How-To: Coil-Over Conversion
Story and photos by Matthew Eddy

Over the last year I have slowly been prepping my 91 Toyota MR2 for NASA (National Auto Sports Association) Time Trials and eventually race in the Performance Touring series.  Last summer I swapped in a 3.0L V6 and over the winter I have built and installed what I am calling a coil-over conversion; converting the stock struts to coil-overs. 

What may not be clear to all my readers is what coil-overs are and what benefits they bestow over struts since they seem very similar.  If you are purchasing a coil-overs, especially good ones, you are pretty much guaranteed that the dampers and the springs are going to be well matched.  Secondly, coil-overs allow for ride height and corner weight adjustments.   Also they have common spring sizes so it would be easy to swap out springs to fine tune the suspension for individual tracks or if you make other modifications.

Measure First

Before doing this conversion check and see how much clearance you have between the front wheels/tires and the strut housing.  On the MR2, the front spring sits above the wheels and the strut housing is only 1/4" from the wheel.  In most cases you will need to either get new wheels with a larger offset or wheel spacers.  I installed 12mm spacers and new longer wheel studs.  Replacing the wheel studs is pretty easy, especially if you have an impact.  I have posted a video on youtube that shows how to do it here.  I bought both teh studs and spacers on ebay.  To find the spacer search for "hubcentric MR2" and they are usually priced at about $60 pair.  The wheels studs were $35 for 10pcs.  Also, keep in mind that you will need to get new front end-links with this conversion because the bracket is being relocated.  You can deal with this one of two ways; buy Powergrid Endlinks which can be ordered at custom lengths (you will need 5.5" center to center), or cut and weld the stock ones but the stock ones are probably so old and rusty you are better off getting new ones anyways.

Powergrid End links.  It might be hard to believe, but these aren't new, they have been on my car for 2-3 years.

Next, measure your current ride height for baseline for future adjustments after installing the coil-overs.  Park the car on a level surface and measure from the ground, through the center of the wheel to the lip of the fender.  Also measure from the center of the hub to the fender lip.  If you have any fender damage or rust that might make these measurements unreliable pick another point to measure to.  WRITE THIS DOWN, or at the very least, text message yourself.  You will want this information later.

New Studs and 12mm Spacer.

Strut Preparation

The following write up is specific to the MKII MR2 but is still applicable across many makes and models.  One major difference you may encounter is that many cars cannot take strut cartridges.   In the MR2, I can remove the actual damper from the strut housing itself but I do not believe this is a common feature.  If you find this is the case with your struts you have one of two options.  Convert the strut to accept the coil-over sleeve or convert the strut to accept cartridges.  I will address this a bit more at the end of the post since my suggestions will make more sense after you see what is involved.

It might be a good idea to buy an old used set of struts to work on that way if you mess them up some how or the project is more involved than you anticipated you can take as much time as you need.

Strut Diagram - parts of the strut.

Strut Diagram - Distances to measure

See the diagram to the left as a reference for the terms I use for the various parts of the strut. 

1. Disassemble the strut and remove the upper mount and spring.  Second, take measurements.  See my video posted on youtube that shows how to disassemble struts HERE. Reference the diagram on the right.  

2.Measure the overall length of the strut, the length of the body, the distance from the end links bracket to the end of the strut (if applicable) (G or F or both), the shock body diameter (A),  length of the strut rod (D), the diameter of the strut end (B), and distances to other brackets you may have on the strut body.  Also measure the length of the spring, and the thickness of the upper strut mount.

MR2 Struts in Before Being Disassembled.

Disassembled Struts with Spring Perches

3. Cut off the spring perches and grind the housing "smooth."  For the MR2 front strut you will also need to cut off the sway bar end-link bracket.


3a (Optional) - To convert struts that don't usually take cartridges you can convert them to cartridges by drill a small hole near the bottom of the strut to relieve the pressure, then cut off the top of strut to remove the guts.

Disassembled Struts with Spring Perches Cut Off.

Struts Close-up; Spring Perches Cut Off. You can see the gland nut just above the grounded down area.

