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Different STA, same measurments numbers?

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Different STA, same measurments numbers?

Old 09-03-20, 08:55 AM
  #1  
8aaron8
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Different STA, same measurments numbers?

In order to maintain the same balance over the bike, yes you should maintain the same setback value. Regardless of the seat tube angle, the bb shell is in the parallel to the ground and your position over it is determined by the setback value. Sometimes, people may have to change from a setback seatpost to a straight seatpost as the ST angle gets more shallow so they can clamp the saddle in the center of the rails. As for the bike feeling different, assuming the top tubes on both bikes measure the same, technically the reach on the frame with a 72.5 seat tube angle will be shorter because as the seat tube moves backwards the head tube (connecting to the seat tube by the top tube) will move back as well. So you may need a longer stem to get your body weight further forward to compensate.
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Old 09-03-20, 11:34 AM
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dsaul
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As 8aaron8 said, the saddle setback and height in relation to the BB should be the same regardless of the STA. If both frames have the same top tube length, using the same length stem will cause the reach to be shorter on the bike with the slacker STA. Just set the saddle in the proper location and measure the distance from the tip of the saddle to the bars and adjust the stem length to match. It still may not ride the same, because the front wheel will be closer to the bottom bracket, but the fit should be the same.
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Old 09-03-20, 03:25 PM
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One prime example Folding bikes with a seat tube that passes Behind the BB you push the saddle mount forward
to get the set back like the bike you are trying to match..

Data: a vertical plumb line to the axis line through the crank BB center, is a baseline,,
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Old 09-04-20, 07:12 AM
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Originally Posted by dsaul View Post
As 8aaron8 said, the saddle setback and height in relation to the BB should be the same regardless of the STA. If both frames have the same top tube length, using the same length stem will cause the reach to be shorter on the bike with the slacker STA. Just set the saddle in the proper location and measure the distance from the tip of the saddle to the bars and adjust the stem length to match. It still may not ride the same, because the front wheel will be closer to the bottom bracket, but the fit should be the same.

Ok my new frame is 60cm and the older one was 57cm I dont know if the top tubes are/were the same. I have the same size stem I think the stack maybe lower slightly. Reason being is I had the balance test from Steve hoggs fitting, and I got an weightlifting sensation when riding around threshold I dont get that on my new 60cm frame
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Old 09-04-20, 07:49 AM
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If you want to match your position on the new bike, you must start with setting the saddle at the same height and setback from the bottom bracket. Measure the distance from the saddle tip(assuming you are using the same saddle on both bikes) to the handlebars on the old bike. Also measure the drop from the top of the saddle to the handlebars. Use whatever length stem and spacers necessary to put the handlebars in the same place, relative to the saddle, on the new bike. That will match your position/body angles from the old bike to the new. The ride characteristics may not be the same, but your body will be in the same position.

This is not really a framebuilding question and you may be better served in the Fitting Your Bike forum.

Last edited by dsaul; 09-04-20 at 07:54 AM.
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Old 09-04-20, 02:42 PM
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Originally Posted by 8aaron8 View Post
technically the reach on the frame with a 72.5 seat tube angle will be shorter because as the seat tube moves backwards the head tube (connecting to the seat tube by the top tube) will move back as well. So you may need a longer stem to get your body weight further forward to compensate.
for every half degree how many cms difference?
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Old 09-04-20, 03:00 PM
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unterhausen
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Did you say what your seat height is?

Do you have a drywall t-square?

On edit: moved here from framebuilders. OP, please don't cross post, request to move.

Last edited by unterhausen; 09-04-20 at 03:07 PM.
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Old 09-04-20, 03:17 PM
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Originally Posted by unterhausen View Post
Did you say what your seat height is?

Do you have a drywall t-square?

