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  #1  
Old 11-24-2019, 12:06 PM
bikser bikser is offline
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Seat Setback in relation to different Frames/SA's

Ok, I'm a geometry geek in general. Probably over study as I try different frames frequently. My current bike has been professionally fitted, and feels great. I have a new frame with different geometry. I am trying to compare them to determine if the new one will fit me well.

Current New
ett 540mm 550mm
SA 74d 73d
HA 72d 73d
Stack 548mm 564mm
Reach 383mm 378mm
Seat Tube 500mm 520mm
Head Tube 148mm 150mm (current has a taller crown to axle fork length of about 380mm, new uses King inset, so add 10mm to bottom of head tube, essentially a wash)

Figure bb drop, fork rake, chainstay length essentially the same.

Current frame uses zero degree seatpost, saddle is roughly in the middle, 18cm setback (center of saddle from center of bb). I've set the setback on the new one with the same 18cm setback, visually this looks like the saddle is about 1cm further back on new frame, and saddle tip to center of stem at steertube seems to add several cm's. Bike fitter suggested I use the saddle tip to center of stem as the saddle set up which would change setback considerably between frames.

I know current frame fit philosophy says to use stack and reach as more accurate way to fit which would suggest the frames would be comparable. I'm just trying to figure out how the different seat tube angles (and/or ett) would effect my overall fit.

I only have the new frame, no fork or headset yet. I put a rear wheel, setback seatpost, and saddle on. This is propped up side by side with current bike.

I know this is a lot of info to put out there, but if anyone has words of clarity for me I would appreciate it. thanks
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  #2  
Old 12-05-2019, 01:31 PM
AndresLD AndresLD is offline
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Location: Bay Area, CA
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Quote:
Originally Posted by bikser View Post
Ok, I'm a geometry geek in general. Probably over study as I try different frames frequently. My current bike has been professionally fitted, and feels great. I have a new frame with different geometry. I am trying to compare them to determine if the new one will fit me well.

Current New
ett 540mm 550mm
SA 74d 73d
HA 72d 73d
Stack 548mm 564mm
Reach 383mm 378mm
Seat Tube 500mm 520mm
Head Tube 148mm 150mm (current has a taller crown to axle fork length of about 380mm, new uses King inset, so add 10mm to bottom of head tube, essentially a wash)

Figure bb drop, fork rake, chainstay length essentially the same.

Current frame uses zero degree seatpost, saddle is roughly in the middle, 18cm setback (center of saddle from center of bb). I've set the setback on the new one with the same 18cm setback, visually this looks like the saddle is about 1cm further back on new frame, and saddle tip to center of stem at steertube seems to add several cm's. Bike fitter suggested I use the saddle tip to center of stem as the saddle set up which would change setback considerably between frames.

I know current frame fit philosophy says to use stack and reach as more accurate way to fit which would suggest the frames would be comparable. I'm just trying to figure out how the different seat tube angles (and/or ett) would effect my overall fit.

I only have the new frame, no fork or headset yet. I put a rear wheel, setback seatpost, and saddle on. This is propped up side by side with current bike.

I know this is a lot of info to put out there, but if anyone has words of clarity for me I would appreciate it. thanks
You're correct that stack and reach is your best bet, but also your current set-up will affect how well it accommodates your position. There's a possibility that your new frame won't work if your current set-up is slammed with a -17, it would make the new frame too tall for your position. This is all assuming you end up with the correct fork for that frame.

