Degrees of Seat Tube Angle = cm of top tube length
Steve Freides
seat tube angle by one degree, from 73 to 74 degrees, approximately how
much closer is one putting the seat tube to the head tube? I assume
that shorter seat tubes would move the seat tube less and longer ones
more.
-S-
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+ For those of you who don't see newsgroup headers, I am +
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+ Steve Freides +
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PPWrench
Peter
JYates605
>
>On the "average" 56cm road bike with a 56cm top tube, if one changed the
>seat tube angle by one degree, from 73 to 74 degrees, approximately how
>much closer is one putting the seat tube to the >head tube?
"Average" does not matter, nor does the top tube length. For a 56cm seat tube
(c-c for instance), changing the ST angle from 73' to 74' moves the c-c
crossing point forward .93cm.
Now, if you stipulate a 56cm top tube, then the seat lug doesn't move closer to
the head tube at all. 56cm measured from the seat lug is 56cm, no matter what
the ST angle. But if you take a given frame design with a 56cm ST, a 56cm TT,
and a 73' seat angle, then change the seat angle to 74', the TT shortens .93cm.
Jon Schaer
jyat...@aol.com
jver...@lucent.com
sj...@idt.net wrote:
> On the "average" 56cm road bike with a 56cm top tube, if one changed the
> seat tube angle by one degree, from 73 to 74 degrees, approximately how
> that shorter seat tubes would move the seat tube less and longer ones
> more.
>
> -S-
>
> --
> +----------------------------------------------------------+
> + +
> + For those of you who don't see newsgroup headers, I am +
> + +
> + Steve Freides +
> + +
> +----------------------------------------------------------+
Draw a right triangle as follows: The (56cm) seat tube is the hypotenuse, a
line drawn from the top tube down straight (vertically) through the BB is one
leg, the section of top tube between the seat cluster and the point on the
top tube through which you drew the first leg is the second leg.
What you see from this triangle is that the measurement you are interested in
is the second leg you drew, the short one between the seat cluster and the
point on the top tube through which you drew the first (vertical) leg.
Specifically, you are interested in the difference in the length of this leg
if the angle down at the BB is 16 or 17 degrees (that's 90 - 74 and 90 - 73
respectively).
Since you have two angles (you drew a right triangle, the angle up on the top
tube is 90 degrees) and the length of the hypotenuse, you can figure out
anything else you want...and what you want is the length of the top leg.
Using SOHCAHTOA to remember high school trigonometry, we see:
Sin 17 degrees = X/56cm where X is the length of the leg we're interested in
for the 73 degree frame, and similarly:
Sin 16 degrees = Y/56cm where Y is the length of the leg we're interested in
for the 74 degree frame.
multiply both sides by 56 to solve for X and Y and we get:
56 Sin 17 = X
56 Sin 16 = Y
|Y - X| is the value you are interested in. I'd calculate it for you, but I
don't have a calculator with trig functions at work with me. Besides, I'd be
robbing you of the joy of going through this on your own. :)
I will tell you this though, it'll be a bit less than 1 cm. Make sure your
calculator is in "degrees" mode rather than "radians" for the purpose of trig
calculations.
John "...teach a man to fish..." Verheul
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Steve Freides
ride with a no-offset seatpost and like the "classic" position of the
plumb line dropped from my knee landing right on the pedal spindle. I
currently use an 11cm stem. My frame builder is proposing a 74 degree
seat tube angle and a 57cm top tube, and I'm concerned it might end up
being a bit too long for me, which is why I'm asking all these
questions. The frame will be fillet brazed.
-S-
JYates605 wrote:
>
> > Steve Freides <sj...@idt.net> asks;
> >
> >On the "average" 56cm road bike with a 56cm top tube, if one changed the
> >seat tube angle by one degree, from 73 to 74 degrees, approximately how
> >much closer is one putting the seat tube to the >head tube?
>
> "Average" does not matter, nor does the top tube length. For a 56cm seat tube
> (c-c for instance), changing the ST angle from 73' to 74' moves the c-c
> crossing point forward .93cm.
