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.
vs Single adjustable shocks
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.
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.
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 its
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
dont 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.
coilover shock should be installed so that at ride height
the shock is installed 2/3 collapsed.
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:
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
6) Using the ORIGINAL 24 of
the extended shock length subtract the 4 like this:
24 4 = 20
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.
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.
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
- With the rearend mocked
up in the car hold this bracket in place on the
- 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
- If one of the shock
holes will work then tack weld a lower bracket
onto the axle tube in the desired location.
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.
& 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 isnt 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
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. Lets say it
2) Count how many coil spring
gaps there are. Lets say there are 10
3) Multiply these numbers together
and this is how far the spring can compress before it
.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
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