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  #1  
Old 12-10-2019, 02:34 PM
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BRad704 BRad704 is offline
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Saddle position/setback on a new frame

I just need a mental double-check on something...

I bought a new (used) bike frame and both bikes are 73-degree seat tube angles. 2009 Fuji Team Carbon to a newer 2014 Trek Madone 4.9

I was fitted on the Fuji before, so does this mean I can take that same seatpost from the Fuji, drop it in the Madone at the proper seat height and everything will be the same?

I feel like I know the answer is YES, but the Madone feels way further back over the rear wheel...
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Old 12-10-2019, 02:35 PM
benb benb is offline
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Yes if they're the same angle.

You'd feel more over the rear wheel if the Fuji had longer chainstays and the Trek had shorter chainstays.

You could probably feel that way if the Trek had a longer reach/lower stack too.
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Old 12-10-2019, 02:43 PM
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BRad704 BRad704 is offline
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Quote:
Originally Posted by benb View Post
Yes if they're the same angle.

You'd feel more over the rear wheel if the Fuji had longer chainstays and the Trek had shorter chainstays.

You could probably feel that way if the Trek had a longer reach/lower stack too.
Thanks for the quick info. I'll look deeper into the geo for both next.

I've already put a 0-setback post on the Madone based on "feeling", but that might have been unecessary...
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Old 12-10-2019, 02:53 PM
benb benb is offline
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You should get accustomed to sticking the bike's rear wheel in a corner and measuring the X/Y position of the BB and the X/Y position of the front/top of the saddle. It allows you to adjust two bikes to fit exactly the same with quite a bit of precision.

You are looking to measure "Y" as the vertical distance from the center of the BB to the top of the saddle.

X is the horizontal distance the saddle is behind the BB.

It takes all the guesswork out of the old fashioned method of measuring up the seat tube.

I think Lennard Zinn or someone has a webpage about it.

But for two bikes with the same STA this should be really easy.
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Old 12-10-2019, 03:36 PM
kgreene10 kgreene10 is offline
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Quote:
Originally Posted by benb View Post
You should get accustomed to sticking the bike's rear wheel in a corner and measuring the X/Y position of the BB and the X/Y position of the front/top of the saddle. It allows you to adjust two bikes to fit exactly the same with quite a bit of precision.

You are looking to measure "Y" as the vertical distance from the center of the BB to the top of the saddle.
On two bikes with the same STA, all three sides of the triangle will be the same length - from center of BB to top of saddle along the STA, the vertical height of the saddle from the BB, and the distance of the horizontal line connecting the other two sides of the triangle.

If you go to a bike with a different STA, the vertical distance should remain the same. To make sure that it does, you can calculate the distance up the STA by putting the following in Excel =(vertical height from old bike) / COS(RADIANS(90-STA of new bike).

If you only know the height along the STA from the old bike and want to know the vertical height, use =(height up the STA) x COS(RADIANS(90-STA)).
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Old 12-10-2019, 03:52 PM
zennmotion zennmotion is offline
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There's probably different (and maybe easier?) ways to replicate saddle setback, but I use a construction bubble level (just a cheap 24 inch plastic thing from Home Depot, is this why my DIY remodel projects are off-square?) and a plumb line (piece of string with a fishing sinker or just a large nut tied to the end of a 4ft long piece of string) and a tape measure. You might need an extra pair of hands-

1) dangle the plumb bob so the string exactly lines up with the BB center
2) rest one end of the bubble level on the tip of the saddle to be sure you're measuring a horizontal line
3)With the tape measure, measure the distance between the saddle tip and the string along the horizontal bubble level

This works if you're using the same saddle on both bikes. If your saddles are different then the next best thing is to measure along a virtual horizontal line to the center of the seatpost to compare setbacks between two frames, but then there is some minor, though imprecise, adjustment to set a different saddle in the "same" position- no two saddle models feel the same. If your top tube is horizontal, no need for the bubble level, just mark where the string touches the top tube for the center point

Like I said, there's probably an easier way, but this uses what I have handy near my work bench.