>
> Now, if you stipulate a 56cm top tube, then the seat lug doesn't move closer to
> the head tube at all. 56cm measured from the seat lug is 56cm, no matter what
> the ST angle. But if you take a given frame design with a 56cm ST, a 56cm TT,
> and a 73' seat angle, then change the seat angle to 74', the TT shortens .93cm.
>
> Jon Schaer
> jyat...@aol.com
--
Tim McNamara
<ppwr...@aol.com> wrote:
> General rule of thumb is 1cm for every degree of seat tube change-
A very suspect rule of thumb if it is without reference to seat tube
length. A 1 deg change on a 61 cm bike moves the top end of the tube a
lot farther than on a 51 cm bike.
jver...@lucent.com
sj...@idt.net wrote:
> My current bike has a 56cm top tube and a 73 degree seat tube angle. I
> ride with a no-offset seatpost and like the "classic" position of the
> plumb line dropped from my knee landing right on the pedal spindle. I
> currently use an 11cm stem. My frame builder is proposing a 74 degree
> seat tube angle and a 57cm top tube, and I'm concerned it might end up
> being a bit too long for me, which is why I'm asking all these
> questions. The frame will be fillet brazed.
>
> -S-
Assuming you put your seat in the same position relative to your BB (why
wouldn't you do this?), the proposed top tube/seat angle will extend the head
tube ~2 cm farther away from your saddle than it is now (the saddle is higher
up than the top of the seat tube).
Unless you've got a bit more than 1cm of rails left on your saddle to move it
back, you'll need a seatpost with more setback to attain your current
position. Along the same lines, you'll probably need a stem about 2cm shorter
than your current stem to attain the same position as you have now.
Perhaps your framebuilder thinks you ought to sit farther forward? Perhaps he
thinks you ought to be more stretched out? Perhaps he thinks stems over 10cm
are the work of Satan? Did you ask him why he proposed these measurements?
Oh, and I'm assuming we're still talking about a 56cm frame, measured
consistently (c-c or c-t), right?
John
Eric Harvey
: My current bike has a 56cm top tube and a 73 degree seat tube angle. I
: ride with a no-offset seatpost and like the "classic" position of the
: plumb line dropped from my knee landing right on the pedal spindle. I
: currently use an 11cm stem. My frame builder is proposing a 74 degree
: seat tube angle and a 57cm top tube, and I'm concerned it might end up
: being a bit too long for me, which is why I'm asking all these
: questions. The frame will be fillet brazed.
Steve, this all depends how you will be fitting the new bike. If you are
going to have the saddle setback the same between bikes, which you will
have to get the identical knee over pedal spindle setup, then I think you
will find some problems.
1) A bike with 73 STA (seat tube angle) and a no-offset seatpost and the
saddle all the way back will not give the same position as a bike with a
74 STA and the same seatpost and saddle. You will be 0.98 cm more
forward (I did the trig, correctly I hope). So, you may need a seatpost
with more offset to achieve the same saddle setback.
2) For a given setback from the BB shell to the tip of the saddle, the
bike with a steeper seat tube will have a longer reach to the stem,
assuming the top tube and stem length are the same between the two bikes.
The easy way to think of this is that for a given setback there will be
less top tube under your saddle (and more in front of you) on the steeper
bike. So in your case, assuming you fit yourself the same in terms of
saddle setback, your new bike will feel significantly longer, because you
will be 'using' more of the top tube (0.98 cm) than your old bike due to
the steeper seat angle, and the top tube is also 1 cm longer. I would say
your reach to the bars (assuming 11 cm stem on both) will increase by
almost 2 cm, for the same relative seat position to the BB. It's up to
you to decide if you need this increase in length. At any rate I would
talk to your builder about this, especially if you're comfortable on your
old bike.
Hope this helps,
Eric.
