Rear coilover shocks have always been a good way to add a lot of adjustability and tuning to the rear of a drag car but few people understand the process to installing them and setting them up.
The below information is a good starting point for installing and setting up rear coilover shocks on a drag car but your car should be tested and adjusted afterwards to find what works best.

Double vs Single adjustable shocks

Single Adjustable: The less expensive units are Single Adjustable shocks.
What this means is that there is One knob that can be turned to adjust how quickly the shock can be extended but the compression doesn't change.

Double Adjustable: The more expensive units are Double Adjustable shocks.
What this means is that there are Two knobs that can be used for adjusting. One knob is used to adjust how quickly the shock can be Compressed and the Second knob is used to adjust how quickly the shock can be Extended. Which shock you use will depend on how much you want to spend on shocks.

Spring Rate
This is one of the things that really throw people off from lack of understanding and can create a difficult installation and traction problems.

For Rear springs I prefer to start with the lightest spring possible that will not coilbind during operation. The reason is b/c under the shock of launching I want the spring to react as quickly as possible and allow the suspension to do it’s thing to maintain traction. If the spring is too stiff then the suspension can not react quick enough and the rear tires can unload. This makes it much more difficult to tune the suspension with shock adjustments because the springs are working against the shock settings.

A lot of cars can start with a 110# spring then go up/down from there once the car is built and tested. Unless the car is over 3500lbs you should not need a spring over 130# so if you have a car that is 3100lbs and your rear springs are more than 110# then you should look at trying softer ones if you haven’t.

Shock Installation
Proper Shock Length
Most people don’t know that shocks are designed to be installed at a SPECIFIC length so they just weld on some brackets, toss on some springs and go racing.

*A coilover shock should be installed so that at ride height the shock is installed 2/3 collapsed.

Example:
Follow along but USE YOUR OWN measurements. These numbers are ONLY for examples.

1) Extend the shock as far as it will go. Measure from the bottom mounting hole to the top mounting hole. Lets say this measurement is 24”.

2) Now Collapse the shock as far as it will go or up to the bump stop. Measure from the bottom mounting hole to the top mounting hole. Lets say this measurement is 18”.

3) Now subtract these measurements: 24”-18”= 6”
This 6” is the total shock travel.

Now comes the tricky part using the 2/3 Rule.

4) Take the 6” and multiply it by 2 like this: 6 x 2 = 12

5) Devide the 12 by 3 like this: 12 / 3 = 4”

Almost done:

6) Using the ORIGINAL 24” of the extended shock length subtract the 4” like this: 24” – 4” = 20”

DONE! Sorta. This 20” is the LENGTH OF THE SHOCK as it is installed in the car at ride height. In other words as the car is sitting on the starting line the Top shock bolt is 20” above the Bottom shock bolt. 

Hint: Once this distance is figured out I make a small fixture that has 2 holes in it at 20” apart. I use this fixture to mock up the shock brackets so both sides are equal. 

Installing shock brackets
For the best installation you should mock up the rearend in the car so the car is at the desired ride height. This way the car will have the proper stance and desired tire clearance front-to-back and side-to-side.

Lower Mounts
Now that you know how long the shock should be when installed you need to install the shock brackets.

Start with the lower mounts on the axle tubes. If you have the brackets that have several mounting hole positions assemble the brackets so the DESIRED shock hole will be in the middle of the settings.

1. With the rearend mocked up in the car hold this bracket in place on the axle tube.
2. Measure up 20” (or use your fixture as noted above) from the desired shock hole and see if there is enough room to mount the upper shock bracket. If not then see if one of the lower mounting holes will work. If not then you need to get a shorter shock or find a way to raise the upper shock mount bracket.
3. If one of the shock holes will work then tack weld a lower bracket onto the axle tube in the desired location.

Upper Mounts
The bracket positions are not critical as long as both sides are even. Naturally it is best to make sure everything is square with the chassis so do the best you can and it will probably be good enough.

When installing the shocks it is best to make sure they are straight up and down but if you need to tilt the shock to gain more clearance then feel free to do it.

Mount the Upper shock mount in the desired location so that it is 20” from the lower shock hole (use your fixture) so you have clearance for wrenches, antiroll bars, fuel tanks, etc. Spot welding the brackets in for now is the best just in case you need to make adjustments.

Tack weld all the upper and lower shock mounts in. Mount the shocks without the springs to verify that everything mounts correctly. Move the rearend up and down through the travel to make sure there is no binding or interference. If everything looks good weld it all in.

Installing & Checking Springs 

Install the coil springs and adjust them with the spanner nuts so that the shock bolts are 20” apart. IF all goes well this should give you the expected ride height that you wanted. If there isn’t enough threads on the shock body to get the shocks at the desired distance then you need to change to a softer/stiffer spring OR a different LENGTH spring.
If you need to you can move the lower shock hole up/down one hole but when you do this you alter the distance the shock can travel before it stacks solid so try to maintain as close to the 20” as possible. Hopefully if you bought a “kit” the springs will be the proper length for your car.

Checking the Spring Rate
When installing coilover springs remember that the springs should be very soft so when they are at ride height there may be very little gap between each coil. This is good although the spring looks like it may coil bind.

The amount of travel the spring has can be calculated very easy.
1) With the car at ride height measure the distance between the coils. Let’s say it is .125”
2) Count how many coil spring “gaps” there are. Let’s say there are 10 gaps.
3) Multiply these numbers together and this is how far the spring can compress before it coilbinds:       Example: .125” x 10 = 1.25”

So with this example the spring can compress 1.25” before coilbind. If you feel that this is enough “bump travel” and the car will never compress the springs this much then test these springs.

Remember above when we were calculating the shock length? The shock ended up at 20" long at ride height. This gives the shock 2" of "Bump" travel. In otherwords the shock can only compress 2" before it compresses solid if you hit a bump.
Our Spring only has 1.25" of travel before it will compress solid. This can be a problem.
You do not want the spring to ever coilbind so if your calculations come out that the spring has less travel than the shock then you need a STIFFER spring. Go up in stiffness just one maybe two spring rates but don't go too stiff. Most distributors selling coilover kits will swap out the springs for free as long as the springs are were never used and are still in good shape.

If on the otherhand there are large gaps between the springs and you have a lot of "spring travel" then you have too stiff of springs and you should get a SOFTER spring.

Kevin Slaby
www.BaselineSuspensions.com
Copyright © 2002 - 2006 Baseline Suspensions, Inc. all rights reserved.
No part of this site may be reproduced without prior written consent.