Note from the editor: This is a new portion of our website, but has been around Dirt Bike since the ’70s. Mr. Know-it-all has captured both the novice and pro rider with his wit, his pertinent information and his nasty attitude. Every week we will be posting some of the best Mr. Know-it-all letters, along with new ones that come in. If you have a query please send it to email@example.com.
Mr. Know It All,
I have a 2008 Honda CRF450, which I will never, ever sell. It works perfectly for me, and so I plan on just rebuilding it over and over. But I’ve noticed that the fork is getting softer with age. I’ve had it rebuilt to the same specifications by the same shop twice, but each time it comes back a little softer. Do springs wear out? I have an extra set of fork springs that have no markings on them; is there a way to tell what the spring rate could be?
From a long time reader
There are three possibilities here. First, yes, fork springs could wear out. But oddly enough, fork springs technically get stiffer as they get older. Some say this is because they get shorter, but when viewed as a torsion bar, a sacked spring is the same length as a new one. The increase in rate comes from work hardening. They might feel softer initially because preload is lessened by a more dramatic percentage. A good suspension shop should have a spring-rate tester and be able to test your extra springs for a small fee. If you don’t have access or time, you can make a calculation based on some accurate measurements. You will need to measure the diameter of the wire (W), the coil’s diameter from center to center (D) and count the number of coils (C). Using millimeters, plug your measurements into this formula: W4x 80400 / (8 D³ x C). The answer is given in Newton-mm. You can divide by 9.81 to get KG-mm.
The other two possibilities are more likely. Your valving shims could be deformed, making the fork feel soft, in which case you can have a competent fork service measure, so replace them with the same shims. Or, you could be getting fatter.