Correctly Measuring For Shocks And Building A Custom Rear Suspension

There are plenty of direct-fit shock and suspension options for popular vehicle platforms, but what if your project isn’t one of those vehicles? Well, that means you need to start pulling pages out of the old-school hot rodding handbook and make things happen. You have to know where you should be measuring for a new set of shocks and how a custom rear suspension will work with your project vehicle.

Our 1994 Volvo 940 wagon known as “Project Swedish Meatball” needed a set of rear coilovers and a suspension fabricated to work with the Moser Engineering M88 we swapped into the car. The team at Big 3 Racing came up with a plan, and we reached out to Max Prusak at QA1 to get some input on what was needed. In this article, we’re going to cover how you measure for custom application coilover shocks, and the parts we used to build our suspension.

It really doesn’t take a lot of parts to fabricate a custom rear coilover shock setup.

A Shockingly Simple Start

Measuring for shocks, selecting springs, and building a suspension from scratch can seem intimidating on paper, but it’s really not as bad as you would think. It all comes down to devising a well-thought-out plan and recording accurate measurements. As long as you have a solid foundation, the suspension you’re going to create will come together correctly.

Before you measure for the shocks, the vehicle’s stance needs to be locked in, and you should have your mounting brackets where you need them. This will be based on your specific application and how the suspension will be set up. Generally speaking, having the shocks sitting straight up is going to work best for most suspensions. If you don’t have all of this figured out before you measure for the shocks, the shocks could end up with an improper stroke length that will cause the shocks to bottom or top out.

Once you know how you want the vehicle to sit and know where the shocks are going to be mounted, it’s time to pull out the measuring tape and get to work.

“We like to measure shocks based on ride height. For example, on a solid axle rear suspension, we would like the axle supported to the desired ride height, and from there we would take a center-to-center measurement on the shock mounting holes. This will give us our ride height, and allow us to find a shock that fits with ample travel in both directions,” Prusak explains.

While it may seem simple to measure for shocks, there are some common mistakes that people make that should be avoided. The easiest mistake to make according to Prusak is using your old shock dimensions at the base for your new shocks. Those old shocks probably won’t have the correct dimensions since you’re making changes to the vehicle. It’s best to get a fresh set of measurements to be safe.

It’s important to make sure your shocks will have plenty of travel. That’s why taking the correct measurements from the right locations is so critical.

“We find a lot of customers are unaware of how shock travel directly impacts ride quality. For example, if a customer were to call in with a shock ride height measurement of 11 inches on their solid axle it would really be in their best interest to alter their mounts closer to 14 inches to allow for more travel in a 5-inchstroke shock. An 11-inch ride height would call for the use of a 3-inch stroke shock, meaning we only have 1.5 inches of travel in either direction as a best-case scenario. This presents many challenges related to ride quality as we typically like to see 2.5 to 3 inches of compression and 2 to 2.5 inches of rebound on a solid axle,” Prusak says.

Here’s how Stefanski measured for the shocks on our Volvo.

“So, what I did before we even removed the old suspension, I measured to the bottom of the frame where I knew we were going to have to mount the shock, to roughly how far below the rear axle it was going to sit. I had a general idea of where the shock was going to sit. So, I just measured that, and that gave us our right height measurement. We obviously need enough range of movement for the shock to compress and extend for normal driving, plus any kind of racing the car would do.”

r you figure out what length the shocks need to be, it’s time to look at springs. This is a two-part process. First, you need to figure out how long the springs need to be. The spring length is dictated by the shock length and what will physically fit, it’s that easy. Prusak recommends looking at a slightly taller spring for drag racing applications to take advantage of the stored energy they can provide. If you’re going to be carving up the corners on a road course, or in an autocross application, a shorter spring is ideal for faster action.

Don’t take a guess on what springs you’ll need. There’s plenty of tools online that can help you make the right choice.

Finding the spring rate for your application is more in-depth, but isn’t difficult.

“Calculating spring rate takes just a little math. As a baseline, we typically aim for 30 to 35 percent spring compression on a drag car and 20 to 25 percent spring compression with weight on wheels for street or road course applications. We have more resources that cover the technical information and formulas that you would use right here on the QA1 website,” Prusak states.

We worked with QA1 to make sure we had all the right parts for this project. They sent us hiem joints, mounting brackets, mounting tabs, shocks, and springs.

Parts For The Project

With the measurements in hand, we needed to select parts to build the suspension. Starting with the shocks, QA1 suggested we use the Proma Star double-adjustable shocks. Big 3 Racing did the math and found we needed a set that was 11.5 inches from eyelet to eyelet. 

