Saddle position/setback on a new frame

[BRad704]

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...

[benb]

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.

[BRad704]

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...

[benb]

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.

[kgreene10]

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)).

[zennmotion]

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.

[Dave]

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.

[Ralph]

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.

[OtayBW]

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....

[grateful]

Is that you Mr. Cox (my high school math teacher)?

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)).

[chiasticon]

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.

[wallymann]

+1 for drywall square.

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....

[Dave]

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.

[BRad704]

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.

[kgreene10]

Is that you Mr. Cox (my high school math teacher)?

Busted! (Not math, but teacher in your fair city.)

[avalonracing]

You have to base all measurements from the center of the BB as this is fixed. A bike with a higher or lower BB will make everything else feel a bit different unless you measure from there. This is the kind of craziness I've gone to get positions matched. It is a negative image of my Merlin layered over my Klein when trying to match the fit. I saw once I lifted the Merlin's lower BB to match the high BB on the old Klein (with vintage criterium geometry) it was making the bar height on the Merlin higher even though the bike height was the same when measured side by side. You can see the difference between the 74º STA on the Klein vs. the 73ºSTA on the Merlin (the Merlin saddle was slid forward to match the Klein)

[OtayBW]

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.
Who says we're doing it on a wavy concrete floor? Geez...

[Mzilliox]

the answer is yes

your saddle will be in the same spot relative to the BB, which is where you want to be for pedaling efficiency.

now does this mean you may feel further forward or backward in relation to the front center? or front wheel? sure, the rear wheel may be further back or further forward on one bike. the fork on one bike may have more or less rake, putting the front wheel further from you, or closer to you, all of these things could make you feel like you are sitting further back on the madone.

so maybe the madone has a rear wheel tucked in closer? or maybe the head tube is more slack?

im kinda surprised theres that much difference

[Dave]

Who says we're doing it on a wavy concrete floor? Geez...

I've never seen a concrete floor that wasn't wavy. They screed the concrete with a 2×4, then hand trowel the concrete to make a smooth surface, but it's never very flat.

Get a 6 foot level and place it on the floor. With feeler gauges, you'll probably find some significant ups and downs.

Working in the machine shop business for many yeas gives me an appreciation for a flat surface.

Building a house showed me how uneven most concrete floors really are.

If you laid a 4x8 of 3/4" MDF on the floor, it would make a more suitable measuring surface.

[benb]

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.

The plumb bob is worse if the floor is not level if you start doing the math for what kind of error is induced if the floor is not level.

The plumb bob is hanging from a long piece of string from the saddle down past the BB. The saddle setback is a small # compared to the length of string, so if the floor is not level the error is a high % of the actual setback.

If you measure out from the wall you might not get the measuring tape/stick level but one side of the triangle (height) is very very small if you're not holding the measuring device level.. you're making a very small change in the hypotenuse of the triangle and that's the error. The error is very small compared to the length from the BB/saddle tip to the wall.

For the height.. I was never advocating measuring that up from the floor. Do the math for your STA and measure up from the BB through the saddle. But realize the distance needs to change on different bikes with different STAs so that you keep the Y distance the same.

I see your point though that if you put the 2 bikes in the same spot on the same floor that reduces the error on the plumb bob. But what if the bikes have different wheelbases and are differently affected by the wavy floor? You're back to not controlling that.

Obviously a lot of this is overthinking things! But plumb bobs also get caught on chainstays and such and don't hang quite vertical too.

I think I only have a 3' level at max and that's enough to easily see the floor is all kinds of wavy and not level in the garage.

[OtayBW]

I've never seen a concrete floor that wasn't wavy. They screed the concrete with a 2×4, then hand trowel the concrete to make a smooth surface, but it's never very flat.

Get a 6 foot level and place it on the floor. With feeler gauges, you'll probably find some significant ups and downs.

Working in the machine shop business for many yeas gives me an appreciation for a flat surface.

Building a house showed me how uneven most concrete floors really are.

If you laid a 4x8 of 3/4" MDF on the floor, it would make a more suitable measuring surface.Who says I'm even on concrete? What makes you think that I have not considered all of of this already? I already use a 6 ft level and I work in a suitable place, and I am aware of the precision and bias in the measurements. I also don't care for the plumb bob method, but that's just me. Geez man....

[Dave]

Who says I'm even on concrete? What makes you think that I have not considered all of of this already? I already use a 6 ft level and I work in a suitable place, and I am aware of the precision and bias in the measurements. I also don't care for the plumb bob method, but that's just me. Geez man....

Excuse me! I must have read that into your post. In that case, it's something for those working in the basement or garage to consider. I've never worked on a bike on any other type of floor.

The spot in my workshop where I make this sort of measurement shows perfectly level over 6 feet, but it's sure got some up and downs over that length.