This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Sun, 23 Mar 2003 12:21:02 -0800 (PST)
Bicycle wheel bearings, as most, require a slight preload so that more
than one ball under the cone (inner race) will support its load. With
proper preload, slight drag should be perceptible. Preload drag is
small compared to drag caused by wheel loads, neither of which are
significant regardless of adjustment. In contrast bearing life is
affected by proper adjustment. Adjusting ball bearings to spin freely
unloaded does not reduce operating friction because a bearing with
proper preload has lower drag when loaded than one with clearance.
For high quality bearings, preload should be just enough to cause
light drag when rotating the axle between thumb and forefinger. Low
grade bearings will feel slightly lumpy with proper preload.
Wheels with quick release (QR) axles present an additional problem in
that closing the QR alters bearing clearance. Closing the lever
requires increasing manual force with a slight over-center feel near
the end of the stroke. This lever force arises from compressing the
hollow axle and stretching the skewer. The ratio of elastic length
change between axle and skewer is that of their cross sectional area
and active lengths.
Although small, axle compression on QR hubs is large enough to alter
bearing clearance and should be considered when adjusting bearings.
Bearings should be adjusted just loose enough so that closing the QR
leaves the bearing with a slight preload. Excessive preload from QR
closure is the cause of most wheel bearing failures not caused by
water intrusion. Clearance, in contrast can be felt as disconcerting
rattle when encountering road roughness.
To test for proper adjustment, install the wheel and wiggle the rim
side-to-side to determine that there is no clearance (rattle), then
let the wheel rotate freely to a stop. If the wheel halts with a
short (indexed) oscillation, bearing preload is too high.
Although adjusting QR force is a safety consideration, it is also one
of bearing life. It should be kept at a constant level once the
desired closure force has been determined. Rear vertical dropouts
require a lower and more predictable closure force than was formerly
required with axles that could move forward from chain tension.
Because vertical dropouts do not rely on friction to resist chain
load, many hubs now have smooth faced jam nuts that do not damage
dropout faces as older knurl faced ones did.