I have an old MTB frame that has the drive side rear dropout partially filled in as if it was part of the original forging of the dropout. And, I have seen this on a few other frames -- even different manufacturers.

Clearly it was done to limit how far back the axle can go in the dropout but the question(s) is(are):

why does the frame maker want to control this detail?

why is it only on the drive side?.

=== edit ===

To clarify...

This is not the bike in question but it is an example of what I mean. My apologies to the site where I found this -- it was not copyrighted.

enter image description here

  • Are they vertical, semi-horizontal, or rear-facing dropouts? – jimchristie Oct 5 '12 at 21:01
  • It looks like it's there to get the best position relative to the derailer, like vertical dropouts, AND allow the "conversion" to horizontal dropouts if wanted. But I'm not sure. – heltonbiker Oct 5 '12 at 22:41
  • I can only imagine that this was to avoid having to install dropout adjustment screws on the semi-horizontal dropout. It is only on the drive side because it accurately positions the cog set above the derailleur guide jockey. (Though, in the photo they're definitely intending to drill out the aft section of that dropout.) – WTHarper Oct 6 '12 at 1:31
  • I'd guess its to shorten the wheelbase slightly too, making for nippier handling. – Criggie Aug 25 '19 at 0:39

What you more often see is that the derailer hanger is a separate piece that bolts into what is the filled-in space in your picture.

But this is for frames that are designed for the axle to fit only that far back. Generally these are less than top quality frames where having both dropouts match exactly would make it obvious that the frame wasn't quite true (since the wheel would not center with the axle all the way back).

So with one side filled in you are forced to tighten that side, then slide the other side forward or back until the wheel is centered. They make it look like a "feature" that facilitates alignment.

  • That sounds like good reasoning, but are you sure about that? It seems that with automation and materials costs it would make sense for a manufacturer to figure out their alignment problem. The frame in question is not great but not junk either. – Arbalest Oct 6 '12 at 17:24

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