Seat Tube Angles

Discuss light weight issues concerning road bikes & parts.
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mr.thi03
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by mr.thi03

Hello,

I was wondering if anyone knows how seat tube angle changes the power or riding charateristics of the frame.
1. I'm currently riding a 74.5 degree seat tube w/ 52cm top tube w/ 71.5 head tube
2. Changing to 74 degree and 52.5 cm top tube w/ 72 head tube
This really should change much?
But with these angles, will thing change a whole lot?
3. 72.5 seat tube, 52.5 top tube, 72.5 head tube?
Looks like a major change in position....but I'm not an expert..help?
worth a try or ?
basically building more of a climb specific bike........IF this amount of info is not enough...Let me know and I'll post what u need....As usual, many thanks.....mr.thi03

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strobbekoen
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Location: BELGIUM

by strobbekoen

STA = Seat Tube Angle
SH = Saddle Hheight ; distance of center BB to top of saddle
SB = Setback (horizontal distance from top of saddle to center of BB)

SB = cos(STA) * SH

HTA angle obviously does not change the position much, but the handling yes. A shallow HTA steers more sluggish (it moves the front wheel further forward).

The more you move the saddle forwards, or the seat tube angle steeper, the more your body should rotate with it to keep a comfortable position or one where you can generate your power. This means lowering the bars as well.

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mr.thi03
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by mr.thi03

strobbekoen wrote:STA = Seat Tube Angle
SH = Saddle Hheight ; distance of center BB to top of saddle
SB = Setback (horizontal distance from top of saddle to center of BB)

SB = cos(STA) * SH

HTA angle obviously does not change the position much, but the handling yes. A shallow HTA steers more sluggish (it moves the front wheel further forward).


So...the only bike I have these numbers are my current ride:
STA=74.5 degrees
SH=695mm
SB=175mm

IS it:
SB= 74.5 x 695 = 51777.5 ?

Now I'm lost when it comes to the other frames...and is this number the Set Back?

thanks...I'm sorry, I might be a bit slow getting this.....mr.thi03

PS. so, to confirm a 71 head tube is more sluggish then a 72.5?

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mr.thi03
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by mr.thi03

strobbekoen wrote:The more you move the saddle forwards, or the seat tube angle steeper, the more your body should rotate with it to keep a comfortable position or one where you can generate your power. This means lowering the bars as well.


Did not see this...I don't think I could lower my bars any more...w/o going from a 6 degree down to 17 degree down....
my current headtube is 105mm:
and the top of my stem (on the steer tube) to the center of the drops are 52.5cm
I believe the next frame might have a longer head tube: 109mm

thanks....thi03

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mr.thi03
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by mr.thi03

Wait a minute:

51777.5= the maximum seat tube angle I can go w/o changing the position?

thanks......thi03

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strobbekoen
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by strobbekoen

Ok check the drawing below.

SB = cos(STA) * SH

with a SH = 69cm and a STA of respectively 74.5, 74 and 72.5 this gives a SB of 18.44cm, 19.02cm and 20.75cm.

So the 72.5 angle puts your saddle 2.3cm further back as compared to the 74.5 angle.

What i mentioned about rotating the body with the STA has more to do with extreme geometry changes like for triathlon or timetrial.
Attachments
s.jpg

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mr.thi03
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by mr.thi03

strobbekoen wrote:Ok check the drawing below.

SB = cos(STA) * SH

with a SH = 69cm and a STA of respectively 74.5, 74 and 72.5 this gives a SB of 18.44cm, 19.02cm and 20.75cm.

So the 72.5 angle puts your saddle 2.3cm further back as compared to the 74.5 angle.

What i mentioned about rotating the body with the STA has more to do with extreme geometry changes like for triathlong or timetrial.


Thanks a lot: Thats crazy...I would of thought the complete opposite...Now I know why I play the violin for a living :roll:

Great...now that means the reach to hoods might be futher way...stem search..... :lol:

thanks again.......thi03

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strobbekoen
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by strobbekoen

Well you also have some adjustment space on your saddle, depends how much you got it back or forwards on your 74.5 STA.

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mr.thi03
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by mr.thi03

it's on the forward side of the rails...but as a general rule...should I always try the keep the SB distant like the way you calculated? SB= STA * SH?
I also think I miss something in that formula (I came up w/ 51777:oops:): cos=? cosign?
The only changes would be the stem height and reach to keep the comfort, but with this formula, my power shouldn't change (IE: hip angle?) or at least a very close starting point?

