I need some help with frame geometry- Mtbr.com

# Thread: I need some help with frame geometry

1. ## I need some help with frame geometry

First off, I have never been accused of being intelligent, so I want to get that out of the way first.

Someone please help me here. I am trying to determine the effect of ST angle on effective TT. Let's say I have two frames very close in geometry except for seat angle. I am trying to figure out the change on effective TT. Here are some specs:

Frame 1 Effective TT = 23", Head Angle = 71 deg, Seat Angle = 73.5 degrees

Frame 2 Eff TT 23", HA = 71 degrees, SA = 72.5 degrees

What would be the differences between the frames as far as pedal to handlebar measurement? Would it vary? I am also thinking the frame with the slacker SA would have the seat mounted further forward on the rails to maintain the same position over the crank.

Could some explain this to me using the frame examples above? Is effective TT constant regardless of ST angle and HT angle? I remember hearing in the roadie world that a 1 degree change on a road frame would equal a 1 CM or so effective TT change.

Thanks to anyone who can help.

Oh, where is the SS Content here? Frame 1 is my current SS' geometry and Frame 2 is a possible SS addition to the stable (different braking systems on frame 2 - but justifying a new bike purchase on here is like preaching to the choir, right?).

2. ## too many decades between me and trig

for a quantitative answer, you would want a trig table and enough unkilled brain cells that still retain H.S. trig class to know how to use it.
but qualitatively, you are correct in realizing that with a steeper seat tube angle, your effective top tube length will be shorter. your butt is going to sit where it wants to sit, regardless of the frame's seat tube angle. so with a shallower seat tube angle, your butt is going to sit farther forward so more of the seat tube will be under you rather than in front of you.
all this will also affect the length of stem you prefer.
whether you climb seated, or standing, is also a factor.
ex. my litespeed obed has a 72.5 degree seat tube angle which is shallower than I prefer. I climb seated, so I end up with my saddle all the way forward for seated climbing.
sorry the only trig I know is how to calculate trail using offset and head tube angle, I wrote it down so homebrew wouldn't kill off the brain cells that retain that information.

3. I believe anvil bikes has an excel spreadsheet on his website to do all this stuff... I'm not great with this stuff either, so I'm no help there! (=
lots of good info on this sort of stuff there, if you've got some time to check it out. he's linked in the hotlinks.
good luck,
Serge

Originally Posted by bulC
for a quantitative answer, you would want a trig table and enough unkilled brain cells that still retain H.S. trig class to know how to use it.
but qualitatively, you are correct in realizing that with a steeper seat tube angle, your effective top tube length will be shorter. your butt is going to sit where it wants to sit, regardless of the frame's seat tube angle. so with a shallower seat tube angle, your butt is going to sit farther forward so more of the seat tube will be under you rather than in front of you.
all this will also affect the length of stem you prefer.
whether you climb seated, or standing, is also a factor.
ex. my litespeed obed has a 72.5 degree seat tube angle which is shallower than I prefer. I climb seated, so I end up with my saddle all the way forward for seated climbing.
sorry the only trig I know is how to calculate trail using offset and head tube angle, I wrote it down so homebrew wouldn't kill off the brain cells that retain that information.

4. BikeCAD is a cool Java applet that works great even tho the pix it makes are road-bikey. It may almost be too powerful though. IYou almost have to have all the measurements of a bike, like BB drop, TT angle, wheels/fork, etc.

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