31.8 mm Front Derailleur Style Chain Keeper

This keeps your chain from falling off a single ring setup.

There are various STLs which documented the cage profile before I trimmed off the extraneous lines, which I kept in case I want to tweak the profile later.

I never use the 52/53 tooth ring for standard road doubles, & I can't fit a 28 mm rear tire in my road bike because of the front derailleur clamp, so I thought I'd go to 1x & get rid of the front derailleur. I don't have a narrow-wide chainring or a clutch rear derailleur, so I'll need one of these to prevent the chain from falling off.

There's a Driveside SLDPRT/STL which is literally just a pole. Hopefully with this close enough of the top, the chain can't even achieve the height necessary to fall off.

How I Designed This

Designed for a 38 tooth SRAM Apex 10 speed ring, which is approximately 160 mm in diameter.

Measuring from the center of my seat tube to the middle of the largest ring (double crankset, so the 1x ring will default to the furthest outboard position), I got 49.50 mm.

I measured the distance between the plates on a Shimano Claris FD-2400 to be 13.44 mm, then I scaled that by 1.25 to get a wide width that could accommodate cross chaining to the big cog.

Designed around a 1.2 mm high, 4.85 mm wide zip tie.

For 31.8 mm seat tubes.

These measurements might make the STL useless to you, but you could always just edit the STLs if you want.

Assembly & Materials

Print a non-driveside piece & the driveside variation of your choice. Clamp around your seat tube & zip tie them together.

Updates

Never tested yet. Hopefully it's stiff/strong enough to resist deflection from the chain. I didn't close the rear end of the cage because I thought I'd be unnecessary.

Category: Replacement Parts