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I just recently replaced a chain that got incredibly beat up in the winter (it was rusted, and practically stiff when I removed it.)

I removed links from the new chain to match the length of the previous chain. The previous chain was the manufacturer installed stock chain.

It is a 3*8 - 24 speed configuration. I usually leave the front derailleur on 3, and make all adjustments on the rear.

If my bike is on the 4th cog or larger (so 4,3,2,1), the chain will "slip down" to the smaller cog if I pedal backwards.

So for example, if I am in 3x4, and I pedal backwards, the chain slips to 3x5. It corrects itself as soon as I start pedaling forward. If I try to pedal backwards any farther though, the chain just wraps around on itself and falls off.

The bigger the rear cog (for instance 3x1 or 3x2) - the slower I need to pedal for it to slip.

All comments & answers are much appreciated :)

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4 Answers

To dissent slightly... It's common practice for mechanics and bike-shop folks to recommend this... It does result in selling more parts...

However, if your chain is all rusted up as a result of sitting out and not as a result of running it in that condition... The cogs and chainrings might be fine.

The skipping while backpedaling might well be due to similarly gunked-up derailleurs. Before springing for a bunch of new parts, I would take everything off. Both DRs, chain, etc. Clean and lubricate everything. I spray the DRs down with WD-40 and let 'em soak for a bit, then attack 'em with a toothbrush and the wire wheel on my Dremel to get off any caked-on lube and road grit. Then just wash 'em in hot soapy water and re-lubricate. Make sure the idlers are free-spinning and unworn on the rear one. I restore old bikes for re-sale, and I've done dozens this way with only a chain replacement and they shift perfectly before I let them go.

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Seconding JDV...I replace chain, cogs and sprockets as a set. These parts mesh together by design. Using parts with different wear patterns accelerates wear on the new parts.

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I consider it standard practice to replace the rear cogs at the same time as buying a new chain. If you don't the rear cog's teeth start to look like shark fins after a while and your chain will "climb". Esp. when pedaling backwards.

Edit: worn cogs, even if they don't cause immediate problems, will stretch your chain causing it to wear out quickly.

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I'll ask my LBS how much longer I can expect out of my rear cogs, I'll also check the price on a new 8 speed cassette while I'm there. –  Robbie Aug 26 '10 at 21:14
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There are a few problems that could cause your chain to de-rail when pedaling backwards:

dirty/stuck Jockey wheels

dirty/stuck link

stretch on the derailleur cable (normal for new bikes or new cables) so either the front or rear derailleur is now a touch out of adjustment and causing it to derail(the LBS you purchased it from should adjust it for free).

Worn chainring or rear cogs

Lastly, if you take care of all those problems, some chains are just a bit temper mental. Try removing one link to increase your tension and see if that helps.

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