Old Truck, New Tricks: Making Custom-Length Shocks With Bilstein

You did it! You finally took the plunge and started the axle swap you’ve been dreaming of. Maybe you bought your dream rig with the tough job of an axle swap already completed or maybe you bought a rig and the previous owner said he’d installed a six-inch lift kit and you are pretty sure it’s actually a three-inch lift. If you find yourself in any of these situations, then the question will eventually come up about shocks. So, how do you really know what shocks are right for your rig’s setup? That’s why we turned to Bilstein.

My 1972 Ford F-250. It started life as a 2WD before the previous owner upgraded it to 4WD thanks to a Bronco drivetrain swap.

I found myself contemplating this same issue with this 1972 Ford F-250. This truck started life as a two-wheel-drive farm truck when the previous owner ripped the legs out from under a Bronco and installed them onto the truck to convert it to four-wheel-drive. After falling in love with the look of the truck, I brought it home to be my new project. I planned to make it into a camping or fishing rig, with the odd run to the hardware store every now and then.

The truck was running and driving, but there were a lot of loose ends to fix before I could get it on the road. While rebuilding the steering on the truck, I found that the axle had almost no droop when jacked up. As it turned out, the old shocks from the Bronco were nowhere near long enough for my truck. So it was time to toss the old shocks in the trash and grab a set that would allow my truck to flex its new 4×4 muscle.

I started into the project with a good idea about how to measure axle travel, but shocks were another matter. For that aspect, I enlisted the help of Junior Africa, a technical expert with Bilstein. His first question was simple – “What style of shock mounts do you have on the truck?”

My F-250 had the same style shocks in the front as well as the rear. The top mount was a stem style mount where a threaded stud extended from the top of the shock. This is a common style for factory setups, and the previous owner had kept them when he did the swap. The bottom of the shocks were eye mount, where there was an eyelet welded to the bottom of the shock. This allowed a bolt to pass through the shock and attach it to the suspenion.

Bilstein’s off-road shocks come with zinc plating to help prevent oxidation.

With the simple part figured out, now I needed to get my hands dirty and put the truck up in the air. I articulated the axles to see exactly how much travel the truck had. Starting at the front, I took the wheels off the axle to allow for more downward travel while measuring for the shocks. Doing this could keep the truck at a safe height. Next, I pulled off the old, worn-out shocks and tossed them straight into the trash.

Start With The Dirty Work

The first step: Junior told us to put the truck’s frame on jack stands, and then remove the springs and sway bars from the axle. “Droop the axle with no springs or sway bar connected. This will give you the full extended measurements,” was the advice from Junior.

Measuring the axle’s droop with the springs and sway bars removed.

My truck has “captive” springs, meaning the springs have retaining clips on the top and the bottom, so the spring doesn’t fall out of place when the axle is drooping. With the springs unbolted, I let the axle droop down under its own weight until it finally rested on its own. This was the max mechanical droop for my truck, so I simply ran a tape measure between the two points – the eyelet holes in the axle up to the upper mounting hole. Thus, I had the max travel that the shock would see.

With the max axle droop numbers in hand, I could now look at the max bump. I did this by simply taking the axle and jacking it up until I contacted the bump stops. This would give me a measurement for the shortest length that the shocks would be at under full compression.

One thing I had to remember was that the rubber bump stops would compress under load, so I subtracted half of the bump stop height from the measurement to ensure I didn’t bottom out the shock from a hard hit.

Pro tip: If you are running air bump stops, air them down and push the axle up until they bottom out. Then you can take your measurements.

The rear of my truck is fitted with leaf springs. This made things a bit more difficult. If you have coil springs, you simply repeat the steps you did with the front; the process is different with leaf springs.

There are two ways to go about grabbing your measurements with leaf springs – the hard way and the easy way. The hard way involves pulling the leaf packs apart until the main leaf remains. From here, you cycle the suspension as I did with the front of my truck, using the main leaf to locate the axle while moving it up and down. This will give up a much more accurate measurement, but there are still some things to consider. The springs sit on top or below the axle, so depending on your truck’s setup, you may lose the thickness of the leaf pack when only using the main leaf.

I made sure to take notes on every measurement we took from the truck. All the extra information helped when talking with the Bilstein tech.

