Question for those who know leaf springs better than me.

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Jan 9, 2011
I had an incident while towing camping gear this last weekend. Saw smoke from one trailer wheel and pulled over to find the leaf spring had over extended allowing the shackle to flip up (not invert, other way) and stay there in that position, the tire was rubbing the fender.

The total load couldn't have been near even 1,000lbs and the angle of the shackles at rest weren't noticeably different from what they are unloaded.

Not sure what the load rating on the springs is. The spring mounts are 29in apart and the spring seems to be 30.5in eye to eye. From what I can find, three leaf springs this size tend to be rated from 650-1250lbs. However, I don't know if the springs were overloaded. Even extended with the shackle flipped up the spring wasn't flattened, wondering if the geometry is just off.

Here are some pics unloaded.



Is that too much angle at the shackles? Solutions?

I found 1-3/4inx26in add a leafs I was wondering about, or do I need slightly shorter springs?
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You mean the shackle travelled all the way round to the right (in the top picture) and the suspension bottomed out?

Or to the left (top picture again).

The former would imply it was overloaded (that trailer looks like it'll easily be 600lb empty), or that the springs are done. If it's catching when up when empty there it'll probably just need greased to allow it to drop again on it's own.

Going to the left would imply they'd deformed pretty badly somehow and need replaced.

I think you probably need stronger springs or to add a few leafs.
And add some will help to diminish the spring travel harshness...I also agree with add a leaf theory

lol.....I think
Shackle angle can play a big role in effective spring rate. Take the two extremes for example; with the shackle vertical compression of the spring results in the shackle eye of the spring moving mostly rearward and very little upward (due the the shackle's arc of travel), that makes the effective spring rate very close to what the spring's rating is. With the shackle laid way over spring compression results in the shackle eye moving up as well as back. How much it moves up vs. how much it moves back is a ratio that is 1:1 at a 45° shackle angle. Less than 45° (more vertical) and it moves back more than up which yields an effective spring rate closer to the rating. Greater than 45° (more horizontal) will reduce the effective spring rate. Long shackles reduce the size of the arc, which reduces the possible degree of change. Short shackles will have a greater range of change.
That's what I was wondering with the shackle angle. When it was bound upwards I used a jack to lift the frame and unbind it, as the shackle moved through the arc back down the spring wasn't flattened out at all (the spring didn't need to flatten out for the shackle to go through the arc back down to normal). That makes me think either the spring is a hair too long or the shackle is a little too short, allowing too much upward movement instead of backwards movement.

The trailer was hardly used when I bought it, just looked a little ruff from primer failure so the springs are still fairly new. The PO had it built by the guys at Iron Pig. There was a build thread on it, I'll look for it.

When I unloaded the trailer from the trip I noticed there really wasn't a change in shackle angle from loaded to unloaded trailer weight, which is why I'm unsure if the trailer was really overloaded.
I'd just cut the shackle hanger blocks loose from the frame, move them back 1"-2", weld them back on, and then plate over their mitered cuts to put some lateral strength in them.

And add shocks as far outboard as you can get them.
This is a fairly common occurrence in your type of configuration (with your conditions - a straight link plate, light load, weakish springs for those size tyres and no shocks).

What happens is that the leafspring compresses as it hits a bump, the tyre then becomes airborne, the stored energy in the compressed spring together with the weight of the axle and the large tyres, then shoots the leafspring downwards, extending it so far that the distance from eye to eye now rotates the rear link plate to it's maximum forward horizontal position. As the leafspring now retracts, it simply continues it's rotation direction and lands up jamming upwards as it has done.

The best remedy for this is to fit a triangular shaped link plate that prevents forward rotation. Some chaps also fit a chain or webbing strap to the axle, arresting the axle's downward movement. (This has a secondary benefit for shocks)

Shocks are excellent, but also only while they are working. Often shocks fail because they are the mechanism that arrest the downward movement of the axle, and when they bottom out, it's good buy to a set of new shocks.

For serious off road you have to limit the downward travel of the axle when you fit shocks (or ensure that the shocks have sufficient extension, not to fully extend.

Simply adjusting the link plate more vertical (as it should be) will only make this happen quicker. The springs are probably also on the light side.

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