Seat Angle Question??



F

FasTrack

Guest
What is the therory why smaller road bikes have steep seat angles?

Most smaller bikes have 74-75 degreee seat angles...

What would a 72 degree seat angle do to a say 50cm or smaller road
frame?

Thanks,

Dave
 
S

Sheldon Brown

Guest
FasTrack wrote:

> What is the therory why smaller road bikes have steep seat angles?
>
> Most smaller bikes have 74-75 degreee seat angles...


Small frames with big wheels often have to resort to various levels of
bogosity in frame design.

> What would a 72 degree seat angle do to a say 50cm or smaller road
> frame?


If you kept the short top tube that should be on such a small bike,
there'd be excessive toe overlap of the front wheel.

As a work-around to this problem, some manufacturers make the head angle
shallower, some make the seat angle steeper, some a little of both.

None of these changes improve the ride or handling of the bike.

The steeper seat angle leads to the rider putting more weight on the
handlebars.

Shallower head angle could be OK if the fork rake were also increased to
provide a desirable amount of trail. Unfortunately this is not always
done. The guys who design such bikes never ride the smaller sizes.

The real answer is smaller wheels, or at least a smaller front wheel.

Sheldon "Georgena Was Right" Brown
+------------------------------------------------------+
| If I have been able to see farther than others, |
| it was because I stood on the shoulders of giants. |
| -- Sir Isaac Newton |
+------------------------------------------------------+
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
 
A

A Muzi

Guest
FasTrack wrote:
> What is the therory why smaller road bikes have steep seat angles?
> Most smaller bikes have 74-75 degreee seat angles...
> What would a 72 degree seat angle do to a say 50cm or smaller road
> frame?


It would make a small 50cm-sized rider (with maybe a shorter
femur than a big bike rider) slide her seat forward on the
post to get back to where she would have been (in relation
to the pedals) with a 73 or 74 degree seat angle.

Or not.

Depends on the rider's femur length.


--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971
 
D

David

Guest
"A Muzi" <[email protected]> wrote in message news:[email protected]
> FasTrack wrote:
> > What is the therory why smaller road bikes have steep seat angles?
> > Most smaller bikes have 74-75 degreee seat angles...
> > What would a 72 degree seat angle do to a say 50cm or smaller road
> > frame?

>
> It would make a small 50cm-sized rider (with maybe a shorter
> femur than a big bike rider) slide her seat forward on the
> post to get back to where she would have been (in relation
> to the pedals) with a 73 or 74 degree seat angle.


Yup. All seat-tube angle really does is position the saddle with respect to the bottom bracket.
If your legs are short, you'll want a steep seat-tube, else not (I suppose--I only have experience
with the former).
 
A

AB/9000

Guest
A Muzi wrote:

> It would make a small 50cm-sized rider (with maybe a shorter femur than
> a big bike rider) slide her seat forward on the post to get back to
> where she would have been (in relation to the pedals) with a 73 or 74
> degree seat angle.
>
> Or not.
>
> Depends on the rider's femur length.


If you belive in KOPS - Knee Over Pedal Spindle - and if shorter riders
in general have shorter femurs. They don't. The average femur/leg
ratio is constant.

---
AndersB
 
Q

Qui si parla Campagnolo

Guest
Fastak writes-<< What is the therory why smaller road bikes have steep seat
angles?
> Most smaller bikes have 74-75 degreee seat angles...
> What would a 72 degree seat angle do to a say 50cm or smaller road
> frame? >><BR><BR>


It's because most smaller riders have shorter femurs, meaning a steeper seat
tube angle to get the knee over the pedal spindle.

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

Qui si parla Campagnolo

Guest
AB writes-<< If you belive in KOPS - Knee Over Pedal Spindle - and if shorter
riders
in general have shorter femurs. They don't >><BR><BR>

Ahhh but they do have shorter femurs. kops is a starting place generally, the
only thing in bike fit that is sort of a constant, but not that 'firm.

You next mention that femur/leg ratio is constant and that is true, shorter
femur and shorter lower leg,

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

AB/9000

Guest
Qui si parla Campagnolo wrote:
> AB writes-<< If you belive in KOPS - Knee Over Pedal Spindle - and if shorter
> riders
> in general have shorter femurs. They don't >><BR><BR>
>
> Ahhh but they do have shorter femurs. kops is a starting place generally, the
> only thing in bike fit that is sort of a constant, but not that 'firm.


Doh, I missed the word "relatively".

---
AndersB
 
A

AB/9000

Guest
Qui si parla Campagnolo wrote:

> It's because most smaller riders have shorter femurs, meaning a steeper seat
> tube angle to get the knee over the pedal spindle.


Not true!

Smaller riders have shorter femurs and shorter lower legs = same
femur/lower leg ratio as larger riders = same optimum seat angle - KOPS
or not.

Steeper seat angles on smaller frame sizes has to do with what Sheldon
says. It's a precaution against toe-overlap and not based on fit and
position.

In the larger sizes you see the opposite: steeper head angles and
shallower seat angles to avoid overly long wheelbases (= slow handling
bikes), but at the same time chain stays are often longer to obtain a
more even weight distribution. This is especially the case with Italian
frame builders

If frame manufacturers should offer proportionally fit frames to riders
of all sizes, it would require proportionally scaled wheels.
And ideally proportionally scaled courses with proportionally scaled
turns (corners!) ......

Take a look at:
http://bikefitting.com/English/Theory/SeatAngle.aspx

Hope you understand. English is not my first language.

