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- From: v206gb6c@ubvmsb.cc.buffalo.edu (My stuffed shark is eating my toenail clippings)
- Newsgroups: rec.bicycles.tech
- Subject: Re: Mathematics of gears and why 24-36-46 is bad
- Message-ID: <C1HJ1o.L1u@acsu.buffalo.edu>
- Date: 26 Jan 93 23:41:00 GMT
- References: <93019.193822ASLXG@ASUACAD.BITNET> <C15M40.Loz@watserv2.uwaterloo.ca> <C15uwz.39t@watserv2.uwaterloo.ca> <1k1v10INNelh@darkstar.UCSC.EDU>
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- Organization: University at Buffalo
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- In article <1k1v10INNelh@darkstar.UCSC.EDU>, haverl@cats.ucsc.edu (Carl A Haverl) writes...
- >
- >When I replaced the cranks on my mountain bike, the gearing changed
- >from 26-36-46 to 24-36-46. With the new chainwheels and chain, shifting
- >from the smallest chainring was very difficult. I generally had to
- >overshift into (if not onto) the largest chainwheel and then back the
- >
- >I investigated. When shifting off the smallest ring, the chain never
- >lined up with the teeth of the middle ring. Here is the problem.
- >The greatest common divisor, gcd(24,36), is 12. This means that
- >there are 24/12 = 2 qualitatively different ways to shift the chain
- >off the smallest ring, depending on which set of 12 teeth the chain
- >leaves from.
- >
- >The solution? Going back to 26-36-46 solved my problems. Since
- >gcd(26,36)=2, I have 26/2=13 different shift positions off the small
- >ring so one of them is bound to line up well. Also, gcd(36,46)=2
- >so there are 36/2=18 different shift positions off the middle ring to
- >the larger ring.
-
- I can't support this logic. I've been riding 24-36-46 for more that 3
- years on multiple bikes and have never had this problem. Are you sure your
- chain is not streched out? I just tried 20 24->36 shifts on the 2 bikes I
- have with this combo (one XT-RFP and one top-mount DX) and neither failed
- to shift onto one of the first 3 teeth to come by.
-
- PauL M SchwartZ (-Z-) | Follow men's eyes as they look to the skies
- v206gb6c@ubvms.BitNet | the shifting shafts of shining
- pms@geog.buffalo.edu | weave the fabric of their dreams
- pms@acsu.buffalo.edu | - RUSH -
-