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Author *Topic: The Mono-Leaf rear suspension: Repairs/Coilover Conversions  (Read 5131 times)
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Twilight Fenrir(4) 3100mpfi(2)
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Twilight Fenrir
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« on: October 03, 2015, 09:25:49 AM »

Hello there,

I picked up '95 Grand Prix 2 days ago. The rear trailing arms were broken off when I bought it (Which is why I only paid $300 for it n.n) But I'm a blacksmith, and knew I could fab up some new mounts easily enough. I noticed the Mono-leaf in the back was split when I was looking at the car, but didn't pay it much attention aside from a curiosity as to why it was there.

I assumed, (wrongly) that there were still coils on the struts, and that this was some sort of performance suspension gimmick that wasn't integral to the operation of the car. Yesterday, I took the wheel off and started working on the trailing arms, only to realize that mono leaf spring is the entire rear suspension.... So.... what do I do now? XD

The ends of the springs have slipped from the little recesses in the knuckle which they are supposed to live in. The spring is intact length-wise, but it's frayed at both ends. It seems to drive and handle amazingly well still, but there's no way this is a good place to be in. Winter is coming. And I need to get this bolted back together within the next month or be royally screwed. So what's the best way to approach this?

What I've got in mind... is making a steel boot to slip over the ends and bind all the little bits together, with some rubber padding inside and out to protect the springs. While I think this would work, I'm not sure how to get the frayed bits in order to do so.

I've heard that there are coilover conversion kits... but I can't find ANYTHING about where to get them >_> Could someone please point me in the right direction? Where my struts attach to the body are completely rust free... actually, so is 90% of the car, it's just the trailing arm mounts that completely dissolved. So I'm not worried about them punching through the car. As I said, I'm a blacksmith, if it comes down to it I'll add a piece of plate steel up there to reinforce it. I just have to get this thing together..

Thanks!

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3100mpfi
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« Reply #1 on: October 03, 2015, 02:59:19 PM »

This is what it is supposed to look like.



As far as real coilovers, there is not a "kit" per se.  People usually do the conversion by going to a junkyard and pulling the rear suspension from a Gen 1.5 w-body (Chevy Lumina or Monte Carlo) and swapping in the complete rear ends.  A lot of people who have done it say its really good, but personally, I've seen doing this to a Gen 1 car fail when the strut punches through the upper strut mount of the body.  Yours being a 95 might be "new" enough to handle the added stress.  Basically you will need the rear struts, springs, and upper strut mount of the coil setup, you will remove the rear monoleaf, and swap out the struts and rear sway bar.

Personally, I would just go to a junkyard and look for a car with a good monoleaf, or look for a replacement one online.  Its not too hard to swap out the spring.  Disconnect the trailing arm, disconnect the 2 lateral links, and remove the 2 hanger guide thingies, and then you can just pull the whole knuckle assembly outwards.  pull the old spring out, then when you put the new one in, I would get the moog leaf spring bushings, put one side all the way in, then use a jack and a piece of 2x4 to push the spring up and get the knuckle back into place.  Once the 2 lateral links are reattached, just let pressure off the jack, connect the trailing arms and the other other small things, and you are good to go (make sure you put the second spring pad in before removing the jack). 

Post Merge: October 03, 2015, 03:01:52 PM
also, going back and looking at yours, I believe your monoleaf is actually totally broken off and its not the mount that you need to fix, its replacing the whole spring.

http://www.ebay.com/itm/CUTLASS-SUPREME-GRAND-PRIX-LUMINA-CAR-REGAL-FIBERGLASS-MONO-LEAF-SPRING-/331670249547?hash=item4d3916604b&vxp=mtr
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Twilight Fenrir
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« Reply #2 on: October 03, 2015, 04:21:44 PM »

This is what it is supposed to look like.

As far as real coilovers, there is not a "kit" per se.  People usually do the conversion by going to a junkyard and pulling the rear suspension from a Gen 1.5 w-body (Chevy Lumina or Monte Carlo) and swapping in the complete rear ends.  A lot of people who have done it say its really good, but personally, I've seen doing this to a Gen 1 car fail when the strut punches through the upper strut mount of the body.  Yours being a 95 might be "new" enough to handle the added stress.  Basically you will need the rear struts, springs, and upper strut mount of the coil setup, you will remove the rear monoleaf, and swap out the struts and rear sway bar.

