From Wayne Burstein:
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.
WDCR SCCA ITS #10
The most current version of my how-to is always here:
The text is copied and pasted below:
SECTIONING DATSUN 240Z STRUTS
For Koni 8610-1437RACE Inserts
Remove stock lower spring perch from all the struts and remove the brake line brackets from the front struts. Use a cutoff wheel to remove all the spring perches and brackets above the welds and then grind the welds off the strut tube. Get it smooth but be careful not to grind off too much of the tube itself and thin it.
From the dished bottom center of the strut to the top lip the overall length should be between 12.875" and 12.938", measured from the inside using a tape measure. The threaded collar weld-on ring height (assuming 10" tall springs and 5" tall threaded collars) measured from the top of the spindle casting (opposite spindle) is 5.250". The strut tube is cut at about 6.5" up from the top of the spindle casting to put the welded section under the threaded collar. From 1 to 2" is cut from the bottom of the top half of the strut tube but you must measure first to be sure of the exact length as specified above. The most important measurement is the overall length of the strut tube (12.875" to 12.938").
From the dished bottom center of the strut to the top lip the overall length should be between 14.938" and 15", measured from the inside using a tape measure. The threaded collar weld-on ring height (assuming 10" tall springs and 5" tall threaded collars) measured from the top of the hub casting (opposite hub) is 7.250". The strut tube is cut at 8.5" up from the top of the spindle casting to put the welded section under the threaded collar. From 1 to 2" is cut from the bottom of the top half of the strut tube but measure first to be sure of the exact length as specified above. The most important measurement is the overall length of the strut tube (14.938" to 15").
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. This bevel is important to ensure proper weld penetration.
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. Its critically important that the strut tubes are welded square.
Welding the Strut Tubes
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. You'll also need to lightly grind down he weld to allow the threaded collar to slide over it.
Fabricate the lower threaded collar weld-on ring from 2" schedule 40 plumbing pipe (2" pipe nipple 6 or 12" long). Cut four .75" rings from the pipe and slide over the strut tube. They should fit over the tube but if not, cut a slot in the ring. A gap in the ring supporting the threaded collar is not an issue.
Slide the threaded collar weld-on ring 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 weld-on ring. Make sure the ring is perpendicular to the strut tube. Tap the ring 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 ring.
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 approximately 1 to 2" tall and 1.5" in diameter out of .120 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 sit 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.
The Koni gland nut for the 240Z strut tubes and 8610 inserts is part number 73.25.01.003.1 (M48 x 1.5p). For 280Z strut tubes (which are physically larger in OD and wall thickness) the Koni gland nut part nubmer is: 73.25.01.007.1 (M51 x 1.5p). You can get these from Truechoice but be sure to give them the part numbers.
You will need a wrench for the Koni gland nut and spring perch. Both can be sourced from McMaster-Carr with the gland nut as part number 5480A13 (1.5" span, 7/32" pins, 5.5" long). The spring perch wrench style will depend on what type of adjustable lower spring perch you bought with your coil over kit.
Tokico Ilumina BZ3099 and BZ3015 Inserts
Basically all the steps are the same except for the measurements and the tightness of fit of the Tokico inserts in the 240Z strut tubes. The Tokicos are a little smaller OD then the Konis and slide in easy.
From the dished bottom center of the strut to the top lip the overall length should be between 13.375" and 13.500", measured from the inside using a tape measure. The perch height (assuming 10" tall springs and 5" tall threaded collars) measured from the top of the spindle casting (opposite spindle) is 5.250". The strut tube is cut at about 6.5" up from the top of the spindle casting to put the welded section under the threaded collar. From 1 to 2" is cut from the bottom of the top half of the strut tube but you must measure first to be sure of the exact length as specified above. The most important measurement is the overall length of the strut tube (13.375" to 13.500").
From the dished bottom center of the strut to the top lip the overall length should be between 14.938" and 15", measured from the inside using a tape measure. The perch height (assuming 10" tall springs and 5" tall threaded collars) measured from the top of the hub casting (opposite hub) is 7.250". The strut tube is cut at 8.5" up from the top of the spindle casting to put the welded section under the threaded collar. From 1 to 2" is cut from the bottom of the top half of the strut tube but you must measure first to be sure of the exact length as specified above. The most important measurement is the overall length of the strut tube (14.938" to 15").
The rest of the process is that same as for the Koni inserts and Tokico includes the gland nut for their shocks. They are hex shaped so a large adjustable wrench (or gland packing nut wrench) is needed for installation.
MORE PROCESS TIPS
from pparaska (Pete):
I cut the struts with a tubing cutter (pipe cutter), chamfered the edges, laid the parts inside of a piece of 3x3" angle iron, inserted an snug fitting strut cartridge into the assembly before any welding to make sure the two pieces were as centered to each other as possible. Used 4 C-clamps (2 on the lower part of strut tube, 2 on the part I was welding back on) to hold the pieces snugly in the angle iron. I then tacked the pieces together at four equally spaced places around the weld area, doing one, then another that was diametrically opposite the first, then the other two tack welds. Remove the strut cartridge if it is a good part you want to use later. If not, leave it in there. Weld around the joint, one side and then move to the diametrically opposed side, so that any warping will be imparted in a symetrical fashion around the tube. Once you've welded a third of the total joint, the insert can be removed. Finish welding. Clean the weld area, and fill the tube with liquid and leave the tube upright for a few hours to look for leaks. This is only necessary if you feel that oil or coolant/water between the cartridge and the tube are necessary.
