Here’s a quick guide on how to measure your bikes front fork sag. I hope this is useful for y’all. For a review on the E-Bay, Amazon, AliExpress adjusters I used, see here: Click Here
Step 1: Extend the fork completely and measure from the two set points. This time I literally lifted the front wheel completely off the ground using the “Ye Olde Sidestand Swivel Trick”. On my bike I have fork gaiters fitted so it’s not possible to measure from the dust seal as is pretty much the standard way to do it. On my bike I put some masking tape in two positions as seen in this picture.
This measurement is what we will call L1.
Step 2: Take the bike off the sidestand, and sit in your riding position. Ideally you should be wearing all the usual gear you wear. Jacket, boots, helmet etc. I’m not trying to set up my bike for MotoGP, so this time I was just wearing my work clothes. With all my riding gear on, I probably weigh an extra 10KG. Get an assistant to balance the bike from the rear or like I did, balance against a wall etc. (very lightly, so as to not upset the weight balance) then PUSH DOWN on the front end and let it extend very slowly.
Where it stops, measure the distance between your markers again. Don’t bounce the bike.
This measurement is L2.
Step 3: LIFT UP on the front end and let it drop very slowly. Where it stops, measure again. Don’t bounce. This measurement is L3.
L2 and L3 are different due to stiction or drag in the seals and bushings.
Step 4: Just as with the front, halfway between L2 and L3 is where the sag would be with no drag or stiction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true spring sag: static spring sag = L1 – [l2 + l3) / 2].
Step 5: Now adjust your newly fitted preload adjusters and do all the above measurements again until you get your desired numbers. I was looking for the magic 30mm…
Street bikes run between 25 and 33 percent of their total travel, which equates to 30 to 35mm. Roadrace bikes usually run between 25 and 30mm.
This method of checking sag and taking stiction into account also allows you to check the drag of the linkage and seals. It follows that the greater the difference between the measurements (pushing down and pulling up), the worse the stiction. A good linkage (rear sag) has less than 3mm (0.12″) difference, and a bad one has more than 10mm (0.39″). Good forks have less than 15mm difference.
Your personal sag and front-to-rear sag bias will depend on chassis geometry, track or road conditions, tyre selection and rider weight and riding preference.
Using different sag front and rear will have huge effect on steering characteristics. More sag on the front or less sag on the rear will make the bike turn more slowly. Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag. Racers often use less sag to keep the bike clearance, and since roadraces work greater than we see on the street, they require a stiffer setup. Of course, setting spring sag is only first step of dialing in your suspension, so stay tuned for future articles on spring rates and damping.