Good! My goal was to take something simple and overcomplicate it. :homer:My head hurts:flipoff2:all I know is that my springs work very well
I don't think there is anything magical about springs going into negative arch. The length, thickness and material really dictates just how far the spring can flex without yielding (permanently sagging). For the same material of the same size, I don't see how a spring with 6" of arch flexed flat would be really any different than a spring with 3" arch flexed 3" past flat.-What happens when the spring is pushed into negative arch.
-What happens when the axle pulls the spring beyond static arch on droop?
Your analysis is pretty interesting alright! Thanks for working it up using my project parameters as an example. What a bonus, as I can benefit very directly from the discussion!Good! My goal was to take something simple and overcomplicate it. :homer:
I like leafs because they are simple and can work great. But I've also been in a number of leaf spring rigs that are all over the road, ride rough, or have otherwise terrible manners. A lot of it has do with things like friction (why a new YJ might drive fine but a clapped out 200k mile one rides like a shopping cart on a cobblestone road) but geometry surely plays a big role too.
I've seen some that work great, some that don't. I think they can be very tunable, and think it's worth understanding what is happening when we move the hangars around to make the shackle angle "look" right.
I don't think there is anything magical about springs going into negative arch. The length, thickness and material really dictates just how far the spring can flex without yielding (permanently sagging). For the same material of the same size, I don't see how a spring with 6" of arch flexed flat would be really any different than a spring with 3" arch flexed 3" past flat.
Assuming you aren't yielding the spring, it appears that the rate will increase slightly because the shackle angle will be headed in a more vertical direction. (The spring end will be moving down while the axle is moving up). Easy enough to add to my sketch and see..
When it droops below static arch the axle is pulling the spring down (stretching it). I think it would have a similar spring rate, but now the force is acting in the opposite direction. I think things like the gaps between the spring clamps and main and the way the lower leafs fit against the main will have a big impact on what that spring rate is.
Anyway the takeaway I have from what I looked at is:
-A flatter (less vertical) shackle angle results in a softer spring rate.
-This also results in a little more travel (just like changing the lever ratio on a cantilever shock, or leaning a coil over on a solid axle).
I still don't understand why this seems to contradict the standard shackle angle diagram.
I don't understand why this particular Rubicon Express spring seems to be too short for a stock YJ. From what I see you'd want the shackle hangar moved a bit closer to the spring hangar to prevent shackle inversion at max droop, regardless of what spring rate you might want. This makes me think I did something wrong, but I don't know what.
I had the link on the original post but it must have gotten lost in the copy and paste so thanks for re-posting that.Your analysis is pretty interesting alright! Thanks for working it up using my project parameters as an example. What a bonus, as I can benefit very directly from the discussion!
The drawing with the "90degree" positioning that your earlier post references comes from Quadratec. Here's a link for everyone: Jeep Parts, Jeep Accessories & Jeep Soft Tops From The Jeep Parts Experts - Quadratec
On your analysis of rate effects:
The referenced Quadratec article discusses both an effective spring rate change from shackle angle but also an effective rate change from a "jacking" effect. It sounds like the shackle angle effect on rate and the jacking effect on rate can offset each other, with one dominating over the other depending on the geometry and operating condition.
Your analysis drawing and data are like the the "case A" jacking effect figure and discussion. Your analysis is showing a softening effective spring rate with greater shackle angle (from vertical) and this is consistent with the "jacking effect" they describe.
You had mentioned previously that your analysis seemed to be running counter to the posted 90 degree / angle effect drawing from the Quadratec article. Looking at your methodology, I'm wondering if what you are modeling is actually this jacking effect, not the angle effect. Your model shows the effective rate change due to the upward rotation of the shackle creating more vertical compliance to soften the effective rate. This seems to be the case A jacking example. The article also indicates that in this geometry, the shackle angle effect is stiffening the rate - countering the the jacking effect to some extent. It is not too clear when one or the other starts to dominate.
I ordered the SAE design & application book for leaf springs and will see if it can shed some light on the two different shackle effects.
It's a great article TGomes1987! . Thanks for your original post link. I couldn't find it in the new thread, and didn't want to lose it. Sorry I didn't give you credit for your original posting of it.I had the link on the original post but it must have gotten lost in the copy and paste so thanks for re-posting that.
It sounded like from that article that the jacking effect tends to be dominant but I am hoping the SAE design book can shed more light on how to determine when each effect dominates.