View Full Version : Calculating the seat tube angle.
erty65
05-14-2005, 12:21 AM
If the center of the seatpost (780 mm from the bottom bracket) is 220mm behind the center of the bottom bracket, what is the seat tube angle?
Needs Help
05-14-2005, 01:26 AM
73.6
73.6
Is there a simple way to do that calculation for the trigonometrically challenged?
-Ray
Climb01742
05-14-2005, 06:40 AM
if two seat tubes left chicago on two different trains, heading in two directions, with seat tube A traveling at 63 mph and seat tube B traveling at 84 mph, which seat tube would order lunch first?
(sorry, i had a flashback to taking the SATs.) ;)
Kevin
05-14-2005, 06:49 AM
Is there a simple way to do that calculation for the trigonometrically challenged?
-Ray
Go to your local hardware store and buy a digital level. They also sell them at Amazon.com With the bike on level ground put the level on the seat tube and it will give you the seat tube angle. If the seat tube has varying widths, just run a broom handle from the bottom bracket to the top of the seat tube and use the level on the broom stick.
The level is also great for verifying saddle position.
Kevin
I agree with the digital level advice if you have an actual bike in front of you. I'd be interested in the conceptual bike calculation, being too lazy to relearn trigonometry. I know it's all about tangents and such, but when I take it to excel, radians pop up and I lose interest.
BTW, it was on your advice a year or so ago that I bought a digital level to keep my saddles consistent. In my nerdiest moments I take it on the road, literally, to measure what seemed to be particularly steep inclines I've ridden.
keno
unless you have a very flat and level surface on which to place the bike. If the bike is not sitting level, the reading is meaningless.
What you are asking for is a simple solution to a right triangle, where the seat tube is the hypotenuse (longest side), a vetical line through the center of the BB is the side opposite the STA and the setback is the side adjacent to the STA.
The cosine of the STA angle is the adjacent side (220) divided by the hypotenuse (780), which equals .282. Using either a calculator with trig functions or an old fashioned trig table, the angle is 73.6 degrees.
To get the answer from a calculator, first perform the division, then hit the inverse cosine button, which is usually the 2nd funtion of the cosine button.
Kevin
05-14-2005, 09:43 AM
unless you have a very flat and level surface on which to place the bike. If the bike is not sitting level, the reading is meaningless.
What you are asking for is a simple solution to a right triangle, where the seat tube is the hypotenuse (longest side), a vetical line through the center of the BB is the side opposite the STA and the setback is the side adjacent to the STA.
The cosine of the STA angle is the adjacent side (220) divided by the hypotenuse (780), which equals .282. Using either a calculator with trig functions or an old fashioned trig table, the angle is 73.6 degrees.
To get the answer from a calculator, first perform the division, then hit the inverse cosine button, which is usually the 2nd funtion of the cosine button.
I am not that bright. I will stick to placing a couple of magazines, if need be, to have the bike on level ground. Then the digital level is 100% correct everytime.
Kevin
e-RICHIE
05-14-2005, 10:03 AM
call me crazy, but why do you need to know
the angle? you'll never accurately read it to
the minute. why not drop a plumbline from
the saddle and measure in mm linearly to the
bb center. that would be hard to measure
incorrectly, and it will give you a control number
with which all your frames can be dialed in.
slowgoing
05-14-2005, 10:44 AM
A digital level is nice, but the the one I have is finicky if the bike is not exactly upright. Plus you have to have the bike completely built up. It doesn't work for just a frame. A plumb line works well, but then again you have to have the bike built up so the plumb line drops verctically. This is a very hard measurement to make if the frame isn't built up.
Dave, thanks for the trig lesson. I'm think I might have known that at one time....
