Distance of seat setback with various seattube angles

[Dylan]

My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
taken from the center of the bottom bracket, or crank spindle to the center of the seattube/toptube
junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame has a seattube angle
of 74 degrees.

Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
fore/aft postion of the seat between the two frames?

Also, I would like to be able to use this formula to find the differences on other frame sizes and
seat heights.

Any help would be appreciated....

Mathematically Challenged

 

[Josh Gatts]

"dylan" <[email protected]> wrote in message
news:[email protected]...
> My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
> frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
> given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
> taken from the center of the bottom bracket, or crank spindle to the center of the
> seattube/toptube junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame
> has a seattube angle of 74 degrees.
>
> Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
> the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
> fore/aft postion of the seat between the two frames?
>
> Also, I would like to be able to use this formula to find the differences on other frame sizes and
> seat heights.
>
> Any help would be appreciated....
>
> Mathematically Challenged

Hopefully someone will check my math, but I think the correct formula is:

difference in setback = (seat height) x (cos angle1 - cos angle2)

... which comes out to roughly 6 mm.

--Josh

 

[Mike S.]

"dylan" <[email protected]> wrote in message
news:[email protected]...
> My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
> frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
> given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
> taken from the center of the bottom bracket, or crank spindle to the center of the
> seattube/toptube junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame
> has a seattube angle of 74 degrees.
>
> Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
> the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
> fore/aft postion of the seat between the two frames?
>
> Also, I would like to be able to use this formula to find the differences on other frame sizes and
> seat heights.
>
> Any help would be appreciated....
>
> Mathematically Challenged

I'm as mathematically challenged as you, but I remember someone telling me that every degree of
difference in the ST means about 1cm at the saddle. The difference between 73.5 and 74 degrees
should be about 5mm.

Mike

 

[Dan Goldenberg]

"dylan" <[email protected]> wrote in message
news:[email protected]...
> My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
> frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
> given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
> taken from the center of the bottom bracket, or crank spindle to the center of the
> seattube/toptube junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame
> has a seattube angle of 74 degrees.
>
> Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
> the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
> fore/aft postion of the seat between the two frames?
>
> Also, I would like to be able to use this formula to find the differences on other frame sizes and
> seat heights.
>
> Any help would be appreciated....
>
> Mathematically Challenged

Either the sin or tan of the angle difference times the seat height will give you the difference in
setback. For a 0.5 degree difference it comes out to about 64mm setback difference at the top of the
saddle (sin0.5deg x 73cm)=0.64cm

Dan Goldenberg Seattle

 

[Sheldon Brown]

> "dylan" wrote:

>>My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
>>frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
>>given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
>>taken from the center of the bottom bracket, or crank spindle to the center of the
>>seattube/toptube junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame
>>has a seattube angle of 74 degrees.
>>
>>Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
>>the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
>>fore/aft postion of the seat between the two frames?
>>
>>Also, I would like to be able to use this formula to find the differences on other frame sizes and
>>seat heights.
>>
>>Any help would be appreciated....
>>
>>Mathematically Challenged

Dan Goldenberg wrote:

> Either the sin or tan of the angle difference times the seat height will give you the difference
> in setback. For a 0.5 degree difference it comes out to about 64mm setback difference at the top
> of the saddle (sin0.5deg x 73cm)=0.64cm

Oops, you dropped a decimal there. .64 cm is 6.4 mm, i.e. 1/4 inch, a relatively insignificant
difference.

Note to "dylan"...while for this calculation it is correct to measure saddle height from the center
of the bottom bracket, for general purposes it's better to measure saddle height from the low pedal.

If you're setting up or comparing different bikes with different crank lengths, you'll wind up with
incorrect saddle height if you don't take the crank length into consideration.

Sheldon "Trig Is Fun And Useful Too!" Brown +---------------------------------------------+
| Whatever you say, say it with conviction. | --Mark Twain |
+---------------------------------------------+ Harris Cyclery, West Newton, Massachusetts Phone
617-244-9772 FAX 617-244-1041 http://harriscyclery.com Hard-to-find parts shipped Worldwide
http://captainbike.com http://sheldonbrown.com

 

[W K]

"Dan Goldenberg" <[email protected]> wrote in message
news:[email protected]...

