Custom Shock Tuning

28 March 2014

Does the rear suspension on your MTB fall short in some way? Well before you drop a ton of cash on a new shock or dismiss the bike completely, make sure you consider your custom tuning options.

People come in all shapes and sizes, and the trails that we ride vary immensely too. Beyond this, you’ll also find many different riding styles and preferences on how we think a bike should feel. Mix it all up and you can see why determining the stock suspension tune for a MTB is a pretty tough thing to do.

Most manufacturers will use the same shock across all frame sizes within a particular model—it simplifies the production process and keeps costs down. Although every brand is different, the standard shock is often tuned to suit a heavier than average rider. A light rider may find the ride a tad harsh but at least a heavy rider won’t destroy the frame by riding with a completely underdamped shock. Occasionally you may come across a brand that changes the suspension linkage according to the frame size in an effort to cater for differing size riders. Other brands may spec a lighter shock tune on their smaller frames.

While it’s nice to see a bike manufacturer going the extra mile by offering different shock tunes according to the frame size, it can still be a relatively futile effort. Take a medium frame for example, one that would suit a 175cm tall rider; a bike of that size could be ridden by a 60kg flyweight or a 90kg powerhouse. That’s a huge weight range to cater for with the one standard shock tune.

Most cross-country and trail bikes come with air sprung suspension. This provides a broad range of spring adjustment that should cater for just about anyone. Decent suspension will also provide external rebound adjustment, as well as some compression or ‘platform’ tuning options. Take time to set the bike up (maybe with some guidance if you’re not particularly suspension savvy) and the standard adjustment range will satisfy most riders, most of the time. However, if you fall towards the outer fringes of the bell curve in regards to weight or riding style, or if you’re very fussy about your suspension setup, you mightn’t get the performance that you desire from the stock suspension tune.

With some shocks the rebound adjustment range won’t be broad enough; heavier riders may suffer with too much rear end bounce, even with the rebound dial wound to the slowest setting. Conversely very light riders could find the suspension returning too slowly and it may ‘pack down’ over repetitive bumps. Hammerhead XC racers may desire a firmer pedalling platform than is offered by the stock tune. Those who like to take on bigger jumps and drops can benefit from a more ‘progressive’ spring rate, preventing harsh bottoming whilst still allowing good suppleness in the early travel. Situations such as these could call for a custom tune to get the desired traits from your suspension.

So what’s involved in a custom tune and who can do it? Beyond the external adjustments (rebound, air pressure, compression and pedalling platform), it’s best to employ the services of someone who specialises in suspension tuning. Rear shocks often require specialist tools, nitrogen charging equipment and more than anything, ripping a rear shock apart requires specialist knowledge. So use a suitably qualified professional if you need a more elaborate custom tune on your rear shock. Here’s a look at what’s involved using the popular Fox Float CTD shock as an example.

Our tune was done by Sola Sport; the official Fox distributor in Australia. They have a fully equipped workshop with full-time staff dedicated to servicing and tuning Fox suspension products. The service or tune should be arranged via your local bike shop and you’ll fill out a ‘custom tune form’ to let them know what you’re trying to achieve. Providing clear information is crucial in getting the desired result with your custom tune. In this example we felt the suspension lacked suppleness in the early travel, even in the softest ‘descent’ mode. We were already running 30% sag but the bike didn’t track the ground as well as we’d like. Additionally, it wasn’t exceptionally plush over mid-sized impacts and we wanted to use more travel more of the time to mitigate this.

With the air released from the main chamber, the air can is removed—the shock eye is fixed in a soft jawed vice and the can unscrews by hand. Inside you may find a volume spacer located on the piston shaft. These white plastic pucks are a common feature on the newer ‘high volume eyelet’ CTD shocks and they make it relatively easy to tune how progressive the air spring is. Mechanically proficient owners should be able to access and swap the volume spacers without specialist help (you’ll find detailed instructions on the Fox website).

Fox makes a number of volume spacers to suit both RP series and the newer CTD model shocks (five sizes are offered for the Float CTD). Adding a bigger spacer reduces the volume of the main air spring and this gives the suspension better bottom out resistance. Alternately, a smaller spacer will make the spring more linear; that means it will be more willing to use all of its travel. Buy a pack of volume spacers and you’ll be able to tune the initial sag and bottom out resistance to suit your preferences. RockShox have a similar system for some of their shocks and many brands offer different size air cans to change the air volume for the same effect.

