Something I naively thought everyone knew how to do, but also the absolute first, basic step in suspension setup is setting sag. And consequently confirming if you do or don’t have the correct spring rates for your weight.
The old saying goes ‘spring for your weight, valve for your speed’. Springs are there to simply hold you and your bike in the correct position for the chassis angles and dimensions to work as the manufacturer intended…and to make the bike steer as you intend it to!
So step one is to get a friend/helper to hold the tape measure and it really is less confusing to write the numbers down.
Step two, get the back wheel off the ground with a work stand. I like to hook the tape somewhere in the chain adjuster. Making a texta mark at a round number also keeps it less confusing.
Record this measurement. In this case it’s 510mm.
Step three, take the bike off the stand and jump on. Sitting or standing in the natural position you ride in. This is where many customers sit too far back which can result in too much preload. Consistency with measuring is the key.
We now have 410mm. This is rider/race sag.
So what do these numbers mean and what numbers are we looking for? Measurement 1 (510) minus measurement 2 (410) is your rider sag.
For a linkage bike, rider sag is ideally in the vicinity of 95-105mm. For PDS (no linkage) we’re looking for 110-115mm.
Using our example, if the difference is more (say 125), you need more preload on the spring (righty tighty). If the difference is less (say 85), you need less preload on the spring (lefty loosey).
Now to adjust your preload either loosen the second locking ring (with a soft punch) or loosen the clamping screw. Avoid the urge to hit the adjuster ring! Take the weight off the back wheel again with the work stand and rotate the whole spring or lever it around. Sometimes doing both is easier. And two pairs of hands also helps. There’s no need to overtighten the lock ring/screw…it’s not going anywhere.
Repeat step three until you get the rider sag in the ballpark. Don’t stress over 2-3 mm.
The next step is more of a double check to determine if you have the correct spring for your bike/rider weight.
With the wheels of the bike back on the ground, holding the bike vertical, measure again. This is static sag (the bike under its own weight). Using our original example we have about 477mm. Take this away from our very first measurement and we have 33mm. For a linkage we’re looking for about 32-38 mm of static sag and PDS about 40-45.
If after setting rider sag you have a static sag number ourside of the above figures, it simply confirms the spring rates suitability. If you had a larger difference, you would ideally go to a softer spring. Conversely if the difference was less (say 20mm) you would require a heavier spring…and let’s face it, most of us do !
So a few important side notes are that I did the measuring and generally do, without gear on (a couple of extra mm with gear on is OK and I’m expecting that).
Obviously if you carry a lot or excessive amount of extra gear, that would need to be accounted for. Or it may be a good time to cull the amount of gear you carry.
Preloading a spring that is too soft for your weight will completely destroy the handling characteristics of your bike and make the suspension action harsh. And equally not enough preload on a heavy spring is not ideal either. Correct spring rates are the basis for proper suspension set up.
As a final note, measuring fork sag on an offroad bike is incredibly difficult, subjective and generally accepted among suspension tuners too be in the ‘too hard’ basket. Fork spring rates generally go hand in hand with a shock spring change and from experience taking into account the rider and bike.
Advice from Endurowe Tech Suspension and Chassis is free! I love bikes and talking about bikes….feel free to give me a call or better yet, shout me a beer at an event.