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Shock Building Guide

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The single biggest factor in your setup aside from your tires is your shock package. This article is intended to help keep your shocks functioning smoothly, freely, and if done right for multiple race days. Additionally I will be showcasing some of the JConcepts shock parts that help with durability, function, and add the ever important bling to your ride.

The first thing to understand about shock function and performance is that you are trying to find “balance”. The goal is to have a shock that is as free as possible while not loosing oil. Like the rest of your car, you want your shocks to be unbound and free. This allows the shock oil, pistons, and springs do their job without obstruction. One shock being bound is like running thicker oil on that one corner of your car, understandably not ideal.

If you I had to choose between a friction free leaking shock or a water tight bound shock, the free shock wins hands down every time. There is however issues with a shock that weeps oil, let’s examine. The biggest issue with loosing oil from the bottom of the shock is that if there is room for oil to be escaping there is also room for air to enter the shock. This combination of loosing oil and taking in air reaps havoc on your rebound and ride height settings. If you check your ride height between heats and notice significant changes, then you may have a sealing issue at the base of your shock.

Additionally, a weeping shock allows more dirt and grit into your shock then a well sealed shock. The issue with this is that the dirt combines with your oil which alters the weight of the oil, again not an ideal situation.

Now that we have looked at the reasons why to focus on a free, well sealing shock, let’s look at some strategies to achieve a well balanced shock. There are two key factors I want to focus on: 1) fitment between your o-ring and shock shaft, and, 2) fitment of shock guts (length of o-ring, center spacer, and hat spacer combined). The first of these was a bigger problem in the past, but with the introduction and availability of x-rings, it’s not much of an issue anymore. Let’s focus on the fitment of the guts / internals of your shock.

If the guts of your shock are too short they will not form a good seal against the threaded shock bottom. You can verify this by flipping your shock upside down and pressing lightly on the hat bushing with a small hex driver. If the hat bushing pushes in with light pressure applied, your shock likely is not making a good bottom seal. At this point, you have two choices. First you can add an additional spacer to the center shock spacer which will lengthen your overall setup. This gets tricky because adding a small spacer can make your setup too long, so some sanding of the center spacer to get it just right is often required. A second option which I have been using with quite a bit of success is to swap out to the top o-ring. The old associated red o-rings were notorious for swelling which would cause your setup to bind, however in some cases we can use this slight swelling to our advantage. By replacing the top X-Ring (AE# 91493) with a Red O-ring (AE# 5407), through intended swelling, we are adding a little pressure to the threaded bottom cap. I recommend keeping an X-Ring at the bottom of your shock as it is freer and seals better. With the slight added pressure from the top o-ring pressing down, this setup keeps your shocks sealed without sacrificing smoothness. It may seem odd to run two different style o-rings, but experimentation is half the fun of RC racing.

If you check your initial build with stock internals and find that the shock is not as free as it could be, it may be binding. If it is binding, simply sanding small amounts off the center spacer will solve this problem. Just remember to sand tiny amounts and recheck your setup, you want to get this “balance” between sealed and not binding just right.

Another technique you can try to get the o-ring dimensions just right is to soak them in silicon oil. While we discussed how the Red O-rings swell considerably, the x-rings will also swell ever so slightly. If you feel like the bottom seal is very close to being right, but it is slightly loose you can try the following technique with your x-rings. Take your shock apart and soak the rings in a small plastic bag with shock oil. Do this for a few hours and rebuild your shocks. The x-rings will not swell much, but it may be all you need to get the balance right. 

Now that we are doing everything we can to ensure good fitment, I like to add the following JConcepts components for looks as well as performance. First, JConcepts Fin Shock Bottom Cap (# 2492); not only does this component add bling to your shocks it also provides knurled portion for added grip and seats the hat bushing perfectly. I use the Fin Shock Caps (# 2490) as they replace the plastic caps with aluminum which helps with durability and allows you to tighten the bleeder screws without worrying of stripping the plastic. Lastly, I use the Fin Spring Cup (# 2493) and Fin Shock Collar (# 2491) to ensure my springs are aligned perfectly and do not rub against the shock body. Also, the JConcepts shock collars have a demarcation notch which allows you to precisely adjust your ride height without using calipers. Combining these shock upgrades with proper building technique is a potent combination.

As you can see, quality shock building is a skill refined over time. If you keep the goal focused on trying to find the “balance” between a good tight seal while maintaining minimal bind, you will be successful. It certainly takes more effort on the initial build, but once you get the balance your right, your shocks should not need adjustment to the internals for a long time. You will also be rewarded by multiple hours of run time and when you re-check your ride height and rebound they should be pretty spot on.