Climb01742
05-14-2005, 11:49 AM
call me crazy, but why do you need to know
the angle? you'll never accurately read it to
the minute. why not drop a plumbline from
the saddle and measure in mm linearly to the
bb center. that would be hard to measure
incorrectly, and it will give you a control number
with which all your frames can be dialed in.
what r.s. said.
ada@prorider.or
05-14-2005, 11:59 AM
i understand that you want to know the seat angle
i have a suggestion if you mwant the optimum posistion
try to find the optimum force position that the posistion where with the least effort have the max output
and the least stress factors on your body
there must be enough people with power meters to find the max power position
but what some pro's also do
is best analyse with film on body point's for muscle stress calc and best resperation exhange rate position
e-RICHIE
05-14-2005, 12:00 PM
what j.o. said.
Birddog
05-14-2005, 01:15 PM
why not drop a plumbline from
Some people are soooo old fashioned. Go to Lowes and buy a drywall square. It's much easier to use than one of those brass pointy things on a string.
Birddog
a worthless level can be turned into something of great value by first determining the reading on the top tube, assuming it has no slope, and adjusting other measurements accordingly. In the case of a sloping top tube, lay a 2x4 or some other flat object between the points where the wheels hit the floor making sure that the flat object touches next to the wheels (i.e. making sure there isn't a hump between the two points). Measure its angle and similarly adjust angles on the bike.
keno
Climb01742
05-14-2005, 01:36 PM
Some people are soooo old fashioned. Go to Lowes and buy a drywall square. It's much easier to use than one of those brass pointy things on a string.
Birddog
but doesn't this presuppose that your saddle is level? yes, a plumb line can be a bit of a pain to use, but it seems like the most accurate (as long as the floor is level). or maybe it's simply the method i'm most used to using. :rolleyes:
Birddog
05-14-2005, 01:59 PM
but doesn't this presuppose that your saddle is level? yes, a plumb line can be a bit of a pain to use, but it seems like the most accurate (as long as the floor is level). or maybe it's simply the method i'm most used to using.
Actually Climb, I used to use a plumb bob all the time to determine saddle setback. Then one day the light bulb went on (when I noticed out of the corner of my eye the drywall square leaning against the wall). Voila' much easier to use, even if the floor is not exactly level. Now I know that most of us are sufficiently "anal" to actually locate a spot on the garage floor that is pretty level and consistently use this area. BTW, if you are using a plumb bob, more than likely, you'll want to place your bike perpendicular to the garage opening (paralell to the door) as most garage floors slope at least 1/4 inch to the foot some even more. If you use a square, it doesn't really matter if the floor is level. I also use a digital level to adjust the seat angle. Ironically, severla years ago I found the level along the road one morning while riding my bike.
Birddog
Needs Help
05-14-2005, 04:47 PM
Figuring out the seat tube angle with the given information is an application of the definition of the sin of an angle. A right triangle is a triangle with a 90 degree angle:
http://www.pbase.com/image/43363399.jpg
If you have a right triangle, then the sin of an angle is defined to be:
1) The length of the side opposite the angle
divided by:
2) The hypotenuse of the triangle, i.e. the longest side of the triangle.
So, if you have this setup:
http://www.pbase.com/pbase222/image/43363400.jpg
then:
sin a = 220/780
To find out the angle a, you have to take the 'inverse sin' of the whole equation:
a = inverse sin (220/780)
You can use the calculator on your computer to calculate the angle a (windows: Start/Programs/Accessories/Calculator and on the calculator click on View/Scientific). First, divide 220 by 780, and then look around on the calculator for an 'inv' checkbox or button, or a sin -1 button. If it has an 'inv' checkbox or button, first click on that, and then the sin button. If it has a sin -1 button, click on that button. The answer should be: 16.382.... But, 'a' is not where the seat tube angle is measured from:
http://www.pbase.com/pbase222/image/43363401.jpg
'b' is where the seat tube angle is measured from. So,
b = 90 - a
= 73.6
By definition, the angle in the upper left corner of the triangle in your drawing is also the STA. The simpler solution is the inverse cosine of 220/780, since cosine equals the side adjacent/hypotenuse.
e-RICHIE
05-14-2005, 05:46 PM
if you know the angle, what will that tell you?
i have a friend who has a friend, and he once
mentioned that the angle was a means to an
end, not "the" end. if you can more easily and
exactly measure setback to the millimeter using
a common ruler, why not lose the trig?
