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Spring sagging with shackle flip?

Discussion in 'OffRoad Design' started by AJMBLAZER, Oct 14, 2001.

  1. AJMBLAZER

    AJMBLAZER Better to be lucky than good. Premium Member

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    Stephen, or anyone for the matter, I need a little info:
    I'm getting close to doing the solid axle swap on my Blazer, so I will need about 4-5" of lift in the rear. I've already got some 1" zero rate of yours back there, so I was thinking the 4" shackle flip and my already flexy stock springs and that 0 rate. However, the owner of the shop doing the conversion says that he shies away from the flips because they move the axle forwards and make the spring move outside of it's normal range of movement. Which causes them to sag prematurely. I was aware of the axle moving forward thing, but not of the sagging. Is this an issue?

    He suggested some 4" or 5" spring packs for the rear instead. This will eventually become the family vehicle, fiance/wife's daily driver, and tow vehicle, so it doesn't need a RTI of 1000, I just want a good, capable multipurpose truck. I can't truly say if he wasn't just trying to get me to buy the more expensive springs from him or not, but it seemed to me that he was knowledgeable about the swap and mechanics in general, and that he was being honest with me.

    I'm not trying to defame your product or anything, I love your stuff and your zero rates. I recommend your stuff to everybody I talk to with a solid axle GM, and then some. I just want to be able to discuss the flips with him from a more knowledgeable standpoint before we decide on the rear.

    1992 Blazer Sport, 350/4L60, Flowmaster, K&N, MSD stuff, 2" Rancho[​IMG], 285/75R16 AT's[​IMG]
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I can't see the flips causing the springs to sag...hit a bump and they have the same forces on them. I suspect you were getting a pitch to buy new rear springs. Just because he was knowledgeable doesn't mean he has your best interests in mind...[​IMG]

    Unless I'm overlooking some obscure spring theory...and if so maybe someone else can shed some light on it for me too.

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
    <a target="_blank" href=http://coloradok5.com/gallery/project_T2> tRusty pics...</a>
     
  3. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    There is some obscure spring theory that comes into play here in the form of basic differences between a tension shackle and a compression shackle. With a tension shackle, as the spring flattens out and starts to go into a reverse arch, the shackle angles and reactions cause the effective spring rate to skyrocket, mathmatically going to infinity. With a compression shackle, you can have a rising rate as the spring compresses, but at some point, the effective rate is going to drop off, basically tending toward zero. I think this is why most trucks run a tension shackle, they're more tolerant of a drastic overload situation.
    BUT..... here in the real world they put big overload leaves on the bottom of the spring pack and it basically become impossible to reverse the arch. I did work with a flip kit and my last set of custom springs with no overload and it all worked out OK. They did go to a decent reverse arch when they were twisted, but never overflexed or "went away". And one consideration of the shackle angle is keeping a rising rate on the spring up to it's usable limit.

    So, the practical answer to your question is that the flip kit will work out just fine for your use. You do have a nice flexy stock spring pack that will continue to work just the same after the flip as it does now. It will actually seem softer due to the raised CG and it's resulting greater leverage of the spring.
    As for the moving the axle centerline, it's definitely possible, and I haven't drawn out the IFS truck rear suspension to see how much it's going to move, but I do know the rear of the spring is above the front in stock form, and the springs are pretty flat, so they're not going to move a lot. and there are ways to move the axle back if you like.

    Making the world better, one truck at a time.
    SW-ORD
    <a target="_blank" href=http://www.offroaddesign.com>www.offroaddesign.com</a>
     
  4. Donovan

    Donovan 1/2 ton status

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    I was told by Deaver Springs the fast way to make a spring go flat is to put the spring in to reverse arch on a normal positive arch spring. Like when you flex the rear suspension. I have no proof of this but that is what I have been told. So take it with a grain of salt or take it for its word.

    Donovan
    <a target="_blank" href=http://www.rustbucket.rockcrawler.com>www.rustbucket.rockcrawler.com</a>
    Got Leafs??
     
  5. Stephen

    Stephen 1/2 ton status Moderator Vendor

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    There's quite a bit more to it than just reversing the arch. Material type, thickness number of leaves and amount of available travel to a bumpstop are the biggest factors. My customs look like they're turning inside out when fully flexed, but are designed for it and live just fine. Obviously a stock spring isn't built for that kind of use or it wouldn't have a big overload leaf to keep it from happening. Most stock springs are stopped by the lack of available weight to keep compressing them rather than a bumpstop. That's what they're supposed to do.

    Making the world better, one truck at a time.
    SW-ORD
    <a target="_blank" href=http://www.offroaddesign.com>www.offroaddesign.com</a>
     

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