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terrabit 05-23-2013 08:06 PM

Progressive vs Linear Coil Springs
Hi guys! Just got a 2013 JKUR and I want to put a 2.5" coil lift on it. I'm leaning toward the Teraflex 2.5" Coil Lift or the Rock Krawler 2.5" Stock Mod. Rock Krawler uses progressive springs, Teraflex uses linear springs. ... What's the difference? I understand conceptually that progressive springs get progressively stiffer as they compress and linear springs should maintain their spring rate as they compress. Beyond that though, what are the pros and cons of each? Is there any "real" reason to choose one over another or is this really more or less a triviality? I'm hoping to hear some nerdspeak! Lay it on me!


Matador 05-24-2013 08:36 AM

10 Attachment(s)
Your right, the linear spring has one set spring rate that is carried through the entire spring. Progressive springs have tighter coil spacing on the top that gradually increases the stiffness as the spring is compressed. The coils collapse onto each other forming a solid ring. But, let me introduce a 3rd option for you, Dual Rate springs. Similar to progressive they have a tighter wound portion on the top, the difference is that they compress independently from the bottom portion and form a two step spring compression. This is a great illustration

Once the top coils are compressed they form a "Virtual bucket/perch" for the bottom portion of the spring

So what are the advantages and disadvantages for each.

Linear: The biggest advantage for these is the price point. They are the entry level spring option. The disadvantage is that its would set spring rate. If its too soft, it will feel great on the road but offer very poor handling, especially on highway speeds and too much travel on the trail. If they are too stiff they might be better on the trail and highway speed handling but will not be comfortable on the road.

Progressive: progressive springs try to address the varying spring rate issue depending on the driving taken place. If your on the highway or on the trail more of the spring is compressing onto each other and firming up the spring rate, providing a stiffer more stable ride. If your going slower and around town driving, less of the spring is compressed and its a softer more comfortable ride. The only disadvantage with progressive springs is that a lot of the stress is put on the last couple of coils. Over time poor quality springs will lose tension on those coils and will begin to sag. AEV offers progressive springs on all 4 corners, RK just offers them on the front and linear in the back. RK's decision to run their kits like that is because they don't segment their kits between 2 and 4 door Jeeps. By not doing that they can't control the weight variables between the two Jeeps.

Dual Rate: Dual rate springs build up on the progressive idea but its like two different springs fused together. The top is softer and will collapse onto itself easier than the bottom. This will help with around the town driving by offering a comfortable and controlled ride. As the drive gets more aggressive the top fully compress and becomes a solid "Bucket" acting like a new spring perch for the bottom perch which is a stiffer spring. Once the bottom spring takes over it offers a stiffer and more controlled ride. The negative with Dual rate springs is finding true dual rate springs. A lot of manufacturers claim to have dual rate springs but in reality they are just progressive. They look identical but work differently. Metal Cloak is a company that offers Dual Rate springs on all 4 corners. They also offer vehicle specific (2 door or 4 door) springs and soon will have a 2.5" spring option for the 2 door.

If your interested in more info, check out this thread

terrabit 05-24-2013 11:01 AM

Excellent, thank you!

Doctorsti 08-20-2013 01:16 PM

just bumping this thread as I think we can delve into this further and get some good dialogue going about the pros and cons of coil spring rates.

My impression that has been transferred from tarmac and off road racing/track suspension is that a constant rate coil spring with the proper spring rate and a damper valved for that rate is far better at being a predictable unit especially near the limits of compression or extension.
Although I really am intrigued by Metalcloaks desire to look outside the box with their engineering and to come up with some unique products that seem to answer questions no one thought to ask yet I also feel the "dual" rate spring is simply a way to keep the spring seated when that side of the axle is nearing full droop travel. This very light rate will bind without a whole lot of force as that side of the suspension begins to compress again leaving essentially a number of coils in bind and the other 2/3rds or so active for what they call the "road rate" Isn't this just a unique way to keep your springs on your Jeep and off the trail when you flex to the extremes. This is one of the roles of a "tender" spring in a dual spring coilover type suspension. The other companies like Teraflex rely on spring retaining clips or limit straps and thus reduce droop travel to not drop springs all over the ground. I would be interested to see some more engineering from Metalcloak on how much absorption comes from the lighter of the dual rates.

AEV and others use a variable/progressive rate which I have read elsewhere that AEV's is triple rate. These strategies I feel are ways to limit compression near the limit of compression to prevent bottoming out of the suspension which in turn would lead to loss of grip and lead to potentially very dangerous situations. Fast speed offroad, or on road for that matter, driving would require something like a progressively increasing coil rate or a progressively stronger bumpstop as any spring rate good for crawling around slow speed over rocks and dirt would by way to light to drive fast and vice versa.
AEV's answer is lower rate sections for slow speed low amplitude compression and higher rates for higher speed or higher amplitude suspension movement. The con as far as I'm concerned here is that the change in rate as the spring compresses can lead to unstable corning and grip as you near the limit of travel.

