Shortening Struts

Submitted by,

John Coffey

Moss Micro, Inc.

jcoffey@mossmicro.com http://www.mossmicro.com

As a result of my parts for sale post I've received a few questions

about shortening struts. The best description of he "why's and how's"

was sent to me back in 1996 by Wayne Burstein:

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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. This is dependent on

the length of the struts but 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.

Finally, Carerra was of invaluable help to me

in figuring out what to do and how to accomplish these

modifications, as well as supplying many of the

parts I used. You can reach them at:

770-451-8811

Sorry to be so long winded, but this is a fairly complicated

task to plan and implement without too much trial and

error.

My 73 240Z has been lowered as well. I bought my car back in 1978 and the original owner before me (I am the 2nd owner) did the special suspension. There is no doubt in all of these years that I have had a lot of nice comments on how the car looks so much better than other Z's because it rides lower than stock.

However, it no doubt can be a rough ride. Yes it handles corners very nicely, but now that it is 24 years later and I am now in my mid 40's (oh god), this ride can get a little too stiff for me. Speed bumps can be killer (I had a friend who weighed 350 lbs and I had to make him get out of the car before crossing speedbumps or we would bottom out).

I am presently looking to get new Struts installed to help get a better smoother ride, yet maintain the performance. But I do not want to raise the car back up. I know... I can't have the cake and eat it too. I'm not expecting a luxery car ride, but I definitely need as much as I can get.

Does anybody have a suggestions or recommendations as to what Struts I should get installed or anything else I should know?

Dan.

Boy, that's an old post Mike. New e-mail address below:

I would like to keep the stock " ride" but would like to lower the over all height. Is it possible to increase the shock tower height? Regards, MikeT73.

I wouldn't attempt anything like that, about the only way possible way to use the stock springs and lower your ride height would be to lower the bottom spring seat on the strut. But you would lose a lot of your suspension travel (not good when using the stock springs). You would run into hood interference if you attempted to raise the strut tower height and if it weren't done correctly you would run the risk of a catastrophic failure.

A better idea would be too use the progressive rate springs from Eibach, it would lower your ride height about an inch and still give you a decent highway ride. This would be your best alternative.

Very cool information on the springs and struts. Thank you.

Thanks for the info. Had an original 72 Z, and lowered it about 1.5 in. The ride was uncomfortable. Now that I`am more " mature" (old) I want more comfort and still have a cool looking car. Thanks again, MikeT73.