What is sag?
Setting sag means setting the bike’s static ride height. There are two numbers typically discussed:
- Free sag: bike half full of fuel, no rider. Generally, the sag in the rear is about 25% of total wheel travel
- Race or Rider sag: bike half full of fuel, rider geared up and ready to race. This number is about 33% of total wheel travel. This is harder to set because you need to be very careful to ensure you are positioned over the bike how you actually ride. For example, if you are a rider who tends to sit or stand towards the rear of the bike, but you set sag sitting or standing forward over the tank, you’re going to end up with sag set incorrectly
Which measurement should I care most about?
Race/rider sag is the important measurement. Your bike doesn’t go ride itself without you on it. The reason people check free sag is it can indicate if your spring rate is correct or not. If your race sag is correct, but the free sag % is too small, you likely have too much preload on a spring that is too soft. Likewise, if the free sag % is too large, you may have a very stiff spring without much preload.
In general, Tuned Concepts focuses on race/rider sag and how much preload it takes to get there. 5-15mm of preload is generally acceptable on fork or shock springs. See the “preload vs. spring rate” article for more information.
Why is sag important?
The chassis of the bike is designed with specific geometry that defines its fundamental handling characteristics – its stability, how it feels in turns, etc. These characteristics all change depending on where the suspension sits in its travel. If you have forks that sit low relative to the shock, for example, you have now reduced the effective rake and trail measurements. You can have excellent suspension under your bike, but if you don’t have the sag set properly, you might never get it to go around a corner or feel stable in chop or whoops. This can get expensive quickly if you try to change valving, buy inserts, or do other major work instead of taking the time to properly set sag.
My owner’s manual gives me a race sag number. Should I use this?
You should use this as a starting point, it will get you close. If you are just trail riding or not trying to fully optimize your setup, it will be good enough.
If the race sag number is just a starting point, how do I dial it in?
There are many opinions on this, but what we have found is that this number is not an end-all. Setting the geometry takes time, and is dependent on the rider’s style and handling preferences. The goal is to make the bike corner properly and predictably for you in the conditions you ride in, and then measure the sag so you have a baseline to set back to.
The primary adjustable variables on most bikes are below. Yes, aftermarket options can add more variables, but all bikes have these in common:
- Fork and shock spring rate
- Fork and shock preload
- Fork height in the triple clamps
Assuming you have close to the correct springs on your bike with a reasonable amount of preload, set your race sag to ~33% of wheel travel at the rear and run the fork tubes 5mm (typically the second groove) above the upper triple clamps as a start point.
Find yourself a large, flat, generally uniform area. This can be dirt, grass, or even pavement, but it must be a firm surface with consistent traction. Gravel, mud, very rough areas, or hilly areas are not great for this. Higher tire pressure can also make the effects of sag easier to feel.
Put your gear on. Your gear is heavier than you think, and it makes a difference. The same is true for the amount of fuel in your tank. Especially if you are setting up a bike with a large tank, try to do this test with it roughly half full.
Now, go ride in circles. Do this at low engine speed so you can easily maintain steady state, around 15 mph. Too slow and your body weight and bar inputs can make too much impact on the bike. Too fast isn’t a horrible thing, but it requires more space. You want to be cornering comfortably, not aggressively ripping around in circles – being relaxed allows you to feel what the bike is doing.
As you maintain your steady state circle, make sure you are maintaining good form. This can be done sitting or standing but is often easier to feel when sitting.
- Does the bike feel like it is wanting to “fall in” to the center of the circle, such that you have to put pressure on the bars to keep it at a constant radius? If so, the rear is too high relative to the front. Reduce shock spring preload, increase fork spring preload, or slide the forks down through the triple clamps.
- Does the bike feel like it is wanting to “stand up” and ride at a bigger circle, such that you have to put pressure on the bars to keep the bike leaned over on its constant radius? If so, the rear is too low relative to the front. Increase shock spring preload, reduce fork spring preload, or slide the forks upwards through the triple clamps.
Doing this will establish “neutral” sag for your bike. Continue to think about these things as you ride or race, as your body position may be different when you aren’t thinking about it. If the bike “falls into” the corner, increase shock sag. If it wants to push to the outside, decrease shock sag.
Don’t forget to record your settings when you get to where you are happy and comfortable! This will allow you to quickly set the bike back to baseline if you service the suspension or change settings and realize your prior setting was better for you.
Once I have my sag set, to I ever need to change it?
Once you’ve got the sag set, no, you don’t need to change it. However, if you change the bike (larger fuel tanks, added luggage, navigation tower, etc.) this can change the balance of the bike enough to impact sag.
Terrain may also prompt you to adjust your settings. Faster, straighter riding or racing in the desert may drive you to add sag to make the bike more stable, where tight woods events may prompt you to reduce sag slightly to make the bike more responsive.
As your riding style changes, you may also find that your new body position changes how the bike handles, and you may need to adjust sag to compensate.
Why don’t people talk about fork sag?
Forks inherently have a lot of stiction. This can result in tens of millimeters of variation in measurement results as the forks come to a stop in different places, and that’s enough to make a big difference in measurements and feel. It’s an important measurement, but if the forks are “close enough”, it’s generally more effective to use the shock preload to dial in the bike’s balance because it’s so much more consistent.