Struts/Springs

The broad definition of "coilovers" are a shock or strut inside a coil spring sharing a common top or bottom mount. The stock 240/260/280Z front and rear suspensions have coilover spring/strut combinations in front and rear. A more common definition is: Coilovers consist of a threaded aluminum collar, a stop (some kind of ring of metal) for the collar to rest on, a threaded perch that rides on this collar (and incorporates some kind of locking mechanism), a spring that sits on the perch (typically 2.5 inch diameter and anywhere between 6 and 12 inches long), and an upper hat for the spring. In addition, threaded body shocks (like Penskes, Carrerras, etc.) eliminate the stop and collar by having the shock body itself threaded.

You install coilovers by removing the strut from the car and the strut cartridge from the strut tube. Then cutoff the stock spring perch and grinding the weld smooth. Then the rest ring is slid down over the strut tube and positioned 5.25 inches above the bottom of the tube (where it is joined to the cast spindle part of the tube). The ring gets welded to the tube, collar slid on top of that. Then the cartridge is re-installed, followed by the new spring, topped by the hat. Re-install the strut.

There are 3 key advantages to coilovers:

1. The 2.5 inch diameter springs are relatively inexpensive and are available in a wide variety of lengths and spring rates.
2. The ride height can be adjusted very easily.
3. The narrower spring will allow for a wider Wheel/Tire combo.

Also, the only way to use Camber plates is to switch to coilovers.

Strut sectioning means cutting out a section of the strut tube and welding it back together. This is done on a lowered car to gain back some bump travel in the suspension. Wayne Burstein wrote up a good description of this for IZCC and its reprinted below with his permission:

Let's start by defining the task at hand. We want to lower the car in order to lower the center of gravity. I'll skip all the analysis on why we want to do this because there are lots of good books on the subject, and confine my comments to what you might run into in performing this on a Z.

First I need to define a couple of terms:

Bump -- suspension travel in the compression direction (i.e. the result of hitting a high spot in the road).

Rebound -- suspension travel in the opposite direction (i.e. the result of going over a hill and the wheels leaving the ground).

The first problem we run into is that when we shorten the springs, we are reducing the available bump travel in the strut cartridges by the same amount we lowered the car. With all the travel available in a stock Z, this is not too much of an issue when we lower the car only an inch or so. For those of us who are racing our cars, we often lower them much more; for instance, in the SCCA's IT class, we are allowed to lower the car until the rockers are no lower than 5" above the ground. This causes a problem because the suspension is almost fully compressed when the
car is sitting at rest. When you hit a bump, the suspension quickly bottoms out (hopefully on a bump stop of resilient material). This is a real problem because in effect, the spring rate increases very dramatically and negates all of our efforts to drive the car smoothly. When driving at or near the limit, this often is the beginning of a very impressive crash.

Well, we now have the car at the desired ride height, but need to increase the travel in bump. The way to do that is to shorten the struts. Now things get pretty messy. Don was correct in stating that this is dependent on the length of the struts; however, this is only partly true. The struts need to be long enough to insert the cartridges of choice. For racing, the ones that I would recommend are Carerra, Koni, or Tokico, in that
order ( this should cause a bit of discussion on its own). If we automatically shorten the strut to exactly fit the cartridge, we might actually shorten it too much. This leaves us without adequate rebound travel. Just in case this does not scare you, it should. I learned my lesson the hard way when
I had the rear wheels pick off the ground while cresting a hill that had a slight turn to it. That made for a looooong full lock slide at 100 MPH!

Ok, now we need to decide just how much we want to shorten the strut housing. The desired end result is to have about equal bump and rebound travel. In other words, when the car is sitting at rest, we want the struts half way compressed. On a street car, this is fairly easy to do, because we generally set the car up once and never play with it. Race cars are another situation entirely. First of all, different tires require different ride
heights -- for instance, switching from 60 series to 50 series tires lowered my car by .75", causing me to have to raise the car by the same amount. We also play with spring rates, and assuming that we are using coil overs, need to keep the spring collar low enough on the strut housing to avoid it interfering with suspension travel. The bottom line is that before cutting anything off your struts, you should carefully think about what you
anticipate doing to the car over the next few years as far as tire/wheel, strut, spring or ride height changes, and then come up with a compromise that works for you.

FWIW, most people shorten struts 1-2". If you figure out that you want to go more than this, recheck everything before cutting. Yes, you can add a section, but speaking from experience, it is much easier to remove than to add. I almost forgot to mention this, but if your strut housing is longer
than the cartridge, you need to put a spacer below the cartridge inside the housing -- typically, these are just pieces of tubing that is slightly smaller in diameter than the inside dimension of the housing.

Just a couple of tips to consider:

1) The best way I have figured out to cut the struts is to use a large pipe cutter. This gives a fairly straight cut with minimal cleanup -- you need to grind the burr off the inside of the housing and bevel the outside edge before welding them together. Be careful not to make the cut
so high on the strut that you hit the threads for the gland nut!

