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Old 06-16-12, 11:23 PM   #1
krobinson103
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8 speed chain + one 9 speed gear - works but I'm not sure if its safe

My smallest gear on the rear cog on my Altus compatible (Sun race m50 - almost identical) got worn down to nubs by my nasty habit of riding it to the limit for 400km every week. When I went to the shop he didn't have any 24 speed cogs, but he did have a 27 speed xt small cog. Looking at them there are almost identical and it fits into the set just fine, shifting is super smooth and predictable, BUT...

The width of the chain and the width of the sprocket are slightly different leading to a very small but noticable chatter under load. I know why, as there is a tiny bit of extra space when the teeth engage the chain. What I'm interested in hearing about is this. Is this going to cause any detrimental long term effects? The xt gear set is stronger than the altus and I can live with the slight irregularity if its not going to cause any problems down the line.
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Old 06-16-12, 11:45 PM   #2
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Sounds like an excellent fix and should be very safe if the cassette lockring is snug and there's no play. As to the width issue, I ride wide 1/8th inch chain on 3/32 width S/S freewheels and no issues and longevity seems okay, but I'm not putting 400km/wk on a small cog.

However, I'm surprised by the chain chatter. Are you sure it's the chain? The fit might be fine but if the spacing is a tiny bit off, you may need to check and adjust the outer H-limit set screw if you haven't adjusted it already. Typically, the chain width should be fairly tolerant of the cog width, especially if the difference is very small. If you are worried about the chain compatibility, different makers will vary in inner/outer widths for a particular chain for a particular number of rear cogs. Among 8spd chains you probably have 6.7mm to 7.0mm outer width. And 9 spd chains go from 6.4mm - 6.9mm. You might be able to find some middle ground on a chain. I couldn't tell you which brand or model right off hand. But at one time, I used to have a table somewhere written down on brands, models and their widths.
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Old 06-17-12, 02:15 AM   #3
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Played around with the limits and tension on the rear derailer. In the process I discovered a misaligned hub and 2 missing bolts on my brake disk. Good thing I did look. I got the drive train smoother, but that gear still 'feeds back' somewhat. Probably not to any degree that someone who hasn't ridden the bike for months would notice.. but I hate it when things aren't 100% perfect. Still I think its good to go and with some time it may wear in a little more.
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Old 06-17-12, 02:42 AM   #4
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Those smaller 11-12 tooth cogs can tend to be a little noisy for the first few miles... if the chain line and cog clearance is not an issue it will get smoother.
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Old 06-17-12, 04:21 AM   #5
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Since this is the last cog on the stack, a slight spacing variance shouldn't cause much issue since you only shift on and off it from one side (limit screw compensation).
Are you sure it is the width causing the noise; and not the fact that you are using a worn out chain on a brand new cog?
if the chain is actually rubbing agasint the 8th gear while on 9th, you might try adding one of those >1mm spacers (found on some cassetes between 6/7th) to increase the gap. should still shift fine.
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Old 06-17-12, 06:40 AM   #6
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I did the exact same repair on a local kid's bike awhile back. The thickness of the cog was not an issue. The spacing was though, because in my case the one nine-speed cog set just a bit closer to the adjacent eight-speed cogs than the derailleur expected. As I recall, I set the high-limit to leave the chain centered on the new cog. Then I allowed a tad bit of slack in the cable that was taken up on the very first downshift. That slack (minimal) allowed the shifter to pull an eight-speed amount of cable while moving the chain across a nine-speed amount of distance.

Xenologer's point about a mismatch in wear between old chain and newer cog is a good one to consider as well.
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Old 06-20-12, 03:19 PM   #7
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I found the solution quite by accident today. Last week I changed two components - both with non standard replacements. The Derailer hanger which needed some finessing to fit, and the small cog. The problem was caused by the hanger being slightly different and the cog having a tiny spacing varience leading to the chain being a few mm out of optimal on the front derailer. Gear trains are tricky. "right or wrong - there is no almost."
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