> For a 0.5 degree difference it comes out to about 64mm setback difference
at
> the top of the saddle (sin0.5deg x 73cm)=0.64cm

I was going to say "do you work for NASA" in reference to their fine use of metric measurements,
but ooooh no.

 

[Kbh]

I have a rather involved spreadhseet that calculates the relative position of every interesting
point on a bicycle frame given the frame's various angles and dimensions.

For your calculation, all you need to do is set the saddle height, and look at the setback for the
two different seat tube angles and voila! - 0.6 cm.

Kyle

http://www.hollasch.com/bike_geometry.xls

"dylan" <[email protected]> wrote in message
news:[email protected]...
> My question has to do with the amount of fore/aft lay back a bicycle seat will have for a given
> frame geometry and seat height. For example, I'm trying to compare two seperate frame sizes of a
> given racing bicycle. One is slightly smaller than the other, 53cm and 54cm (this measurement is
> taken from the center of the bottom bracket, or crank spindle to the center of the
> seattube/toptube junction). The 53cm frame has a seattube angle of 73.5 degrees. The 54cm frame
> has a seattube angle of 74 degrees.
>
> Now the question is, what formula(s) (given my proper seat height of 73cm taken from the center of
> the bottom bracket to the top center of the seat) can be used to find the difference in horizontal
> fore/aft postion of the seat between the two frames?
>
> Also, I would like to be able to use this formula to find the differences on other frame sizes and
> seat heights.
>
> Any help would be appreciated....
>
> Mathematically Challenged

 

[Archer]

In article <ec9qa.341051$OV.370338@rwcrnsc54>, [email protected] says...
> I have a rather involved spreadhseet that calculates the relative position of every interesting
> point on a bicycle frame given the frame's various angles and dimensions.
>
> For your calculation, all you need to do is set the saddle height, and look at the setback for the
> two different seat tube angles and voila! - 0.6 cm.
>
> Kyle
>
> http://www.hollasch.com/bike_geometry.xls

Do you have a diagram indicating what some of these measurements are? Such as "virtual seat tube"
and "virtual top tube"?

....

--
David Kerber An optimist says "Good morning, Lord." While a pessimist says "Good Lord,
it's morning".

Remove the ns_ from the address before e-mailing.

 

[Kbh]

That would be a useful addition, it's a very rough spreadsheet right now. Virtual Seat Tube and
Virtual Top Tube are basically what the dimenions (center to center) of these would be for a
traditional horizontal top tube frame. This allows you to size a sloping top tube frame "as if the
seat tube continued beyond its actual top, to an imaginary intersection where the top tube would be
if it were parallel* ". So if you set the top tube slope to 0, virtual seat tube and actual seat
tube are equal. I never bothered to calculate actual top tube, and the only reason I calculated
actual seat tube was to check on the required seat post length. There are few comments when you
hover hover various cells that help explain the spreadsheet. Also, look here:

* http://www.sjscycles.com/700solobrochure/cyclo2.asp

This is my bike by the way - great bike.

Kyle

"archer" <ns_archer1960@ns_hotmail.com> wrote in message news:[email protected]...
> In article <ec9qa.341051$OV.370338@rwcrnsc54>, [email protected] says...
> > I have a rather involved spreadhseet that calculates the relative
position
> > of every interesting point on a bicycle frame given the frame's various angles and dimensions.
> >
> > For your calculation, all you need to do is set the saddle height, and
look
> > at the setback for the two different seat tube angles and voila! - 0.6
cm.
> >
> > Kyle
> >
> > http://www.hollasch.com/bike_geometry.xls
>
> Do you have a diagram indicating what some of these measurements are? Such as "virtual seat tube"
> and "virtual top tube"?
>
> ....
>
> --
> David Kerber An optimist says "Good morning, Lord." While a pessimist says "Good Lord, it's
> morning".
>
> Remove the ns_ from the address before e-mailing.

 

[Kbh]

I added a short explanation and a diagram.