Last edited by zennmotion; 12-10-2019 at 03:56 PM.
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Old 12-10-2019, 03:57 PM
Dave Dave is offline
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You should have the your body positioned at the same position, relative to the BB. If I have two bikes with the same saddle, just drop a plumb bob from the saddle nose and measure how far back it is from the crank center line.

If something feels off after that, then it's the reach or drop that need changing.

If you have your body too far forward, which will probably be the case with a zero setback post, you should feel too much weight on your hands.
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Old 12-10-2019, 04:26 PM
Ralph Ralph is offline
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Quote:
Originally Posted by Dave View Post
You should have the your body positioned at the same position, relative to the BB. If I have two bikes with the same saddle, just drop a plumb bob from the saddle nose and measure how far back it is from the crank center line.

If something feels off after that, then it's the reach or drop that need changing.

If you have your body too far forward, which will probably be the case with a zero setback post, you should feel too much weight on your hands.
I do it this way also. Plumb Bob (actually a socket with a string which I tie to seat frame and hang off nose of seat. Then measure how far back from BB center) I lean the bike over sideways.....and prop it up with a piece of furring strip. Tie an old pedal strap around front wheel to hold wheel straight. make sure floor level.
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  #9  
Old 12-10-2019, 04:41 PM
OtayBW OtayBW is offline
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I do all of this (^, ^^, ^^^) with a drywall square held with the short end on the floor. Gives you an accurate saddle position behing BB center accounting for floor slope as well....
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  #10  
Old 12-10-2019, 04:48 PM
grateful grateful is offline
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Is that you Mr. Cox (my high school math teacher)?

Quote:
Originally Posted by kgreene10 View Post
On two bikes with the same STA, all three sides of the triangle will be the same length - from center of BB to top of saddle along the STA, the vertical height of the saddle from the BB, and the distance of the horizontal line connecting the other two sides of the triangle.

If you go to a bike with a different STA, the vertical distance should remain the same. To make sure that it does, you can calculate the distance up the STA by putting the following in Excel =(vertical height from old bike) / COS(RADIANS(90-STA of new bike).

If you only know the height along the STA from the old bike and want to know the vertical height, use =(height up the STA) x COS(RADIANS(90-STA)).
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  #11  
Old 12-11-2019, 07:18 AM
chiasticon chiasticon is online now
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Quote:
Originally Posted by benb View Post
You should get accustomed to sticking the bike's rear wheel in a corner and measuring the X/Y position of the BB and the X/Y position of the front/top of the saddle.
this is my approach too. measure from wall and floor to three things: BB center, saddle tip, bars. this gives you three X/Y's. subtract the BB center from the other two; this makes the BB center the origin (i.e. coordinate 0,0) of an X/Y plot. if two bikes are setup the same, these two will be the same. if not, you know by how much you need to adjust.

BUT: if I know the two seat angles are the same, and I'm using the same post setback and saddle, I don't do any of this. just use the lines on the saddle to get them in the same fore/aft spot, then measure from BB up the seat tube to get the height the same.
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  #12  
Old 12-11-2019, 07:51 AM
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wallymann wallymann is online now
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+1 for drywall square.

Quote:
Originally Posted by OtayBW View Post
I do all of this (^, ^^, ^^^) with a drywall square held with the short end on the floor. Gives you an accurate saddle position behing BB center accounting for floor slope as well....
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  #13  
Old 12-11-2019, 09:15 AM
Dave Dave is offline
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Wavy concrete floors make for unreliable measurements with a drywall square. With a plumb bob, the floor doesn't have to be level or free of waviness, if the two bikes being compared are placed in the same spot for comparison.
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  #14  
Old 12-11-2019, 09:30 AM
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BRad704 BRad704 is offline
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Yeah I did the plumbbob method when I originally set this bike up, but it never "felt" right. I'll get my measurements from my fitter and make sure to put the setback back to where it should be.
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  #15  
Old 12-11-2019, 09:31 AM
kgreene10 kgreene10 is offline
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Quote:
Originally Posted by grateful View Post
Is that you Mr. Cox (my high school math teacher)?
Busted! (Not math, but teacher in your fair city.)
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