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  #16  
Old 12-02-2020, 09:24 AM
Dave Dave is offline
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Join Date: Nov 2004
Location: Loveland, CO
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The most common mistake when comparing reach between two frames is ignoring the difference in stack. For example, I just bought a frame with a 10mm shorter reach, but the stack was 18mm shorter. If I used a 20mm spacer on the new frame, instead of none, the real difference in reach is about 16mm, not 10. I ended up with a 15mm spacer under the stem and 5mm on top. That choice allowed the use of a 110mm, -17 stem that I already had. A 120mm would have been too long.

Using the spacers may improve the ability to sell the frame in the future. If I choose, I could get rid of some of the spacer with a -12 stem or all of it with a -6. The -6 might require a 120mm stem.

Seat tube angle only affects the amount of seatpost setback that might be required. In my smaller size, the STA is usually 74-75 degrees. My new frame was advertised as having a 75, but it wasn't. It was 74.5.
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  #17  
Old 12-02-2020, 11:32 AM
flying flying is offline
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Join Date: Dec 2018
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Quote:
Originally Posted by Dave View Post
The most common mistake when comparing reach between two frames is ignoring the difference in stack.

Seat tube angle only affects the amount of seatpost setback that might be required.
Another sometimes overlooked/unreported Stack difference to watch for is if frame that uses an external headset like CK & its additional stack is not included in geometry chart

Yes About SA/setback posts..... not always a simple fix..
In my case I use a 20 degree setback post on a 73.5 seat post angle frame
My setback I run is 8.25cm saddle nose behind center of BB so saddle is nearly fully slammed back even with setback post

So....for me Seat Tube angle is very important

But all this highlights for the OP the need to ride & learn what his own needs are over time & keep track of those numbers
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  #18  
Old 12-02-2020, 09:14 PM
Imaking20 Imaking20 is offline
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Join Date: Oct 2016
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Quote:
Originally Posted by Dave View Post
The most common mistake when comparing reach between two frames is ignoring the difference in stack. For example, I just bought a frame with a 10mm shorter reach, but the stack was 18mm shorter. If I used a 20mm spacer on the new frame, instead of none, the real difference in reach is about 16mm, not 10. I ended up with a 15mm spacer under the stem and 5mm on top. That choice allowed the use of a 110mm, -17 stem that I already had. A 120mm would have been too long.

Using the spacers may improve the ability to sell the frame in the future. If I choose, I could get rid of some of the spacer with a -12 stem or all of it with a -6. The -6 might require a 120mm stem.

Seat tube angle only affects the amount of seatpost setback that might be required. In my smaller size, the STA is usually 74-75 degrees. My new frame was advertised as having a 75, but it wasn't. It was 74.5.
I'm not sure you've stated this all correctly. If you were using that 110mm -17 stem before with no spacers and still using it now on a frame with 10mm less reach, 18mm less stack, and using 15mm of spacers under your stem - your effective reach is now 15mm shorter than what you had before and 4mm lower stack.
I guessed some of your angles, but red frame has +10mm reach and +18mm stack compared to green bike in the attached photo.

Also, STA does not just impact saddle setback. You can't ignore seat tube angle if you're looking at top tube length. As I showed in a picture earlier, STA does have an effect on reach - not just on saddle setback. Your saddle setback should be fixed relative to the BB centerline, not the seatpost clamp centerline.
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