Why two axles???? - Page 6 - Fiberglass RV


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Old 02-06-2013, 05:21 PM   #71
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One of the things that got me to thinking about this whole subject was a striking few seconds of video I came across in a tutorial about trailers' lug nuts .

Side note: We DO all torque-check those regularly, right?

The video's actually pretty instructive as a whole, but I was shocked when I saw the strange distortion that occurs with tandems' tires when cornering.

Check out the distortion of the tires at between 0:48 to 1:09:

Video: http://www.youtube.com/watch?feature...&v=NpOWLMA2NpE


Francesca
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Old 02-06-2013, 06:50 PM   #72
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Instant center!
All wheels with 2 bearing assemblies in it have an instant center.
ALL LOADS subjected to the tire are are received by the bearings
As vertical loads. If there is as side load on a tire one of the bearings receives it as up pressure and the other as down pressure. On a simple bump both bearings receive a vertical load.
Trailers with 2 axles have an instant center between the 2 tires. The trailer turns on the instant center and scrubs both tires placing vertical loads on all bearings.
Bearings fail for 3 reasons, lack of lubrication, adjustment or overloading.

Fred
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Old 02-06-2013, 07:03 PM   #73
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I think your "both tires scrub on a turn" assertion to be perhaps correct only if the trailer is so perfectly balanced as to have exactly the same load on both axles. I recently watched a tandem axle RV trailer do a fairly tight turn on a dirt/gravel surface, and not only was the front axle's tire visibly rotating as expected, the "scrub" track from the rear axle was clearly visible once the maneuver was complete.

Francesca
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Old 02-06-2013, 10:56 PM   #74
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The instant center is an invisible point that moves with the weight each wheel carries. And yes, unless the wheels are mounted to the trailer on a walking beam the load on each wheel changes during road irregularities and turning.
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Old 02-06-2013, 11:07 PM   #75
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This is great stuff!
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Old 02-07-2013, 03:31 AM   #76
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Conventional springs. I don't remember if they were mounted individually or linked. I traded the trailer to a welder for welding work as I felt the design to be flawed.

The amount of stress on the axles was alarming to me, and that in conjunction with the failure led me to believe it was poorly designed. This was a relatively short trailer--maybe 25' long with the tandem axles more centered. A longer tongue would have helped.

This was a car hauler and heavily loaded. The axles seemed to be sting enough. I had the tires aired up to the max to haul a heavy K5 Blazer.

It was probably a combination of factors however I suspect the most critical factor was side loads in tight turns. In the future, I would avoid backing and making tight turns with tandem axles in the future, or else consider airing down tires to provide more flexibility. Now if the axles are set back far, as is typical of larger trailers, I would not be as concerned. Even a triple axle Avion, where the axles are 22' back from the tongue.

One final point. Getting stuck hauling a trailer means backing up and sharp turns. For this reason I am firmly convinced 4 wheel drive tow vehicles are preferred. It is much better to pull forward than back awkwardly. I would not use anything but a 4x4 for heavier trailers.
My car trailer is a 16', 20' with the tongue. It has leaf springs, and has hauled plenty of loads. It's rated for 7500, and the trailer is 2000 by itself, leaving a 5500 capacity. It has been over capacity many times, for many, many miles. ZERO bearing failure, with a much sharper turn than your trailer. Again, it was not the turns that ruined your bearings. They were improperly lubed, too lose, too tight, spalled, or had moisture in them.



This trailer has grossed at 11k pounds, 3500 over max (not on purpose). It was also grossed around 9k pounds, for 800 miles (dad went to pick something up that was MUCH heavier than he was told, didn't have much choice at that point).

Oh, and we lived in North Dakota. We never had a 4wd tow vehicle, and never needed it.

It hauled this over 1200 miles.



Quote:
Originally Posted by Francesca Knowles View Post
One of the things that got me to thinking about this whole subject was a striking few seconds of video I came across in a tutorial about trailers' lug nuts .

Side note: We DO all torque-check those regularly, right?

The video's actually pretty instructive as a whole, but I was shocked when I saw the strange distortion that occurs with tandems' tires when cornering.

Check out the distortion of the tires at between 0:48 to 1:09:

Video: How To: Maintain Proper lug nut torque on a Keystone RV - YouTube


Francesca
When turning, it's not uncommon for the outer edge of one tire to be almost even with the center of the other tire on the same side of my trailer.
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Old 02-07-2013, 09:57 AM   #77
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Quote:
Originally Posted by Francesca Knowles
One of the things that got me to thinking about this whole subject was a striking few seconds of video I came across in a tutorial about trailers' lug nuts .

Side note: We DO all torque-check those regularly, right?

The video's actually pretty instructive as a whole, but I was shocked when I saw the strange distortion that occurs with tandems' tires when cornering.

Check out the distortion of the tires at between 0:48 to 1:09:

Video: http://www.youtube.com/watch?feature...&v=NpOWLMA2NpE

Francesca
Thanks for sharing Francesca.

That torquing side load effect is greater for shorter heavily loaded trailers.

I feel this trailer was flawed in that it was too short for a car hauler carrying a heavy load. Tires with less pressure would have helped.

In any event it is an interesting topic.
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Old 02-07-2013, 12:36 PM   #78
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Instant center???

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Originally Posted by FRED SMAILES View Post
Instant center!
All wheels with 2 bearing assemblies in it have an instant center.....snip....
...
Trailers with 2 axles have an instant center between the 2 tires.

Fred
I'm having some trouble grasping this concept, at least as it relates to wheels on one side of a trailer.

Everything I've been able to find on the topic seems to discuss "instant center" as a point somewhere between opposite sides of the vehicle (see image below).

Can you elaborate/simplify for this novice the meaning as it relates to the tires on one side ?

Thanks!

Francesca

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Old 02-07-2013, 01:10 PM   #79
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I think we are over thinking the topic....
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Old 02-07-2013, 01:20 PM   #80
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I think we are over thinking the topic....
Ya think?
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Old 02-07-2013, 01:23 PM   #81
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I think we are over thinking the topic....
It is a sure sign that spring is coming. Over at the tractor forum it is usually a long drawn out argument about synthetic vs Dino oil.
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Old 02-07-2013, 01:25 PM   #82
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Well, I'm interested, so here's a (literally) big thanks to all who're contributing to my free education!

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Old 02-07-2013, 01:34 PM   #83
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Hey, I really have no problem with all the analyzing, to whatever degree you wish. Everyone has been playing nice, so do carry on. : )
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Old 02-07-2013, 03:13 PM   #84
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I think we are over thinking the topic....
I think so too. I've read so much... now I'm thoroughly confused. I think this guy was confused too...
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