About Dented Motorcycle Steering Races

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

As part of any motorcyclist’s periodic tech inspection, it makes sense to put a bike on its centerstand, rock it back to hold the front wheel off the ground, and turn the handlebar from side to side. Is the motion perfectly smooth, as it should be, considering that the head bearing races and their rolling elements were very smooth when manufactured? Or do you feel some degree of bump-bump as you turn the bars? If you do, you probably have dented steering races. That notchy feeling can interfere with smooth steering and occurs as the balls alternately drop into dents in the bearing races and rise again to the undamaged surface between them.

How does denting happen? Proposed causes are overtightening of the head bearings (there is a procedure in the service book), a sudden impact, or long periods under combined load and vibration. This denting of bearing races is sometimes called brinelling, after Swede Johan Brinell (1849–1925) who devised the Brinell hardness test for metals and the Brinell hardness scale.

In Brinell hardness testing, a small and extremely hard ball is pressed with a standard force against the metal to be tested. The diameter of the resulting permanent dent is an inverse measure of hardness.

There are two steering-head bearings, upper and lower. Typically, each consists of an upper and lower grooved race with roughly 20 hard steel balls distributed around the grooves between them. In some steering heads, heavy-duty taper rollers are used instead of balls and races.

Heavy-duty tapered bearings are most commonly used on modern motorcycles. (Motorcyclist/)

New bearing races and balls are the only remedy for dented races. Replacement is a job for an experienced and mechanically competent person. The basic mechanics are to place the bike on a stand with the front end supported in the air, then remove the top fork crown, unload the head bearings by unscrewing the threaded steering-head preload adjusting nut near the top of the steering stem, and removing the rest of the fork. The races in the steering head can be removed, by careful use of a drift pin and hammer, and replaced with new. This was a familiar task for older motorcyclists, who well remember sticking the new bearing balls to the upward-facing ball races with grease, then ever-so-carefully reinserting the steering stem into the steering head, installing the top race, and securing all in place with the preload adjusting nut.

Another set of causes can produce the same notchy steering effect and are called “false brinelling.” In this case the shallow pockets in the races that form at each ball contact are caused by corrosion, electrolysis, or fretting, and are the reason why collector cars must either be moved from time to time or be supported as static exhibits by other than their own wheels.

In the 1980s it looked as though a “forkless revolution” was about to do away with the simple telescopic fork. But ultimately it was the very easy and smooth controllability and “feel” of a fork pivoting on rolling bearings that has so far kept the telefork as dominant as it remains. Best, therefore, to preserve that essential control smoothness through properly adjusted and dent-free steering-head bearings.

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[Bender]

To sum up. 

 

If broke fix, if not don't. 

[Guest]

4 minutes ago, Bender said:

To sum up. 

 

If broke fix, if not don't. 

That's part one where two and three?

[Bender]

19 minutes ago, goat said:

That's part one where two and three?

They broke