It has been a running joke lately that all I think about at work, home, on the trails, or otherwise is volume spacers. This is not really the case, but I have been given to tinkering with my suspension a good amount this winter. Volume spacers are shaped bits of plastic you can put in a suspension air sprung fork or rear shock to alter its progression curve. The progression curve of an air fork or shock is simply the relationship of the air spring pressure in the fork created as the fork is compressed though its travel. It is this changing air pressure that makes the fork feel different as it moves through its travel. The oft-advertised but difficult to achieve ideal is a supple top end, supportive mid-stroke, and progressive bottom out. This means a well-tuned fork for all-mountain riding will have sensitive small bump compliance and a mid-stroke near ideal sag that is neither too floaty or too harsh and will come within a couple millimeters of the bump stop at full compression without actually hitting it.
Tuning guides are often short and a little too vague if they even provide any helpful information at all. In regards to volume spacers, Fox’s tuning guide for their 2018 Fox 36 fork on my Santa Cruz Hightower LT is limited to the following:
If you have set your sag correctly and are using full travel (bottoming out) too easily, then you could install one or more spacers to increase bottom out resistance. If you have set your sag correctly and are not using full travel, then you could remove the spacer to decrease bottom out resistance.
While this is true, it leaves out some important exposition on how the relationship between different amounts of volume spacers and lower air pressure can significantly affect not just bottom out resistance but create a more supple top end and supportive mid stroke as well. As can be seen in the graph, running four volume spacers allows me to run an air pressure significantly below what is recommended for my weight by Fox. The smaller air chamber created by adding volume spacers run at a lower air pressure provides less resistance because less air volume when moving through the first 30% of travel. This is important for small bump sensitivity. At around 30%, the resistance to moving through the travel starts to ramp up, creating a supportive mid-stroke that is important for pushing through turns or navigating rock gardens. In my experience, it is also actually more supportive than with fewer spacers where technical terrain can feel floaty and vague. Despite running air pressure almost 20 psi less than recommended, I can still hit all my favorite jumps and drops without bottoming the fork more than once a ride. By exaggerating the progression curve in this way, I do not have to sacrifice any part of the travel for concern of another (e.g. low pressure for small bump sensitivity but also bottoms easily, or harsh ride to prevent bottom out).
Every fork model is a little bit different, especially when taking into account different amounts of travel, wheel size, and riding style within each model. These are the conclusions I have come to so far for a Fox 36 at 150mm travel for my riding style. Later today I will be replacing the air spring assembly to stretch out the travel to 160mm, primarily to adjust the headtube angle for a slightly slacker ride. We will see what perceptible effect, if any, that has on the progression curve as it now stands.
If you have any questions about your suspension setup and want to talk through how to get more out of it, come on by and let’s have a conversation. I am happy to answer any questions I can and work together through anything I don’t already know.