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Geary Geary is offline
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Join Date: Feb 2003
Location: Kailua, Bend, & Tamarack
Posts: 1,573
What you say about rotational mass is basically correct, and I oversimplified my last statement, so let me try to explain another way. Let’s compare two 2nd gear ratios: a low 2.19 (large diameter idler) and a 1.500 (this is a typical range of 2nd gears used in 915 racing). The weight of all parts on the pinon shaft (except for the idler gear that you are shifting into) can be negated, because this shaft is already spinning at the targeted rotational speed. The 2.19 is about 100 grams heavier than the 1.50, and about 3/4” larger in diameter. Weight removal from the 2.19 therefore, is more important (than from the 1.50) because of where that weight is located in relation to the center point.

Secondly, because shifting into a low ratio idler on the pinion shaft is less influenced by the rotating mass of the input shaft (than with a taller ratio), weight removal from the physically larger 2.19 idler is relatively more important than from the 1.50.

Racers have always found that removing weight from the idler gear translates to a more quickly energized synchronizer -- not only because of the above, but also in part because it take a nano second for all the other rotating pieces to “catch up”, due to backlash clearances. The importance of this initial energization really becomes more pronounced with the later 6-speed transmissions, where the gears are huge in comparison to those of the 915, and where almost an additional half-pound can be removed from a 2.25 2nd gear idler with careful profiling. We get a fair amount of positive feedback in this area.

Beyond this layman’s explanation, we really need input from a guy like Hayden, who not only is a 915 transmission pro, but has an engineering background. (a lot smarter than me, because I am/have neither)
Old 10-14-2005, 10:12 AM
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