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Old 02-16-2005, 08:26 PM   #1
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Leaf Springs & New Axle

Hi, We have had our 52 Fly'g Cloud for about a year and am finally going to start working on it. First thing I plan on doing is replace the axle. Currently it has a "Mobile Home" type axle with those wheels that are "rim only". I plan replace with a Dexter. My existing Leaf springs are 35" long (centerline of bolt to centerline of bolt) and am not finding Leaf Springs that length . . . and am starting to think I will only replace the axle. Does anyone know the stock length of the leaf springs from that time period or is 35" stock, if not then I'm guessing that the whole assembly was from a mobile home . . . also, does this infer that the existing assembly is probably too "stiff" for my +/-2,500# trailer? Anyway I can't find any evidence of "old" locations for the leaf spring "brackets" - they appear to be fine (there are 10 "leaves" . . . and it's a single axle trailer). I guess my questions are - how do I evaluate the leaf springs and if it turns out I should replace them, is the 35" going to be problematic.
Thanks, Mark
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Old 02-16-2005, 09:35 PM   #2
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Mark,

You won't find those springs at a trailer shop or at Dexter, but any spring shop should be able to inspect them and adjust them to what you want.

First, make sure you know the weight of your trailer, with any added stuff you will be adding such as new tanks, batteries, and likewise. The springs on my '59 tradewind were the same length as yours, but they only had 7 leaves and the empty weight is 3200#. I added a leaf to increase the weight capacity by 800#.

There is a formula you can use to calculate the spring rate that you have now. Then you need to determine if you want to adjust the number of leaves to make the springs softer. Any spring shop can take your springs apart, clean them, remove a few leaves (or add new ones, like i did), and put it back together. Here's a web site that has the spring rate formula: http://www.4wheeloffroad.com/techart...58/index1.html

If you are getting a new Dexter axle, it may be a different diameter than what you have now. If that's the case, you will need to get new mounting plates, U-bolts, shackles, bushings and bolts. You might find the eyes on the original springs are larger than todays, and bushings are harder to find, but they are still available from a good spring shop.

Here are a few more web sites i found useful for figuring out how to measure leaf springs: http://www.suspensionspecialists.com/techinfo_fs.html http://www.landrumspring.com/racing/technical.html
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Old 02-16-2005, 10:54 PM   #3
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Don, Can you help me understand how to use the results of the Leaf Spring Formula you sent - the example problem's answer is 148 lbs/in, how do I translate that into "capacity". Also since your '59 Tradewind has 35" Leaf Springs could that mean I have "stock" length springs? Thanks for your reply.
Mark
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Old 02-17-2005, 08:55 AM   #4
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Mark,

First, the 'stock' length of the springs is hard to measure. If you look at the second web site above, it gives methods for measuring the springs. Since you said you springs were 35" eye-to-eye, they could well be 37" long if measured along the arc. It depends on the arch of the springs. The mounting brackets on the frame of my '59 are 36" apart. So if the springs are 'unloaded' they will be 35" long, when they are fully loaded (flattened out) they will be 37" long.

For the capacity calculation, there are a lot of conditions. My springs are 0.36" thick. The spring rate on my Tradewind was 1170 lbs/in in the original set up. It increased to 1330 lbs/in when I added a leaf. I don't know how thick the leaves are in your Overlander, that why I gave you the spring rate calculator.

If the thickness of the individual leaves in your Overlander are 0.25" thick, even with more leaves, the spring rate will only be half of what I have, or about 560 lbs/in. It would not suprise me if your springs were thinner than mine, as they were built several years apart and may have been built in different factories. Airstream was not famous for standardizing parts back in the 50's. Of course if the springs were the same thickness as mine, the spring rate would be very high (1700#/inch) and the ride would be very harsh for a 2500# trailer. In that case, I would say you probably do have a poorly done modificiation. However, I do remember seeing someone else's axle replacement project and I think he may have had a 9 or 10 leaf spring. So you should find out the thickness of the leaves.

So once you know the spring rate, what can you do with it? A lot depends on the geometry of your axle, wheel wells, shock absorbers, and frame. And what do you want to do with it? My plan was to add a considerable (500#) amount of weight, and a new, higher capacity axle. With the heavier load, and the thicker axle, I was uncomfortable with the total amount of suspension travel I would be left with. I didn't want the axle to be banging against the frame if I went over a big bump.

