lbs. Our A/S is rock solid, NO running gear vibration, (thanks to Centramatics),( Oh ya, Centramatic, I hear may have a demonstation at the Homecoming Rally) the axle arms flex easily and smoothly. Riding in the trailer as an experiment, it is smooth as silk except for an occasional clank from a forgotton mini blind not being secured.
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Old 05-27-2005, 02:31 PM   #51
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Looks like the new wheels/tires are going to work great. I've put the 15x6 OEM Airstream wheels up for sale in the classified section.
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Old 05-27-2005, 03:33 PM   #52
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Where are the spinners for those new great looking wheels?

Now that would be a way to go!!!

Andy
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Old 05-27-2005, 04:26 PM   #53
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My head hurts.

I guess like Roger, I too have never had (knock on wood) and issue with my ST tires. I plan on replacing them every 3 years (approx) and each tire is rated for I think 2360lbs last time I checked...either way, it's over 2k rated per tire. Now the Safari has a GVWR of 6300lbs meaning that **IF** I load the coach to the max, I'd still be 3,140lbs under the max rating.

I too plan on getting my wheels balanced and also am considering John Irwins suggetstion of these:

http://www.centramatic.com/

In the end, for me, what has been suggested might be more safe, but it also might be trying to hit a fly with a sledgehammer rather than a flyswatter.

But this is good info.....guess I shouldn't hook my 18' runabout to the back of the Safari.
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Old 05-28-2005, 11:38 AM   #54
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Silvertwinkle, I'm with you about this thread's effect. I guess I'll go take the bike rack off my '79 Excella 31'. Now I'm paranoid about SLEEPING back there.
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Old 05-29-2005, 07:31 PM   #55
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After considerable thought, I've decided to get the Centramatics. Even though the tires are balanced now, they will start getting out of balance almost immediately. And the running gear may be a bit out of balance also! Problem is, as you travel along, it is hard to be aware of how much things are out of balance. A trip of 8000 miles could see substantial changes in the balance. Centramatic balancers will take care of all that.

In the past we've have almost no indication there was an out of balance problem, but then I also had that big flat spot on one of the tires I took off. It looked like the tire had a ripple in the casing, but then I can't be sure it wasn't caused by a little out of balance in the tire or running gear. It looked like a belt shifted, but I can't be absolutely sure. Better to be safe!
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Old 05-30-2005, 05:29 AM   #56
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Balancing the tires and running gear

Here is another solution to the question of "how do I keep my tires in good shape so that they don't explode"... www.innovativebalancing.com has these little ceramic beads that you dump into the tire to balance out the load. Looks interesting, I'm getting some for the tow vehicle.

Andy,

How many ounces out of balance is the typical running gear? Thanks for the input in advance.

IceKing02
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