I'm pretty sure I know the answer. I did a long ride on my mountain bike this afternoon that included a steep, sustained climb on rocky, loose singletrack that took about an hour (the entire ride was about 2 1/2 hours). During the climb, my lower back (right side specifically) hurt and had mild spasms.

On my road bike, I regularly ride climbs of similar duration and intensity with no back issues. My road bike fit is dialed, and has been for years.

My mountain bike fit ... let's call it a work in progress. I'm thinking the frame is too long for me ... toptube is 23.62" (according the geo chart) and the stem is 120. Long and low. When I set it up, I was riding exclusively dirt/fired roads in the midwest, no sustained climbs. Now, I ride front range singletrack and this bike just feels like it doesn't fit. It needs to be shorter; or, I need to be longer. Neither are likely to happen.

Any input appreciated.

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*****
"In case of flood climb to safety"