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  #1  
Old 09-24-2020, 04:24 PM
Nick12 Nick12 is offline
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Seat tube angle impact on reach

Hello All,

Can someone help me understand how seat tube angle impacts reach?

I've been playing around on bikegeo.net and noticed that as I increase the seat tube angle (moving from 74' to 75' in my case), with all else being equal, the reach also increased. I would have thought that the reach would have decreased as I'm effectively moving the saddle closer to the handles. Is this not how I should think about it?

Sorry if this is glaringly obvious, but I've only recently started spending a bit more time paying attention to bike geo's and how it impacts handling, fit, etc.

Also, related, if I'm moving from a bike with a ST angle of 73' to 75', all else being equal, can I calculate how much setback I should get on my seat post? Would you say 10mm is enough or should I aim for closer to 20mm? It seems like every 0.1' change in ST angle results in a direct 1mm change in reach, but I'm not sure how that translates in setback measurements (as its behind the BB).

Thanks in advance for your help!
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Old 09-24-2020, 05:25 PM
robt57 robt57 is offline
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It only effects saddle setback limits technically.

I think I have a link for range effects of different STA. will look for it...


"can I calculate how much setback I should get on my seat post?"

Using a plumb bob for exact set back pretty much.

Also can put the rear wheel against the wall and measure to seatpost clamp.


Linl: http://www.csgnetwork.com/righttricalc.html
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Last edited by robt57; 09-24-2020 at 05:41 PM.
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  #3  
Old 09-24-2020, 05:51 PM
Nick12 Nick12 is offline
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Originally Posted by robt57 View Post

Also can put the rear wheel against the wall and measure to seatpost clamp.


Clever! Hadn’t thought about that.


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  #4  
Old 09-24-2020, 06:35 PM
robt57 robt57 is offline
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Clever! Hadn’t thought about that.


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This is close enough for guv work: For a 67cm seat height, every 0.5 degrees difference is approximately 5mm difference in setback. So, for every 0.5 degree steeper, you'll be in the ballpark of moving your saddle setback backwards by ~5mm.
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Old 09-24-2020, 06:41 PM
robt57 robt57 is offline
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Originally Posted by Nick12 View Post
Clever! Hadn’t thought about that.


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Just remember chain stays vary, so compare the mearsurement from the wall to the BB/Center to the wall to clamp range on saddle.

I use a plum bob on level ground personally. My set back across many bikes is certainty.
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  #6  
Old 09-24-2020, 07:08 PM
Nick12 Nick12 is offline
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Thank you! Very helpful!


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  #7  
Old 09-24-2020, 10:10 PM
slambers3 slambers3 is offline
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Quote:
Originally Posted by Nick12 View Post
Hello All,

Can someone help me understand how seat tube angle impacts reach?

I've been playing around on bikegeo.net and noticed that as I increase the seat tube angle (moving from 74' to 75' in my case), with all else being equal, the reach also increased. I would have thought that the reach would have decreased as I'm effectively moving the saddle closer to the handles. Is this not how I should think about it?

Sorry if this is glaringly obvious, but I've only recently started spending a bit more time paying attention to bike geo's and how it impacts handling, fit, etc.

Also, related, if I'm moving from a bike with a ST angle of 73' to 75', all else being equal, can I calculate how much setback I should get on my seat post? Would you say 10mm is enough or should I aim for closer to 20mm? It seems like every 0.1' change in ST angle results in a direct 1mm change in reach, but I'm not sure how that translates in setback measurements (as its behind the BB).

Thanks in advance for your help!
If you’re increasing the seat tube angle while the top tube length is constant, reach will increase due to the fact that now less of that top tube is behind the bottom bracket. Hope this makes sense.
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Old 09-24-2020, 11:53 PM
giordana93 giordana93 is offline
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Quote:
Originally Posted by slambers3 View Post
If you’re increasing the seat tube angle while the top tube length is constant, reach will increase due to the fact that now less of that top tube is behind the bottom bracket. Hope this makes sense.
in a nutshell, this is the answer, but it assumes (correctly) that you are controlling for your own setback. So if you keep the same setback--let's say saddle nose 6cm behind the bottom bracket center--more of the top tube is behind that point on a slacker sta, effectively shortening the top tube.

the real question is why you would want to make a pretty radical jump from 73 to 75. Unless you are pretty short or trying to adopt a time trialist super forward position, that's pretty steep (and will require a pretty offset seatpost, like 25mm, unless you are on a zero post now on the 73, in which case you might be ok)
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  #9  
Old 09-25-2020, 02:50 PM
Nick12 Nick12 is offline
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Thanks @slambers3 and @giordana93 .. it is a little odd to visualize the TT getting longer when the ST is getting straighter, but I think I'm following!
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  #10  
Old 09-25-2020, 02:53 PM
Nick12 Nick12 is offline
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Quote:
Originally Posted by giordana93 View Post
the real question is why you would want to make a pretty radical jump from 73 to 75. Unless you are pretty short or trying to adopt a time trialist super forward position, that's pretty steep (and will require a pretty offset seatpost, like 25mm, unless you are on a zero post now on the 73, in which case you might be ok)
And a very good question indeed. I'm really interested in a Bishop but its a size 52, which sounds like its too small when compared to my ideal size 54 (taken from the Specialized Allez), but when mapping the geo side by side, the Bishop has a pretty tall HT and fork A2C, which helps the stack, and the steep ST angle helps the reach (although I'd need some set back so the reach would be shortened).
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  #11  
Old 09-26-2020, 11:27 AM
giordana93 giordana93 is offline
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what is your current setback #? leg length? Height?


it happens all the time that people fall in love with a frame and try to rationalize a possibly ill-fitting geometry. just make sure you're not one of them. if you end up with a short tall stem and scouring the internet for an extra setback seatpost, you'll make an ugly duckling out of that pretty frame

not saying that is the case here, just be careful
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  #12  
Old 09-26-2020, 06:26 PM
Nick12 Nick12 is offline
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No... you’re probably right. I am trying to justify it, and run the risk of having a wonky set up or, even worse, injuring myself.

My setback is 67mm on my current bike, which has a 74’ ST angle, my inseam is 87cm and my height is 177.8cm.

The prospective bike has an ETT of 52.5 and with the ST angle at 75’, it looks like it might have enough reach. With a HT length of 130mm and 45mm of headset and spacers, should also be enough stack.

Thoughts?


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  #13  
Old 09-26-2020, 07:34 PM
giordana93 giordana93 is offline
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it's too small. I'm 1.75m with short legs, about 80cm. I ride a 52 and probably about as tall as one could fit on a 52. I also have a pretty aggressive position, which a small-ish frame works for.

It will be a challenge, if not impossible, to get 67mm of setback on that bike, depending on seat and post. I had a 52 with a 75sta and even with a 25mm setback post and saddle pushed all the way back, the setback was around 45 and the rails of the saddle were way past where they should have been for proper support (i.e. rails risked being bent). You'll get a little more from the higher saddle but not enough (I'm about 71cm saddle height from bb to saddle top).Your seatpost will be jacked up sky high and 2 inches of spacers under the stem will scream the bike is too small for the rider. If you're really lucky it won't handle like crap, but with that steep angle, it was designed to get someone shorter closer to the bottom bracket and unlikely to feel balanced under someone of your dimensions. I would keep looking--another beauty is just around the corner.
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  #14  
Old 09-26-2020, 08:00 PM
Nick12 Nick12 is offline
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Thanks for the feedback and insight. Its very helpful! You’ve saved me from making a tragic mistake!

Will keep looking... there’s a 54 with my name on it out there!


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