I’m trying to fathom out the confusing world or rear mech cassette limits.

In short I want to run my Roadrat as 1×10. I’ve already got Tiagra 4600 rear mech (short) and flat bar shifters running on an 11-30t cassette. I am wondering how low (big) a cassette I could run. I could swap the mech for a 4601 rear mech that allows up to 32T on a 2x setup. Would a 1x allow me to go bigger still? i.e. 34 or even 36T?

As far as I can tell the cage length affects the length of chain one can run. More chain = a wider range from small sprocket to big sprocket. Or is there more to it than that?

The maximum sprocket size is limited by the geometry of the mech body.

The difference between chainrings plus the difference between top and bottom sprocket is related to cage length.

So no, not really. A longer cage in itself doesn’t let you have a bigger large sprocket. Unless there are other changes to the mech geometry. Or restrictions placed on sprocket size by chainring difference.

32t is the biggest a road mech will handle regardless of single or double ring.

I and a fair few others run their cx bikes with a 38 or 40t front and 11-32t out back.

Consider a Lindarets roadlink?

They claim it’s able to stretch a mid or long cage road mech to 11-40, or a short cage mech to 11-36… Worth a punt?

Thanks that clears things up.

The new Tiagra 4700 rear mech goes up to 34T. However, it won’t work with my existing 4600 shifters. Given my “lets see if I can a cheap roadrat frame off ebay” project has turned into a new frame, fork and wheels arriving tomorrow I’m not going to be able to splash out on a new set of shifters too.
I’ll see what the gearing is like once I’ve built it up. I only really need a low gear on a couple of hills round here when I have a child in tow.

Any 9 or 10 speed road mech (Other than 4700) or any 9 speed MTB mech (might be a better bet) should work with your shifter it doesn’t need to be tiagra, doesn’t have to be new… I am sure you could find something compatible for peanuts…

there you go That will probably stretch to 36t.

Short cage 105 and 11-36 here and shifts perfectly.

The critical thing is chain length, one link too many or too few and it won’t work.