4.  Put the strut housing into a vice and remove the gland nut with a monkey wrench.

Removing Gland Nut From Strut.

5.  For the front struts, weld the front sway bar bracket just above the knuckle bracket.  Then weld a bead around the strut for the coil-over sleeves to sit on.  For the rear struts, the sleeve will also sit on the sway bar bracket, and you should weld a bead around the strut at that location. 

Weld a bead around the struts for the coil-over sleeve to sit on like this.

6. Cut the bump stock bump stop in half.   More than likely you will be lowering the car which means you will be decreasing the travel of the shock so cutting the bump stop in half will give you a little more travel.

7. (Optional) Sand blast and paint the strut housings.  I would also suggest grinding off any sharp features.  That will reduce the chances of the paint chipping or peeling.  For instance there are little tabs on the knuckle bracket that should be ground down.  After painting them, us caulk or silicon to fill in the gap around the top of the knuckle bracket.  This will prevent water and debris from collecting there and causing corrosion in the future.  I used a special caulk that is applied before powder coating (see the second picture below).

Stock Struts Sand Blasted.

I Have Applied Caulk to the Above Area.

Strut Assembly

I have compiled parts list with prices so you can have an idea of what you need and how much it will cost.  I have not included the cost of the struts or dampers (aka shocks).  For certain parts I have included links to where you can buy them. 

Parts List:

Coil Over Parts - (Left to right) Gland Nut, Lock Nut, 1" Washer, Strut Cartridge, Strut Housing, Top Mount, o-ring (center of pic), Coilover Sleeve, lower Spring Perch, Upper Spring Perch, Spring.

Coil-over Sleeves; 2.038" ID, 5" Length (x4): $15.53 ea

Lower Spring Perch (x4): $12.60 ea

2.5" Coil Spring Tops - Upper Spring Perch(2 sets): $64 total (with powder coating)

Springs – Fronts (Set of 2) (Eibach, 250# 8”): $103 – bought as Edelbrocks on Summit Racing

Springs – Rear (Set of 2) (King, 400# 10”): $43 – bought on ebay, used

O-rings (8 total): $4 (local hardware store)
18mm Washer (6): $2 (local hardware store)
1" Washer (2): $1.50 (local hardware store) 
Light Weight Oil (engine oil, shock oil): $4 (local autoparts store)
Total: $334

Part List Notes:

Spring Rates: Not sure which spring rates to use?  Check out my blog posting about choosing spring rates HERE.  For my MR2 - which I drive to a from the track/autoX, I choose 250 in/lbs front, and 400 in/lbs for the 

Coil-over Sleeves:  I used 5" sleeves because I am using sway bars and that's probably as low as you can move the front sway bar bracket.  I would not suggest going sway barless because there might be a time that you actually want a sway bar.  Also you shouldn't need to get more than 5" of travel that I can think of.  However, on the rears, you can move the sway bar bracket down 2" and get a 7" soil-over sleeve.  

2.5" Coil Spring Tops:  You will need to measure the diameter of the strut cylinder and the guy will customize the inner diameter for you.  I would also suggest getting the parts powder coated to protect them from debris.




Assembly


1. Put (1) o-ring on the strut housing (about 2-3" from the top of strut housing)
2. Slide coil-over sleeve over o-ring.  The sleeve should slide over the o-ring and be moderately tight.  If the o-ring is just to thick, wrap the strut tower with electrical tape about 1" above the weld bead.  The purpose of the o-ring or electrical tape is to remove play between the sleeve and the housing and prevent the sleeve from spinning when adjusting ride height. 
3. Take a second o-ring and force it into the gap between the strut housing and sleeve from the top. See pic below.

Using a flat head screw driver to insert o-ring.  Here the gland nut is already on, but its a lot easier to do this before installing gland nut.

