The new 27.5 sizing looks interesting. At 5’9” I am sort of between sizes. I am usually a medium but all the measurements are larger than I would expect. Should I go with the medium or small?

Medium, definitely. I’d say that 5’10-11″ is the borderline between medium and large.

Just ask Cy

Medium definitely
I’m 5’10” my medium is spot on
Read Cy’s blog on website about the geometry, dimensions will make more sense then

Doing a comparison with my other bikes (a 15” orange blood) would suggest I go for the small (it fits perfectly with a 50mm stem) the main difference is the seat angle which will make it feel longer. Just concerned a medium will be a gate. There sizing is a bit odd or did they go long and slack before it was fashionable?

Not often you’ll see this but Jamby is right (is that twice today?)

This is from the Cotic website:

If you would like us to help you out with sizing, or are on the cusp of two sizes then drop us a line at size@cotic.co.uk or give us a call on 07970 853531 and we’ll be happy to discuss set up based on what you’re riding at the moment.

Funnily enough, Paul and Cy know their shit. they will talk you through it I know as I’ve done this myself. And so has my mate. They won’t laugh (or bite), they just want to sell you the right sized frame (for you).

I’m 5’9/ 5’10 on a medium. Reverb or inline Thomson post with a 35mm stem.
Fits nice, just as comfortable on an all day ride as it is for an hour ragging local woods.

Doing a comparison with my other bikes (a 15” orange blood) would suggest I go for the small (it fits perfectly with a 50mm stem) the main difference is the seat angle which will make it feel longer. Just concerned a medium will be a gate. There sizing is a bit odd or did they go long and slack before it was fashionable?

You’re probably comparing ETT but it’s reach that’s much more relevant – ETT is how long a bike feels when sitting down. Reach is how long it feels when standing up.

15″ Blood, ETT 575; Seat Angle = 75 deg.
17.5″ BFe, ETT = 614; Seat Angle = 71 deg.

For each degree increase in seat angle, you gain about 10mm reach for a given ETT, so the 15″ Blood and 17.5″ BFe have the same reach.