[Swat]

Here's a good article: http://www.afcoracing.com/tech_pages/leaf.shtml

And a diagram to splain it:

The article is related to race cars and suggest a 90 degree starting point for your shackle angle. Wheelers prefer it angled back, position "A" in the diagram, so as to get some travel out of the shackle. The shackle drops down which adds to droop when extended and rises to stuff the wheel up higher in compression. However the article does say and shows in the diagram above that it adds spring rate in that position.

I hope this is helpful and can't quite get my mind around how the angle back adds spring rate? Perhaps it is just the force required to push the shackle back? I do know if you angle them too far back the shackle won't extend thus not allowing the spring to lengthen as it compresses.

 

[Redwagon]

I think at position A you run a bigger chance of the shackle rotating the wrong way (up instead of down).

 

[Swat]

I think at position A you run a bigger chance of the shackle rotating the wrong way (up instead of down).

I think your seeing the diagram wrong. Position A has the top of the shackle rotated forward and the bottom back as we like to run. The diagram is not the best as it shows the top of the shackle rotating which would actually be fixed in the hanger. It's the best diagram I have seen though. Think of position A as mounting the top of the shackle forward of the bottom of the shackle. Confused me too! Position B could allow the shackle to rotate up, get inverted/stuck and actually lift the spring a little as it droops!

 

[Redwagon]

Your right, now that I look at it position A is the direction I was thinking.

 

[reptillikus]

I hope this is helpful and can't quite get my mind around how the angle back adds spring rate? Perhaps it is just the force required to push the shackle back? I do know if you angle them too far back the shackle won't extend thus not allowing the spring to lengthen as it compresses.

From the page you linked.

The vertical movement of the rear spring eye causes a jacking effect. If the shackle movement forces the rear spring eye downward, the leaf will deflect and exert an upward force on the chassis that will add stiffness to the rear suspension. Conversely, the shackle will reduce suspension stiffness if t causes the rear spring eye to move upward during suspension travel.

The stiffening effect occurs during suspension deflection whenever the rear spring eye is ahead of the upper shackle pivot and the shackle is moving rearward (see illus. 4, example B). In this position, however, the shackle also produces a softening effect by reducing the effective rate of the leaf spring (due to the large shackle angle). The overall effect to the stiffness of the rear suspension is determined by the greater of the two shackle effects. Under opposite conditions, you can expect a reversal to the above effects. If the rear spring eye is located behind the shackle pivot (illus. 4 example A) the shackle effect will tend to reduce suspension stiffness whenever the shackle moves rearward. However, the small shackle angle will tend to stiffen the spring's rate. The overall effect to the suspension's stiffness is determined by the more dominant of the two shackle effects. Keep in mind that the movement of the rear spring eye (from its static position) is mostly forward under racing conditions.

Example A is what we all set out trucks to, not Example B so ignore that half of the diagram.

Ok so at 45º to 90º, the shackle angle causes the suspension to stiffen. Anything steeper than 45º begins to cause it to soften up, and anything with the shackle angle past 90º also causes it to soften up.

Remember though, this is talking in reference to drag racing, in which both shackles are generally at the same angle. In our case, when the truck is twisted up the shackle angles are usually opposite so depending on the angle of both shackles you could actually be doing either, altho it seems that most of the shackles arc consists of softening up the suspension, so most likely thats what wed be experiencing. And then of course the longer the shackle the more the leverage, so the more it will soften/stiffen the suspension.

Clear as mud? :toothless

 

[Swat]

Ok so at 45º to 90º, the shackle angle causes the suspension to stiffen. Anything steeper than 45º begins to cause it to soften up, and anything with the shackle angle past 90º also causes it to soften up.

In our case, when the truck is twisted up the shackle angles are usually opposite so depending on the angle of both shackles you could actually be doing either, altho it seems that most of the shackles arc consists of softening up the suspension, so most likely thats what wed be experiencing. And then of course the longer the shackle the more the leverage, so the more it will soften/stiffen the suspension.

Clear as mud? :toothless

I get it. I thought they were trying to say it gets stiffer past 90 degrees. I misunderstood. If it is in compression, moving say from 40 degrees to 50 degrees, bottom toward the back, as the springs flatten and elongate then the effective spring rate goes down with the leverage of the shackle increasing as the angle increases. The opposiite would be true as the spring droops. The first diagram I posted is a bit misleading and I think they wrote "stiffen" in meaning it would plant the wheel for mo better traction in a drag.

I saw your post on lengthening shackles, good job! I too went with a longer shackle to try and soften up the springs some. Alonger shackle would make sense for springs that have more arch than stock, more arch means more elongation as the leaves compress.


So what say ye all. Is 40 degree back best?