TEIN Flex Z is the successor to the Street Flex. The Flex Z is a non-rebuildable system compared to the Street Flex, which contributes to the lower manufacturing cost. 

High-End Top-Quality Ride Height Adjustable Shock Absorber at Amazingly Low Price! 

Specifications

Full-Length Ride Height Adjustment Feature …… Ride Height Adjustment with Little or No Change in Ride Quality Twin-Tube Structure …… For Low Resiliency & Securing Sufficient Stroke Damping Force Adjustment System …… 16-level Linear Damping Force Adjustment System Complete Kit with Upper Mount …… Exclusively Designed Upper Mounts are Included (Except Some Models) 1-Full-Car-Kit …… Complete Suspension Kit of 4 Shock Absorbers & 4 Springs

Spring Rate:

Here are some of the concepts that Flex Z is following: 

1. Incredibly Low Price & Made in Yokohama! 
2. Twin-Tube System for Comfortable Ride on Streets! 
3. Full-Length System for Ride Height Adjustment w/ Minimal Change in Ride!
4. Damping Force Adjustable to Change Ride to Desired Settings! 
5. Compatible with EDFC Series to enable In-Car Damping Force Adjustment!
6. Upper Mounts Included for Easy Installation!
7. Highly Durable & Reliable! 
8. Thoroughgoing Rust-Proofing!
9. Highly-Rigid Bracket & Shell Case!
10. Smooth Ride Height Adjustment & One-of-A-Kind Adjusting Wrench! 
11. No-Sag Springs!
12. High-Spec Damper Oil!
13. Replacement Service to Bring Used Shock Absorbers Back to Life! 

Twin-Tube System for Comfortable Ride on Streets!

Considering the use of larger tires/wheels and the lowered ride height, FLEX Z uses twin-tube system, in pursuit of ride comfort on streets. In order to maintain the ride quality on streets with rough spots here and there, a shock absorber has to have sufficient stroke, be low-frictional and move smoothly. As an oil chamber and a gas chamber are not arranged serially in a twin-tube system, it is easy to secure enough stroke. Also, low gas pressure keeps friction low. Its smooth stroke movement makes it possible to follow rough and bumpy road surfaces quite well, offering supple ride at low speed and stable ride at high speed.

3.Full-Length System for Ride Height Adjustment w/ Minimal Change in Ride!

Full-length ride height adjustment system, used for FLEX Z, enables adjustment of ride height by adjusting the case length, without changing the damper stroke length and/or spring pre-load. This has the advantage that the change in ride quality is kept minimal when the height is adjusted. This system not only makes it possible to maintain good balance between the low-down appearance and ride comfort, but also provides more setting choices for sport driving as well.

4.Damping Force Adjustable to Change Ride to Desired Settings!

FLEX Z has ever-popular 16-level damping force adjustment (comp./rebound together) feature with the use of needle valve. By turning the adjustment knob, damping force (stiffness) is adjusted linearly step by step, to change the ride and handling of the car to suit driver's needs/preferences. Use of well-reputed "ADVANCE Needle" at the needle valve for oil flow control secures sufficient damping force adjustable range. Whether it's for the handling characteristics of sport driving or for the ride quality according to numbers of passengers, finding the most desirable setting is easier than ever. Damping Force Adjustment Knob

5.Compatible with EDFC Series to enable In-Car Damping Force Adjustment!

With EDFC (Electronic Damping Force Controller) series installed, damping force can be controlled from the cockpit with ease. Stepping motors attached in place of adjustment knobs control the damping force quickly and accurately. Use of EDFC turns 16-level adjustment into finer 32-level or 64-level adjustment mode (64-level only available with EDFC ACTIVE / EDFC ACTIVE PRO), to provide the optimum performance by fully utilizing the damping force adjustable range drastically widened by the use of "ADVANCE Needle". With EDFC ACTIVE / EDFC ACTIVE PRO, damping force can be controlled automatically according to changes in G-force and/or vehicle speed, to offer very smooth and slick ride like never before. (Some models are NOT compatible with EDFC series.)

6.Upper Mounts Included for Easy Installation!

FLEX Z kit includes upper mounts exclusively designed for each vehicle model; either reinforced rubber mounts or pillow ball mounts. Pillow ball upper mounts for front strut suspensions have camber adjustment feature. (Some vehicle models use OE mounts.)