Originally Posted by
Russ Roth
Stack is an easily understandable thing. I've never seen a good explanation of the importance of reach vs top tube length. Care to elucidate?
Reach is a better reference as you know how far forward you are going to have to reach relative to the BB i.e. where your feet are fixed. Effective top tube length is a bit more vague as it doesn't relate directly to the BB, which should be the centre of your whole bike fit. That's why reach & stack are now generally considered the primary frame dimensions as they both relate directly to the centre of the BB.
For the OP, the 54 Felt is a non-starter with a stack 42 mm lower than a known comfortably fitting frame! The reach is the same and the reason the effective top tube is shorter is because the head tube is shorter, so it's being measured at a lower reference point. When the stack heights are so different you can't really compare reach directly. If you added 42 mm of stem spacers in the smaller frame to make the stack height identical, then the effective reach would be about 13 mm shorter because of the head tube angle.