e-GANDINO
Needs Help
05-14-2005, 06:48 PM
By definition, the angle in the upper left corner of the triangle in your drawing is also the STA. The simpler solution is the inverse cosine of 220/780, since cosine equals the side adjacent/hypotenuse. Good point. However, I probably would have had to draw an additional line on my diagram, and gone into a discussion of the theorem that states: "if you have two parallel lines and another line intersects them, then opposite interior angles are equivalent".
seat tube angle is just a convuluted and stupid way to describe setback. the jerk blames the english for no good reason except that they gave us lots of other stupid ways to measure things like yards, degrees fahrenheit and hogsheads. anyway, taking one measurement on its own doesn't tell you squat about the bike's design anyway.
jerk
Dr. Doofus
05-14-2005, 07:03 PM
17 cm setback doofus
130 itm big one and old shoestring
with knot
plumbs bob nicely
in bed
sinus
need pipe cleaners
water
drano
whole body
hurts
season gone
again
jerk's allergies are killing him too. don't let it phase you, couldn't see, couldn't breathe...the fast kid still guilted the jerk into riding all week. doofus, get on your bike.
jerk
e-RICHIE
05-14-2005, 07:28 PM
how fast is the fast kid these days?
Kevin
05-14-2005, 08:07 PM
****ing fast.
I can do that fast, sometimes too fast. :crap:
My problem is being fast on the bike. ;)
Kevin
Good point. However, I probably would have had to draw an additional line on my diagram, and gone into a discussion of the theorem that states: "if you have two parallel lines and another line intersects them, then opposite interior angles are equivalent".
By defintion, the three angles of a right triangle add up to 180 degrees, so the angle in the upper left hand corner is 90-a, which is the STA. No extra lines required.
Please define, precisely, "setback". I have heard that term used in relation to the nose or back of saddle, but all saddles are different as to nose and overall length. How do you use the term?
Thanks.
Climb01742
05-15-2005, 04:47 AM
"saddle setback" is, as i've been taught, the distance the tip of the saddle is behind the center of the bb. the tip of the saddle is used precisely because saddles vary in length.
"saddle setback" is, as i've been taught, the distance the tip of the saddle is behind the center of the bb. the tip of the saddle is used precisely because saddles vary in length.
When you see the term "setback" used on geometry charts and the like it refers to the distance from the center of the seat tube to the point on the top tube that that intersects perpendicularly with the bottom bracket (on the plomb line). Not to speak for RS but this is the number to which he refers. "Saddle setback" is another one of those confusing terms but if it is used at all, then Climb has it right. Still, this is not a common term and not the one you see used by builders.
dbrk
Climb01742
05-15-2005, 05:22 AM
as always, douglas is correct. there are two (i believe) uses for the term setback. one is a framebuilding term, as douglas says. and there is the one used to position your saddle.
so perhaps there is "frame setback" and "saddle setback". one used to build a frame, and one used after a frame is built. where i might (at my peril :rolleyes: ) diverge from dbrk is how often the term saddle setback is used. i think it's a pretty commonly used term among riders, if not builders. because it helps you position your saddle the same from frame to frame, and on frames with varying STA.
but i'm sure richie will speak for himself soon. he's not shy. ;)
Saddle setback, I believe, IS more commonly used than Douglas says (even if badly) by many riders, I think. But my point, now that I see the two, is that I have always considered saddle setback as a bad measurement masquerading as a good one. Saddles vary quite a bit in how long the nose is from where one sits, so unless this measurement is being used for identical saddles it is prone to all kinds of error.
e-RICHIE
05-15-2005, 09:19 AM
of course i'm assuming that identical saddles are used
for this measurement. i meeeeeannnnn...
saddles, like h'bars represent 1/3 of your contact area
on the bicycle. fit, proper fit, is predicated on that contact
point being correct. you can nitpick this to death, but if
all you want to do is play around with your ruler and your
saddle and are not "that" concerned with proper fit, then
you can find the difference in your many saddles/many
bicycles and use the differences in your math correction.
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