Personally, I would just go to a junkyard and look for a car with a good monoleaf, or look for a replacement one online.  Its not too hard to swap out the spring.  Disconnect the trailing arm, disconnect the 2 lateral links, and remove the 2 hanger guide thingies, and then you can just pull the whole knuckle assembly outwards.  pull the old spring out, then when you put the new one in, I would get the moog leaf spring bushings, put one side all the way in, then use a jack and a piece of 2x4 to push the spring up and get the knuckle back into place.  Once the 2 lateral links are reattached, just let pressure off the jack, connect the trailing arms and the other other small things, and you are good to go (make sure you put the second spring pad in before removing the jack). 

Post Merge: October 03, 2015, 03:01:52 PM
also, going back and looking at yours, I believe your monoleaf is actually totally broken off and its not the mount that you need to fix, its replacing the whole spring.

http://www.ebay.com/itm/CUTLASS-SUPREME-GRAND-PRIX-LUMINA-CAR-REGAL-FIBERGLASS-MONO-LEAF-SPRING-/331670249547?hash=item4d3916604b&vxp=mtr

Oh I see, thank you, that's very informative n.n

Reading a post from someone else made it sound like the entire rear end had to come appart to change out the mono-leaf, but that doesn't sound that bad... Especially if I bring a sawzall with me to the salvage yard tongue No one in their right minds would go to the salvage yard for one of the POS stock trailing arms X3 I do like the idea of keeping the monoleaf, it's an.... interesting setup.

Ah crud... I just went out to look again, and you're right, the spring is (mostly) broken off... about half of the spring, top to bottom, still reaches into the slot for it. The other part is broken off...

I'm still going to give forging a boot for it a try, though. It won't cost me anything but a half hour, and a few pennies worth of steel... If/when it doesn't work, I'll do what you suggest. Thanks :3
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« Reply #3 on: October 03, 2015, 09:16:27 PM »

YW.  Don't let some people tell you that the monoleaf setup is crap.  If it was crap, GM would not have used it in the Corvette. 

Personally, what I would do, is if I swapped out the monoleaf, I would upgrade the trailing arms themselves as well.  The upside down U shaped ones tend to bend.  You can get tubular ones online for cheap that are stiffer.  Its an easy upgrade to do.  Another thing to look into, if you go to the junkyard, is a thicker rear sway bar.  Look for a Lumina Z34 (93-94~ish).  If memory servers, that is the years that got the thickest ones.

GMPP used to have the trailing arms, but seems they are discontinued.  They say the  2005-2008 Pontiac Grand Prix GXP has them, so might want to search for them on ebay, or in a junkyard.  They are a lot stiffer, and a direct bolt-on.
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« Reply #4 on: October 05, 2015, 05:23:45 AM »

YW.  Don't let some people tell you that the monoleaf setup is crap.  If it was crap, GM would not have used it in the Corvette. 

Personally, what I would do, is if I swapped out the monoleaf, I would upgrade the trailing arms themselves as well.  The upside down U shaped ones tend to bend.  You can get tubular ones online for cheap that are stiffer.  Its an easy upgrade to do.  Another thing to look into, if you go to the junkyard, is a thicker rear sway bar.  Look for a Lumina Z34 (93-94~ish).  If memory servers, that is the years that got the thickest ones.

GMPP used to have the trailing arms, but seems they are discontinued.  They say the  2005-2008 Pontiac Grand Prix GXP has them, so might want to search for them on ebay, or in a junkyard.  They are a lot stiffer, and a direct bolt-on.

I already have new trailing arms on their way. The bolts were rusted to the bushings, and I got some closeout arms for the same price as the bushings would cost n.n

I had to rebuild the trailing arm brackets, and the part of frame to which they attached. I've got my passenger side in, need to do the drivers side yet. They are hilariously overbuilt now XD 3/8" square tubing and 1/4" plate steel. Bolted, and welded to the body. The front welded to that crossmember under the front of the rear seat.
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« Reply #5 on: October 08, 2015, 10:27:56 AM »

And I pulled a new monoleaf from a salvage yard for $40 n.n Only took about 20 minutes to take it out, so thanks  for the info! Saved me a lot of trouble! biggrin It was harder carrying my tools across the yard than pulling the part...

 My Prix should be on the road by the end of this weekend n.n
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