seat tube angle trig

[eddief]

my worst sport in school was math.

my Curtlo has 72.25 seat tube angle.
my Roubaix has 73.5 seat tube angle.

What is the difference in setback in mm? Top of my saddle is 74mm above the middle of the bb.

Is this even a valid question?

I have done many things within my power to duplicate my position on the Roubaix to be as comfortable as the Curtlo. So far, not so good. I know the Roubaix has steeper angles and a bit less bb drop too but thought I'd consider more setback first since that is in my control to some degree.

[tuscanyswe]

http://yojimg.net/bike/web_tools/stem.php

You can use this for The seattube angles as well even tho designed for stem.

I get 16mm diff in setback at 74cm

[donevwil]

1697990597

[eddief]

it's a beautiful thing.

[mhespenheide]

Eddie,

The posts above are correct.

But the easiest way to set up the two bikes identically (if they have the same length of crankarms and the same saddle) is to:

1. Set the Curtlo up with the rear wheel against a wall.
2. Measure the distance to the bottom bracket.
3. Measure the distance to the front tip of the saddle.
4. Subtract [3] from [2]. Call this the "Curtlo setback".
5. Put the Roubaix against the wall.
6. Measure the distance to the bottom bracket.
7. Subtract the Curtlo setback. This is the distance the tip of the saddle needs to be away from the wall for the Roubaix.
8. Move the saddle on the Roubaix foreward or aft until it matches what you calculated in step [7].
9. You may need to adjust the saddle height slightly, then iterate steps [8] and [9] to get everything within a millimeter or so.

If that doesn't make sense, let me know what I can help clarify.

[eddief]

that makes good sense...and I did that this afternoon. interestingly enough the saddle on the Roubaix is all the way back on a setback post and I still feel bunched up. it's a WTB saddle and I think has relatively short rails. so i need to try a different saddle or find a post with more setback. think i have a lead on a post. that will be the next experiment. I need about another 1.5 cm to get it where I want it.

Eddie,

The posts above are correct.

But the easiest way to set up the two bikes identically (if they have the same length of crankarms and the same saddle) is to:

1. Set the Curtlo up with the rear wheel against a wall.
2. Measure the distance to the bottom bracket.
3. Measure the distance to the front tip of the saddle.
4. Subtract [3] from [2]. Call this the "Curtlo setback".
5. Put the Roubaix against the wall.
6. Measure the distance to the bottom bracket.
7. Subtract the Curtlo setback. This is the distance the tip of the saddle needs to be away from the wall for the Roubaix.
8. Move the saddle on the Roubaix foreward or aft until it matches what you calculated in step [7].
9. You may need to adjust the saddle height slightly, then iterate steps [8] and [9] to get everything within a millimeter or so.

If that doesn't make sense, let me know what I can help clarify.

[dddd]

that makes good sense...and I did that this afternoon. interestingly enough the saddle on the Roubaix is all the way back on a setback post and I still feel bunched up. it's a WTB saddle and I think has relatively short rails. so i need to try a different saddle or find a post with more setback. think i have a lead on a post. that will be the next experiment. I need about another 1.5 cm to get it where I want it.


I'm thinking that maybe you just need to raise the saddle on the Roubaix.

The most accurate indicator of saddle height being too low for me is when I am tending to slide back past the rear edge of the saddle.
Intuitively I want to move the saddle rearward, but raising it is usually the correct action.

[marciero]

http://yojimg.net/bike/web_tools/stem.php

You can use this for The seattube angles as well even tho designed for stem.

I get 16mm diff in setback at 74cm

Not sure how you obtained this but it's too far off the correct amount to be rounding error.

[fignon's barber]

I think the easiest way to remember this calculation is: one degree equals one centimeter. If it's a minus one degree, it's 1cm setback. If it's plus one degree, the reach is 1cm longer. Same for HT angle. There are so many variables in bike manufacturing and assembly, as well as the human body, that make the .5mm irrelevant.

[tuscanyswe]

Not sure how you obtained this but it's too far off the correct amount to be rounding error.

Hmm interesting.

I just know from experience that it would be approx one and a half cm so when i entered the numbers into the calculator and they came out as 16mm i just assumed there was nothing i missed as it made sense but like u say its weird and likely something i did not think of at play.

[marciero]

If you are trying to replicate positions; that is, have the saddle in the same position relative to bb and handlebars, when going from the shallower to the steeper seat tube angle you need to lower your saddle, as well as slide it backward on the rails. For your 74 cm saddle height the amounts, rounded to one decimal are

lower saddle 5.0 mm. (this is a distance along the seat tube and not vertical height; that is, simply sliding seatpost into the seat tube this amount)

slide saddle back 16.8 mm. This assumes that your rails are horizontal. If you have the saddle tilted it will be different. Notice that this amount is larger than in donevwil's very nice diagram. That's because you've lowered the saddle and so the top endpoint of the right hand seat height in the diagram moves down and a little to the right.