--
--------------------------------------------------------------------------
Eric Harvey email: eha...@med.phys.ualberta.ca
Dept. of Medical Physics eric....@cancerboard.ab.ca
Cross Cancer Institute
11560 University Avenue phone: (403)432-8618
Edmonton, Alberta
T6G 1Z2
Steve Freides
rails - I think you were assuming I was. In fact, I'm more than halfway
forward on them. I believe that using a slightly steeper seat tube
angle will allow me to put the seat further back on the rails, or maybe
even use a setback seatpost, which are more common and easy to come by,
a plus from my point of view.
The new bike will also have a slightly short seat tube than my current
bike, which again makes me a little bit further forward. I think my
frame builder's goal here is to allow me a range of adjustment, which I
currently do not really have, since I'm through more than half the rail
length and the post has no setback.
-S-
Bruce Frech
H * (cos(A)-cos(B))
To compare seat cluster difference for a 56 cm frame and 74 versus 73 cm
you get: 56*(cos(74)-cos(73)) = 0.937
But if your seat position is determined by the hardware (using the same
post and same position on the seat post, all the way back on the rails
for example) then H is not 56 but perhaps 75cm. This would give a
difference of 1.245 cm.
You also mention the new frame is spec'd with a 74 seat angle and a 1 cm
longer top tube. If you use the same stem then the bars will be 2.245
cm further from the saddle. Thus you will either have a very different
reach to the bars or you should change something.
If the frame is already built you should use a seatpost with at least
1.25 cm setback to get the same saddle to BB horizontal distance.
-Bruce
SB
This is from the Rivendell site in the article about seat post offset. It
doesn't exactly answer the question, but does give some figures you should
be able to use, and also brings up the question as to which seat post will
you be using as SPO is also a factor.
Rivendell at : http://www.rivendellbicycles.com/
"On a 56cm road bike, the difference at the seat cluster between a shallow
72-degree seat tube and a steep 74-degree one is about 8mm. But most 56cm
frame riders I know sit about 17cm above that, or 73cm above the bottom
bracket. By the time you're up there, the two degree difference between 72
and 74 degrees amounts to 20mm, and a sensitive chimp can notice that. If
the 74-degree rider also has a No-SPO post, and the 72-degree rider has a
2.7cm SPO-er; then the difference is 47mm--an inch and three quarters".
Steve B.
Steve Freides wrote in message <36A4B4...@idt.net>...
>On the "average" 56cm road bike with a 56cm top tube, if one changed the
>seat tube angle by one degree, from 73 to 74 degrees, approximately how
>much closer is one putting the seat tube to the head tube? I assume
>that shorter seat tubes would move the seat tube less and longer ones
>more.
>
>-S-
>
JYates605
>
>
>I'm using a no setback post, but I'm not pushed all the way back on the
>rails - I believe that using a slightly steeper seat tube
>angle will allow me to put the seat further back on the rails, or maybe
>even use a setback seatpost, which are more common and easy to come by,
>a plus from my point of view.
>
>
Without asking more questions or seeing the frame drawing it's hard to tell
what the builder has in mind, but I would suggest considering something else
besides just the fit issue.
Having a fit (or frame) that leaves the saddle centrally mounted (or even
slightly forward) is much preferable in terms of parts fatigue. A lot of
setback is very hard on saddle rails, seatposts, and even seat tubes. If you're
in the habit of using a lot of lightweight components, this could make a big
difference.
If you already have a saddle-to-BB position you like, I'd suggest having the
builder start with that. Figure out what ST angle puts the seatpost clamp of
common offset in the middle of the rails. This gives you the greatest
flexibility in picking both fore-aft saddle position and seatpost brands.
From there you should discuss with the builder what you like or dislike about
your current position, and they can design the frame accordingly from there.
Jon Schaer
jyat...@aol.com
Mark Hickey
>Draw a right triangle as follows: The (56cm) seat tube is the hypotenuse, a
>line drawn from the top tube down straight (vertically) through the BB is one
>leg, the section of top tube between the seat cluster and the point on the
>top tube through which you drew the first leg is the second leg.
