spring shackle design question

[48Rob]

KC and Chris,

Here is a quick sketch of what I have in mind.



It may be that throwing everything away and building a trailing arm system would be easier...but I have a good brake axle, I have good springs, and I really want to try to make this work.
If none of the ideas are worth a darn, then I will be wrong, and will have to say I should have listened to those guys the first time...


The concept is that the trailer keeps the leaf spring.
The shackle, and shackle mount are modified.
The shackle will bolt to the spring as is standard.
The rest of the shackle will have a slot machined most of its length, be around 10" long, maybe a half inch thick for strength, and will connect to a shoulder bolt or machined pin that runs though the frame (in a welded sleeve, with greasable bushing.

A set of air bags near the frame, with independent air fill valves will be used to lift/lower the frame from the axle.
The bags would connect to the frame rails, or to a 2 x 2 support between the frame rails.
The shackles, being connected to the frame with a single bolt, will be able to move up and down, and front to back to be able accommodate the see/saw motion of the leaf and air bag springs.

When traveling, the shackles do nothing but keep the springs from moving side to side, the air bags support the weight, the springs just hold it all together.
When lowered, the rear spring eye rests against the frame.
In an emergency, if the bags failed, the compressor failed, or whatever, the trailer could still be towed because it still has the leaf springs, which could still move forward and back.
It would be rough, but possible.

Rob

As you've noted, air bags have a sweet spot, a level/psi/height at which they offer the best ride.
It would be easy to buy a set that offers the ride height I need, at the middle ground of pressure.

[KCStudly]

I think there will be a whole lot more 'action' going on than you are envisioning and the slotted shackle won't work the way you think. There will be side loads involved and the constant up and down motion from going down the road will result in a lot of wear along the slots. It might allow the spring to twist, too.

If you want to keep your existing leaf springs as a matter of cost savings and so as not to reinvent the wheel (so to speak) then you might want to just turn your existing leaf springs into your new trailing arms by cutting them off behind the existing U-bolt plates (do this with an abrassive wheel or reciprocating saw so as not to build too much heat, don't use a torch). Add a simple panhard bar to address lateral loads (look to 67 thru 72 Chevy and GMC 1/2 ton trucks for inexpensive track bar parts LMC Truck Catalog). Keep the panhard bar as close to parallel with the axle at ride height as possible. You could even leave the rear portion of the spring a little longer and mount your air bag there (however that would likely put a lot of the air bag profile outside of the frame, so maybe not, it would be better inboard over the axle tube).

It wouldn't be that difficult to replace the front half of the spring with a rigid tube member, but there would be more material involved and so there would be some cost associated with that. These guys have busings with mandrel (DOM) sleeves Ruff Stuff Bushings that could be welded to the front of some square or rectangular stock, then just drill a hole near the other end for the spring locator pin, cap the rear end of the tube and you're done with the trailing links. Probably wouldn't even have to replace your u-bolts if the tube size was a similar thickness to your old leaf spring.

Heck, I'm having second thoughts on my purchase of YJ leaf springs now. for the price of those and half of my leaf spring mounting hdwr I could have come close to having air bags myself!

[48Rob]

Thank you.

So, if I install a simple panhard bar



I have prevented all side to side potential movement of the axle and no longer "need" the rear shackles to prevent side to side movement, is this correct?
If so, the air bags can be used to adjust ride height, with a travel limiter, and when deflated, the rear end of the leaf spring can come to rest against the bottom of the frame, to prevent the dropped floor from extending down too far/touching the ground?
Shocks will be required also correct?

Rob

[Martiangod]

with panhard installed your springs will act as trailing arms
shocks required with airbags and can act as travel limiter on air up

[droid_ca]

what about the way a motorbike has a swing arm...more fuel for the fire

[KCStudly]

OP has indicated that he wants to minimize cost impact and build effort by keeping as much of his existing set up as feasible, and still get to a point where he has an easily adjustable lowering capability for storage purposes.

