Mountain Bike Reviews Forum banner
1 - 8 of 8 Posts

·
resident crackpot
Joined
·
483 Posts
Discussion Starter · #1 · (Edited)
I think I remember this whole thing right...chain length that is affected by changing the chainring by four teeth (+ or -), will be able to attain a like chain tension when X number of links are removed or added...is this correct? And on a side bar, changing two teeth on the chainring is like changing one tooth on the cog...is this correct?

I am concerned about an upcoming race this weekend. I usually ride a 2:1 ratio, exclusively. But lack of actual trail time has me slightly worried about my race-readiness to ride in an area that I know holds many more climbs than I am used to. Factor in some wet weather as of late, and I would like to change to a slightly lower gear.

My ride, a Bontrager Race. Gearing; 36/18, sans tensioner. I have been through many trial and error with finding a magic ratio over the years and seem to remember someone having a Bontrager-friendly solution to this. Yes, ENO is the light at the end of the tunnel, but I don't have one now, nor will I by week's end.

Anyone out there willing to help out with this gearing issue...directly related to a Bontrager??
 

·
Registered
Joined
·
3,931 Posts
If 36:18 is magic for you then 38:20 will also be magic, but that's "only" a 5% change in gearing. (2.0 -> 1.9). To keep the magic perfect you should add 2 to each or subtract 2 from each (cog and chainring).

36:22 is somewhat less perfect magic, the chain will sag >1/2".

And on a side bar, changing two teeth on the chainring is like changing one tooth on the cog...is this correct? No.
 

·
resident crackpot
Joined
·
483 Posts
Discussion Starter · #3 ·
Been working on it...Using a new chainring of 32t, same 18t cog, and different chain of similar wear, a 32/18 combo works. I can tell by the sound of the chain links 'growing' that all is not exactly the same, but the chain tension is similar to what it was before...with a bit more tension. And after running it through FixMeUp, I see that it is slightly shorter in gear inches...when compared to a new chain with zero wear (I hate the term chain stretch).

OK, pacman, if the two in front = to one in rear is wrong, then what is it that I am thinking of? I know from experience that changing one tooth on the chainring is not equal to changing one tooth on the cog...
 

·
Registered
Joined
·
168 Posts
loonyOne said:
OK, pacman, if the two in front = to one in rear is wrong, then what is it that I am thinking of? I know from experience that changing one tooth on the chainring is not equal to changing one tooth on the cog...
The reason they're different is because changing teeth changes gear ratios, not gear sums or anything like that. So 36/18 is 2:1, 37/18 is 2.0556:1 and 36/17 is 2.118:1, which gives a 211% greater change in gearing than the 37/18. Now for 42/21 (2:1), 42/20 (2.1:1) gives a 210% greater change in gearing than 43/21 (2.048:1). Not as significant as I would have hoped (doing this all on the fly), but the difference that changing a single tooth at either end and the difference that they make relative to each other vary depending on the number of teeth involved.
 

·
you said.. Member
Joined
·
228 Posts
loonyOne said:
Anyone out there willing to help out with this gearing issue...directly related to a Bontrager??
Not sure about the Bontrager stuff but this thing works well for calculating all there is to know about gear ratios [for those of us who don't calculate gear inches in our heads :D] : http://www.teambigtime.com/software/ssConvert.htm
 

·
Registered
Joined
·
3,931 Posts
loonyOne said:
Been working on it...Using a new chainring of 32t, same 18t cog, and different chain of similar wear, a 32/18 combo works. I can tell by the sound of the chain links 'growing' that all is not exactly the same, but the chain tension is similar to what it was before...with a bit more tension. And after running it through FixMeUp, I see that it is slightly shorter in gear inches...when compared to a new chain with zero wear (I hate the term chain stretch).

OK, pacman, if the two in front = to one in rear is wrong, then what is it that I am thinking of? I know from experience that changing one tooth on the chainring is not equal to changing one tooth on the cog...
What you're thinking of is that if you added two teeth to a cog or two teeth to a chainring you would need to add only a half-link to the chain (one more hole) This is because the chain only "sees" half the cog or half the chainring (this is exact only if the cog and chainring are the same size).

If you want to be more exact the chain wraps more than half-way around the (bigger) chainring and less than half-way around the cog. A change in chainring size affects the chain length a bit more than a change in cog size. If you add two teeth to each (= one chain link) the effects cancel to keep the fit exact.
 

·
resident crackpot
Joined
·
483 Posts
Discussion Starter · #8 ·
Thanks for the 'help' fellas, really. At the time I posted, I was using too much thinking and not enough doing. Six or so years ago when I first embarked on the SS trail, I had been through all of this. Constantly changing cogs, adding an offset link, and so on. It was the viscious, SS cycle with vert dropouts. The only real reason I happened onto the magic Bontrager gear was because I happened to remember reading a post from someone here who had converted a Bontrager with this gearing. Then, I thought I remembered someone else here with a Bontrager (with rear disc brakes...) that used 32t, 36t, and 40t chainrings with the same cog. That's when I farted into the 'rule of four' thing.
 
1 - 8 of 8 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top