Rear Shakle Position Theory, Trailer Axles

[andyfletcher]

The drawing included shows the way I design my shakle
position both for single and double axle (Example 1). The shakle for a double differs in that more degree tilt to the right is required at frame attach point to compensate for the added total horizontal spring length. Point C should not go beyond 15 deg in my view, and that's a ballpark figure, otherwise the fulcrum effect would occur.

A shakle tilted to the left of vertical in design and welded to frame as shown Example 2 creates an added force decreasing load limit as the loaded trailer works the spring in compression (horizontal from semi-circular). The added force comes from the spring itself by creating a fulcrum point on the shakle at the spring/shackle eyelet D.

What do you think?

Fletch

 

[MicroPilot]

What's a shakle?

 

[RobS]

Not quite sure what to think but if you check the Dexter Axle web site they have a ton of information for trailer fabricators /forums/images/graemlins/smile.gif

 

[JD4610]

I would agree with the basic principles you are stating with regard to shackle position versus spring deflection.

However, I would be more inclined to question the effectiveness of the overall design you have presented. By having the trailing eye of the front spring directly connected to the equalizer bar, you are inducing in the equalizer the very forces you are trying to neutralize via the shackle angle in the rear spring. As the front spring is loaded and flattens, it will cause the equalizer to rotate counter-clockwise (as viewed in your diagram.) The vertical force of the rear spring will also induce a counter-clockwise movement in the equalizer thus pulling the front spring even straighter. The further the equalizer moves off center, the greater this effect would become.

I had always thought it was an industry standard to have the shackles of both springs at the equalizer in a tandem-axle setup.
I'm not saying it can't be done any other way, just that I haven't seen it.

Or maybe I'm missing something? /forums/images/graemlins/confused.gif

Jay

 

[theonlybull]

i second JD4610's post. most tandums have the shackles on the equalizer bar, and will only work that way.

 

[andyfletcher]

JD:

If I read you right, you are describing the standard setup. In fact double axle shackle kits are sold this way. Install standards would be to attach front spring eye to frame bracket as shown, then attach rear spring eye the same as the front, then have the remaining 2 eyes of each spring shackled to the equalizer.

On load, the front wheel would moves rearward, rear wheel forward.

Looking at it from above, wouldn't an uneven load place each axle askew of each other causing tire wear and stress on the running gear for that side.?

In retrospect, my setup isn't 100% either. An uneven load sets the angle of each axle askew the same in respect to the opposite side, causing tire wear and stress, except both tires point in the same direction.

I guess your method is the industry standard, but I still say the principle of angle attachment in my original post still stands and that is the shackle attachment at the equalizer should be angled slightly forward of the spring, not back. Plans show it being installed vertical, which is passable in my opinion.

A good way to test out shakle angle is with bent saplings. Cut them the running length of the spring and bend them to curve of spring, minus trailer dead weight deflection 1/4in +-. Then let them straighten out and measure the distance again.

 

[JD4610]

</font><font color="blue" class="small">( Looking at it from above, wouldn't an uneven load place each axle askew of each other causing tire wear and stress on the running gear for that side.? )</font>

On a "standard" setup, uneven loading would cause an articulated-tractor-like effect causing the trailer to want to turn continuously in a gradual arc. This would cause continuous tire scrub, the amount being proportional to the unevenness of the load on each end of the axles. On your design, each axle would skew in the same direction, although not identically. This would cause the trailer to dog-track, again with the amount being proportional to the offset of the load. So, yes, I think I understand your point.

Have you built a trailer with the design you have shown? I would be interested to see how it works in real life.

Jay

 

[andyfletcher]

No, I've seen many built the way I've drawn. Whenever I can, I watch the trailer if it pulls up to a curb or obstacle.

What usually happens is when the first axle spring compresses due to lift, the rear eye moves the equalizer rearward, that moves the rear spring first without compression rearward toggling the rear shakle back.

Then when load is further carried to the rear spring the shakle moves back a little further due to rear spring loading.

The whole effect is to transfer movement to the rear as one unit for that side of the trailer.

Though now with the dynamics you describe, I'm not so sure anymore.

Food for thought, thanks.!

 

[SPIKER]

I haven't chimmed in here yet but though I should too.


the front spring eye is mounted to the trailer frame, then the center rocker / piviot is mounted to the frame similar to the front eye. the front spring compresses under load and transfers rearward movement into the rear spring by two methods, one it slides the rear spring back slightly, 2nd it moved the front of the rear spring UP closer to the trailer frame. the amount it moves UP is equal to the amount back under normal light loads under heavier loads the rear srpinf front mount actually moves UP farther than it does BACK. As you can see this effects the rear shackel in a few different ways.

Under high loads it acts as a damper and keeps weight even on both sets of springs bu the streach form the first axel spring pushing back the 2nd axel and loads them more onto the truck. now when reaching max load you can actually cause a bit more damage to a front spring than the back spring as once the center pivioting section bottoms out rearward there is no where for the front springs extra mass to GO, under this senerio the max back and spring have to be dead level so if the spring wants to "Reverse bend" some it can. if the bottom out spot still has an arch in the front spring then that spring load is transfered right into the springs eye and a spring can then break the eyelet or the mount.

the rear spring & shackel should be pretty much vertical when loaded with the trailer weight to slightly facing forward. if the shackel is facing rearward when the trailer is NOT loaded then there is some mounting geometry that is poor. when the front spring & center schackel is fully compressed the rear springs rear shackel should be no more than about 45 degrees rearward. "this would be under trailer MAX LOAD."

again this leaves room for the rear spring to flatten out the arch and if the shackel is over 45 degrees then it is over stressing the rear spring by again attempting to raise the loaded trailer through lift & PUSH/EXTENSION.

the resulting is a heavy loaded trailers center of weight on the wheels can MOVE backwards several inches. remember this when you mount the wheell mud flaps/covers. /forums/images/graemlins/blush.gif

the spring eyes and schackels all need to be IN-LINE to avoid any binding of the mech when it moves up/down and for/aft.

there should also be a slight bend in the trailer axels so that the tires have a caster/camber effect to counder act the twisting forces of the axel & drag of the tires. front of the tires should face the inside by 2~3 degrees and the top should be out by no more than 5 degrees.

anyhow just my 2 cents worth.

Mark M /forums/images/graemlins/crazy.gif /forums/images/graemlins/tongue.gif

 

[Nat]

My question is how do you connect the spring eye to the equaliver eye? I have built many trailers and I can't see how you can connect eye to eye. The secondary propose of shackles is a means to connect the spring eye to the equalizer eye, and the spring hanger is made to accept the spring eye, you would need a post to connect to the shackles. Can you explainthe way to use them as you have? Thanks, Nat