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How much further forward in the dropout will 48:18 be compared to 48:15? At the moment the axle is pretty much slammed forward so I need a longer chain too - possible to extend the one I have or not worth bothering?
⅜" or about 10mm. If you have some offcuts from the original chain and you can get the appropriate master link, you can just add a spare inner link and a second master link, that's what I do on my race bike to allow a wider range of gearing. Obviously if the chain is old and cheap, you might as well just throw a new one on there.
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Maybe, depends how long the chain is at the moment. You certainly need to re-check that it's possible to engage the big:big combination.