With Fox shocks, delving deeper than the air canister requires specialist knowledge and proprietary tools—Fox doesn’t want the average punter fiddling with the internals. The damper oil is pressurised with a few hundred psi of nitrogen which is held behind an internal floating piston (often referred to as the IFP). Once the nitrogen has been released, the main shaft and piston is removed.

This is the main piston—the heart of your rear shock. It contains a series of small holes and a stack of thin flexible shims. When the shock moves, oil is forced to pass through the piston and this provides resistance and damping force; without it you’d be riding a very bouncy pogo stick. For the CTD shock Fox offers three tune options for compression and three for rebound. The stock tune on your shock is marked on the air can just near the main wiper seal. The red sticker is for rebound and blue is compression; ‘L’ means a light tune, ‘M’ is medium and ‘F’ is a firm tune. There are also different springs that vary the firmness of the Pro Pedal platform damping. In this case we had the medium compression swapped to a light tune.

Some Fox Float shocks are equipped with a ‘Boost Valve’ which provides yet another means for adjusting the compression damping. The Boost Valve tune is altered by varying the pressure of the nitrogen within the shock. Pressures can range from 150 to 350psi in 25psi increments and this number is also marked on the outside of the shock. The Boost Valve provides ‘position sensitive damping’. The idea is that it provides lighter damping in the early travel for small bumps but progressively offers firmer damping deeper into the travel to handle the bigger hits. Varying the Boost Valve tune on a Float RP or CTD will affect the firmness of the shock in both the open and platform settings.

All the components are cleaned, new seals are fitted and the internal floating piston is set to the correct position within the shock. The damper is then filled with fresh oil and bled so that there’s no air in with the fluid. Fox only offers one damper fluid for their rear shocks but with some other brands you may be able to vary the weight of the oil to provide more or less damping force. Some custom tuners may choose to run non-standard weight oil in Fox shocks, but in most instances enough variation can be obtained by altering the shim stacks and boost valve setting.

The appropriate volume spacer is fitted under the washer and O-ring. We wanted a plusher feel over mid-sized bumps without having to run additional sag, so a smaller volume spacer was fitted to make the travel more linear. Around 5ml of ‘Float Fluid’ (a really thick oil) is placed down near the volume spacer and the air can is pushed back in place and threaded on.

High pressure nitrogen is then pumped into the shock behind the IFP. We weren’t experiencing harsh bottoming when running 30% sag and wanted a softer ride to suit a lighter rider, so the Boost Valve pressure was dropped from 200psi to 175psi. All shocks need periodic maintenance and the Boost Valve itself can lose pressure after a year or two of riding. If you find the suspension performance dropping off over time, send the shock in for a service and get the Boost Valve reset.

So how did our custom tuned Float CTD shock perform—did we get the performance traits that we were chasing? The compression tune was changed from medium to light, the Boost Valve dropped by 25psi and we went down one step in the size of the volume reducing spacer. The damper tunes made a difference but it was quite subtle and hard to discern. In contrast, swapping the volume reducing spacers had a very distinct effect. They serve as a great tuning device and are comparatively ease to change. Make the air chamber smaller (a bigger volume reducer) and the shock will be more resistant to bottoming. This allows you to make the air spring softer to help with small bump compliance and traction. If you rarely use all of your travel (even with sufficient sag), fitting a smaller volume reducing spacer will let you access more travel over mid-to-large bumps with the more linear action making better use of the available travel. Combine all of these elements and you can see that there’s plenty of scope to tweak and tune the feel of your suspension.

At the end of the day I still wasn’t entirely enamoured with the suspension feel on this bike and the same shock performed beautifully on a completely different bike. It goes to show that the suspension design is equally important in determining the ride feel. In this case, the ‘regressive’ leverage rate in the early part of the travel was compromising the suppleness (this also made the bike efficient to pedal, even in ‘descent’ mode). A bike with a more ‘progressive’ leverage rate would make the same shock feel more compliant at the start while still resisting bottom-out on bigger hits. Other elements such as the axle path will also have a bearing, so it’s important to remember that suspension performance is a package deal and not about one element. In any case, taking the time to setup your suspension will allow you to make the most out the bike that you’ve got and it can vastly improve the ride.