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Yes. I think you are right about steeper seat tube angles being an accidental discovery. I sense the longer reach longer reach call increased top tube measurements which we all used to find a bike that fit so seat tube angles were steepened accordingly. That also allows room for wheels and suspension travel and the short chain stays we are told we love. DH out front, BMX out back. The ultimate mullet. I thought the longer reach allowed a rider to keep their weight more evenly balanced between the wheels. Slack angles and higher trail measurements give a more stable ride plus with a short stem allows us to weight the front end more without fear of going over the bars. Long stems of yesteryear forced us way back with little weight on the front and less traction than we wanted on the front wheel. Longer reach really helped that OTB thing go away but still I think the short rear centre is not really matching that front end.
On many bikes you could keep the same wheelbase but slacken the seat tube angle by simply moving the bb location forward. Like a Pole. What would that do to weight distribution through the feet? We all move our weight back relative to the front end to allow for the forces of hitting things like braking bumps, roots, braking forces, big rollers etc. To me that would seem to place more weight on the back wheel and if the chain stays are super short, too much weight bias to the rear. Move the bb forward and that changes that.
Short rear centres are not as good for climbing. There’s a reason why hill climb motor bikes have ridiculous swingarms. In the olden days we used to say we needed a long stem to keep our weight over the front wheel on steep climbs but that was stated wrong. We didn’t want the weight the front wheel, we wanted our weight ahead of the rear contact patch. When we lower our body climbing, we aren’t just getting our centre of mass in a better place we are also pulling back on the bars to offset pushing a bit forward at the top of the pedal stroke. Look at just about anyone hammering a steep pitch and you’ll see forearms parallel to the ground. Sitting more upright doesn’t work for hard uphill efforts. For slow easy climbs sure, it may be more comfortable to sit more upright. But I don’t think it’s more efficient.
Steeper seat tube angles will change your seated balance and may place more weight on your hands leading to comfort issues like Andrew mentions. You’ll probably need to raise your bars, or move your feet forward or saddle back to achieve balance. Shorter cranks will affect fit too. Lots of people moving to shorter cranks now. That moves to your saddle up to keep the same leg extension. All these things affect fit.
I don’t know the answer. I’m not saying super steep is wrong but I’m just questioning it.
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