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Old 02-27-08, 03:56 PM   #1
jccaclimber
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Making a better rear derailleur

Cross-posted in Recumbents
Background:
I'm at Rose-Hulman making a faired recmbent for an American Society of Mechanical Engineers Human Powered Vehicle Competition. It's our third year doing this and we'd like to shrink down our deraileur this year.

I wanted a Dahon Neos rear derraileur but after two months of promises of these at wholesale, and promises of these at retail from places that just never have them in stock I'm thinking of just making my own. Ideally it will work on 9sp spacing although I'm only going to run 5 gears. Unfortunately I'm travelling on break and don't have any bike parts/callipers/whatever with me. So:
1:What is the center to center spacing on shimano 9sp cassettes. I know this is posted somewhere but a search didn't turn it up.
2: What is the cable movement per gear for Shimano 9sp shifters?
I thought it was a 1:1 ratio but that seems incorrect, perhaps 1.5:1? This should also be measurable, especially if you have a bike with any exposed cabling.

I would greatly prefer if these measurements were in Metric since I believe that is what they were designed with.

If anybody has any special wisdom for me in this area that would be appreciated.

Thank you in advance.


If all else fails: Does anybody have a Dahon Neos IN STOCK that you would be willing to sell cheaply in exchange for a sponsor sticker on our bike? Also, I remember something about these using SRAM grip shifters. What is the cable pull ratio for those?

Last edited by jccaclimber; 02-27-08 at 04:26 PM.
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Old 02-27-08, 04:26 PM   #2
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Since it came up over in recumbents I should add that this is a front wheel drive/front wheel steer vehicle. I have used standard derailleurs the past two years and will not have space for one with this year's design.
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Old 02-27-08, 04:48 PM   #3
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This is what I have for cog/spacer thickness in MM.
7- 1.85/3.2
8- 1.80/3.0
9- 1.78/2.56
10- 1.60/2.35
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Old 02-27-08, 05:26 PM   #4
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> 1.78*9+2.56*8;
36.50mm
So that leaves a cassette width of 36.5 MM? I guess that is a
Now, does anybody know the cable movement?
That leaves a total movement of34.72mm
> 1.78*8+2.56*8;
34.72mm
I managed to find some evidence that the pull ratio is ~1.67:1 which would result in a cable movement of 58mm. Can anyone confirm that?
> 34.72*1.67;
57.9824

Thank you again.

Edit: If I did that wrong and it's 34.27/1.67 then that's ~20.52mm.

.

Last edited by jccaclimber; 02-27-08 at 05:37 PM.
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Old 02-27-08, 05:38 PM   #5
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1. CtoC spacing of the 9sp is 4.34mm

2. Cable pull is 2.5mm per gear, and the RD mech. advantage is ~1.7:1

Values from this very useful site:
http://www.ctc.org.uk/DesktopDefault.aspx?TabID=3946
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Old 02-27-08, 05:39 PM   #6
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Shimano
9-speed 4.34 mm 1.78 mm 2.56 mm

I thought Shimano was 2:1. At least I've seen "compatibile with SRAM 2:1" when refering to Shimano rear shifters and derailers. But it could be that the 2:1 is nominal.

I visited Rose-Hulman for a week when I was in high school (1978?). It was a thing called Camp Retupmoc. Do they still run that?

Last edited by Gonzo Bob; 02-27-08 at 05:47 PM.
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Old 02-27-08, 07:58 PM   #7
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They still run Catapault for the high school students between junior and senior year if that's what you mean. They also run Fast Track for incoming freshman. Fast Track is Calc 1/2/3 in 5 weeks and is apparently the worst, but most useful experience you can go through with math unless you're a math major.
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