Double-adjustable shocks made the most sense for our build since the car is going to be used in multiple applications. There are 18 clicks of adjustment available between the rebound and compression knobs, which gives us 324 different valving combinations at our disposal without having to remove the shocks.

“Highlighting the benefits of a double-adjustable over single-adjustable shock really comes into play when we are at the track. For example, on the front of a drag car we are going to want some high compression force and low rebound to promote weight transfer off the line, but then be able to support the vehicle when the nose comes back down. This type of valving isn’t achievable with a single-adjustable as we are only able to adjust our compression and rebound forces simultaneously versus independently,” Prusak says.

Double adjustable shocks give you more options for how the vehicle will ride, and how it will perform at the track.

For springs, a set of 220 pound-per-inch springs that measure 9 inches in length got the call. These springs are made of a high-tensile, chrome silicon alloy spring wire that QA1 specially designed for high-performance applications. The springs have a reduced number of coils with a smaller wire diameter. The performance of the springs is optimized by the reduction in unsprung weight, which allows them to react faster. All of these attributes make these springs a perfect fit for our application since it will be used in different track environments, and driven on the street.

Mounting Brackets And Making The Suspension

The brackets you use to mount your shocks and suspension parts shouldn’t be treated as an afterthought. These are important links in the suspension system, so you need to select brackets that are strong enough for your application, and will work with how you need to mount everything. We used some of QA1’s standard mounting tabs for our upper shock mounts. For the lower shock mounts, a set of QA1’s HD Pro Coilover Conversion Brackets were welded to the Moser rearend.

“Our shock mounts use a double-shear design, meaning we are sandwiching the shock mount in between the brackets instead of simply bolting to one of them. The HD Pro Coilover Conversion Brackets have a good amount of adjustability. This is something to consider, as most brackets will have multiple mounting locations which allows the user to alter their stance without sacrificing shock travel in either direction,” Prusak explains.

It could be tempting to use cheap rod ends for your build, however, this will impact how the suspension performs and its longevity. The control-arm-style suspension that Big 3 Racing created for the Volvo uses rod ends at each mounting point. We used QA1’s X Series rod ends for strength, and to allow the suspension to move freely. These rod ends are self-lubricating thanks to the Teflon liner that lies between the ball and the race of the rod end.

With the shock measurements in hand, Stefanski moved on to deciding the style of suspension the car would use. Based on how the Volvo’s chassis is designed, and where the rearend sits, Stefanski elected to go with a control arm-style suspension that used a torque arm and panhard rod.

“We wanted to use the control arm setup because it would fit in the car perfectly. This allowed us to put the front mount for the lower control arm where we needed it, so we could make the control arm the exact length that would work with the suspension. We really didn’t want to put any kind of angle to it from side to side, we wanted it to be as straight as possible. Since the car was now using a Moser M88 rearend, we had to create a new panhard rod and torque arm to get the right fit,” Stefanski says.

Don’t skimp on the rod ends. If you do, you’ll regret it later.

“Determining the correct rod end for your application is very important as some are intended for low-load applications like throttle linkages, while others may be better served in high-load situations like a torque arm, trailing arm or tie rod. It is important to check over rod ends on a regular basis on your performance car, if they are making through suspension travel, feel tight in the housing or have noticeable slop then it is probably time to replace them. We have a lot of information available about how to select the right rod ends for your application here on the QA1 website,” Prusak says.

The Volvo’s Suspension

Big 3 Racing went into this suspension project with a rough idea of what they wanted to create. The goal was to build a rear suspension that would function well driving on the street, but also be able to work at the drag strip or autocross course. This meant building a suspension that would have some compromises, however, it would still perform well in any environment.

The final result is a suspension that’s similar to the OEM Volvo setup, but much stronger, and with plenty of adjustability. Since the Volvo is going to be used as a street cruiser, as well as make the occasional trip to the drag strip and autocross course, Stefanski had to be mindful of how the build was approached.

“It’ll drive phenomenally on the street and will handle anything a daily-driven vehicle encounters. But, we also set this car up so it can perform well at the track. If we wanted to build a suspension that was optimized for track use, the back half of the car would have needed to be cut up, and that would have impacted how it drives on the street,” Stefanski explains.

Measuring for custom shocks and building a custom suspension is a task that can be accomplished by just about any DIY’er in their home shop. As long as you measure everything properly and follow a good plan, you’ll be able to achieve the desired outcome.