Hey CannondaleTony, think this is a good idea, as I and others,keep answering the same question, over and over. Please delete this opener as it serves no purpose in the long run, but can we make this a sticky?
I wrote most of it this AM in another thread, seems to cover most of the common ????????
Bearing migration, what, why, how, etc.
It's like setting the gap on spark plugs, it's not something that "happens" it's something that is, and can be changed till it fits the parameters set by the manufacturer.
Also, it's not an "issue" really, it's part of any needle bearing device, be it a MTB fork, or a large industrial machine. Bearing preload changes are simply a function of fine tuning
It's the inner races job to create the preload. They are available in thickness differences of 1/1000th. Most forks can be built up with perhaps even 4 different thicknesses resulting in a super stiff acting fork, to a super smooth one.
The trick is to get it in the range that feels good, without allowing migration to happen too quickly.
The problem is, certain riders and certain conditions, or combos of the two, make the benchmark that they are built to, not be quite acceptable to that individuals needs. Other factors include too much, too little, or too stiff of a grease, races and bearings getting more polished with use which will allow the bearings to stuff along the race more easily upon a heavy impact, a bit of extra plastic flashing that was on the bearing cages when new, going away, running a lower pressure than normal on a rough ride, oil leaks breaking down the grease viscosity, etc etc etc.
What it is is simple. The inner leg has four races, as does the outer, there's a strip of needle bearings in between them, creating a sandwich, great, now I'm hungry...
When a hit occurs, the lower leg moves into the upper. If it is a particularly rapid or hard hit, it stuffs the lower leg up a touch, much like taking a PB+J in your hands, one on topone on bottom, and pushing your upper hand away from you, the top piece of bread, slides on the jelly, hence, jelly migration
If your migration happens very quickly, like several inches in a few rides, you can have the fork's preload changed, by getting it rebuild, and requesting that the preload be bumped up, or, at least, looked into. It is done, by swapping out say, two of your .021 inner races, for .022's, or a tad more, say .023 if it was really on the light side.
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