Wednesday, February 15, 2012

How many speeds is that?


When non-cyclists see our bikes they either take no interest at all, or typically ask, “How many speeds is that?”  A question like this of course dates back to the days when bikes like ours were all called “10-speed” bikes, because they all had two gears (aka “chain rings”) in the front and a cluster, or cassette, of five gears in the back—two times five equals 10 possible gear combinations.  As technology (and competition in the bicycle market) progressed, manufacturers started adding a 6th gear into the cassette in back, or a 3rd chain ring in the front, thus making what would be known as a 12-speed or a 15-speed bike (which presumably would be better than a 10-speed bike and therefore would justify a higher price tag.) 

But as this trend continued, the whole idea of referring to a bike by the total number of possible gear combinations sort of fell apart on a couple of counts.  For one thing, even if it made sense to determine the total possible number of "speeds", the thing that would count would be the total number of distinct gear ratios, that is, the ratio of the number teeth on the front gear to the number of teeth on the back gear.  It is these distinct gear ratios that would determine the number speeds that a bike would have.  For example, if you put the chain on a 39-tooth chain ring in the front, and a 15-tooth gear in the back, that’s a gear ratio of 39:15 or 2.6.  Likewise, if you put the chain on a 52-tooth chain ring in the front, and a 20-tooth gear in back, that would be a gear ratio of 52:20, or, again, 2.6.  So if you have a 39-52 chain ring set in front, and your cassette in the back includes both a 15-tooth and a 20-tooth gear, you can put you chain in either of those two different gear combinations, and each of them would each give you the exact same gear ratio, or same “speed”.  This means that even though a bike might have 18 or 20 or 27 different possible gear combinations, there will actually be far fewer distinctly different gear ratios, or speeds. 

And for another thing, there are some gear combinations that a rider will want to make sure to never use.  A bicycle chain is designed to bend in one axis only—that is, it’s not like a rope, which can flop in any direction.  Each link of a bike chain has those side plates that kind of restrict its side-to-side floppiness.  So it’s really non-preferred to put your chain on the outer-most chain ring in the front, and on the inner-most gear in the back, or vice-versa.  A combination like this forces the chain to take more of a side-to-side bend that it would otherwise want to, and that stresses the joints on those side plates causing them to loosen up a little.  Too much of this makes the chain start to get kind of sloppy, and the next thing you know shifting gears starts to become a bit of an adventure.

So how many speeds is my bike?  I have no idea.  But it seems to be enough to let me keep up with Brenda, and that’s what counts!

1 comment:

  1. I have a 4:1 low range in my Jeep's transfer case and a 4.46:1 first gear in its transmission. That, coupled with my 4.10:1 gears in my axles, gives me a final crawl ratio of 73:1. I have 7 distinct gears (including reverse) in the transmission and two speeds in my transfer case. I guess my Jeep is a 14 speed. :)

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