Kyle

"archer" <ns_archer1960@ns_hotmail.com> wrote in message news:[email protected]...
> In article <ec9qa.341051$OV.370338@rwcrnsc54>, [email protected] says...
> > I have a rather involved spreadhseet that calculates the relative
position
> > of every interesting point on a bicycle frame given the frame's various angles and dimensions.
> >
> > For your calculation, all you need to do is set the saddle height, and
look
> > at the setback for the two different seat tube angles and voila! - 0.6
cm.
> >
> > Kyle
> >
> > http://www.hollasch.com/bike_geometry.xls
>
> Do you have a diagram indicating what some of these measurements are? Such as "virtual seat tube"
> and "virtual top tube"?
>
> ....
>
> --
> David Kerber An optimist says "Good morning, Lord." While a pessimist says "Good Lord, it's
> morning".
>
> Remove the ns_ from the address before e-mailing.

 

[John Everett]

On Thu, 24 Apr 2003 14:59:18 -0700, "Josh Gatts" <[email protected]> wrote:

>> Also, I would like to be able to use this formula to find the differences on other frame sizes
>> and seat heights.
>>
>> Any help would be appreciated....
>>
>> Mathematically Challenged
>
>Hopefully someone will check my math, but I think the correct formula is:
>
>difference in setback = (seat height) x (cos angle1 - cos angle2)
>
>... which comes out to roughly 6 mm.

Or as Sheldon pointed out in another response, about 1/4". I've often wondered about this myself,
but not because I was concerned about a particular frame's geometry. I was more intrigued about what
importance can really be placed on seat tube angle.

I just went down and pulled out a random seatpost and random saddle (in this case a Vetta TT
Trishock) out of my spares bin. I wanted to check how much fore and aft adjustment was available.
Turns out it's about 4cm (1.5"+).

So how important really is seat tube angle when three degrees or so can be adjusted for just by
moving the saddle?

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3

 

[Josh Gatts]

"John Everett" <[email protected]> wrote in message
news:[email protected]...
> On Thu, 24 Apr 2003 14:59:18 -0700, "Josh Gatts" <[email protected]> wrote:
>
>
> >> Also, I would like to be able to use this formula to find the differences on other frame sizes
> >> and seat heights.
> >>
> >> Any help would be appreciated....
> >>
> >> Mathematically Challenged
> >
> >Hopefully someone will check my math, but I think the correct formula is:
> >
> >difference in setback = (seat height) x (cos angle1 - cos angle2)
> >
> >... which comes out to roughly 6 mm.
>
> Or as Sheldon pointed out in another response, about 1/4". I've often wondered about this myself,
> but not because I was concerned about a particular frame's geometry. I was more intrigued about
> what importance can really be placed on seat tube angle.
>
> I just went down and pulled out a random seatpost and random saddle (in this case a Vetta TT
> Trishock) out of my spares bin. I wanted to check how much fore and aft adjustment was available.
> Turns out it's about 4cm (1.5"+).
>
> So how important really is seat tube angle when three degrees or so can be adjusted for just by
> moving the saddle?
>
>
> jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3

Right, it doesn't make much of a difference unless you're pushing the limits. I like to ride with my
saddle pretty far back, though. Even on my Trek, which has a relatively slack seat tube angle of
73.5 degrees, I'm riding with my saddle almost all the way back on the rails, so I might actually
have a problem getting my desired position if the seat tube angle were 74 degrees. Of course, a
different seatpost with more setback would likely fix that problem.

--Josh

 

[Pat Norton]

W K wrote:
>I was going to say "do you work for NASA" in reference to their fine
use of
>metric measurements, but ooooh no.

NASA is one of the most metric organisations in the US, along with military and medical
organisations. A subcontractor (Lockheed Martin Astronautics) failed to comply with the NASA
specification. NASA should have picked up the mistake of their subcontractor.

Errors of this kind (but not so spectacular) must happen in the US every day.

