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ZJ coils vs BB

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bb cheap lift zj
2K views 14 replies 7 participants last post by  Pine_Cat 
#1 ·
This is a 2 part question. I have read several threads on using the ZJ coils to lift my TJ. I have seen people claim anywhere from 1"-4" but I know that the differences can be attributed to the many variable such as engine size, extra weight, saggy springs, etc. I am hoping for 2-2.5". My questions are regarding this type of lift vs a 2" spacer lift:

1. Which would have more articulation
2. Which would have better ride quality

I know both would require new shocks and that would generate a whole new batch of questions...
 
#3 ·
Yes, 2" is roughly what I am expecting. That is not the question. Question is regarding articulation and ride quality of the full coil lift from a ZJ vs a spacer on the stock coils. Assuming the same shocks on both which would be best?

Same height, same money, which has better articulation and which has better ride?
 
#4 ·
I bought a set of new springs online. they were zj fronts and BDS 2" in the rear. The front lifted about 2.5'. Would of been perfect but I have a 2.5l. I added 3/4" spacers in the rear and now it sits perfect.. The ride is good. I used ZONE gas shocks. I wish I nutted up for the Bilstiens, the ride would of been closer to stock.. No complaints for the cost though..
 
#7 ·
scox said:
Yes, 2" is roughly what I am expecting. That is not the question. Question is regarding articulation and ride quality of the full coil lift from a ZJ vs a spacer on the stock coils. Assuming the same shocks on both which would be best?

Same height, same money, which has better articulation and which has better ride?
If you have the same shocks, why would the articulation be different between the two?
 
#8 ·
Having the same shocks sorta takes them out of the equation. That leaves the difference between the 2 as:
Lift 1---all coil spring with the ZJ coils
Lift 2---Spacer with stock spring.

My guess is that the longer coils would have more articulation but thought I would check with someone who had some experience with that.
 
#10 ·
that would depend if the shocks were the limiting factor.

overall you are probably right, the shocks probably do limit articulation.
that is not always the case though, longer springs are less likely to unseat.
longer springs are less likely to go into coil bind, than a shorter spring with a spacer.

assuming the spring rate is not completely wrong, i think longer is "usually" better.

i cant think of alot of situations that a 2" inch lift spring isnt better than a stock spring with a 2" spacer puck.

a properly set up suspension should not use shocks as bumpstops, or limit straps in a perfect world.
all to often that is not the case, they are used to limit travel.

in that case, you are correct, without a shock change, the travel will most likely be the same.

the other question is what do his springs do during flex?
do they coil bind?
do they unseat?
is their rate good?
 
#12 ·
In almost all well done setups the amount of available travel in the shocks is the limit to articulation.
Replacement springs are certainly better than a budget boost. The replacement springs will give you a higher rate so you can drive faster offroad. If you keep the factory springs, then you'll still be rocking to full compression when you're offroad.

Consider getting Moog springs for both front and back, so they'll be balanced on the freeway. I used Moog instead of OEM ZJ springs, so I can't tell you what rear springs would be balanced with OEM ZJ springs. I think some of the people who did the ZJ/ Crown Vic combo were disappointed to discover it was not balanced on the freeway.
 
#13 ·
Replacement springs are certainly better than a budget boost. The replacement springs will give you a higher rate so you can drive faster offroad. If you keep the factory springs, then you'll still be rocking to full compression when you're offroad.
All the hype on this site that occurs about spring rate is hilarious. Variations in springs that are made for TJ just aren't that great. When you are tuning a suspension spring rates that make a difference are multiples of each other, not 10-20%. That small a rate difference is swamped by shock setup as it relates to ride in a TJ.
 
#14 ·
Pine_Cat said:
Replacement springs are certainly better than a budget boost. The replacement springs will give you a higher rate so you can drive faster offroad. If you keep the factory springs, then you'll still be rocking to full compression when you're offroad.

Consider getting Moog springs for both front and back, so they'll be balanced on the freeway. I used Moog instead of OEM ZJ springs, so I can't tell you what rear springs would be balanced with OEM ZJ springs. I think some of the people who did the ZJ/ Crown Vic combo were disappointed to discover it was not balanced on the freeway.
What do you mean not balanced?
 
#15 ·
In a nutshell, rear springs and shocks need to be slightly firmer than fronts for the vehicle to have a stable response to bumps on the road. The front hits the bump first, so the rear needs to be a bit firmer to catch up.

It helps to have a test road where the bumps are evenly spaced, because that exaggerates the effect of unbalanced springs, such that you can feel the lack of balance even if your shocks are new. There's a test mile close to home on I-295, where the bumps are evenly spaced, so I got experience with balanced versus unbalanced springs. I can tell the difference between (1) factory-balanced springs/shocks (e.g., newer car), (2) close to factory-balanced springs/shocks (e.g., install Monroe/Tenneco shocks/struts, and you'll be close to factory-balanced, but not exactly), and (3) very unbalanced springs (e.g., worn out rear leaf packs at 90k miles).

When they design an OEM suspension, they decide on front firmness based on handling and comfort (e.g., "drop" testing, etc.), and then the rear is mathematically determined to be the right amount firmer than the front.

Instead of me just writing about it, here's what the AEV website says, since that's where I first learned about the concept:

". . . Since there are front and rear springs, the forward and rearward halves of the Jeep actually represent two spring-mass systems that must interact with each other. To understand the concept of frequency-based spring rates, think of a shock-less vehicle driving over a single speed-bump. When the front end hits the bump it starts to oscillate up and down at a certain speed. This is the front’s ride frequency. The rear encounters the same bump at a time delay determined by wheelbase and vehicle speed. The key is that the rear needs to react faster than the front so that the oscillations of the rear can catch up to the front in about one cycle (from ride height to some amount of ‘up’, then ‘down’, and back up to ride height). This is important because if the vehicle doesn’t naturally tend to level out quickly after a bump, the shocks will be overtaxed with trying to control body position/motion instead of their real purpose of simply getting rid of the oscillations.

"So to ensure the best possible combination of ride and handling, the front and rear spring rates must be derived to create the proper front and rear frequencies relative to one another. . . . To further enhance the spring’s ability to maintain proper frequencies under varying load conditions, a suspension engineer will design a progressive-rate spring (especially for the rear), which will keep the frequencies closer to constant over the expected load range."

I think people who tried the ZJ/ Crown Vic combo found the Crown Vic springs to be a bit too soft for the rear of a TJ that has OEM ZJ springs up front. There's probably a spring that's suitable for the rear of a TJ that's balanced with front OEM ZJ coils, but I haven't done the research/testing to know what it would be. To save time and money, I stopped looking at springs after I found my second replacement set to be balanced and otherwise satisfactory.

It's worth switching to dual-rate springs or progressive springs. You get to have a huge improvement to offroad performance without compromising ride quality. The spring rates published by Moog for my combo are more than 40% higher than rear OEM TJ springs, and more than 25% higher than front OEM TJ springs. With factory springs, I was always bouncing to full compression when offroad. With my current springs, I haven't touched the bumpstops yet, even though I'm now driving a lot faster when offroad. At ride height, my dual-rate springs have a very soft frequency, so I got best results with Bilstein 5100s. I like the combination of firm shocks and soft springs (at ride height).
 
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