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Is a chain tensioner (tug nut or spring) needed for horizontal dropouts ?

I assume that in case of horizontal dropouts, chain tensioner is not needed.

Nevertheless, does it provide any added value ?

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Chain tension adjustment in frames with horizontal dropouts can be perfectly well achieved without a tensioner by means of sliding the axle forward or backward. This can be done either without or with adjustment screws (such as those found on most motorcycles).

If the adjustment screws are used and the opening for the axle faces backward (as is the norm in track bikes), it can be cumbersome or even impossible to remove the rear wheel without manipulating adjustment screws shall you have a flat tire. It is not a big concern for track riding but it is rather a dealbreaker for everyday use. A chain tensioner can save the day here. Note, however, that this problem is irrelevant if the dropouts face forward.

A more important point, however, is compensating for chain wear. Regardless of the maintenance level, every chain will elongate over time due to the wear of rollers and pins. As a result, the chain will slack more and can start to skip the cogs under load, which, apart from being extremely unnerving, is also a safety issue. To compensate for chain wear, the axle position should be adjusted every so often. In practice, however, most people will likely not know about it or simply not bother. Moreover, moving the axle longitudinally alters the bike geometry (if only by a microscopic amount). A chain tensioner, just as the jockey wheel system in a normal rear derailleur, prevents both problems.

That being said, you can do without a tensioner if you know how to properly maintain and service your bike and do it regularly. After all, most chain-driven motorcycles do just fine without it.

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