Seat angle geo question

[SoCalSteve]

Hi all,

Ok, so I'm really bad with geometry. My brain just doesn't work spatially like that. That being said...here is my question:

If you have 2 bikes with the same seat post ( 2 cm setback) and same saddle...but, one bikes seat tube angle is 73* and the other is 72*, does using a zero setback seat post and moving the saddle back 1 cm on the rails on the 72* bike, will I be sitting in the same place as the bike with the 73* seat tube angle?

Or, am I missing something here?

Also, I'm not so concerned with reach to the bars, that part I can figure out on my own.

Please, please don't answer with geometry illustrations, links to CAD websites or algebraic equations. I am a simple guy with a simple brain, please answer as simply as possible.

Thank you all in advance!

[jamesau]

Well, it depends on your seatheight. For a seatheight of 78cm, the difference is 1.3 cm. (So, if you've the 2cm setback post on the 73 deg bike, you'd have to move the seat .7 cm back on the 0 offset post on the 72 deg bike).

If you're feeling brave, you can multiply your seatheight by 0.0166 to get the number you're looking for.

[bicycletricycle]

What is your seat height? BB to top of seat? Also easiest way to do this is a practical measurement anyone can do at home.

Step 1. Attach something heavy (some big washers) to the end of some string.
Step 2. Hold string onto the very front of the nose of your saddle.
Step 3. Make sure string is falling straight down, lean the bike slightly to the side if you need to, make sure washers are slightly above the ground and not resting on the ground.
Step 4. Measure distance from string to bottom bracket center Holding the ruler parallel to the ground.

[CNY rider]

Roughly, yes.
To verify: Tie something heavy to the end of a string. Hang it down from the saddle nose and measure how far that falls behind the BB> Should be equl on both bikes.
Oh yeah and E=mc^2

[lhuerta]

Steve. one degree diff in seat tube angle (from 73 to 72) will yield approx 1 cm more setback, but the exact amount will depend on your seat tube length and seat height. If you are trying to match setback on two different bikes, best approach is to use a simple plumb line to match exact position.
Lou
PS-Reynolds wheels are working out great

[SoCalSteve]

Well, it depends on your seatheight. For a seatheight of 78cm, the difference is 1.3 cm. (So, if you've the 2cm setback post on the 73 deg bike, you'd have to move the seat .7 cm back on the 0 offset post on the 72 deg bike).

If you're feeling brave, you can multiply your seatheight by 0.0166 to get the number you're looking for.

Is that something like math? I'm pretty allergic to math!

But, thank you so much, I'll give that a whirl.

[Louis]

If you're feeling brave, you can multiply your seatheight by 0.0166 to get the number you're looking for.

Maybe I'm not visualizing this properly, but 1* = 0.01745 rad.

[Chief]

Hi all,

Ok, so I'm really bad with geometry. My brain just doesn't work spatially like that. That being said...here is my question:

If you have 2 bikes with the same seat post ( 2 cm setback) and same saddle...but, one bikes seat tube angle is 73* and the other is 72*, does using a zero setback seat post and moving the saddle back 1 cm on the rails on the 72* bike, will I be sitting in the same place as the bike with the 73* seat tube angle?

Or, am I missing something here?

Also, I'm not so concerned with reach to the bars, that part I can figure out on my own.

Please, please don't answer with geometry illustrations, links to CAD websites or algebraic equations. I am a simple guy with a simple brain, please answer as simply as possible.

Thank you all in advance!

When you ask if you will be sitting in the same place, I assume that you mean relative to the BB. If so, then rigorously the required setback depends upon the length from the BB to the top of the seat post. If the length is 60 cm, then 1 cm is the correct setback; if the length is 65 cm, then the setback is 0.9 cm; if the length is 70 cm, then the setback is 0.8 cm; if the length is 75 cm, then the setback is 0.75 cm; if the length is 80 cm, then the setback is 0.67cm; if the length is 85 cm, the length is 0.59 cm. For lengths greater than 90 cm the setback for all practical purposes is 0.5 cm. Hope this helps.

[jamesau]

Maybe I'm not visualizing this properly, but 1* = 0.01745 rad.

The number, .0166, is sin(18) - sin(17) where the argument is in degrees and represents the angle a plumb bob would make with the seattube (90-72, 90-73 respectively). Right, guys?

[SoCalSteve]

The number, .0166, is sin(18) - sin(17) where the argument is in degrees and represents the angle a plumb bob would make with the seattube (90-72, 90-73 respectively). Right, guys?

You lost me at "The number"... :(

[cody.wms]

If you have 2 bikes with the same seat post ( 2 cm setback) and same saddle...but, one bikes seat tube angle is 73* and the other is 72*, does using a zero setback seat post and moving the saddle back 1 cm on the rails on the 72* bike, will I be sitting in the same place as the bike with the 73* seat tube angle?



Other way around - if your saddle is in the same place on both (same position on the seatpost), on the 72 it will be further back. If you took the saddle/seatpost combo out of the 72 frame and installed it on the 73, you'd need to move it BACK about a cm. (edit from earlier post)

And since people are throwing math around, heres a picture I drew for another thread:

http://farm6.static.flickr.com/5027/5735072742_7a67fa6a55_z.jpg (http://www.flickr.com/photos/52811793@N02/5735072742/)
Geometrication (http://www.flickr.com/photos/52811793@N02/5735072742/) by Cody Wms (http://www.flickr.com/people/52811793@N02/), on Flickr

[Louis]

The number, .0166, is sin(18) - sin(17) where the argument is in degrees and represents the angle a plumb bob would make with the seattube (90-72, 90-73 respectively). Right, guys?