With a lot of work on Autocad and Excel, I was able to calculate how much suspension travel I had originally, then calculate how much weight I wanted to add. When I got finished, it looked like adding a single leaf was the best way to go. Because of some other changes (new larger axle and larger brake drums) I also went to a gas filled shock absorber that had longer travel than the original.

I'm happy with what I got. The nice thing about having leaf springs instead of a Henschen or Duratorque axle is you can change the spring rate in a couple of hours and at minimal expense. All you need to do is take the spring pack off and add or remove a leaf or two.

Good luck, and keep us posted.
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Old 02-17-2005, 10:31 AM   #5
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okay, I'm a little slow, sorry . . . I understand how to use the formula but I still haven't figured out how you get from the "Pounds per Inch" answer to the capacity or weight it can carry? And the 35" I was measuring was horizontally from fixed/welded "bracket" to "bracket".
Here's the formula w/my data (I also have .36" thick leaves):

[(1.75x10)/12] x [(1000x0.36)/35]cubed = 1,587 #/in

Does that mean I have 3,174#'s of capacity (2x1587)?
And does the example problem really only have 146#'s of capacity?

I really appreciate your help/patience.
Mark
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Old 02-17-2005, 11:39 AM   #6
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Mark,

Getting from the spring rate to the capacity is a little tough. First, your springs are definitely different than mine, being they're only 1 3/4" wide and mine are 2". On the other hand, I was looking through some old photos and saw a '52 flying cloud that had a spring pack that looked like it had 10 leaves in it. I think you should assume you have the original springs.

How does the spring rate translate to capacity? It doesn't, really. Capacity is based on the frame, axle, bearings, wheels, and tire load rating. What the spring rate tells you is how the trailer will react as you drive it over bumps. Springs absorb the shock of driving over potholes, curbs, and other road hazards.

What the spring rate tells you is only that you have 3,174#'s of capacity per inch of axle travel relative to the trailer. The important part is not how much your springs sag when you overload the trailer, but how it will react when you run over a 24 at 70 mph.

The energy that's placed on the wheels when you run over something needs to be absorbed by something, otherwise it goes directly to the trailer and breaks all the crockery. The springs absorb the force and dissipate it as heat. How much the springs travel as they absorb the energy is the spring rate. If you have a very light load, and lots of room (say 6 inches) between the axle and the frame, you can use very light spring rates, and get a very smooth ride, like grandpa's old Oldsmobile.

But in an Airstream, where you might only have a total of 2" of travel between the frame and the axle, you need to have a heavier (and stiffer) spring rate, or else you might have the frame and axle bumping into each other.

As for the example calculation, that was probably the rear spring on grandpa's Olds. It may have an unloaded arch height of 9" or more. Installed, it might have an arch height of 3", so a pair of springs that size could easily support the back half of an Oldsmobile.

Hope this is helpful.
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Old 02-17-2005, 12:05 PM   #7
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wow . . . tell me there won't be a test on this stuff.

I think I'm understanding - am I correct in thinking this is a fairly stiff ride?
Like the trailer probably "gets air" when it goes over that 2x4?
And because of the limited travel distance, it has to be this way - correct?
Thanks again,
Mark
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Old 02-17-2005, 12:28 PM   #8
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Mark,
It's a tough call. Like I said before, I was adding weight and upsizing my axle, so I went a little stiffer than the original.
I don't know what the original design conditions were for your '52 FC. Does it have plenty of clearance (>3") between the axle and frame? If it does, you could probably soften the ride a little, expecially if most of your travel is 'highway'.
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Old 02-17-2005, 01:31 PM   #9
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Mark,

I knew I'd seen a Flying Cloud with a 10 leaf spring pack. Here it is: http://www.vintageairstream.com/floy...es/brakes.html
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Old 02-17-2005, 01:50 PM   #10
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While on the subject - can a spring shop determine if a spring should be replaced or not? My plan was to just replace them with new modern springs, brackets etc. It would probably be easier just to keep the springs I have if they are good - however - I want reliable running gear...

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Old 02-17-2005, 02:12 PM   #11
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Ken,

Definitely. I would recommend taking them in for a 'rebuild' just to have someone take them apart, check for cracks and corrosion, and freshen up the hardware.

Some shops might want to 're-arch' them. I've heard that this is only a short term fix. The shop that did my springs said they looked fine.
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