Provided you keep the leaf spring front halves (or simple tubular trailing link replacement, my vote) firmly clamped in the leaf spring u-bolt plate with the original pin locator engaged, I see no reason to keep the back portion of the spring. The front half of the leaf (or link) will locate the axle lengthwise and prevent axle wrap as before (you might get a little more axle wrap under braking than previously, but as long as you don't get any hopping under hard braking, you will probably be ok). With the rigid tubular link option clamped between the axle spring perch and u-bolt plate, axle wrap would be more fully controlled, virtually eliminated. A true ladder bar (with triangular tie in to the top of the axle, as well as a connection at the bottom) is probably not needed due to relatively low braking loads and no driving/accelerating force passing through the axle.

The panhard bar eliminates all axle side to side movement (except a tiny bit due to the arc path of the bar...minimize this by making the panhard as long as practical and by keeping it as close to parallel with the axle as possible...if the frame end is a little higher than the axle end that would be better than the other way around since the axle traveling down the road will tend to push up more often than it drops down). Using Pythagorean theory for a right triangle (A^2 + B^2 = C^2) for a 3 ft panhard (= C) and a 3 inch change in suspension height (= B), if the panhard starts level the side to side movment of the axle will only be 1/8 inch (where axle movement = C minus A). The front spring bushings can easily accomodate this. The longer the trailing links the more side to side travel can be accomadated by the front bushings.

The further away the panhard bar starts from horizontal the more sideways travel of the axle will occur if it continues to travel in the same 'farther away from horizontal direction. It is similar to dwell in a crankshaft piston around top dead center (TDC)...when the crankshaft and connecting rod are in a line the piston doesn't move very much with modest rotation of the crankshaft, but when the crankshaft throw and connecting rod are at 90 degrees apart the piston is moving at its highest speed relative to crankshaft rpm. Think of the panhard bar as a crankshaft.

Let me see if I can whip up a sketch to go with the math.

[KCStudly]

Ok, here's what I'm talking about.


Hope you can see it. The Panhard bar length never changes so when it is horizontal it is 36 inches, and displacement of the axle is zero.

When the axle drops 3 inches (B) the panhard bar becomes the hypotenuse (C) and the long side of the triangle is 'A'. The difference between 'A' and the starting point is how far the axle has moved sideways.

The closer the panhard bar gets to straight up and down, the more sideways displacement you will get per degree of rotation, or inch of drop/rise.

[48Rob]

Thank you guys, for the help.

This is sounding incredibly easy...
Of course anything is easy once you know how.

Yes, I understand the displacement theory, nice drawing!

Measuring my setup, I find that between rest, and typical/final ride height, I have 5 inches difference.
That is, I have 5" between the bottom of my frame and the top of the axle.
Using this diagram,


It appears that the bracket that attaches to the axle will place the panhard bar 2-3" above the axle.
So, a bracket welded to the frame, hanging down 2-3" should put the bar horizontal when the trailer is being towed.

"simple tubular trailing link replacement".

Okay, I'm willing to consider this option.
What I've found on line so far looks like this,
[img]
http://thumbs4.ebaystatic.com/m/mLwYIpg ... xw/140.jpg[/img]

Is there some pre made arm that would work, or is it a build your own proposition?
Currently, when at rest, the axle, or more precisely, the "U" bolts contact the bottom of the frame.
If I clamp a trailing arm on top, I will lose clearance.
If it bolts on the bottom, as the spring now does, it could work.
Are you talking about one of these?


Using the shocks as a bag limiter seems reasonable, thanks.

Rob

[KCStudly]

Basically it would be like those truck trailing arms, but those are much too long and specific to that application. The perches on those truck rear axles have a really big single u-bolt that passes thru the perch and down thru the middle of the trailing arm. Those are actually the genesis for modern NASCAR rear suspension. The arms go all the way up to the middle of the vehicle right near the transmission tail shaft. Not what you want. If you are a NASCAR fan you have seen them before whenever somebody gets upside down.

