Some interesting things I learned
Here are some things the model helped me see. When I refer to fit relationships such as knee-over-pedal, I’m not necessarily endorsing them, just examining what happens to them. You probably have to click on the screenshot to read it.
1. My time would have been better spent riding. Here is the model showing 175mm and 150mm cranks with a 70mm setback, and 73 degree seat tube angle. The differences are mathematically not that much. However, the feel is significantly different. Hence, to figure out what short cranks do, go riding, not calculating.
2. Shortening the cranks does not mean raising the saddle the same amount as measured along the seat tube. This is intuitively true because the circles of possible saddle positions converge to either side of the vertical line through the bottom bracket. The difference is slightly less (here 23.75mm instead of 25mm), because some of the change is accounted for by the fact that moving the saddle up also moves it back in relation to the pedal at the lowest point. But, I think that's important: I believe that I can feel changes of 1mm in my saddle height, and I know I can feel 4mm (the difference in sole thickness between my two regular pairs of shoes).
3. It is possible to deal with the entire crank shortening by pushing the saddle back without raising it. In this case, I would need to double the setback to about 137mm. Not recommending this, even if it’s possible, but you could. More realistically, you can counteract some of the effects that short cranks will have on reach and drop in relation to the handlebars with different combinations of saddle height/setback changes.
4,
Shorter cranks don’t necessarily push the knee forward of the pedal, especially if this wasn't a problem with the longer cranks. I was always convinced they would, so I burst at least one bubble here. In this case, with a constant setback of 70mm, shortening the crank by 25mm will cause my knee to move back 21mm once I’ve adjusted the seat post up the necessary 23.75mm.
5. If I really want to maintain the same knee over pedal relationship, then I would need to move the saddle up 21.94mm and back an extra 10.4mm.
6. In fact, even with 10.56 mm cranks (random nonsense small number), the knee remains behind the pedal. Seeing this was why I decided against buying an expensive seat post with extra long setback. I decided on a much cheaper option with 1cm more set back, see no. 5 above.
7. Bikes with slacker seat tube angles will require less change in saddle height than bikes with steeper seat tubes. This makes intuitive sense, of course. Here are three screenshots for 73, 75, and 71 degree seat tubes, all else constant. What is interesting here is that each will have different ramifications for reach and drop in relation to the handle bars, but that is beyond the model right now.
73:
75:
71:
Hope this was interesting!
Last edited by Frkl; 02-04-22 at 04:51 AM.