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Spring Rate Test Results for 2006-11 Civic Spring Kits

by Chris Shenefield, RedShift Motorsports - UPDATED 7/20/12

RedShift's Recommendation as a result of this testing:

Best Slight Drop Springs:  Tein High.Tech -  1/2" drop.  Perfect for stock or Koni STR.T (orange) shocks.  A little stiffer than stock spring, but more a comfort spring than anything.

Best Medium Drop Springs:  Swift Sport Mach - 1" drop.  Very comfortable performance spring.  Easily the most comfortable and best engineered 1" drop spring.

Best Performance Springs:  A 3-way tie - 1) Tein S.Tech - 1.3" drop.  My favorite all around lowering springs.  They work great with Koni STR.T (orange) shocks, and we pair these together for our "HFP Killer" kit.  VERY popular and other agree it's a great combo.  2) Neuspeed Sport - 1.6" drop.  These are a great performance spring with a great drop.  Aggressive but not too low.  A great street performance option.  You have to use Koni Sport shocks in the rear with these springs.  3) Swift SPEC R - 1.2" drop.  These springs are also excellent!  They have a stiffer front spring and softer rear spring as compared to the Neuspeed Sports.  These Swift SPEC R spring can be used with stock shocks... but they like Koni Sports.

Best Slammed Springs: Since the Neuspeed Race spring are discontinued, the only spring I can recommend is the Truhart Lowering spring - 2.0" drop.  Interestingly, in talking to Truhart, they used the Neuspeed Race springs as a goal in creating their Lower Springs, and they look and feel more or less identical.  Plus they are very inexpensive... if you want low, these are the best ones out there.  You absolutely need to run the Truhart shortened shocks with these springs.

Springs Tested:

1) Stock Civic Si

2) Civic Si HFP

3) Eibach Prokit

4) Swift Sport

5) Eibach Sportline

6) Skunk2 Lowering

7) Progress Sport

8) Neuspeed Sport

9) Neuspeed Race

10) KW V3 coilover

Test rig:

It uses a basic press and is quite manual, but we are using $900 professional force meter; so the data is accurate and repeatable.  The strut used has been drained of all oil and no bump stop is used; so the spring is kept securely in place but only the spring provides force.

Front Swift Sport spring uncompressed.

Front Swift Sport spring partially compressed.

Rear Swift spring uncompressed.  Rear springs were tested using the front strut rig and alternate perches.

Front Neuspeed spring uncompressed.  Note the difference in progressive coil application (in the middle) as compared to the Swift springs at left.

Front Eibach Prokit fully compressed.  These springs definitely hit coil bind earlier than any of the others.

This is the top mount.  It's a custom welded setup that is securely fastened to the press and the strut's top mount.

The force gauge is analog but high quality; so the results are accurate.  Note the ball bearing that provides consistent force application to the gauge.

Now on with the information..... interesting stuff.


This is total compression force in lbs.  This is the raw data.
This is incremental spring rate in lb/in.  This is the "Spring Rate", and it is derived from the raw data above.

Front springs.... y-axis is lb/in.:

A few important observations on the front springs:

1) This front spring data shows an interesting difference between the Stock/HFP springs and the other "drop" springs in that the stock and HFP springs are more linear.  This is attributable to the drop springs needing to lower the car; in other words, the first few inches of compression must be softer to lower the car.  The Neuspeed Sport, Swift Sport, and Neuspeed Race are the most linear of the drop springs.

2) Prokit, Sportline, and Skunk2 all bind early..... essentially at coil bind before 7" of compression where all the others go beyond 7" of compression, but I don't know how big a concern that is.

3) Related to item #2 above, there is a very non-linear increase in spring rate of the Prokit, Sportline, Skunk2, and Progress springs deep into their compression.  This could be designed into the springs, but the reality is that they will feel harsh under heavy compression (big bumps).

4) The KW V3 front spring is a higher rate, which I would expect from a coilover type setup, but it's not overly stiff and should be a very nice street coilover setup.

Rear springs....y-axis is lb/in.:

Each rear tire has 560 lbs on it, and that does not equate to spring compression force exactly because the spring is far inboard of the wheel (any wheel force is multiplied at the spring).  The distance on the lower control arm from the inner pivot to the spring is 12 inches, and the distance from inner pivot to wheel/tire contact patch "vector" is 21 inches.  This means that a wheel force must be multiplied by 21/12 to equal the force at the spring; so 560 lbs at the wheel is 980 lbs at the spring.  980 lbs is a between 4 and 5 inches on most of these springs (See raw data).  The KW V3 is obviously higher, which makes sense because it is a coilover type setup, but it is interesting that the Neuspeed Sport springs "primary" coils are in the same range as the KW V3.

A few important observations on the rear springs:

1) As with the front springs, the Prokit, Sportline and Skunk2 all had binding issues and could not be measured to 6" of compression.  And again the Stock and HFP springs are ultra linear (very nice!).

2) Stock springs and Swift springs are very similar in back.  Prokits are very similar to HFP.  Sporline and Progress are similar rear rates at static ride height.   Skunk2 is higher than all but Neuspeed Sport and Race, which are in a class of their own.  Neuspeed Sport has a more linear performance in the primary coils than the Neuspeed Race.  The Neuspeed Race have the most drop of any spring; so the spring rate increase may be related to keeping the car from bottoming out as easily, which the Neuspeed Sport doesn't have to worry about so much.