4. Screw the lower spring perch onto sleeve (can be done later).
5. Put the strut housing into a vice, and protect the paint with rags.  Pour about 3oz of oil into the housing.  The oil fills the gap between the cartridge and the strut housing to prevent heat building up in the shock.  I prefer not to use tranny fluid or gear oil because of the smell.  I used some engine oil because it was convenient.
6. Insert strut cartridge.  Check oil level, and fill as necessary.  You want the oil to be about .5-1" from the top.
7. Apply some anti-seize to the gland nut and screw it on.  First by hand and then tighten with a monkey wrench.
8. Put spring on. 
9. Put bump stop on the shock cylinder.
10. Put top spring perch on
11. Put (1) 1" washer on (FOR THE FRONT STRUTS ONLY).  This is acting as a spacer so that the top spring perch contacts the bearing portion of the front strut mount.  See below photos.  I also added bearing greases here to prevent corrosion and wear.  This section will be turning with the wheels.  Though I have two pictures, each showing a washer, you only need to use one washer.  The pictures below are just to demonstrate where the washer will sit. 

Washer sitting on spring top.

Where the washer will sit on the strut mount.

12. Put the strut mount on.
13. Put (3) 18mm washers on (FRONT ONLY).  The stock upper strut mount would usually sit under the mount and without it there is some play between the mount and the locking nut that holds this all together. 
14. Screw on the locking nut on the end of the shock shaft.  The hardest part about this is that the shock will want to spin.  You best bet it to snug it up by holding the shaft with your fingers through the coil.  Once it starts to spin you can try wrapping a piece of rubber around the shaft and grip it with vice grips.  Get them as tight as you can.  Don't worry, you won't be able to get it super tight, but it should be good enough. 


When you are done they will look something like this.

Coil-over Conversion.  Fronts for a MKII MR2 with 10" springs.  I later switched to 8" springs.

Here is a pic of them mounted in the car.

Front Coil-overs mounted on the cars.

For struts without gland nuts you can weld on the strut without removing the inner shock but take a few precautions such as welding in short durations.  Do not weld weld near the valves (toward the top or bottom of the strut). But the better option will be to cut off the top of the strut, remove the damper and then weld a threaded collar that will allow you to swap out strut cartridges.  You will probably have to do some research and find a cartridge that is the proper diameter and length to fit. 

Adjusting Ride Height

 When I installed mine I moved the lower spring perch up to hold the spring against the top mount, then mounted the coil-overs on the car.  Measure the ride height from the ground to the fender as described above.  Remember to do this on a level surface!  If, by magic or math you got the ride height perfect on the first go then make sure everything is tightened up and take it for a test drive.  If you want to make adjustments then jack up the car, remove the wheel and measure the distance from the lower spring perch to some point on the strut.  Then adjust the position of the lower spring perch to change the ride height. Below you can see how I measured using a metric measuring tape.  I find metric to be much easier than figuring out what fraction of an inch I was looking at. I was using the knuckle bracket as my reference point.  In this cse I was at 73mm.  I wanted to reduce my ride height by 1".  25mm = 1" so to lower the car 1" I need to moved the lower spring perch down until I measure 48mm.
 
 

Measuring the Distance of Lower Spring Perch to Reference Point on Coil-Over.

Sure enough, I dropped the car and I was at exactly where I wanted to be.

If you have questions, please feel free to ask them in the comment section below.  


Fine Print: You are responsible for your own safety and modifications to you car.  I am not responsible for any damage, injury or death that may result.  Follow these steps at your own risk and if you aren't sure you are doing something safely then don't do it. Always use proper safety equipment.  Modifying or changing existing products on your car is risky and not suggested by the manufacturer so any damage, injury or death is your own responsibility.  Do not attempt if you don't feel you can accomplish this safely.  Make sure to check all the bolts are tightened properly and test the car before driving it.

Suspension Series Part 1 - Shocks and Springs

Selection of coil-over springs.

To avoid confusion, I want to be clear that I am not a suspension expert.  I am preparing my MR2 for NASA HPDE (High Performance Driving Events) with the intention of working toward my time trial and competition license.  Over the last couple months I have been reading articles and books about suspension design and tuning so that I can make my car as good as I can.  The more I learn the more I will share.

