Bearing play stems primarily from two sources in combination: 1) a necessary tolerance between the axle’s outer diameter and the bearing’s inner diameter (axle tolerance); and 2) a necessary tolerance between the bearing balls and the groove or raceway (raceway tolerance).
Many myths are circulating in the longboard world about bearing play. A properly functioning bearing requires some play of both kinds. It is a mistake to assume that “the less play the better.” Also, during actual rolling motion, the momentum of the balls tends to minimize axial play compared to what is observable when the bearings are at rest.
A small amount of play between the axle’s outer diameter and the bearing’s inner diameter is normal and necessary. Otherwise it would be impossible to slide bearings onto axles by hand. This is usually the main source of play once wheels are assembled and snugly fastened to a truck
Seismic Tekton bearings are designed to correct for most sources of misalignment. However, all 8mm bearings have play on standard 5/16-inch (7.9375mm) axles. This is simple mechanics, not an issue of bearing design or quality. Even higher-precision axles have minute high and low spots that add to play, depending on how the bearings’ inner races are “clocked” on the axle when the wheel nuts are tightened down.
For greatest wheel and bearing precision, use trucks that have axles around 7.95mm – 7.97mm in diameter. Avoid trucks with axles that claim to be exactly 8.0mm in diameter – bearings are likely to get stuck on them.
Inside an individual bearing, a small amount of play between the inner race and the outer casing is the result of a necessary tolerance between the balls and the raceway. Minimizing the gap between the balls and the groove decreases internal play, but it also increases friction, reduces speed, and shortens bearing life.
Loose bearings (with a lot of internal play) are more durable, but they result in wheels that roll less precisely, which compromises grip and sliding characteristics. Tight bearings (with very little internal play) are slower and prone to damage. Optimal bearing performance is a matter of engineering the appropriate amount of internal play. Not too much and not too little.
Note that packing a bearing with thick grease can create the illusion of a “tight” bearing. However, lubricating with grease that is too thick (or too cheap) can make for a slower bearing. In addition, thick grease heats up and grows thinner when the bearings spin at high speeds for a sustained period of time. Then the inner race and outer casing may exhibit more play than when the bearings are “cold.”
Furthermore, over time bearing lubricant evaporates and spins out, particularly if very light oil is used or if one side of the bearing is unsealed. As this happens, the gap between the balls and the grooves grows larger, since less lubricant is present. In turn, the amount of internal play increases, possibly to the point of compromising wheel performance. Clean and re-lube your bearings as necessary.
It goes without saying that bearings damaged during cleaning (bent seals or cages) may exhibit excess play, as well as noisy, diminished performance. Also, like any other product, bearings naturally wear and degrade over time, particularly if ridden intensively.
Other factors that contribute to bearing play include variations in the ID of the bearing seats. During production, they may shrink more on one side of the wheel hub than the other.
Even a properly assembled and mounted wheel with high-quality bearings has at least 0.15-degrees of play on a standard axle. But for most bearings under load, 1 degree of torsional deviation between the inner race and outer casing will cause damage resulting in loss of speed and increased noise. Beyond poor axle tolerances, a number of other factors can easily contribute to this type of damage: loose axle nuts; absence of spacer support between the two bearings in a given wheel; and crooked or flexible bearing pockets in the wheel hubs.
The effect of loose axle nuts should not be underestimated. Wheels that are not snugly fastened are free to lurch onto the threaded portions of the axles, which have a substantially smaller diameter. In turn, this creates a significantly higher potential for excess play leading to permanent bearing damage and slow, noisy bearing performance.