Cutting into the frame to accomodate longer coil springs for a rear 4-link (1 Viewer)

[truggier]

I'm looking for opinions on a proposed design to allow a Proffit's rear 4-link design to keep the full length springs that were sent with the kit. It involves cutting into and modifying the frame substantially so I want to be sure of what I'm doing before committing. Cutting the springs so that they fit under the frame means that at least two active coils would be lost (meaning I could carry considerably less load) as well as changing the spring rate and ride characteristics.

Before anyone says 'why in the *#$%#^ would you do that, it's not worth it, just cut the spring'...this is a project car and I'd like to do something different. It's an interesting change and I'm having fun with it...enough said, this is the way I want it.

I also realize that I would need to raise the rear floor on the body tub to make this work.

I have parts modeled up in AutoCAD and was able to get some screenshots of what I am thinking of:

- The first two are of the uncut spring compressed to ride height as it sits under the stock frame. As you can see, with the way the upper spring buckets would sit under the frame, I'd lose at least two active coils.

- The next shot shows the proposed cut into the frame to accomodate the full length spring, along with an added rail on top (blue) and a 1/4" thick plate for the upper spring bucket (also blue). Some of new structure that reinforces the area isn't shown in this shot but is shown in the next three shots. The rail on top of the stock frame would be 3/16" and fully boxed.

- The fourth screenshot shows some ribs extending lower than the upper spring bucket base plate to carry the load around it.

- Shot 5 is from underneath showing the upper spring bucket recessed into the frame.

- Shot 6 shows the fully built up modification, with the added parts in different colors for clarity.

- Finally shot 7 shows clearances for the rear tire and shock absorber. The rear tire is at the stock location and is a little close but I was going to add wheel spacers anyway. What is modeled up is a 35x12.5R16.

So...anything I'm forgetting? Any pitfalls / things to be aware of when altering the chassis like this, like fatigue cracking in the years to come? It will be much stronger than stock. Thanks.

 

[truggier]

The rest of the attachments.

 

[ibinazbass]

I personally think that looks awesome! I've always wanted to go 4-link front and rear eventually

 

[TCinVA]

If you can fabricate half as well as you can render it on the computer, you're golden. That looks like a solid plan to me. Some day I want to 4 link my rear and stretch the wheelbase and body about 6-8 inches. I want the wheelbase without the comp cut corners, they are so wranglery.

 

[truggier]

One more showing the extent of the cut into the frame...this is what gave me pause, it's almost cut right through at one point.

 

[ibinazbass]

That is a lot of frame missing but I think as long as you reinforce your work it should be ok in my mind.

 

[BC40]

strength wise I think you would be fine. But do you have enough clearance between the tire and the spring bucket/frame mod to cycle the suspension/articulate? And why not just move the springs further inboard and avoid most of that frame work?

 

[Chamba]

Not to mention how much work you'll likely need to do to the floor to clear the raised frame-which in turn means less room inside and altering the jump seats if you're using them. Good on ya for tackling it but man, what a lot of work!

 

[sadam_husain]

as already mentioned above is there some reason you want to mount the springs directly under the frame rails that's a pretty wide part of the frame, shifting them further inboard would be a lot easier and have better tyre clearance with the suspension cycling

 

[69LC]

Inboarding the coils would also aid in forced articulation of you care about that kind of thing.

 

[bikersmurf]

Why don't you use cool overs? Maybe there's something I'm missing, but I'd think you could have more travel with fewer cuts through the frame.

The frame will be more prone to cracking where it is welded... Or at least that's the theory,

 

[want2wheel]

I would rethink the design that you have for the frame for two reasons. The coild bucket is too big and will have clearance issues with the tire and wheel, and the frame is a c at that spot and not boxed for strength. I would look at and study how the low rider guys redo their rear frame sections. At the very least I would cut it where you are going to start in front of the bucket, box the frame and replace the whole frame from that point. This will give you more strength over all by giving you a complete rear boxed frame. This added strength would also lesses the amout that you would have to push the floor up except for the areas for the shock towers. Rob

 

[truggier]

Thanks for all the commentary...as I suspected, it's making me rethink the design. I love thinking up these things but that's why I'm still not driving it. Inboarding the springs sounds promising, except for two things: 1) the lower spring buckets are already fully welded up on the axle housing (see below), and 2) I'm a little cautious about moving the springs inboard since this would seem to make the body less stable.

I rotated the axle and tire 10 degrees around the axle center point, and even with 2" wheel spacers it seems like it would hit the reinforced parts of the frame.

Another idea I guess is to simply get different springs that will fit under the frame without cutting. If they were thicker they'd be able to carry the same load with less active coils.

 

[peteinjp]

Are you so sure that these spring rates are perfect for your load requirements? I'd probably go ahead with the springs under the frame and just see how it works out unless you know empirically ( from someone using these rates with your load requirements) that the springs will A meet the requirements and B will require that much space. They may put your rig at just the right height when under load when mounting under the frame. Your drawings show the uncompressed length but when under load it will no longer be uncompressed. You may loos exactly the 2 coils lengths just with load. Pete

 

[truggier]

Are you so sure that these spring rates are perfect for your load requirements? I'd probably go ahead with the springs under the frame and just see how it works out unless you know empirically ( from someone using these rates with your load requirements) that the springs will A meet the requirements and B will require that much space. They may put your rig at just the right height when under load when mounting under the frame. Your drawings show the uncompressed length but when under load it will no longer be uncompressed. You may loos exactly the 2 coils lengths just with load. Pete

Thanks Pete. What I have in the pictures is the compressed spring - I used a spring deflection calculator and estimated the weight. Below is a picture of the uncompressed spring - it's at least one active coil higher. The springs were supplied by Proffit's so I'm assuming they're good for the application.

I'm looking into coilovers and inboarding the springs. Anyone care to comment on what inboarding the springs does for stabilty? Seems like you'd be more prone to body roll the further inboard they go.

 

[69LC]

The only 4 link "kit" I recall ever seeing for 40s(Black Diamond?) had the coil overs in boarded and narrowed towards the front and top. If you are concerned about excessive body roll for road driving, I'm sure a simple, disconnecting sway bar could be added.

 

[sadam_husain]

if you look at 80 series, 70 series etc that are factory coil sprung you'll see they are all inboard at the rear, its a wider part of the frame so you'll probably find they aren't much further in than the front coils are

 

[truggier]

The only 4 link "kit" I recall ever seeing for 40s(Black Diamond?) had the coil overs in boarded and narrowed towards the front and top. If you are concerned about excessive body roll for road driving, I'm sure a simple, disconnecting sway bar could be added.

Proffit's sold a kit for a long time. This is what I'm using.

I think I'm leaning towards moving the bottom spring buckets on the axle inboard a bit, so that I can move the top bucket near the tub floor. This should give the clearance I'm looking for.

I'm new to coilovers...still checking out how the springs on these stack up against what I have now, and fif they can handle the loads I might have (two fuel tanks, full roll cage, hard top, back seats, rear bumper (with spare, high lift, and cooler), two adults, and possibly towing as well.

 

[truggier]

Testing the deflection of the current set of springs - 35 gallons of water, 2 cinder blocks, and my 2 teenage boys (about 550 lb total) compressed both springs 1.5 inches. I'd definitely have to cut these if I put them under the frame.

 

[bikersmurf]

Random thought... have you considered airbags? They'd more or less work with the bucket's you've got, with positioning shackles radical travel is possible, and the sky's the limit for load capacity.

Not to mention adjustable ride height and possibly even forced articulation with a compressor.