It seems that a lot of people confuse the purposes of the shocks and springs.  The spring should be determining the “stiffness” of the suspension not the shock.  That is not to say that their functions aren’t interrelated and as such it is very important that they complement each other in order for them to be effective.  Primarily the function of the spring is to keep the wheels in contact with the ground.  In short the springs allows the wheels to move vertically over bumps and into divots to maintain contact with the road.  The spring is actually the load bearing component of the suspension and is a boundary between what is referred to as “sprung” weight and “unsprung” weight (see future articles).  The shock’s, or the damper’s, sole purpose is to control the motion of the spring.

When a spring is compressed and then released, it will bounce or oscillate.  The same can happen to an unrestrained spring in a car and in such a case you will see the wheel literally bouncing after it hits a bump or pothole.  It’s not very often you will see this on the street, but I have seen it a few times.  The damper is designed to control the spring to prevent this from happening.  Ideally, when a wheel encounters a bump it will travel over it causing the spring to compress.  As the bump tapers off, the wheel will follow the profile of the bump back down to the level surface of the road and then its vertical motion will cease.  In other words the natural tendency for the spring to oscillate will be limited to one up and one down motion also known as a cycle.  This is achieved by a properly designed and matched set of springs and dampers. 

Another way to think of this relationship between the spring and the damper is to consider this system separate from the car.   If the spring is compressed then released it will want to bounce up and down a few times.  As I described above, paired with the right damper under the same test, the spring will be compressed and when released will rebound back to its uncompressed length and stop.  Now imagine that we replace the spring will one that is much stiffer but we don’t change the damper.  When we compress the spring we will need much more force to do so.  The spring is now storing or absorbing much more energy and when we release it the spring will rebound with a lot more force.  Now the spring is too strong for the damper and it cannot be controlled.  The spring will oscillate a couple times more than we would like it to because the damper is being over powered by the spring. This is an example of an under-damped car.  Now if we instead swapped out the spring for one that is much softer than the original, then this condition would be called over-damped.  When the spring is compressed and released the spring will rebound slowly; too slow to react to changing road/track conditions.  In a car, imagine hitting a bump and the spring can’t compress because the damper is to stiff.  This artificial increase in spring rate isn’t beneficial to the handling of the car because even minor bumps will be  

Adjustable Shocks

Considering the above examples, if you get stiffer springs for your car but use the stock dampers, then you may be under-damped depending on how much stiffer your new springs are.  This is precisely why adjustable struts/shocks/dampers are available.  You can buy adjustable dampers and throw on a new set of springs and tune the damper to them.  If you decide you need to change out the springs, then you can quickly make adjustments.  Some racers change their spring rates to suit specific tracks or in the case of an Auto-x’er they may change out their springs for different lot surfaces.  Theoretically, once you have set the dampers to the correct stiffness for your springs, you shouldn’t have to change them, but that isn’t so in the real world.  Different tracks and surfaces will need fine tuning.  However, it’s not likely your adjustments will vary much from the theoretical “ideal.”  Even adjustable dampers have a specific working range.  If you choose springs outside that range then you will need to change them out. 

One major drawback of adjustable dampers is that the adjustments can be very inconsistent and aren’t usually repeatable. Dennis North, a highly successful Auto-X’er has tested thousands of shocks and repeatability.  Unless you pony up big bucks for Penske’s, the adjustability is pretty much useless.  No two shocks from the same manufacturer are alike.  Sometimes adjusting them a little stiffer has the complete opposite effect and vice versa.  His suggestion, for the serious yet budget conscious racer, is to get Bilsteins which are re-valvable. This means you can disassemble the shocks fairly easily, change out the internal valves which control the compression, and rebound to tune the shock.  Obviously having to disassemble the shock is not as convenient as an adjustment knob, but it is more customizable. 

Another drawback of most adjustable dampers is that they will only adjust rebound, compression, or both simultaneously.  Compression is how much resistance the damper has when the spring/damper is being compressed and rebound is when the spring/shock is extending.  If you can only adjust one you might find yourself under/over damped in compression but OK in rebound or vice versa.  Higher end dampers will allow you to adjust both independently but they also tend to be quite a bit more expensive.  So the end of the story is, if they aren’t going to be consistent, it probably isn’t worth spending the extra money.