Got a bit of a situation here, especially with weeks to go before a certain endurance based event.
This bike gets ridden 4ish times a week 80-100ish miles maybe, commuting, bombing about, errands - it fits me perfectly, and I love it.

This bike is my 'proper' bike.
I ride this once or twice a week for my proper MTB rides. Not had it long, only recently starting to use it on big days out.

This one, is starting to give me horrendous lower back pain after an hour.
Both bikes have the same -
BB-saddle height
Saddle-bar distance (effective reach)
Bar width
Chainset
Pretty much everything.
The only thing I can work out is that the effective seat tube angle on the Heckler is much, much slacker.
For the same reach, the Heckler is running a 50mm shorter, and a layback post, whereas the Kona has a straight post (a Thomson, not whats in the pic) and a 80mm stem.
So this, combined with the naturally slacker angles of the Heckler, means Im sitting much further back in relation the the front wheel (you can see this with both bikes lined up).
Or rather more importantly, further behind the crank. A lot further, maybe 50mm, maybe even more.
Is this it?
Is it stretching the back of my legs/lower back too much?
Whats the best solution - longer stem + straight post?

