i beleive the first number is the the angle at the top of the helix in respect to the base. The last number is the finish angle for the bottom of the helix. In between it transfers through the two angles.
You are correct. The first number is for the initial part of your run and the last number is for the top rpm's. Some are progressive and others are stepped. The stepped multi angle helixs have cut angles and will shift out slower (keep the shifting in each angle longer). Each set up has it pros and cons. The smaller the number the higher the rpm's too. lol
Just to add my .02. Anyone trail riding, as opposed to racing, who is thinking of changing their helix should stick with a progressive cut. As Deanm mentioned some helixes are stepped and the result is diminished smoothness during upshift as you can imagine. Just to reiterate as well - steeper angles (larger numbers) = faster upshift at the expense of slower backshift while shallow angles = slower upshift and faster backshifts. If anyone has purchased a clutch kit you'll typically find a steep initial angle to improve low to mid range acceleration that progresses to a shallower angle as speed increases to allow for full shift out.
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