The official report says: [begin quote] The MCO MIB [Mars Climate Orbiter Mishap Investigation
Board] has determined that the root cause for the loss of the MCO spacecraft was the failure to use
metric units in the coding of a ground software file, "Small Forces," used in trajectory models.
Specifically, thruster performance data in English units instead of metric units was used in the
software application code titled SM_FORCES (smallforces). The output from the SM_FORCES application
code as required by a MSOP Project Software Interface Specification (SIS) was to be in metric units
of Newton-seconds (N-s). Instead, the data was reported in English units of pound-seconds (lbf-s).
The Angular Momentum Desaturation (AMD) file contained the output data from the SM_FORCES software.
The SIS, which was not followed, defines both the format and units of the AMD file generated by
ground-based computers. [end quote]

http://www.hq.nasa.gov/office/codeq/risk/mco_mib_report.pdf

 

[Tim McNamara]

In article <[email protected]>, John Everett
<[email protected]> wrote:

> So how important really is seat tube angle when three degrees or so can be adjusted for just by
> moving the saddle?

Seat tube angle is one of those incredible mumbo-jumbo aspects of frame design. People believe that
1/2 degree makes a significant difference in the vertical compliance of the frame, for example, or
that a 73 degree seat tube means a touring frame while a 73.5 degree seat tube means a racing frame.

For probably 99% of the population, a 73 degree seat tube will work perfectly well given the
generous amount of adjustability that a good seapost and saddle combination will permit. This is
because the variability of human proportions is small, with some people being statistical outliers.
Note that almost all mountain bikes come with a 71 degree head tube and a 73 degree seat tube-
regardless of frame size- and almost no one complains about it.

Steering geometry is important and front and rear centers are important for how the bike handles;
clearances are important for how versatile the bike is for various uses; the relative positions of
the saddle, pedals and bars are important for how the bike fits. IMHO saddle height is more
important than saddle setback; if the seat tube is at a 73 degree angle, it will work for the vast
majority of people.

 

[Tim McNamara]

In article <[email protected]>,
"Josh Gatts" <[email protected]> wrote:

> Even on my Trek, which has a relatively slack seat tube angle of
> 73.5 degrees,

Which used to be considered a *steep* seat tube angle- 72 having been the norm for "relaxed" angles
in the days of my youth. Frames with 70 or even 68 degree angles could be had.

 

[Qui Si Parla Ca]

dylan-<< Now the question is, what formula(s) (given my proper seat height of 73cm taken from the
center of the bottom bracket to the top center of the seat) can be used to find the difference in
horizontal fore/aft postion of the seat between the two frames?

Put your knee in the proper place on the 73.5 degree frameset, and on the 74 degree frameset, move
it aft 5mm...

Peter Chisholm Vecchio's Bicicletteria 1833 Pearl St. Boulder, CO, 80302
(303)440-3535 http://www.vecchios.com "Ruote convenzionali costruite eccezionalmente bene"

 

[Mark Hickey]

[email protected] (Qui si parla Campagnolo) wrote:

>dylan-<< Now the question is, what formula(s) (given my proper seat height of 73cm taken from the
>center of the bottom bracket to the top center of the seat) can be used to find the difference in
>horizontal fore/aft postion of the seat between the two frames?
>
>Put your knee in the proper place on the 73.5 degree frameset, and on the 74 degree frameset, move
>it aft 5mm...

I don't know about you, Peter.. but my knee doesn't seem to be adjustable. Moving the saddle back
5mm might be easier. ;-)

Mark Hickey Habanero Cycles http://www.habcycles.com Home of the $695 ti frame

 

[John Everett]

On Sat, 26 Apr 2003 10:13:02 -0500, Tim McNamara <[email protected]> wrote:

> In article <[email protected]>,
> "Josh Gatts" <[email protected]> wrote:
>
>> Even on my Trek, which has a relatively slack seat tube angle of
>> 73.5 degrees,
>
>Which used to be considered a *steep* seat tube angle- 72 having been the norm for "relaxed" angles
>in the days of my youth. Frames with 70 or even 68 degree angles could be had.

I almost forgot, for more extreme saddle adjustments check out:

http://home.earthlink.net/~jeverett3/cycling/nishiki2.gif

I believe that chunk of metal between my saddle and seatpost was called a Torque Platform. I further
believe these are no longer being manufactured.

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3