On edit: moved here from framebuilders. OP, please don't cross post, request to move.
81.5cm centre bb to top of saddle and I do not have a T square
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Old 11-11-21, 06:17 PM
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Originally Posted by 8aaron8 View Post
In order to maintain the same balance over the bike, yes you should maintain the same setback value. Regardless of the seat tube angle, the bb shell is in the parallel to the ground and your position over it is determined by the setback value. Sometimes, people may have to change from a setback seatpost to a straight seatpost as the ST angle gets more shallow so they can clamp the saddle in the center of the rails. As for the bike feeling different, assuming the top tubes on both bikes measure the same, technically the reach on the frame with a 72.5 seat tube angle will be shorter because as the seat tube moves backwards the head tube (connecting to the seat tube by the top tube) will move back as well. So you may need a longer stem to get your body weight further forward to compensate.
Revisting this...

"So you may need a longer stem to get your body weight further forward to compensate."

If my old measurement was 60cm from tip of saddle to the center of stem on the 73seatube would I be ok with a new measurement of like 62cm on my new frame with the 72.5 seat angle to keep same torso/arm angles? or would I be longer,/more stretched out dispite everything else staying the same?

Because measurement 60cm on my new frame feels shorter..... compared to the 73 degree seat angle on the older frame.. if i get to like 62cm it feels like more the same...

Last edited by Sam12345; 11-11-21 at 06:22 PM.
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Old 11-12-21, 09:39 AM
  #10  
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Sam12345

If you want the same set back from the BB as your other bike, then you need to imagine a vertical off the BB and then measure where the saddle is to that. Then reach and bar drop get corrected from that.

However if this is a different style bike for different type of riding... IE, leisure vs high effort fitness, then I'd dispute any thought that saddle setback to the BB needs to be the same.
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Old 11-12-21, 11:50 AM
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Originally Posted by Iride01 View Post
Sam12345

If you want the same set back from the BB as your other bike, then you need to imagine a vertical off the BB and then measure where the saddle is to that. Then reach and bar drop get corrected from that.

However if this is a different style bike for different type of riding... IE, leisure vs high effort fitness, then I'd dispute any thought that saddle setback to the BB needs to be the same.
It was originally a forme victeur either 56cm, 57, or a 58 frame I cant remember... now I own a planet X RT80, 60cm frame....

It was 9cm behind the BB on the forme victeur 81.5 cm saddle height, 1.5mm of spacers, and 140 stem, it had the additional 1.5-2m spacer underneath the mainspacers...
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Old 11-12-21, 12:07 PM
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I'm really not sure what you are trying to do or duplicate. If you are trying to pick a frame size before you buy, then best of luck to you. I try them out first. But there are websites that can let you compare bike geometries in a visual way and see one image superimposed on the other. I've played with one years ago, but it wasn't all the help I expected it to be. And I had to put in all the exacting measurements for each frame.

As for saddle setback, I don't try to base that on the other bikes, I just ride the bike and move it till it's where I want it. If when I pedal hard I slide too far forward in an unintended way, then I try to decide if that's the dynamic forces of pedaling trying to keep me in the right relation to the BB or if my reach is off. Might be tilt for some if they angle their saddles, I keep my saddle pretty level.
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Old 11-12-21, 12:16 PM
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Originally Posted by Iride01 View Post
I'm really not sure what you are trying to do or duplicate. If you are trying to pick a frame size before you buy, then best of luck to you. I try them out first. But there are websites that can let you compare bike geometries in a visual way and see one image superimposed on the other. I've played with one years ago, but it wasn't all the help I expected it to be. And I had to put in all the exacting measurements for each frame.

As for saddle setback, I don't try to base that on the other bikes, I just ride the bike and move it till it's where I want it. If when I pedal hard I slide too far forward in an unintended way, then I try to decide if that's the dynamic forces of pedaling trying to keep me in the right relation to the BB or if my reach is off. Might be tilt for some if they angle their saddles, I keep my saddle pretty level.
Yeh I'm aiming to get the same position that allowed me to get "unweighting sensation" when pedaling hard that Steve Hoggs talks about..

I got it my forme victeur... But cant seems to get it on my planet X frame... which is why I'm trying to get the same measurements... using old number measurements to the planet X
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