How seat angle affects handlebar location

Inspired by the discussion surrounding Why is a slack seat angle so rip-able?.

With a diagram this time.

PS: Slackening your seat angle also raises your bars in relation to your seat. That’s another possible explanation for the rip-ability of slack seat tubes.

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12 replies
  1. fanbox says:

    Slacker seat tube shortens the wheelbase? I would think with a slack seat tube the chainstays have to be longer to provide rear tire clearance, so it probably is the same, if not more.

  2. Matt says:

    Think of it this way, if the top tube length and chainstay length remain the same (and headtube angle, etc), a slacker seat tube would pull the top tube more towards the rear end of the bike (and steepen the down tube), thus shortening the wheelbase overall.

    Obviously, there are limits to how slack a seat tube can be with a set chainstay and top tube length without the seat tube running into the rear tire, but with those fixed, a slacker seat tube will shorten the wheelbase.

    If you try overlapping the two pictures, you’ll see this a little better.

  3. Scott says:

    I’m still not convinced this is a valid way to compare the effects of slackening the ST. If the TT stays the same (“everything else remains equal”) then the HA/HT changes, which obviously has a big effect on handling. Besides, TT is irrelevant in my book — it’s the effective TT that matters.

    Seems to me that the best way to compare ST angles AND keep most everything else the same is to try layback posts, offset clamps, and/or sliding saddle back on rails. The bike geo doesn’t change, but the effective ST and effective TT change. Also, the slacker the ST, the greater the changes to effective TT, etc when saddle height is changed.

    The critical dimension, IMHO, that’s central to this discussion, is the horizontal difference between the BB and saddle. That’s what influences which muscles get used, etc.

    I’ve found that the closer I can get to “knee over pedal spindle” (KNOPS), the more efficient my pedaling. Then, couple that with a longer TT and shorter stem, and well, BRAP. To me the sensation is that I’m ‘in’ the bike, rather than ‘on’ it. There’s plenty of chatter out there about how KNOPS is bs, but I’ve tried both ways and too slack a ST just felt like pushing instead of pedaling, and it was harder on my knees. Then again, didn’t Lee establish that quads are pretty worthless when it comes to power output? Then again, if it’s endurance you’re after it helps to recruit more muscles.

  4. Scott says:

    Whoa, qualification. I saw that it was the wheelbase was changing, despite ”everything else remains equal” — maybe not as big of an influence on handling as HA, but still too significant to dismiss…

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