I normally try and get frames w/ the same seat tube angles 74.5 or 74, very close so I know it works...but just incase w/ something like this...I always wonder how the pros set up such different bikes year after year of moving to different manufactures......

Very informative: mr.thi03

Briscoelab
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Joined: Thu Jan 26, 2006 6:01 pm

by Briscoelab

mr.thi03 wrote:
strobbekoen wrote:Ok check the drawing below.

SB = cos(STA) * SH

with a SH = 69cm and a STA of respectively 74.5, 74 and 72.5 this gives a SB of 18.44cm, 19.02cm and 20.75cm.

So the 72.5 angle puts your saddle 2.3cm further back as compared to the 74.5 angle.

What i mentioned about rotating the body with the STA has more to do with extreme geometry changes like for triathlong or timetrial.


Thanks a lot: Thats crazy...I would of thought the complete opposite...Now I know why I play the violin for a living :roll:

Great...now that means the reach to hoods might be futher way...stem search..... :lol:

thanks again.......thi03

You should really adjust your seat so you have the same effective seat tube angle as before... that is if you have your knees where you wanted them, in relation to the cranks. When don't that, a shallow seat tube angle results in shorter reach to the bars.

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mr.thi03
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by mr.thi03

Briscoelab wrote:You should really adjust your seat so you have the same effective seat tube angle as before... that is if you have your knees where you wanted them, in relation to the cranks. When don't that, a shallow seat tube angle results in shorter reach to the bars.


This is exactly what I was hoping to do..but didn't know how...without having both bikes next to each other :?

Thanks.....thi03

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strobbekoen
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by strobbekoen

If you want to keep your position as you have it now, you need to move the saddle to have it in the same position relative to the BB. You may have to adjust your position still though as there will be differences in reach (the distance to the bars). It's really up to you.. if you want to the same position relative to the BB, move your saddle accordingly. Then adjust the stem if needed to get the reach right.

If two frames have the same top tube length, but the seat tube angles are different, then their reach will also be different. This makes sense, since with a shallow STA more of the top tube will be behind the BB.

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mr.thi03
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by mr.thi03

strobbekoen wrote:If you want to keep your position as you have it now, you need to move the saddle to have it in the same position relative to the BB. You may have to adjust your position still though as there will be differences in reach (the distance to the bars). It's really up to you.. if you want to the same position relative to the BB, move your saddle accordingly. Then adjust the stem if needed to get the reach right.

If two frames have the same top tube length, but the seat tube angles are different, then their reach will also be different. This makes sense, since with a shallow STA more of the top tube will be behind the BB.


NOW I'm really confused :oops:
that means move the seat forward? slacker STA = More SB?
Thank god I'm not an engineer.....I'll be starving more then I'm now :lol:
Thanks for being so patient....mr.thi03

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strobbekoen
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by strobbekoen

Ok chech another drawing.

This shows two frames with the same top tube length, but with different seat tube angles.
Since you are coming from a 74.5 STA now, your current setup is the right one (steeper STA).
If you move to a frame with say a 72.5 STA but the same top tube, your saddle will be further back behind the BB if you dont move it. This is the frame on the left in the drawing.

If you move your saddle forwards to get a similar saddle position relative to the bottom bracket as on your old frame, the new frame will shorter in reach and you may need a longer stem to compensate for this.
Attachments
DSC06946.JPG

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mr.thi03
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Joined: Tue Feb 21, 2006 3:59 pm

by mr.thi03

strobbekoen wrote:Ok chech another drawing.

This shows two frames with the same top tube length, but with different seat tube angles.
Since you are coming from a 74.5 STA now, your current setup is the right one (steeper STA).
If you move to a frame with say a 72.5 STA but the same top tube, your saddle will be further back behind the BB if you dont move it. This is the frame on the left in the drawing.

If you move your saddle forwards to get a similar saddle position relative to the bottom bracket as on your old frame, the new frame will shorter in reach and you may need a longer stem to compensate for this.


AWESOME: 'music' hallelujah!! hallelujah!! :D

Do you think I should go with a zero set back seat post with that much change in STA?
I'm on a Record w/ set back post right now...

thanks.......mr.thi03

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