How to Adjust Preload and Ride Height on Threaded-Body Coilovers

It doesn't take a certified genius to adjust preload on ride height adjustable coilovers or struts, but we still see a lot of people hesitant to take that leap. So today, we're demystifying the simple science of preload adjustment.

What Is Preload?

Preload is how much the spring is compressed in the assembly compared to its free, fully extended length.

The more you compress the spring, the more preload you have, and the more the spring pushes to extend the coilover or strut, raising the vehicle. The less you compress it, the lower the vehicle sits.

That can feel counterintuitive, so here's the one rule to remember for the rest of this article:

Two Styles of Preload Adjustment

Preload adjustment is preload adjustment. No matter how you get there, the end result is the same, but there's more than one way to get it done.

Circlip Adjustment

Many entry-level and mid-tier struts, like the Bilstein 6112, use circlip adjustment. The strut body has a few preset grooves, and the spring seat is held in one of those grooves by a circlip or snap ring. You get a handful of fixed ride height settings to choose from.

Threaded-Body Adjustment

Then there are threaded-body coilovers, like the Eibach Stage 3R 2.5″ coilovers we're running on our Ford F-150. You're still moving the spring seat up and down, but the threaded body gives you near-infinite adjustment within a set range.

Do You Need a Spring Compressor?

Circlip-Style Struts: Yes

On any circlip-style strut, you won't be able to move the clip while spring force is holding the seat in place. Unless you're a magician, you'll need to mount the strut in a spring compressor and compress the spring until there's no load on the spring seats. From there, you can move the seat wherever you need it.

Threaded Coilovers: It Depends on the Body

On threaded-body coilovers, it comes down to what the body is made of.

Aluminum-body coilovers require a spring compressor. That includes select Bilstein 8112 models and the Fox 2.0 and 2.5 Performance Series and Performance Elite. Aluminum threads can't handle the full spring load during adjustment. We've tried it, and you'll damage the threads.

Steel-body coilovers are a different story. Steel is strong enough to handle the spring force during adjustment, and many, including Eibach's 2.0 and 2.5 series, include a slip washer between the spring and the seat. That lets the spring rotate freely, so the pigtail doesn't bind against the seat or the adjustment collar.

The result: you can adjust these with the coilovers installed on the vehicle. Just get the front end up on jack stands, like we have with our F-150, so you're not fighting the weight of the truck. And a nice bonus with Eibach: they include a spanner wrench in the box.

Preload adjustment by coilover and strut type

Which Direction to Adjust

The Eibachs on our F-150 are a standard-orientation coilover, meaning the body is on the bottom and the shaft is on top. To raise the truck, move the spring seat up. To lower it, move the seat down.

Many high-end coilovers use an inverted design, with the body on top and the shaft on the bottom. The principle is identical; you're just moving the seat in the opposite direction.

Spring seat direction by coilover orientation

That's why we stick with the simple rule: compress the spring to go taller, back off the spring to go lower. It works no matter which way your coilover is mounted.

How Much to Adjust

Knowing how much to adjust depends on the manufacturer.

Eibach: Turns per Inch

Eibach makes it easy. They give you a baseline for where the vehicle will sit out of the box, and they tell you exactly how much ride height change each turn produces. On our F-150 kit, one full turn of the spring seat equals about 1/8″ of ride height. Across multiple sets of Eibach 2.0 and 2.5 coilovers, we've found that to be very accurate.

Bilstein: Reference Measurements

Bilstein takes a different approach. They provide a measurement, typically from the bottom of the spring seat or the top of the top hat to a reference point, along with several recommended settings, so you can estimate where the vehicle will sit.

No Guidance? Use Your Motion Ratio

Without that kind of guidance, the easiest method is to calculate your motion ratio. It's what I rely on most of the time.

Calculating Your Motion Ratio

On an independent front suspension:

1. Measure A: the lower control arm, from the center of the inner pivot bolt at the frame to the center of the ball joint.
2. Measure B: from that same inner pivot to the coilover or strut mount.
3. Divide A by B. That's your motion ratio.
4. Divide your desired ride height change by the motion ratio. That's your starting preload adjustment.

It won't be perfect every time, but it's a solid starting point, and the goal is to get you close.

Tools for the Job

Spanner Wrench or Punch

An included spanner wrench is mostly an Eibach thing. Few brands outside of Eibach and Dobinsons include preload adjusters with their coilovers. Without one, you can usually use a punch in the slots of the spring seat or lock collar to make your adjustments.

We've been using the same Harbor Freight punch set since Shock Surplus started. They're a little bent at this point, but they still get the job done.

Jack Stands and a Spring Compressor

Even on coilovers like the Eibachs that can be adjusted on the vehicle, taking some load off the spring makes the job easier. If you find yourself fighting it the entire time, don't try to prove anything. Grab a spring compressor and avoid damaging your parts.

• Spanner wrench (included with Eibach) or a punch for the seat and lock collar slots
• Jack stands to get the weight off the front end for on-vehicle adjustment
• Spring compressor for circlip struts, aluminum-body coilovers, or any spring that's fighting you
• Tape measure for motion ratio and before-and-after ride height