So, what does all this mean?  Listed below approximately from from least to most drop, our findings are as follows:

The HFP springs seems to be a very nice subtle drop option, and they can be had for roughly half the cost of a good set of shocks and springs; so for the budget minded that don't want to be slammed and don't need the best performance, they are a great choice.  But they only drop about 1/2"; so it's not super low or super good handling setup....  but still a very good budget setup.

Our favorite mild drop springs are the Swift Sport spring.  UPDATE 7/20/12 - Swift Sport Springs are discontinued.  So the our pick for best medium drop springs are now the new Swift Spec R (1.2" drop and not reviewed here because it was not available at the time of this test).  The Swift Spec R is rated at about 10% over the HFP springs; so the stock shocks will have a hard time keeping up with them, and the HFP shocks will be borderline and at their limit in trying to control the Spec R springs.  The Konis are without a doubt the best match to the Swift Spec R.  Also, the Swift springs are as light as the stock springs, where most of the other spring sets are heavier.  (For example, Eibach Prokit = 27.5 lbs and Swift Spec R = 21.5 lbs....big difference.)  However, if you have stock Si or stock LX springs or even stock HFP shocks, you may have better results with the Eibach Prokit.  It's spring rate is low enough that the stock shocks will be able to handle them; however, there is definitely an issue in my mind with the Prokit binding early in it's compression stroke on the front and back (see charts and data).  Before this test I recommended the Prokit, but now I have a harder time recommending them.  Some Prokit owners have reported a harshness over very deep bumps which I have to assume is related to early spring bind that causes the spring rate to skyrocket and go infinite (solid spring with no compression left).  Still, I think this is rare; so most people will never have a problem with it.

The Progress Sport springs are a good spring.  In fact, it represents a nice spring which sits in between the Swift Sport (1" drop) and the Neuspeed Sport (1.6" drop).  The Progress springs offer an advertised 1.3"F1.2"R drop with a spring rate roughly in between the Swifts and Neuspeed Sports.  Only issue is we have heard of noise issues related to the Progress springs having no isolators on the tight coils; so you may need to get spring isolators from Tein or get rubber/poly tubing from McMaster and fit it to the tight coils before installation.

The Skunk2 Lowering springs are very Eibach-like in my mind, and they may be made by Eibach.  What is most noticeable is that the Skunk2 springs have the same "coil-bind" characteristics as the Prokit and Sportline, which none of the other springs have.  I would stay away from these Skunk2 springs for this reason... I don't freely recommend the Eibach Prokit or Sportlines either.

Our favorite performance drop spring are the Neuspeed Sport spring.  It has a fairly soft ront rate, but the rear rate at anything beyond static compression (how the car sits at a stop) is more performance oriented than the others.  Also, the Neuspeed springs don't have any coil bind issues.  And lastly, the price is right.  This is what I would put on my car if I had an 06 Civic because I am a father but also a performance enthusiast who likes to have some fun, and the Neuspeed Sport springs are far from harsh but will make the car faster through the turns.  Definitely a shock upgrade is necessary with these springs.... or if you don't do the shock upgrade when you install these, you'll need them within a few months or your be bitching about how bouncy your car is because the stock shocks are toast.

The others:

The Eibach Sportline sits higher in back than front and binds under heavy compression.  We don't recommend this spring at all.  If anything, the front should sit a bit higher than the rear because it's better to not lower the front too much because of performance dynamics of a FWD car and front strut geometry issues.  More specifically, 1) you want more "compliance" (softer) in front for better grip because the fronts do both turning and accelerating and too stiff a front end causes lack of traction and 2) because lowering the "strut" front of the Civic will cause the car to act weird and to loose grip in turns faster due to your having moved the "roll center" and and increased length of "roll couple" (the distance between the center of gravity and the roll center).....complicated stuff, but slammed in front is not good and why do that and keep the rear higher....doesn't make any sense.  And several have said it looks a bit strange that way... like a RWD muscle car.  Not recommended.

The Neuspeed Race have the most drop of any spring (2.0").  Even so, the spring rates are not so high that you will rattle your teeth.  Still, an upgrade shock (like Koni) would be important with these so that you don't go bouncy bouncy down the road.  Also, roll center adjusters are recommended because these lower the front so much.

The KW V3 uses a good spring choice for a coilover on a street car, but they are a little bit softer than I like for track.... which makes sense because they the V3 is meant to be a street coilover system... and it does that very well.  Incidentally, we offer a conversion kit to allow these to run linear race springs and camber plates for those track-minded drivers; so they are upgradeable to higher spring rates and camber plates now if needed.

Here are some graphs of the same data listed above, but these separate the aggressive springs from the less aggressive springs.  And these can be easier to read.

Stock, HFP, Prokit, Swift:


Neuspeed Sport, Progress, Sportline, Skunk2, Neuspeed Race:


Reference info:

Edmunds.com has the 2006 Civic Si listed at 2877 lbs.  Weight distribution is 61% Front and 39% rear..... so, that in pounds is 1755/1122, or 880 lbs per front tire and 560 lbs per rear tire.