I have a thick factory leaf pack as well as the factory lift blocks that came on some trucks, so I needed to account for these once they were removed. To use the hard method, I measured the thickness from the axle mounting pad to the side of the leaf spring that the axle will sit on when disassembled. That thickness will then get added or subtracted to the measurements, depending on where your axle sits relative to the main leaf. Due to the fact I had stock springs that are nice and stiff for hauling, I knew I was not going to get a great deal of flex with the rear end, so I opted for the easier method of educated guessing.

First, I pulled the shocks and sway bar off and jacked the truck up by the frame and let the rear axle droop. Next, I took the shock length measurement for full droop. With my full droop measurement in hand, I set the truck back down on level ground and grabbed a level and a plumb bob. I used a right triangle in order to calculate the shock length at full bump.

I lined up one end of the plumb bob with the upper shock mount; the other end hung over the lower shock mount, keeping the level, well, level. Then, I took my plumb bob and held it against the level and lined it up with the bottom of the shock, making a right triangle. Once my right triangle was established, I took the measurements. The first measurement was along the level, at the point of the upper shock mount at which the plumb line intersects it. The second measurement was along the plumb line from our level to the point at which the plumb line meets the lower shock mount.

The horizontal line represents your level, where as the vertical line represents the placement of the plumb line.

I knew two sides of the right triangle thanks to the plumb bob and level, so I could calculate the third with math. Unless you remember how to solve for a hypotenuse from grade school, you will want to jump on Google and search “solve for hypotenuse”. The hypotenuse of a triangle is the side opposite of our right angle (where the plumb bob meets the level), meaning the hypotenuse is the shock’s length.

I ran the numbers I measured from the truck to see if the calculations matched reality. I measured the shock length at ride height and compared it to what the online calculator showed me. So with 11 inches back and 13 inches down, the calculator spat out 17.03 inches for the hypotenuse; in the real world, I got 16.5 inches for the shock length. This was a good enough guess, so I called it close enough, knowing I would need a shock that gave me at least a half inch (plus or minus) of additional travel, which would account for any bad math or measuring.

A right triangle or right-angled triangle is a triangle in which one angle is a right angle. The side opposite the right angle is called the hypotenuse. The hypotenuse of the right triangle represents the theoretical length of the shock, while the other two sides are the measurements I gathered from the actual truck. I used this information to calculate the shock length at full droop.

Now, I measured the distance between the axle and the center bump stop rubber, and then took that distance and subtracted it from the vertical measurement that I took. I plugged in the new height measurement back into the online calculator and the new hypotenuse was my shock length at full bump. I had 6.5 inches from the center of the bump stop to the top of the axle, so I took 13 minus 6.5 and got 6.5. Back in the calculator, I put one side of the triangle at 6.5 and the other at 11, giving me the shock length of 12.78 inches at full bump.

Now, I had all of the shock lengths for both compression and droop. I gave Bilstein a call and talked with their tech team to decide on what shocks to run on the truck. I gave them the measurements, as well as the shock mounting style for the top and bottom of each shock. Then I let them know what I planned to do with the truck, as that would dictate what series shock fit my needs.

Having the right shocks on the truck will make a big difference to its performance.

Bilstein decided that the B8 5100 series shock would be perfect for the light off-roading and fire roads over which the truck would most often be driven. The 5100  line of shocks are specifically built for lifted trucks running larger-than-stock tires. With digressive valving patented by Bilstein, they are perfect for on-road and off-road use like my truck sees. I received a recommendation on part numbers, and from there, it was as simple as order, install, and enjoy.

Now the easy part was the install, knowing that the shocks would be a perfect fit for the truck. First impressions were great – the shocks were smooth and not stiff. I felt connected to the pavement in a way I had never been able to feel before. The real test was a small jump off of the pavement. That let the hard work that Bilstein puts into their shocks really shine. A weekend trip took us down fire roads and down a few trails. The truck felt 40 years younger and handled the bumps and ruts like a champ. The best part was now the axles could fully articulate and flex the way they were designed to. There were no more creaks and groans from the frame due to the lack of travel.

If you need new shocks for your truck or trail rig, head over to Bilstein’s website or give them a call to discuss your application and see how much better your truck can ride with a new set of Bilstein shocks. Stay up to date with Bilstein by following their Facebook page to keep up to date on new products news!

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About the author

Seth Hervey

Seth has been shooting professional quality video for years. He also started and ran all of the social media outlets for FatMan Fabrications while working for them. Seth enjoys shooting still photography of cars and events for fun.
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