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Old 08-11-2012, 03:12 PM   #1
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Axle Placement?

I am in the process of welding up a new frame for our 31 foot airstream. Basically I copied the original ladder design. I increased the axle plate to 1/4".
I removed the old front axle to get a measurement for the axle placement. Measuring from the back side of the jack the street side of the u shaped cut out for the axle measures 1/4" longer than the street side. Is this correct ? This is assuming the A-frame and the attachment of the jack is centered.
Where does one measure for axle placement ?

Thanks for the Help,
Jack
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Old 08-11-2012, 03:54 PM   #2
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Some trailers from the 70s had the axles offset slightly, mine are. I'm not sure if it was done deliberately or not. It might have been to compensate for the crown of the road. This offset causes my trailer to travel at a light angle going down the road. I notice it in my mirrors, but it does not effect the travel.

I have not done a frame, but I would make sure that the axles are exactly the same distance on both sides from the ball.
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Old 08-11-2012, 04:08 PM   #3
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Soo I took the jack off and measured from the front center of the tongue back to the axle plate u shaped cut outs.
curbside=205 11/16"
streetside =206"

I've heard wives tales about the curb side axle forward of streetside to take into account for the crown in the road, but I didn't believe it, maybe a 1/16" but 5/16" ?
Opinions ?
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Old 08-11-2012, 04:39 PM   #4
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Take a look at this thread: http://www.airforums.com/forums/f437...ect-54805.html
Sounds like the same thing to me...
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Old 08-11-2012, 05:56 PM   #5
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Thanks for the post. In research online large trailers measure from the kingpin back to the spindles and the measurements should be within .01 degree.So I think I can get a little bit closer than the 5/16" difference.
This is a 1975 model trailer and the manufacture date on the axles is 1970. So I assume these are original.
I'm sure the 5/16" difference was within specs for Airstream in the 70's.
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Old 08-20-2012, 07:04 PM   #6
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Jack,
I doubt that the asymmetry was intentional. I suspect that the design specifications were to place the axle(s) at a certain distance from a reference point with a tolerance. AS probably had a frame weldment jig that would work for a variety of models (year models and lengths). This jig could have had something like axle assembly reference points or even some type of registration device to locate the axles with some ease and precision (precision is a relative term).

I am a retired engineer having worked 35 years for a general aviation aircraft manufacturer and I can tell you that everything has tolerances that are frequently not as tight as one would hope. In rebuilding and strengthening the frame on my 1991 AS trailer (along with floor and sheet metal work - reference this thread if interested), I can tell you that I have found many aspects that I consider to be out of tolerance and excessivly asymmetric.

As a former tractor-trailer mechanic, I can tell you that axle asymmetry will result in a trailer that does not track straight behind its tow vehicle and abnormal tire wear. I'd make things as even, straight and true as I could!

All my best,

Steve

Please take some pictures and post them to document your journey and pave the way for others who follow!
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Old 08-20-2012, 07:23 PM   #7
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You will always be dragging a tire and it will blow out a lot. A buddy of mine has replaced the same tire a half dozen times before he figured that one out.
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Old 08-21-2012, 09:15 AM   #8
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I have built many trailers over the last 35 years. I try to keep the tolerance for the entire trailer within 1/8". I build them in a jig and when I put the hitch on, I run a string line down the entire trailer to make sure the coupler is exactly straight with the frame. I will have a centerpunch mark in the center of the rear crossmember and the center of the front crossmember and that string line will give me a perfect alignment for the center of the coupler, whether it is a ball hitch or a pintle hitch. When building something like a trailer, your first mistake usually gets magnified as you progress. That is why I try to keep them as close as possible. With a good jig, it is hard to mess them up. As far as axle placement, they should be within a 1/16". A misaligned axle will cause some serious towing issues. A well aligned trailer can run at high speeds and you won't notice it much at all. A misaligned trailer can produce many more issues than the lack of a WDH.
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Old 08-21-2012, 11:27 AM   #9
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Quote:
Originally Posted by 195Pilot View Post
Soo I took the jack off and measured from the front center of the tongue back to the axle plate u shaped cut outs.
curbside=205 11/16"
streetside =206"

I've heard wives tales about the curb side axle forward of streetside to take into account for the crown in the road, but I didn't believe it, maybe a 1/16" but 5/16" ?
Opinions ?
ANY difference in those dimensions will cause the trailer to tow dog legged.

That in turn, causes excessive tire wear.

Make that dimension to be exactly the same.

3/8 of an inch off, will cause a 31 foot Airstream to tow dog legged 9 inches.

Ask me how I know, as I towed one for almost 3 years.

Andy
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Old 08-21-2012, 12:03 PM   #10
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Steve,
thanks for the reply. I to was schooled in mechanical Engineering Although I started a Millworks business right out of The University of Alabama(ROLL TIDE).
I have found so many tolerance issues with this old frame I question if the were specifications. I now realize that the frames were constructed in such a manner that when the plywood was installed on top of the frame, the frame could be pushed and pulled to square up it up, and the plywood and the fastners are the major strong points.
By the way I have rebuilt hundreds of aircraft out of Wichita. I started out as a line boy at the local airport, bought my first wrecked Cub at age 16. Never bought a flying airplane always bought a wreck and rebuilt.Had a nice BE-58P a few years ago. Have an original Cessna wing jig. My call sign 195Pilot refers to my 1952 Cessna 195.
Thanks Andy and M2HB I did some research on trailer axle alignment and found where the kingpin and the axles should be the same distance from side to side.
On a funny note I had a buddy come by and try to explain the difference in axle placement was to compensate for the crown in the road.
I will try to post some pictures today.
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Old 08-21-2012, 01:36 PM   #11
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Her are some pictures of WHY I built a new frame.
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Old 08-21-2012, 01:39 PM   #12
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Here are some pictures of my new frame
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Old 08-21-2012, 01:52 PM   #13
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This has been a real challenge. I retired in 2007 and sold my business along with most of my tools. I kept some for aircraft work but most of the metal working equipment went with the business. Although all was industrial, heavy, three phase equipment, that is of little value out here on the prairie. We don't have a land line phone available in our area. I am lucky the co-op electric company gave me 200 amp single phase service.
So all the shaping, cutting and forming of the frame was done with a Harbor Freight welder, 9" grinder, jig saw, and cut off saw. I spent less than $ 500.00 on tools. I had to use FCAW as all the welding was outdoors. As a side note I have been worried about prairie fire as we are in a drought and all the grass has turned brown.
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Old 08-21-2012, 04:18 PM   #14
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195Pilot, nice radial airplane.

You did a great job on the frame. If I was doing a shell off restoration on an AS trailer, I would build a new frame, even if mine wasn't rusty. It seems that AS relies very heavily on the shell as being part of the structure. I don't believe that is the proper way to design and build a trailer to last. It is the way you may want to build an airplane, to keep everything as light as possible, but with a trailer an extra 150-200# of material and axle can easily get you an extra 1000-1500# of extra GVW. I want to error on the strong side.
I can't wait to see you progress on the trailer. Nice job.
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