Bike size and Back pain?

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Hi All
I have been lucky enough to afford to purchase two New Bikes( Medium Frame), over the years. One Road and one 26er MTB.
Anyway I've also bee lucky enough to pick up two Large frame Road bikes from hard rubbish. The large i ride with ease and it seems I'm probably boarder line between Medium and large as I'm 180 cm Height. Anyway I've picked up a small Frame MTB 17inch 43 cm. It rides like a dream and like the bike but I'm noticing i have a bad lower back pain. Also have ridden a small Road bike and yep I get lower back pains.
Anyway are the smaller bikes The problem here as i have put the seat up a long way and still stretch a little to ride?

[foo on patrol]

It's all about position. Do the measurements between the two frames and see what difference there is.

Foo

[4 1 3 0]

It's all about position. Do the measurements between the two frames and see what difference there is.

Foo

Sounds good, I'll give that a try.