What I was thinking was just a straight piece of square or rectangular tube with the Ruff Stuff bushing sleeve I linked to earlier welded across one end, drill thru the back of the tube where the axle spring perch is and use a cylindrical socket head (Allen) bolt to act like the leaf spring alignment pin, and clamp the existing u-bolt plate on the other side (or better yet, weld it on the top of the tube and get new shorter u-bolts). Cap the open rear end of the tube by welding on a piece of steel plate or flat bar (to tidy the end up and keep things from growing in there). Yes, I am assuming that your current spring is located under the axle, and that the new link tube would also be in that location.

Do you have welding skills or someone competent to do this for you? I wouldn't think twice about doing this for myself without too much expense for the steel and minor parts, but I would also understand that for inexperienced fabricators this kind of thing could be a little daunting, and as soon as you have to farm something out somebody else's profit gets involved and muddies things up...plus all of the little bits tend to add up in the end, price wise (ask me how I know that to be a fact).

The advantage to splitting the height of the panhard bar mounts is that neither of them would have to be especially long (tall). The taller they are the more beef and gusseting necessary to counteract the leverage that would result; best to keep them both shorter. The bar does not have to be directly over the axle. In fact it would probably be better if it was just behind the axle, giving more room for the axle to move up.

Although I have no direct experience with air bags, from what I understand shocks are a good idea, otherwise they can be bouncy. From what I have read here the consensus seems to be that shocks are a good idea for any type of suspension on every TD or TTT, so...I'm planning to use them on my build.

[Martiangod]

Outside of control issue using shocks, you have to limit the extension of the bag, if you over extend a bag it will tear, maybe not the first time, but eventually it will.
When I go on a lift, according to firestone, my rear axel must be supported so not to over extend the bag, so I always insist my truck go on a wheel lift

Heres a nice post of a home made trailing arm setup, panhard, also showing a C notch to get you lower if needed
Pics and everthing

http://67-72chevytrucks.com/vboard/showthread.php?t=451111

[KCStudly]

Axle Limiting Straps should be used to keep the axle from drooping out too far and causing damage, not the job of the shock which could be damaged itself.

Bump Stops should be used to keep an axle from bottoming out too hard and/or damaging other components, such as shocks.

Shocks should not be used as suspension stops. They should be sized to have slightly more travel in both directions than the suspension limiters, otherwise change the suspension limits using another method, such as the linked devices. Various types and sizes of bump stops are available ranging from cheap rubber to big money hydraulic and/or air damped units.

Of course this is overkill for most common trailer suspensions in our categories, especially for on road service, but then we are not talking about common here.

[Martiangod]

Any coil sprung car I had, 68 gmc, 74 torino only limit was the shock, undo the shock, drop out the spring

I'll drop out of this convo now, as you know all

[KCStudly]

Didn't mean to offend, just trying to share what I know (or at least think I know).

Like I said about most applications and on road applications, not really needed.

I agree, the designers of your old truck and car never intended for anyone to try and stretch the shock with an air bag, and jacking a car up to pull out a spring isn't quite the same as rock crawling, which is what the limit straps I linked to are intended for (sorry, I always seem to have 4x4 stuff on my brain).

Peace.

I thought I was wrong once, but I was mistaking!

[48Rob]

Once again, thanks to both of you

My welding skills are good enough for farm and equipment field repair.
I'm tacking it all together and have a pro coming to finish, as I would prefer to have a legitimate company with insurance to back up the work should something ever happen.

Chris, that "C" notch is quite a neat way to get a few more inches of clearance

I appreciate all the links.
I'm ready now to start building.
Pictures soon...

Rob

[angib]

Chris, that "C" notch is quite a neat way to get a few more inches of clearance

Also an excellent way to remove a lot of the strength of the frame..... I guess welding in the really thick-walled half-pipe gets some of the lost strength back, but not much.

Mind you, I don't think there is that big a load on the frame at that position.

I think it would be simpler to make up a trailing arm airbag suspension with the two end 6" of the current axle (plus spindles, brakes, etc) cut off and welded to the trailing arm to re-use them. But it's still a major exercise.