As for FA, you can likely run a zero offset depending on where your saddle sits on the rails and still replicate it (assuming same saddle). Worst case, you'll have to run a 20 or 25mm setback, which in theory might be more "comfortable" but in practice, you likely won't notice.
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  #3  
Old 12-05-2019, 03:06 PM
fa63's Avatar
fa63 fa63 is offline
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If the current bike has a 74 deg STA and you run a zero-setback seatpost on it, you will need to do the same on the new bike with 73 deg STA AND push the saddle forward by about 1 cm (as a rule of thumb, 1 deg difference in STA is roughly 1 cm). This should put the saddle in the same position on both bikes. Then from there, you can go about adjusting the reach to the handlebar.
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  #4  
Old 12-05-2019, 05:50 PM
AndresLD AndresLD is offline
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Originally Posted by fa63 View Post
If the current bike has a 74 deg STA and you run a zero-setback seatpost on it, you will need to do the same on the new bike with 73 deg STA AND push the saddle forward by about 1 cm (as a rule of thumb, 1 deg difference in STA is roughly 1 cm). This should put the saddle in the same position on both bikes. Then from there, you can go about adjusting the reach to the handlebar.
It's best to just measure FA to transfer FA, a person with a 725SH moving the saddle forward 1cm will end up with a different SA than the person with a 825SH.
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  #5  
Old 12-19-2019, 07:30 AM
Road Fan Road Fan is offline
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Quote:
Originally Posted by AndresLD View Post
It's best to just measure FA to transfer FA, a person with a 725SH moving the saddle forward 1cm will end up with a different SA than the person with a 825SH.
Hi, what do you mean by "FA?"
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  #6  
Old 01-07-2020, 05:28 PM
Old School Old School is offline
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Quote:
Originally Posted by bikser View Post
Bike fitter suggested I use the saddle tip to center of stem as the saddle set up which would change setback considerably between frames.
You need a better bike fitter.

You should be using, at a minimum, nose of saddle to a "yardstick" placed across the "hoods".

This will take more variables into account.
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  #7  
Old 01-07-2020, 05:34 PM
Old School Old School is offline
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For anyone who is a "bit of a geometry geek".

http://www.csgnetwork.com/righttricalc.html

for a 33 inch (83cm) inseam, the seat moves 0.58 inches (15mm) if the STA changes from 73deg to 74deg.
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  #8  
Old 01-08-2020, 04:40 PM
flying flying is offline
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Quote:
Originally Posted by Old School View Post
For anyone who is a "bit of a geometry geek".

http://www.csgnetwork.com/righttricalc.html

for a 33 inch (83cm) inseam, the seat moves 0.58 inches (15mm) if the STA changes from 73deg to 74deg.
Thank You

This is useful info
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  #9  
Old 01-08-2020, 06:17 PM
Dave Dave is offline
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Location: Loveland, CO
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I've never used a zero setback post and can't imagine one with a 74 degree STA, at least on a road bike. I've got a short torso, but still use a 25mm setback.

As long as you don't feel too much weight on your hands, I guess you're OK?

http://kirkframeworks.com/2009/06/19/riding-tip-3/

Last edited by Dave; 01-09-2020 at 12:40 PM.
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  #10  
Old 01-08-2020, 06:57 PM
Dave Dave is offline
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A 1 degree difference in STA will create a 1.2 mm change in the saddle position.
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  #11  
Old 01-08-2020, 08:00 PM
Old School Old School is offline
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http://www.csgnetwork.com/righttricalc.html

Double check using that ^
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  #12  
Old 01-09-2020, 08:42 AM
Dave Dave is offline
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I use my personal knowledge of trig. Draw a vertical line through the center of the BB. Draw another line at an angle of 17 degrees from the center of the BB with a length of 73cm. The horizontal setback is sin17 x 73 = 21.34cm. Repeat with the same 73cm saddle height and 16 degree angle sin16 x 73 = 20.12cm. The difference is 1.22cm.

I use 73cm as the saddle height for a person with an 83cm cycling inseam like me.

A long standing rule of thumb is that frame setback changes at a rate of about 1cm per degree. At saddle height, it changes even more. That alone tells you that .68cm is wrong.

The online calculator gives the same answer, you just have to put in the right numbers.

Last edited by Dave; 01-09-2020 at 11:31 AM.
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