Teraflex coils are likely lighter then the stiffer sections of the AEV coils but for the pre-runner kit, made for high speed off road, uses a progressive rate bumpstop instead of progressive rate coil to prevent that bottoming out and loss of grip. The benefit here is when the speedbump is nearing its full compression, lets say in a tight turn or off camber section of dirt, the coil spring still has every one of it's coils "active" and able to absorb imperfections in the road/dirt surface better then a coil that has half of itself bound up into a solid cylinder. Coil bind is not a good situation whether it be some or all of your coil spring if you are concerned about cornering grip which I would imagine figures into offroad driving some how.

My opinion (not an engineer) is that:
Veriable/dual rate coils:
pro -great for off road allowing a lot of droop travel without need for limiting straps so great for articulation and grip offroad at slow speed and various amplitude compressive loads.
con -not ideal for higher speed driving on or off road due to unpredictable rate changes and coil bind resulting in a far shorter "active" spring. Shock valving can't match all the rates so is a trade off or compromise situation results. Adjustable shocks are an option but still can only be set to be better suited for one of the multiple rates of the springs.

Linear rate coils(with speed bump or progressive bumpstops):
pro -better on road and at limits of compression predictability due to fully "active" coils. Shock valving can be matched to the spring rate without having to split the difference with variable rates.
con -could be too hard or too soft depending on weight of Jeep, more expensive as you would also need some progressive bumpstops like speedbumps.

Faith and Trust 08-20-2013 01:35 PM

A wise man once said "everything works in theory." Give me an appropriate weight bearing linear coil set up.

Doctorsti 08-20-2013 01:50 PM


Originally Posted by Faith and Trust (Post 4100643)
A wise man once said "everything works in theory." Give me an appropriate weight bearing linear coil set up.

appropriate or ideal?
Ideally you would base your custom rate off the weight of "your" jeep and then adjust preload with corner weighting but thats a coilover setup. Mass production doesn't lend itself to having customers send in curb weights and custom order spring rates to match so yeah it is theory but the attributes of a linear spring are more predictable and better suited to custom valving of a shock then a dual or multiple rate spring where everything is a compromise. At least on a linear rate the only compromise is the manufacturer is guessing at the curb weight and distribution based on it's average customer.

onetraveller 08-20-2013 04:20 PM

The advantage of the linear spring on a race car or race truck is predicated on a known constant weight of the vehicle, with minimal change for fuel consumption. This allows the race team to dial in the spring and shock combination.

A daily driven, weekend expedition/tow vehicle has a very wide load range and a correspondingly wide vehicle weight range. A progressive spring is an attempt to deal with this fluctuation in the vehicles weight. The problem with a progressive or multi-rate spring is the difficulty in tuning the compression and rebound damping rates in the shocks. At any point in the shocks travel you can have either too much or too little dampening.

The front stock spring is linear, but the rear stock spring appears to be progressive as evidenced by the varying diameter of the spring coil. There are 3 ways that I know of to make a spring progressive: 1. vary the spring wire thickness, 2. vary the coil spacing, 3. vary the diameter of the coils, thereby changing the moment arm that the load uses to compress the spring.

Here are a set of stock JK rear and front springs.


mforce 07-13-2014 02:47 PM

Old thread but very good information

spinlock 07-13-2014 07:38 PM


Originally Posted by mforce (Post 11935914)
Old thread but very good information

Good to know that someone actually uses Search function instead of just posting an age old question first and doesn't think the old laws of physics are stale. :D

terrabit 07-13-2014 09:38 PM

I settled on 2.5" TF linear springs and Rancho RS9000XL shocks. I'm happy with the combination so far.

BentMetal 09-08-2014 04:48 PM

Doctorsti states:
"...Metalcloaks desire to look outside the box with their engineering and to come up with some unique products that seem to answer questions no one thought to ask yet I also feel the "dual" rate spring is simply a way to keep the spring seated..."

That is exactly what the MetalcCloak diagram shows; MetalCloak only states that the dual-rate coil compresses the lighter rate at ride height and ONLY extends at full droop thus allowing the coils to stay seated with a a much lighter force at extension extremes. This very light spring force keeps the vehicle from unloading on off-cambers as a progressive or linear rate coil would. The MetalCloak coils ARE a "true" dual rate set up and are engineered to have a linear rate during the "road rate" with a very long extended length with a light energy carrying capacity-the coils stay in the bucket with out limit straps or coil retainers; Now with that being said the MetalCloak coils are designed to work in conjunction with Duroflex Joints- heim joint type control arm systems need not apply. ( that is a whole other conversation )

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