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Coil recommendations for 3.5-4" lift + stretch

PatrickXJ

NAXJA Forum User
Location
Dover AFB
I'm starting to parts collect for my wife's '98 4.7L XJ. For the street we want low lift and decent size tires, so we're thinking 285 or 295/75R16's and less than 4" of lift. I have experience with Rusty's 4.5" coils on our last '96 XJ but they are a little tall for this project since we'll be raising the fender flares and cutting the wheel wells up a bit, whereas before with 33x10.50's we didn't cut anything.

I have an 85lb winch bumper (minus winch) and 12,000lb winch that needs to be supported, along with an Optima Red Top and some minor frame plating up front. Granted, we're also removing the front bumper which weighs what...30-40lbs with all bracketry included?

The key question here is, how soft will the springs get when placed directly atop the axle tube, compared to being in the factory position in front of the axle tube? Ideally I'd like to find a 3.5" coil with a 175-190lb spring rate. Does such a beast exist? I'm stretching the front just enough to place the coil directly atop the tube. That should be what...2.5" or so?

Side note: the rear is getting stretched with Blazer ZR2 leafs. That will relocate the rear D44 back about 1.5" and net 2.5" longer leafs, plus a 4" lift with greater load capacity. I'll relocate the shackle if need be to compensate for the longer leaf.
 
You want 3.5" springs that are super soft to support your XJ with a ton of weight on the front end, and net 4" of lift? I think your math is off.

Since you are technically moving the axle forward and not the springs backwards, that won't affect the leverage put on the springs by the bumper.

What is more important to you when picking springs. Ride height? Ride quality? Are you crawling, street driving, or blasting through the desert?

I had Rusty's 3" springs in the past (maybe 170-180 lb/in) and they were wayyyy too soft, even with just a ~60lb aftermarket bumper up front and no winch. The RE 4.5" springs were still a bit soft with the bumper and winch up front, so I upgraded to BDS 4.5" springs which are much stiffer (around 240 lb/in) and they ride great and support all the extra weight. You just have to get good shocks to match.
 
Metalcloak tj 3.5" dual rate springs. I run the 5.5" springs and they netted me about 6" with no front bumper. They handle very well on the street and the additional free length of the dual rate set up keeps them seated in the buckets at full droop. They have been great for me.
FEB297BE-4269-4AFE-8885-19A40178AD2A-15228-00000A6D71DC0983.jpg

They are just starting to unseat at 8" of downtravel.
 
You want 3.5" springs that are super soft to support your XJ with a ton of weight on the front end, and net 4" of lift? I think your math is off.
I think what you're missing is the variable (read: unknown) effect the axle placement will have on the springs. Stock springs are in front of the axle tube. This softens the spring. So yes, I'm wondering if 3.5" springs will feel stiffer with a front stretch.

Since you are technically moving the axle forward and not the springs backwards, that won't affect the leverage put on the springs by the bumper.
Do you have any higher education in physics? If so, perhaps draw out a free body diagram and analyze the moment of inertia regarding the two setups you are comparing.

What is more important to you when picking springs. Ride height? Ride quality? Are you crawling, street driving, or blasting through the desert?
If we drove 10,000 miles with this Jeep, 25 might be off road at some play park, but 1000 will be driven in adverse weather. So in order of importance, ride quality & street driving characteristics I group together as #1 priority. #2 would be ride height so the weight of our bumper, winch, lights, canoe, passengers, gear and extra frame plating don't cause the sheet metal to dig into the tire read. #3 would be flex off road while rock crawling followed by #4, blasting through the desert and being able to handle the high speed bumps. #4 also matches #1, ever seen a girl drive down the highway in a lifted Jeep going 80+ hitting potholes? Yep, that's my wife. I try to build to wife-proof everything I can. :shhh:

You just have to get good shocks to match.
This is a must. Bilstein 5100's are in the build plan after we determine ride height, bump and droop.

zachandandy said:
Metalcloak tj 3.5" dual rate springs. I run the 5.5" springs and they netted me about 6" with no front bumper. They handle very well on the street and the additional free length of the dual rate set up keeps them seated in the buckets at full droop. They have been great for me.
I will have to check these out. Thank you.
 
Interesting features of those MetalCloak dual rate coils. I sent them an email asking for more info about putting the 3.5" TJ coils into an XJ.
 
Wouldn't moving the spring back effectively soften the rate due to increased leverage between axle and frame? But, still there will be slightly less movement the further rear you move it. Then I'm no physics wiz, or even a physics dunce ... I'm just a jeep guy.
In all honesty I don't think 2" is really going to make that much difference in felt rate IMO. But I can second the BDS 4.5" coil suggestion, mine ride smooth even with just a stock bumper on the front. And I found the RE 4.5" coils to be waaay too soft for me (IIRC they are ~160lb/in rate).
 
Do you have any higher education in physics? If so, perhaps draw out a free body diagram and analyze the moment of inertia regarding the two setups you are comparing.

I'm a mechanical engineer, so I get it.

Think of what you are telling me. I know the bottom of the spring sits a few inches in front of the axle centerline on the stock brackets. You are saying you want to push the front axle forward a bit, but then center the spring perches over the axle tube. Essentially, you haven't moved the spring at all in relation to the body of the jeep (where all the weight is that you are describing).

If you moved the upper spring perch forward or backwards, you'd be changing the force put on the spring because the leverage would change (assuming we're talking about pivoting about the rear axle). But you aren't.
 
Deavers? OME + ACOs/spacers? RE4.5 springs around 90$ shipped from serious.
 
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