---
AndersB
Denmark
 
Q

Qui si parla Campagnolo

Guest
fastrac-<< What is the therory why smaller road bikes have steep seat angles?
>
> Most smaller bikes have 74-75 degreee seat angles.. >><BR><BR>


Because most smaller riders also have shorter femurs, so to get KOPS, a steeper
seat tube angle is required. If it were 72 degrees, and to get KOPS, a reverse
setback seatpost would often be required.

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

Meccanico di Bici

Guest
> Steeper seat angles on smaller frame sizes has to do with what
Sheldon
> says. It's a precaution against toe-overlap and not based on fit and
> position.


A shallow head tube angle will move the front wheel forward to mitigate
toe overlap, but the seat tube angle doesn't matter. The bottom bracket
is still in the same place if the seat tube angle was 72 deg. or 76
deg. A steep sat tube angle will move the riders **** and knee further
forward, but their feet down on the pedals don't move forward at all.
--Jim
 
S

Sheldon Brown

Guest
An anynymous poster wrote:

> A shallow head tube angle will move the front wheel forward to mitigate
> toe overlap, but the seat tube angle doesn't matter. The bottom bracket
> is still in the same place if the seat tube angle was 72 deg. or 76
> deg. A steep sat tube angle will move the riders **** and knee further
> forward, but their feet down on the pedals don't move forward at all.


Depends how you look at it. If you keep the top tube length constant, a
steeper seat tube angle will move the bottom bracket back with respect
to the front wheel, the handlebars and the rider's butt.

Sheldon "Relativity" Brown
+-----------------------------------------------+
| Who has deceived thee as often as thyself? |
| -- Benjamin Franklin |
+-----------------------------------------------+
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
 
M

Meccanico di Bici

Guest
Sure, but when you fit someone up, don't you get their saddle, knee,
KOPS stuff delt with first, then move forward and work on tt lenght?

It's a strange concept, we're presuming tt has to be constant yet we're
ok with changing the seat tube angle. My feeling is one really can't be
tinkered with too much before the other has to be adjusted also.

I definetly don't envy shorter riders, as their bikes always seem to
have a compromise somewhere, but I also think way to much is made of
toe overlap. I once had a 58cm Italian frame with overlap, and although
it was a very quick handeling bike, the overlap was never an issue.
--Jim
 
M

Meccanico di Bici

Guest
Sure, but when you fit someone up, don't you get their saddle, knee,
KOPS stuff delt with first, then move forward and work on tt lenght?

It's a strange concept, we're presuming tt has to be constant yet we're
ok with changing the seat tube angle. My feeling is one really can't be
tinkered with too much before the other has to be adjusted also.

I definetly don't envy shorter riders, as their bikes always seem to
have a compromise somewhere, but I also think way to much is made of
toe overlap. I once had a 58cm Italian frame with overlap, and although
it was a very quick handeling bike, the overlap was never an issue.
--Jim
 
M

Meccanico di Bici

Guest
Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
my ssn?
 
S

Sheldon Brown

Guest
"Jim" wrote:

> Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
> my ssn?


We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato, Jim
DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?

Sheldon "Mystery" Brown
+------------------------------------------------+
| Too often we... enjoy the comfort of opinion |
| without the discomfort of thought. |
| -- John F. Kennedy |
+------------------------------------------------+
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
 
On Tue, 25 Jan 2005 14:02:58 -0500, Sheldon Brown
<[email protected]> wrote:

>"Jim" wrote:
>
>> Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you need,
>> my ssn?

>
>We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato, Jim
>DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?
>
>Sheldon "Mystery" Brown
>+------------------------------------------------+
>| Too often we... enjoy the comfort of opinion |
>| without the discomfort of thought. |
>| -- John F. Kennedy |
>+------------------------------------------------+
> 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


Dear Sheldon,

Mysteriously, there seem to be no Daves in your wanted
posters, despite their teeming numbers here on
rec.bicycles.tech:

http://sheldonbrown.com/harris/index.html#articles

Does Harris Cyclery discriminate on a first-name basis?

The closest Shakespeare came to a Dave is a trivial servant
named Davy in the second part of Henry IV.

Carl Fogel
 
M

Meccanico di Bici

Guest
Sheldon Brown wrote:
> "Jim" wrote:
>
> > Anynymous? Anonymous. No I ain't, I signed it "Jim" What else you

need,
> > my ssn?

>
> We have three "Jims" here at Harris Cyclery. Are you Jim Ammirato,

Jim
> DaSilva, or Jim Wirtanen? Or perhaps a different Jim altogether...?

A different Jim altogrther is correct.

Jim "Jim" Potter
 
S

Sheldon Brown

Guest
Carl Fogel wrote:
> Dear Sheldon,
>
> Mysteriously, there seem to be no Daves in your wanted
> posters, despite their teeming numbers here on
> rec.bicycles.tech:


"Dave's not here!"

Sheldon "C&C" Brown
+--------------------------------------------------+
| Some of my brother's paintings may be seen at: |
| http://junila.com |
+--------------------------------------------------+
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
 
Q

Qui si parla Campagnolo

Guest
ander-<< Smaller riders have shorter femurs and shorter lower legs = same
femur/lower leg ratio as larger riders = same optimum seat angle - KOPS
or not. >><BR><BR>

Ya can argue this but if you believe KOPS is a place to start, not a hard
constant(as most do, including the likes of DR Andy Pruitt), it is unlikely
that a short person can get his KOPS with a 72 degree seattube angle even with
a no set back seatpost.

ander-<< Steeper seat angles on smaller frame sizes has to do with what Sheldon

says. It's a precaution against toe-overlap and not based on fit and
position. >><BR><BR>

Sorry, don't see how not do I agree.
<< Take a look at:
http://bikefitting.com/English/Theory/SeatAngle.aspx

Hope you understand. English is not my first language. >><BR><BR>


Theory is right....

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