It's way easier to cut and weld the threaded portion back onto the cut down tube than to lathe or tap the tube. If you are careful, a straight, sectioned strut tube will be your result. If I can do it, anyone can!
ADDITIONAL THINKING REGARDING STRUT LENGTH AND THREADED TUBE PLACEMENT
from blueovalz (Terry):
Deciding on the strut length (where the top of the strut housing, or the gland nut location) as well as the placement of the threaded tube can only be successfully determined after you know where you want your ride height to be in relationship to where it was prior to the sectioning. For example, if your suspension was set up at a 7” ride height, and the current struts were positioned exactly half-way in their travel window when the car is at rest, you have a reference point to work from in deciding how much to section and where to place the threaded sleeve.
Let’s suppose you wish to lower the car to a new ride height of 5”, and you wish to use the same exact springs previously used on the car. To do this correctly you’ve got to reduce the height (top of, or the position of the gland nut) of the strut tube by the same amount that you’ve lowered the car (2”). Doing this allows the suspension to once again sit at the midpoint of the strut’s travel window. The only way to do this though is with a shorter strut insert because the OEM insert will be too long to fit into the shortened tube. So what you need to know now is “how long is the body of the new shorter strut insert?”
If it is at least 2” shorter than the OEM insert, you’re in luck because you can use a spacer to make up the difference at the bottom of the tube. If the new insert body is less than 2” shorter than the OEM insert body, than you must increase the strut tube length to at least match the new strut insert’s body length. This may mean that you cannot cut the full 2” off the tube, and must instead settle with a longer than desired tube (strut length), but one that will still work within the parameters of the suspension travel.
All of the above is only a starting point though. What if the new strut insert has a shorter stroke than the OEM strut had?
If the OEM strut had an 8” stroke (just for the sake of this explanation) and the new insert has a 6” stroke, this means that the center of the travel window must be moved up 1” (1/2 the difference between the two strokes). This means that instead of cutting 2” off my strut tube in preparation for lowering the car, you only need to cut 1” off the strut tube, because now you must raise the strut up 1” to compensate for the shorter stroke of the new insert. So now you’ve added another factor to consider before you cut.
One note I would like to add is that it is always better to error on the side of a too long a strut tube. If you cut the strut tube even .200” too short in relationship to the new insert’s body length, you will not have enough thread engagement in the gland nut to secure the insert. In fact, I will go so far as to say even .100” too short can present a problem. With that said, .100” too long will allow the use of a washer or shim to tighten the new insert with proper gland nut engagement. You can always add a shim, but to lengthen a too short a strut tube will take a lot more work.
Guess what, you’re not finished yet. If you install camber plates, or any part that changes the strut rod mounting point’s relationship with the top of the strut tower, then you’ll need to find out what this change is. Will the installation of camber plates allow the strut to sit higher in the tower, or will it push it down lower in the tower?
This change will impact where the car sits in relationship to the strut’s stroke window (the desired midpoint of strut movement). A camber plate with no rubber spacer will allow the strut rod to be over-extended at the new ride height if this factor is not considered. Thus in this situation, you’d need to lengthen the strut tube additionally for this situation. How long are your bump stops? Will you use bump stops? Will the upper assembly reduce the available stroke on the new insert, and if so, by how much? All of this needs to be considered because it will dictate where the gland nut is positioned (the length of the strut tube)
Threaded Tube Placement
Now let’s look at the location of the threaded tube. Nearly 100% of all coil-over conversions involve a change in springs. Your planned ride height change of 2” must take into consideration the spring rate change as well.
For example, lets assume the reference ride height (original pre-work ride height) used 100 lb/in springs (again, a value used only for this discussion) on the struts, and you wish to use 150 lb/in springs with the coil-over set-up. This means that the springs will not compress as much as the OEM springs did. At rest, if your corner weight (un-sprung weight) is 600 lbs, then the new spring will only compress 4” as compared to the OEM spring which compressed 6”. If these two springs were the same length (OEM and new), this means the bottom of the new spring must be raised 2” (again, Â½ the difference between the two rates) to keep the same ride height. But rarely are these springs the same length, so now the spring length (more specifically, the spring compressed length) then becomes a factor as well.
I only mention all of the above to cause you to think about all that goes into this project. In simple terms, if you know the spring rate, and you know the corner un-sprung weight (not total car corner weight), the you can determine the length of the new compressed spring. Then take this “compressed length” value and measure down from the upper spring perch, mark the strut tube at this length (this is with the suspension assembled and installed on the car, and the car sitting at you intended ride height), and then use this location as a good estimate on where the center (midpoint) of the threaded tube should be (actually, this is where the spring perch would be located midway on the threaded tube). Then once you know where the threaded tube should be positioned, the location of where you wish to weld the tube seat is a no-brainer.
Lastly, you need to consider what the use of the car will be. If the car is to be used on the street, but occasionally used on the track, then you may wish to lower the threaded tube so that you can raise the car a small amount for the street, and the lower is a good bit for the track. Regardless, take into consideration whether the car will be occasionally raised or lowered depending upon future use, and then place the threaded tube accordingly. Usually, if the threaded tube is place correctly, and the lower spring perch is used midway up the tube, you should have plenty of room to move the perch for varying ride heights (provide your threaded tube is of sufficient length).