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YST needle headset race damage

YST 8311N headset lower race damage

  
From https://softsolder.com/2015/05/13/tour-easy-new-front-fork as of 2021/06/04

14 years of use; other usage details not reported.

This headset was used on a recumbent with the head tube sloped more than a conventional upright. In turn, typical loads are on the front of the lower race. Compared to a typical upright bicycle, the fork slope means more leverage, but the long wheelbase means less total weight on the fork.

The slope may also mean that the headset is more facing the wind, so debris may be more likely forced in to the bearing.

More information on the headset at http://www.yst-corp.com.tw/products-view-hp-8311n.php as of 2021/06/04.

  
From https://softsolder.com/2015/05/13/tour-easy-new-front-fork as of 2021/06/04

Most needle-bearing headsets, including this one, use conical races with cylindrical rollers rather than tapered rollers. When the headset turns, the difference in rolling diameter means some part of the roller has to slip or "skid". Compare this to tapered-roller bearings, in which the roller is conical so the bearing diameter is matched to the rotation diameter, which avoids skidding.

Note this headset's damage is concentrated in the small-diameter region. It is likely fork forces mean the highest load is on the large part of the diameter. Thus, when the headset turns, the large diameter has high traction and does not skid. In turn, the lightly-loaded small diameter skids, leading to the observed damage.

This failure is interesting in part because people sometimes assume the main cause of headset damage is wheel impacts leading to bearing overlaod. Roller bearings have more bearing contact area so should be resistant to overload damage.

However, tire pressure tends to limit the peak bearing force. At the same time, wheel loads can cause the steer tube to flex, leading to bearing misalignment. Also, the lower headset race is often bathed in hard road debris thrown off the front tire. Even tiny hard particles can dramatically cut a bearing's service life.

A bearing with a higher load rating should be more resistent to damage from hard debris. However, a needle bearing is more sensitive to misalignment than typical ball bearings, and the steer tube flexes under load, so perfect alignment at installation does not lead to perfect alignment in use.

Further, this failure suggests rollers are also more sensitive to asymmetric laoding. If the bearing rating is high enough, then skidding should not be a problem.

However, headsets often support load without rotating -- only small steering corrections. That is, a back-and-forth or oscillating motion. In this use, the lubricant can get pushed out from the ball/race or roller/race contact. In that case, skidding is unlubricated and wear is much faster.

Eventually, the fork/headset turns far enough that the lubricant is pulled back in — for a while, but with more "straight line" riding, lubricant can get pushed out again, leading to more wear.

Unlubricated skidding is likely one part of what led to the failure shown here.

In some uses, the needle roller "skidding" drag may be an advantage. Some riders report that bicycles which are prone to "shimmy" can have the shimmy reduced or eliminated using a high-friction headset. A needle roller headset is one way to achieve that. Although over-tightening a ball headset can also reduce shimmy, it may be hard to keep the headset over-tight, and so shimmy may re-appear with riding.

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