How to put number on bike.

Seat post is a good option if it's long enough. I put mine on the seat tube, below the top tube, and just staple or stick the top corner, leaving the back bottom corner "open" so that it straddles the tire. (That make sense?)

Personally, with the sort of bike number you usually get, I'll put it on the top tube in a position where it isn't in the way when I reach for my water bottle. I'll usually use the twist ties they inevitably provide to hold the corners together, but I usually secure it to the top tube with electrical tape.

(Black electrical tape = the "duct tape" of triathlon...)

Top tube seems to be the only place I can secure it properly.

I use top tube on my P2C.  I also use zip ties.  They are harder to get off but secure the number better.

I take off my saddle bag for short races and attach it to the seatpost with staples.  Also on the top tube in the little v with twisty ties is an option.

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I am thinking about puting it on the angled post.  Not sure what  you call it.  I use an aero drink so I was thinking about removing bottle holder and wraping it tight.  This tube is very wide on the P2C.  I would think a loose number would cause some drag.  Probably very minimal but it just bugs me

I'm a seatposter...with tape...scotch brand.....