Understanding mountain bike suspension: Sag

What is sag?

Sag (as a measurement) is the amount that the suspension collapses into its travel in a static state. The measurement is usually given as a % of total available travel.

It’s the most generic of suspension measurements – which makes it both the best and worst starting place for setting suspension up.

Sag makes a lot of sense for a situation like a demo or rental fleet where setup is happening in just a couple of minutes and it’s not the riders bike. The goal here is to get the bike close to a good riding setup but there’s very little concern given to what is ideal for an individual rider.

If you leave all of the settings in neutral positions and get sag close to 20-30%, the bike will almost certainly ride okay.

However, if a rider is focused on setting a bike up to truly work well, this is a mediocre starting point.

The main reason for this is that sag is a trailing indicator, not a leading one. If you nail sag, but you don’t have the right pretext, it’s all a moot point anyway… and it can leave you working backwards chasing problems elsewhere in the system…

Before we get into the meat of the discussion, it’s also worth noting that sag is a highly variable point of measure. Just how do you measure it? Standing? Seated? Neutral position? How much pressure on the bars? How do you dismount? Do you rock the bike back and forth? Is there stiction in the whole system? What about brakes – grab them or no? Obviously this highlights the issue with having blanket industry-wide recommendations for what % sag should even be – let alone using it for the entire basis of suspension setup.

How to use sag

First: check out the setup overview here on the DOJO for some context. It helps outline the whole setup process.

Accept for a moment that there is an order of operations to setting suspension up. Setting sag first makes it harder to get everything else right.

If you have everything else correct, sag will naturally land in the exact spot that you want it.

Start with spring rate. Only after spring rate is sorted out should you even check sag. Make adjustments to the air spring volume to adjust sag, not air pressure. Air pressure should only be adjusted to affect the desired spring rate – NOT to chase a sag %. This same advice goes for coil springs. Don’t add a ton of pre-load to chase a sag number. Stick to just a couple turns of pre-load on the spring and then adjust the physical spring rate up or down as needed.

If there is a checklist of mandatory items to include in your setup – sag isn’t even on the list. Think of it as a tool to help read the list.

Once you’ve settled on a spring rate and adjusted air volume to get into the estimated sag window, then move on to selecting all of your compression and rebound settings.

After this is done and you begin to test-ride the bike, pay attention to sag only as it relates to the other changes that you begin to make.

Used properly, sag is a useful tool and a complex understanding of how sag changes, given other inputs will allow a rider to make the best use of the tools at their disposal.

I’ll leave you with one parting thought:

It’s almost impossible to have everything working well with the suspension on a mountain bike and have sag be way outside the generally accepted window of what works. It is however, very possible to have exactly the “correct” amount of sag and have an overall system that is working terribly. That should be a clear example of where sag falls in the order of importance for configuring baseline settings.

As always, if you have questions, leave them below!

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