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Index Steering Movie
by Damon Rinard

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While the wheel and stem were off my wife's Merlin I noticed she has an indexed headset. Because of the discussion in about 36 degrees being (or not being) the next alignment spot, I checked. It definitely has secondary alignment stops near 36 degrees (by eye). The secondary stops are about 40% as distinct as the straight ahead position. Knowing there are secondary spots of indexing is mostly of academic interest, since steering motions during riding never turn that far.

I made a short (5 seconds) MPG video of the fork snapping into position. In the video I can hear the brake's barrel adjuster rattle as the fork drops into each position. I adjusted play out of the headset about two weeks before this video, not realizing at the time it was pitted. The action is fairly rough all around.

index steering
index_steering.mpg, 136KB
  • Frame: Merlin Road
  • Fork: Kestrel EMS
  • Headset: Dura-Ace HP-7401? (All-steel crown race, not earlier bi-metal)
  • Age: in use since 1992; estimated 30,000 miles

Jobst Brandt describes the cause of this indexing in the article "Indexed Steering" in a newsgroup posting, and also has this comment to make:

"The motion of the balls is related to (pd+bd)/(pd-bd) [ball pitch diameter and ball diameter] because the ball advances at less than the ANGULAR rate that the race does and therefore does not exactly reach the coincidence of race dimples at 36 and 72 degrees (with 20 balls)."

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