No argument with your math, but the one thing you didn't consider is
the added length of the exposed portion of the seat post itself. This
should also be factored in, since the combination of the seat tube
PLUS the exposed seat post must be determined to calculate the total
effect of the change in seat tube clamp position. In round terms,
it's about 1/2" (1.25cm) per degree of seat tube angle change,
assuming a 56cm frame with 10cm of seatpost showing. It's a linear
function as the bike gets bigger or smaller, so it's not all that much
different for a 50 or a 62.
I agree with the others who question the wisdom of the builder's
choice - it will cause a 2cm further reach to the bars unless they're
planning on using a very short stem for a 56cm bike, or unless they're
planning on changing Steve's position for some reason.
Mark Hickey
Habanero Cycles
http://www.cynetfl.com/habanero/
Home of the $695 ti frame
jver...@lucent.com
mhi...@cynetfl.com wrote:
> jver...@lucent.com wrote:
>
> >Draw a right triangle as follows: The (56cm) seat tube is the hypotenuse, a
> >line drawn from the top tube down straight (vertically) through the BB is one
> >leg, the section of top tube between the seat cluster and the point on the
> >top tube through which you drew the first leg is the second leg.
>
> No argument with your math, but the one thing you didn't consider is
> the added length of the exposed portion of the seat post itself. This
> should also be factored in, since the combination of the seat tube
> PLUS the exposed seat post must be determined to calculate the total
> effect of the change in seat tube clamp position. In round terms,
> it's about 1/2" (1.25cm) per degree of seat tube angle change,
> assuming a 56cm frame with 10cm of seatpost showing. It's a linear
> function as the bike gets bigger or smaller, so it's not all that much
> different for a 50 or a 62.
Well, Steve's original question was in reference to the seat cluster, not up
at the saddle. Now that he has told us why he asked the question, we see that
he actually needs the measurement up by the saddle as you describe.
No matter though, the math is the same, you just substitute the BB-to-seat
measurement for the 56cm seat tube in the original equations. That's the
beauty of explaining to him _how_ to do it with high school math instead of
just giving him the answer.
> I agree with the others who question the wisdom of the builder's
> choice - it will cause a 2cm further reach to the bars unless they're
> planning on using a very short stem for a 56cm bike, or unless they're
> planning on changing Steve's position for some reason.
>
> Mark Hickey
> Habanero Cycles
> http://www.cynetfl.com/habanero/
> Home of the $695 ti frame
Yes, now that we're a bit further in the discussion, it seems that the
builder wants Steve to be a bit more stretched out or he wants him to use a
short (8 or 9) stem. It also seems that Steve has some short femurs, and thus
the steeper seat angle will put his particular seat position in a more
mid-adjustment range as far as the seat and post are concerned.
I'd be interested in hearing who the builder is, and how he responds to the
questions we have posed (assuming Steve asks them). The seat angle is
starting to make more sense, but I'm still leery about the top tube length.
John Verheul
p.s. - Mark, if you're gonna post _and_ e-mail, then say so somewhere. Just a
small point of etiquette, thanks.
Steve Freides
With a no setback post, I'm still forward of center on the rails, and
changing the angle 1 degree will put my in the center of the rails,
still using a no setback post, which sounds like the ideal. The no
setback post isn't quite "common" but, all other things being equal, I
think it's a better solution and it's available enough that I'm going to
consider it. I'm currently riding a Dean Ti no setback post now and am
very happy with it.
-S-
PPWrench
length. A 1 deg change on a 61 cm bike moves the top end of the tube a
lot farther than on a 51 cm bike.
Why I said 'general'-all else being equal-gosh-go take a pill-
Peter
ma...@abtcorp.com
about 1 cm. i'd be shocked if the frame angles are measured that precisely
anyway. all you have to do is change headsets or forks to start messing that
stuff up. (or even just change tires in the cas of mtb's, where fatter front
tires are often mixed with skinnier rear tires).
mw
In article <19990121101950...@ng37.aol.com>,
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