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Coil Overs


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This message was posted to the list by Wayne Burstein. I thought it needed to be captured and put into our database permanently.

Mitch asked about coilovers on a street car (including shortening the struts and how to get a compliant suspension):

Let me address that in two parts. First, getting a suspension that performs well in a street car, but is not too stiff is not particularly easy. In a Z that is driven daily, I would recommend lowering the car less than an inch to avoid problems with speed bumps, etc. Also, I would recommend keeping the springs/struts fairly soft and running stiff sway bars. That keeps the ride acceptable but minimizes body roll in corners. If you want a race car type of suspension, expect to feel every pebble on the road (that is what I want on my street car, but it is not a daily driver).

It is very important to get the right springs and dampers (strut cartridges in the case of a Z). Basically, you want to match the springs and struts. The springs do the work of holding the car up and allowing the wheels to react to bumps in the road. The dampers are there to keep the springs from bouncing the car up and down for a long time after hitting a bump. If the dampers are too weak, it is like the old commercials for worn out shocks -- the car does not quickly settle after hitting a bump. Too stiff dampers keep the springs from doing their job and the car reacts to the bumps instead of the wheel moving up and down in relation to the car.

I am wondering why you are going to coilovers in a street car. Racers use them to be able to raise and lower the car, and particularly to set corner weights. Setting corner weights lets each wheel do the maximum work keeping the car hooked up in corners. The only reason I would see for using coilovers in a street car is for someone who wants to change ride heights -- possibly lowering the car for autocross or track events. And even this use brings into question how you would re-align the car every time you changed the ride height. Maybe you could come up with some kind of compromise alignment that would work for both ride heights?

I am a little concerned about your comment that your current suspension is too stiff, but bottoms frequently. Suspension dampers are not designed to handle frequent bottoming and you should fix this problem. A couple of years ago I posted a fairly long explanation to the IZCC list about shortening struts and the trade-offs that need to be made. Here it is:

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.

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:


Sorry to be so long winded, but this is a fairly complicated task to plan and implement without too much trial and error.

Wayne Burstein





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, Wayne Burstein wrote (alot, but I'm addressing just one point):

>I am wondering why you are going to coilovers in a street car. Racers

>use them to be able to raise and lower the car, and particularly to set

>corner weights. Setting corner weights lets each wheel do the maximum

>work keeping the car hooked up in corners. The only reason I would see

>for using coilovers in a street car is for someone who wants to change

>ride heights -- possibly lowering the car for autocross or track


Well, there are a few more I can think of:

1) You want to put wider wheels and tires one, but not flare the fenders. By using coilovers, you can use more backspacing in the rear and put wider wheels on. In fact, you can put a 17x9 wheel on with 255/45-17 tires if you use just the right backspacing, 8 inch coilovers, etc.

2) You want to try different spring rates with a wide selection. Once the coilovers are one, you can swap in springs in rates that vary by 25 lb/in over a wide range.

3) One unforeseen benefit is that you no longer need a spring compressor to change springs, etc. Just lower the spring perch to unload the spring and disassembly can begin.

4) The prices for coilovers have come down. I paid a pretty penny to Carerra for mine, but you can get them cheaper at places like http://fonebooth.com .

I see more and more people going to coilovers for reasons 1, 2, and 4 above.

BTW, use the search function on HybridZ.org on this topic. There is alot of good info posted there on coilovers.



Pete Paraska <pparaska@home.com> <http://members.home.net/pparaska/> IZCC#15 73 240Z under marathon body restoration, V8 swap, suspension & brake mods

Check out HybridZ.org for Datsun Z cars modifications, no holds barred!


Internet 240z Club - http://240z.org

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