2) To remove the original spring perch, the quickest way I have found is to cut through it just above the housing with a grinder or cut-off tool, and then grind the remaining metal off. I found it much easier to do this before cutting the strut because even though I was not cutting the section with the perch off, it did interfere with the cutter.

3) After lowering the car, you need to align the suspension because you have added negative camber at both the front and rear wheels. Of course, you should probably do this any time you remove suspension components anyway.

Wayne Burstein
WDCR SCCA ITS #10
IZCC #214, NVZCC
wburstein@mountainmotorsports.net
www.mountainmotorsports.net

[Note: For Koni 8610 Inserts. Other inserts (Tokico, etc.) require different lengths.]

FRONT

From the dished bottom center of the strut to the top lip the overall length should be between 12.875. and 12.938.. The perch height measured from the top of the spindle casting (opposite spindle) is 5.250. assuming a .250. thick spring perch. The strut tube is cut at 5. measured the same way as above. Approximately 2. is cut from the bottom of the top half of the strut tube but measure first to be sure of the exact length.

REAR

From the dished bottom center of the strut to the top lip the overall length should be between 14.938. and 15.. The perch height measured from the top of the hub casting (opposite hub) is 7.250. assuming a .250. thick spring perch. The strut tube is cut at 7. measured the same way as above. Approximately 2.250. is cut from the bottom of the top half of the strut tube but measure first to be sure of the exact length.

PROCESS

The Koni 8610 inserts are a very tight fit inside the strut tube. The inserts typically have an OD of 1.725 and the strut tubes typically have an ID of 1.730. All cuts must be precise and perpendicular to the strut tube centerline. Use a lathe or a tubing/pipe cutter. Bevel both cut edges at 45 to 60 degrees leaving a flat of .030 to .060 at the bottom of each bevel.

Physically remove all paint and chemically clean (with Acetone) 3. to either side of the weld area. The strut tubes must be clamped into a large piece of angle and a tube (simulating the insert) of 1.720 diameter and 18. in length should be inserted into the assembly to help ensure straightness.

Tack weld the assembly in at least 6 places making sure the inserted tube still slides in and out easily. After tack welding, alternate 1. beads back-stepping around the circumference. Make sure no weld bead extends inside the strut tube and frequently check to be sure the inserted tube moves easily. Be careful not to weld the inserted tube to the strut tube.

Slide the spring perch over the strut and tack weld it to the strut tube on the underside of the perch. This tack weld should be on the back of the strut with the top of the perch 5.250. from the top of the spindle casting on the fronts and 7.250. from the top of the hub casting for the rears.

Measure down from the top of the strut tube to 3 places on the top of the perch. Make sure the perch is perpendicular to the strut tube. Tap the perch into position with a hammer before adding 3 more tack welds. After tack welding, alternate 1. beads back-stepping around the circumference on the underside of the perch.

INSERT INSTALLATION

Try installing the inserts into each strut tube. They should slide all the way in with nothing more then a light push. Most likely they won.t. Using any or all of the following, clean out and open up the ID of the strut tube:

• 36 grit 1.750. diameter flap sander
• 36 grit 1.5. diameter drum sander
• Christmas tree shaped carbide bit
• 1.735. diameter reamer
• 1.750. diameter wire wheel

You can also sand the paint off the Koni insert and you will probably have to slightly grid down the weld at the bottom of the insert.

When you can easily slide the insert into the strut all the way to the bottom, make two spacers for the rear struts that are 2.250" tall and 1.5" in diameter out of .125 wall steel or aluminum (6061 T6) tube. Drop in and center these in the bottom of the rear strut tubes and install the inserts. Measure to make sure the inserts site at the correct height. You'll probably have to shave a bit off the spacers. Once the spacers are correct, pour a little synthetic oil into the tube, install the spacers and the inserts, tighten the gland nut down, and torque to spec.

Suspension Mods

Chassis Strengthening

Running under the foor pans of the Z car, on either side of the transmission tunnel, are 2 folded sheet metal rails, or floor supports, spot welded to the floors. These rails extend from the front frame rails, but, in the stock Z Car, they end just under your butt (if you're sitting in the car).

Sub Frame connectors continue these rails all the way to the back of the car and connect up to the rear suspension's sub-frame.

Chassis or Frame flex is bad because it is hard to control. The job of the suspension is to respond to irregularities in the road as well as lateral forces from turning and the fore/aft forces of acceleration and braking. Chassis flex introduces an "unkown" and fairly uncontrolable element into the tuning of the car. If the chassis is stiff enough to eliminate flex, then the movements of the car and the forces acting upon it will be entirely dealt with by the springs, shocks, sway bars and tires.

Also, and just as important (if not more so), chassis flex will cause the metal to fatigue over time. This is a very bad thing.

Rust