I agree.

Or the way I visualize it, if L = ST distance from BB to saddle:

delta X = L x (cos(73) - cos(72)) = L x -0.0166
delta Y = L x (sin((73) - sin(72)) = L x 0.0052

[jlwdm]

Other way around - if your saddle is in the same place on both (same position on the seatpost), on the 72 it will be further back. If you took the saddle/seatpost combo out of the 72 frame and installed it on the 73, you'd need to move it BACK about a cm.



Not the other way around. What you quoted from Steve is generally correct (fine tune based on bike size) and what you said is correct. Steve suggested a non-setback post which moved the saddle farther forward on the 72 degree frame.

Jeff

[e-RICHIE]

If you have 2 bikes with the same seat post ( 2 cm setback) and same saddle...but, one bikes seat tube angle is 73* and the other is 72*, does using a zero setback seat post and moving the saddle back 1 cm on the rails on the 72* bike, will I be sitting in the same place as the bike with the 73* seat tube angle?


it depends on the size of the bicycle atmo.
the change will be way more on larger frames.
ps it would be a lot easier to solve if you used setback rather than angles.
linear measurements are quite easy to take and transfer.

ps

arrange disorder

:o :o :o
:o :o :o
:D :D :)

[ultraman6970]

Isn't faster just aplomb the distance from the tip of the saddle in reference to the BB center and move that to the newer bike with whatever st angle has and then adjust the reach with a longer or shorter stem? The shallow angle bike will have less reach than the one with longer angles.

In my opinion the seat back in the seatpost doesn't matter that much if you do that, if the saddle rails are long enough you should have room to move the saddle 1 or 2 cms to the front or back anyways. Obviously if you need a humongous seat back it could make a difference but with regular size people in my opinion isn't that critical if the mods are not more than 2 cms back and forth. That's why i dont understand that much what's the science of zero seat back seatpost, after all if you keep the distance behind the BB center; to have more, less or zero seat back in the seatpost it means nothing. Doing my assumptions with regular size people ok? since im not 7 feet tall can't tell if it does some difference ok? Maybe comfort?

Cheerz

[jlwdm]

Isn't faster just aplomb the distance from the tip of the saddle in reference to the BB center and move that to the newer bike with whatever st angle has and then adjust the reach with a longer or shorter stem? The shallow angle bike will have less reach than the one with longer angles.

In my opinion the seat back in the seatpost doesn't matter that much if you do that, if the saddle rails are long enough you should have room to move the saddle 1 or 2 cms to the front or back anyways. Obviously if you need a humongous seat back it could make a difference but with regular size people in my opinion isn't that critical if the mods are not more than 2 cms back and forth. That's why i dont understand that much what's the science of zero seat back seatpost, after all if you keep the distance behind the BB center; to have more, less or zero seat back in the seatpost it means nothing. Doing my assumptions with regular size people ok? since im not 7 feet tall can't tell if it does some difference ok? Maybe comfort?

Cheerz

As mentioned above the plumb bob is the best way to get the exact position and it is much easier with the same saddle on both bikes like Steve has. I use the same saddle and same pedals and shoes on all three bikes so it is easy to set up.

In Steve's example you do not know which bike will have the longer reach because you do not know the top tube lengths. With all of the different reach bars these days it is best to look at different bars and stems to get the best reach.

Not all saddles have enough rail length to cover all situations. Ideally I have a 25mm setback post on a 72 degree seat tube angle - both customs. But on my 73 degree seat tube angle bike I can't adjust on the rails and have a 35mm setback seat post.

Jeff

[Dave]

The formula to use is (cosA-cosB) x times the saddle height. Plug in the numbers for 72 and 73 degree angles and you get the previously mentioned .0166 value. For a 75cm saddle height, that would be 1.25mm.

If you put a nonsetback post on the bike with the 72 degree STA, the seat rail clamp would be .75cm further forward than the bike with the 73 degree STA and 2cm setback post.

[SoCalSteve]

Besides all the math and algebra and everything else I dont understand, thanks to everyone so far for your great experience and knowledge!

The one thing I failed to mention is that the SI Prolink saddle has very, very short saddle rails with hardly any room for adjustments whatsoever. With that being the case, I assume that I have to use seat post set back (or lack of) to get the saddle where I want it.

And yeah, it gets more complicated as my bikes are all pretty big with long seat tubes...probably relate to about 62cm bikes ( sloping top tubes).

The reason I asked the question is because my new HSG with it's 73* seat tube angle and 2cm setback seat post is extremely comfy. My Ottrott and Kirk have 72* seat angles and I can't seem to get as comfortable on them with 2cm set back posts. I figure if I can replicate the 73* position of the HSG on the other 2 bikes I can get as comfortable on them. I am very close to replicating the reach and height of the bars on all the bikes.

Again, thank you all so much!

[happycampyer]

The plumb bob works ok, but I find that a drywall square is a little easier to work with.

Set the square on the floor on the drive side of the bike you want to measure first (here, the HSG), and set the back edge of the long edge of the square against the tip of the saddle. Measure the distance back from the center of the bottom bracket to the back edge of the square. Also, measure (or mark on the square) where the tip of the saddle meets the square, as well as the bottom bracket height. Move the square to the new bike, set the square the same distance from the center of the bottom bracket, and play with the saddle height and setback until it meets up at the same point on the back edge of the square as the previous saddle (taking into account any difference in bb height). Not super duper precise, but it gets you pretty close. This method works best if you are keeping crank length, pedals, and saddle the same (which you are).