The reason stock big frames have super-shallow seat angles is not because the bike handles better with the rider’s weight cantilevered out over the rear wheel, causing them to pull wheelies on steep climbs, but because the cranks are not proportional in length the length of the rider’s legs. In order to get the knee over the pedal spindle with a crank that is disproportionately short for the rider, you have to have a shallow seat angle to move the saddle further back. If the cranks were proportional to the leg length, the seat angle could be standard. On a custom bike, it would only be based on the ratio of thigh length to lower leg length (longer thigh, shallower angle, shorter thigh, steeper seat angle).The same holds true for tiny frames with super-steep seat angles, of course. Steep seat angles are used to get the knee over the pedal with a stock crank that is overly long for the rider, and to avoid the toe hitting the front tire. With a proportional-length crank, a small rider could also ride a normal seat angle without having the knee way behind the pedal, and pedal-overlap issues would be reduced as well, hence no need for a steeper seat angle to pull the crank away from front wheel as well. And then their bars would not need to be as high, because their knees would not be coming up so high hitting their chest and tugging on their hamstrings.
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