Sunday, 29 March 2009

Gearing up for trouble

Even by the intolerably low standards of this blog, this one is going to be obscure...

I want to talk about gears. Bicycle gears. For ordinary people, rather than, say, drug crazed Americans who have recently broken their collar bones. So, what does a reasonable rider want from his or her gears?
  • A bottom gear that is low enough to get up most hills. In practice unless you are really fit that means a gear of 42* or lower.
  • A top gear that is high enough that you can pedal going down moderate hills. 100 is plenty.
  • A fine spacing of gears in between.
  • And in particular a relatively gentle change from the little to the big ring at the front.
It doesn't sound like much, does it? Yet pretty much all standard gear set-ups from the large manufacturers fail on one or more of these criteria. 39/53 or 39/52 at the front gives far too big a change. You want at most a difference of ten cogs, I would suggest, or the change up is too jarring.

To get a bottom gear of 40 with a front ring of 42, you need a big cog on the back of more than 27. You can't buy one. So that means that 42/52 at the front doesn't work either.

By this point we have eliminated all of the standard front gears available. What does work is 38/48 on the front, and 13/25 or 13/27 at the back. This gives a top gear of a shade under 100, a bottom gear around 40, and a relatively gentle change between rings. But that requires custom front rings. Why does it have to be so hard?

* The gear in inches is given by dividing the number of teeth at the front by the number at the back and multiplying by the wheel size (usually 27) in inches. The biggest gear is therefore the largest ring at the front through the smallest at the back: the lowest is the small ring at the front through the big ring at the back.

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4 Comments:

Blogger Charles Butler said...

David,

With a 7-speed hub (13-14-15-16-17-19-21), a 48-38-30 chainset gives you mostly 1-tooth changes at the back and 2-tooth equivalent changes on the front.

Gives you 38-100 inches without losing cadence when shifting rings.

8:07 pm  
Blogger David Murphy said...

Charles

Thank you. I agree that the triple has attractions, but (unless things have got rather better since I last looked) you don't have nearly as many gears as you think. Only the bottom 4 out of 7 work with the lowest ring in a triple, and only the 4 highest with the big one. That means more changes on the front that you need with a double. I'd rather keep front changes to a minimum, and a double seems to me to work better for that.

David.

10:10 pm  
Blogger Charles Butler said...

But the number of gears wasn't the issue. You basically ride it on the middle ring and pick up two changes on each side with the large and small. None of that half-step double shifting.

Oh, and the tighter than usual chainset avoids much dropping of the chain.

10:38 pm  
Blogger David Murphy said...

Hmmm, yes, I can see the point of that. I might well consider a triple on my next frame. There are some Russians who used to make fighter engines who now turn out custom Titanium frames rather reasonably...

8:18 am  

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