The P-38 has virtually no flex in the crank tube under pedal load. The crank tube is braced back to the stem and with four frame sizes, riders can pick a size that more nearly matches their leg length.
This is, in my opinion, one of the reasons why it has a reputation of a good climber. The other reason it climbs well is the closed rider position and moderately high bottom bracket.
Whenever flex is raised as an issue on Forums, someone will claim that, as the flexed frame returns to it's neutral position, the power lost is returned. This is not correct. When the bottom bracket (BB) is forced down and sideways on recumbents with frame flex, it 'pulls' the chain a microscopic amount. But when the BB rises again, the power side of the chain is slackened. When a BB flexes under pedal load, power is lost.
Some recumbents have as much as 25 mm (1 inch) of flex under static load. My experience and brief tests with a trike before and after a BB brace was fitted, suggest that flex can make a trike 10% slower on average - most of the speed lost is in climbing where the most pressure is applied to the pedals.
Some of the latest carbon frames have very little flex at the BB. They are made with a deep rectangular section crank tube. It's no surprise to me that they climb well.
Riders who are short in stature and who have a high cadence may not notice much power loss form BB flex. Likewise, the very rare riders that have a perfectly smooth pedalling action, with power being applied equally around the pedal stroke, will hold a frame in it's stressed position and no power will be lost.
I have a slow cadence and a very long x-seam (distance from back of seat to pedals.)
As a result crank tubes on my bents are extended a long way forward and unless they're braced like the P-38, they flex a lot.
Lack of flex in the P-38 BB is one of the reasons; probably the main reason, that it climbs faster than many recumbent designs.
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