Stem sizing question for bike fit experts

[[email protected]]

Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem.
Bike B has 54cm top tube, 73° head angle, 74° seat angle.
Bike B has a seatpost w/sufficient setback to push the saddle back as
far as necessary to duplicate the KOPS of bike A, and slightly longer
chainstays/wheelbase to compensate for shifting the weight distribution
rearward.

What stem length is needed for bike B to replicate the upper body
extension of bike A? Is it (a) 115mm, (b)110mm, or (c)105mm?
Thank you for your help.

 

[Jasper Janssen]

On 1 Sep 2005 16:22:02 -0700, "[email protected]" <[email protected]>
wrote:

>Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem.
>Bike B has 54cm top tube, 73° head angle, 74° seat angle.
>Bike B has a seatpost w/sufficient setback to push the saddle back as
>far as necessary to duplicate the KOPS of bike A, and slightly longer

Which is how far? You need both the seat angle and the seattube length to
do that calculation (actually, you need seattube length plus exposed
seatpost length for the exact number). Assuming a 50 cm seat tube, the
difference that the 1 degree of angle makes is about 500 * (sin 1) mm, aka
8.7 mm. So your seatpost lug will be nearly a cm further forward, by the
time you're at the saddle it'll probably be a bit more than a cm, and you
should probably go to a 105 stem to compensate.


Jasper

 

[IMKen]

I have a hard time worrying about 3/8 of an inch difference in reach. I'd
go with a 110 MM so I would not be any less reach or just stick with the 115
mm.

Ken


<[email protected]> wrote in message
news:[email protected]...
Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem.
Bike B has 54cm top tube, 73° head angle, 74° seat angle.
Bike B has a seatpost w/sufficient setback to push the saddle back as
far as necessary to duplicate the KOPS of bike A, and slightly longer
chainstays/wheelbase to compensate for shifting the weight distribution
rearward.

What stem length is needed for bike B to replicate the upper body
extension of bike A? Is it (a) 115mm, (b)110mm, or (c)105mm?
Thank you for your help.

 

[Qui si parla Campagnolo]

[email protected] wrote:
> Bike A has 54cm top tube, 73° head angle, 73° seat angle, 115mm stem.
> Bike B has 54cm top tube, 73° head angle, 74° seat angle.
> Bike B has a seatpost w/sufficient setback to push the saddle back as
> far as necessary to duplicate the KOPS of bike A, and slightly longer
> chainstays/wheelbase to compensate for shifting the weight distribution
> rearward.
>
> What stem length is needed for bike B to replicate the upper body
> extension of bike A? Is it (a) 115mm, (b)110mm, or (c)105mm?
> Thank you for your help.

having the seattube 1 degree steeper roughly translates to 1cm longer
top tube,as you move the saddle back to get KOPS, so a 1 cm shorter
stem than on Bicycle A.

 

[Ron Ruff]

Jasper Janssen wrote:
>
> Which is how far? You need both the seat angle and the seattube length to
> do that calculation (actually, you need seattube length plus exposed
> seatpost length for the exact number). Assuming a 50 cm seat tube, the
> difference that the 1 degree of angle makes is about 500 * (sin 1) mm, aka
> 8.7 mm. So your seatpost lug will be nearly a cm further forward, by the
> time you're at the saddle it'll probably be a bit more than a cm, and you
> should probably go to a 105 stem to compensate.
>
Actually, the exposed seatpost does not matter (since he has already
located his saddle), only the ctc seattube length. An important factor
is whether or not the two bikes have horizontal or sloping toptubes...
I'm assuming they are both horizontal with 55cm ctc seattubes (just
because that is pretty average)... and if we wanted to be precise (for
no good reason) we'd have to know a bunch of other things, too... so
I'll assume that the following are all the same between the two bikes;
bottom bracket height, headtube length, headset stack height, fork
length, stem clamp height above the headset, stem angle, and the
handlebar reach (can you tell I recently wrote a spreadsheet to deal
with all this stuff?).

Difference = 55*(sin(17)-sin(16)) = .92cm which means you'd like a 1cm
shorter stem (105cm) if you wanted it to be the same as before. But
like Ken said, 1cm on the reach is not much... maybe you'd like it a
bit longer than before?

 

[Jasper Janssen]

On 2 Sep 2005 14:15:24 -0700, "Ron Ruff" <[email protected]> wrote:

>Actually, the exposed seatpost does not matter (since he has already
>located his saddle),

Uhm.. doh.

You're right.


Jasper