[marciero]

I think the easiest way to remember this calculation is: one degree equals one centimeter. If it's a minus one degree, it's 1cm setback. If it's plus one degree, the reach is 1cm longer. Same for HT angle. There are so many variables in bike manufacturing and assembly, as well as the human body, that make the .5mm irrelevant.

Yes and no. If we've considered all the variables, why not have an accurate baseline to start, as well as a way of understanding bike geo? I typically round to one more decimal than I would use in practice, then round accordingly. So 16.8 would get rounded to 17mm. Most of us can feel a 1mm difference-16 vs 17mm. On the other hand, manufacturing tolerances do come into play (though I would hope that they are within 0.5mm!), as does our ability to measure these dimensions. So in that sense the 0.5 mm does not have as much practical relevance. I think the rules of thumb such as you cite are fine, or even preferable, for making adjustments in practice. I certainly fine tune my adjustments by feel rather than by measuring.

[dddd]

Another detail is that if two different saddles are on these bikes, the reference positions along the length of each saddle could be very different, corresponding to where the rider's body is actually bearing it's weight.
This brings the saddle's width into play, as well as the saddle's flare angle and top radii along both directions, not to mention the saddle's dimensions under any compressible padding.
So don't expect any measurement points to be accurately transferred with two different models of saddle.

Since I have several different saddles in play on various bikes, I rely on my ability to make relatively gross changes initially by feel, so I recommend simply experimenting with the adjustments since you might end up with better positioning than on the other bike!
But I always start with a look at the bike's geo numbers AND bring the bb center up to a vertical wall corner inside of my house, from where I can at least measure forward to the handlebar center. From there I can then approximately position the saddle by measuring back from the bars.

Don't forget to also consider differences that may exist between different bends of handlebar, and perhaps of how the bars position the levers.
For these sorts of reasons (saddle, bar and frame differences), I tend not to have any two bikes that are really quite identical in terms of "measured" fit. I finalize things by doing plenty of riding on the same bike for several days, then noticing how (for better and/or for worse) that the fit and comfort differ from the next (already fitted) bike that I ride. Often as not, it is one of these other bikes that ends up getting the final tweak!
All of this sort of back-and-forth seems to eventually result in a fleet of better-fitting bikes, even though they are not set up dimensionally identical.

[cyan]

Eddie,

The posts above are correct.

But the easiest way to set up the two bikes identically (if they have the same length of crankarms and the same saddle) is to:

1. Set the Curtlo up with the rear wheel against a wall.
2. Measure the distance to the bottom bracket.
3. Measure the distance to the front tip of the saddle.
4. Subtract [3] from [2]. Call this the "Curtlo setback".
5. Put the Roubaix against the wall.
6. Measure the distance to the bottom bracket.
7. Subtract the Curtlo setback. This is the distance the tip of the saddle needs to be away from the wall for the Roubaix.
8. Move the saddle on the Roubaix foreward or aft until it matches what you calculated in step [7].
9. You may need to adjust the saddle height slightly, then iterate steps [8] and [9] to get everything within a millimeter or so.

If that doesn't make sense, let me know what I can help clarify.

What if the saddle is the same but the crank length is different? For example, if the crank is shortened, shall we:

a. Raise the saddle by the shortened length
b. Move the saddle towards the rear by the same shortened length or something else (shortened length minus the horizontal shift caused by raising the saddle)?

[mhespenheide]

What if the saddle is the same but the crank length is different? For example, if the crank is shortened, shall we:

a. Raise the saddle by the shortened length
b. Move the saddle towards the rear by the same shortened length or something else (shortened length minus the horizontal shift caused by raising the saddle)?

If the crank length is shortened by 5mm, you would implicitly
a. Raise the saddle by 5mm
b. Which inherently moves the saddle back 5mm*cos(STA)
c. So you should move the saddle forward...
d. But the pedal has also moved back by 5mm already relative to where the saddle used to be. So to keep the same position over the pedal, you want to move back 5mm. You've already moved back 5mm*cos(STA), so you need to move back more by 5mm - 5mm*cos(STA).

To a decent first approximation for seat tube angles between 72 to 74 degrees, changing the crankarm length by 2.5mm requires a change in the saddle setback by ~1.8mm: back for shorter cranks, forward for longer cranks.

[dddd]

Shortening crankarm length might not necessitate a change in saddle height, rather the shorter crankarms permit a wider range of saddle height, such that the exact height setting becomes more forgiving in either direction.

I ride a 30" saddle height (center to saddle top) and really prefer 170 arms myself.