Quick Tip: Check Internal Clearance When Replacing Wheel Bearings

By Rick Schultz

A short tech tip to consider if you install your own replacement wheel bearings and they don’t spin as well as the original bearings.

The bearing manufacturers understand that the industry uses different tolerances, so they have come up with a solution called Internal Clearance.

From the manufacturer SKF:

Bearing internal clearance (figure below) is defined as the total distance through which one bearing ring can be moved relative to the other in the radial direction (radial internal clearance) or in the axial direction (axial internal clearance).

In almost all applications, the initial clearance in a bearing is greater than its operating clearance. The difference is mainly caused by two effects:

• Bearings are typically mounted with an interference fit on the shaft or in the housing. The expansion of the inner ring or the compression of the outer ring reduces the internal clearance.
• Bearings generate heat in operation. Differential thermal expansion of the bearing and mating components influences the internal clearance.

Sufficient internal clearance in a bearing during operation is important. Preload (clearance below zero) is possible for certain bearing types.

To enable selection of the appropriate initial internal clearance to achieve the desired operational internal clearance, bearings are available in different clearance classes. ISO has established five clearance classes for many bearing types. SKF uses designation suffixes to indicate when the bearing internal clearance differs from Normal.

Most manufacturers try and stick with CN and that is what most bearings are spec’d. So, if you need to replace your bearings, give the manufacturer a call and ask what clearance bearing to use since some wheel manufacturers will oversize the axle and undersize the outer race causing the bearing to be squeezed from both sides causing excess friction so that it won’t turn very easily.

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Coach Rick Schultz is an avid cyclist who trains, races and coaches in Southern California. Rick is an engineer by trade, and in addition to being a coach, he’s a bike fitter and prolific product reviewer. He’s the author of Stretching & Core Strengthening for the Cyclist in the RBR eBookstore. Check his product reviews website, www.biketestreviews.com, and his coaching site, www.bikefitnesscoaching.com. Click to read Rick’s full bio.