# Effective Seat tube angle??? Sta in low position????



## dirtrider7 (Oct 24, 2009)

Figured I would come to the guys that really understand frame specs for some advice.
Effective top tube length is pretty obvious, i.e. the horizontal distance from seat post to head tube centerline at top of head tube, but can someone explain the difference in seat tube angles...actual versus effective?...and effective in low position?
Does low position mean with the rider on the bike and compressing the front shock.
Below is the geometry chart for a Specialized 29er hardtail.
Can someone explain why there are two spec's, actual and eff. low position...for seat tube angle?
Many thanks.


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## rustola (Jan 15, 2008)

Looks to me like they've done some fancy footwork with the seat tube to give extra tire clearance. In this picture it appears to me that the tube is both curved towards the bottom bracket, and is also offset towards the front of the bike at the BB junction. The effective seat tube angle will therefore change as the seat is raised and lowered. For a more extreme version of this, look at a Jones space frame.


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## dirtrider7 (Oct 24, 2009)

Thanks for the pic Rustola.
Can I infer from your pic and comments then that the seat tube emerges from the BB shell at a more upright 73.5 degrees(effective) but then curves as you say around the tire for clearance (promoting a shorter chainstay) ending up at the seat itself with a 72 degree angle to the ground?
Is this your take? 
My take looking at the pic is the curvature of the seat tube occurs lower on the bike and up near the top including seatpost insertion section, the angle does not change. In my opinion, the angle can not change Rustola between low and high saddle position because that would imply the seat tube has a radius and that would cause a bind to the seatpost where it travels.
I don't know what they mean by "low" unless it pertains to the rider sitting on the bike with 
front suspension depressed.
Let me know any further thoughts if you would.
Thanks.


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## rustola (Jan 15, 2008)

forget the curvature for the moment. The issue is that the (straight) axis of the seatpost does not run through the bottom bracket, because the seat tube is offset. Since the BB is where the effective seat tube angle runs through, the vertical position of the seat will affect the effective angle. 

I spent too much time on these so-so drawings, hopefully they make sense. Kind of an exaggerated example - the second one shows a dramatically offset seat tube (ala walt's 36er), and shows that the effective seat angle is different at different seat heights.


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## dirtrider7 (Oct 24, 2009)

Thanks so much for the clarification Rustola. I appreciate your explanation and taking the time to develop the drawings and explain it to me. This issue is such a drawing is really the best way to illustrate the point of the design. Could you conjecture something for me since your grasp is better than mine and your Cad program maybe scalable? With a saddle height of say 78 cm from center of BB to top of saddle, could you speculate what the equivalent seat tube angle would be on the size 21 frame from the geometry chart above? My sense is this offset seat post design that Specialized offers accentuates effective or virtual seat tube angle the higher the seat post position thereby making it easier to achieve KOPS with long femurs like I have relative to specific frame sizes.
The quandry from many however is to be able to calculate what their actual seat tube angle or equivalent setback is relative to the height they set their seat. Much easier if the seat tube runs through the BB centerline as you know.
Thanks again.


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## Blaster1200 (Feb 20, 2004)

Actually I believe the term "low" is a carry-over from their suspension bikes that often have the ability to make adjustments to the suspension. The "low" setting slackens the head angle and seat angle as well as the bottom bracket, whereas the "high" setting makes the angles steeper and the BB taller. They often just carry over the "low" from their template. Look at the geometry of some of their suspension bikes, and you'll see what I mean.


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## dirtrider7 (Oct 24, 2009)

That's what I thought as well Blaster but now I'm not sure. The offset seatpost design does makes sense but really makes figuring out net amount of setback more difficult. My sense is the "low" reference has nothing to do with the position of the seat as you state...but rather with the front suspension of the bike compressed with rider weight applied.
As to determining amount of setback with this design, it can I believe be done with a simple equation, Y=mx +b, whereby m is the angle or slope of the seat tube and b is the amount of offset of seat tube relative to BB center. One would have to know the amount of offset to calculate the net setback for any given seat height. I am presuming the actual seat tube angle into the bottom of the frame is 72 degrees, but because it starts forward of the BB centerline, the effective seat tube angle is actually 73.5 deg as an "average" only and therefore difficult to determine what your net setback is because of the inital offset.


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## rustola (Jan 15, 2008)

1) If you're determined to calculate the exact setback, angle, or whatever, grab yourself a paper, pencil, protractor and straight edge. It's really not that hard

2) Not totally sure why you want to calculate this anyway ... realize that there is quite a bit of adjustment of effective seat angle simple by moving your seat fore and aft on the seatpost, and even more with the choice of a post with an offset or non-offset clamp. Without actually doing any calculations, I would guess that this adjustment supercedes the effect of a slightly offset seatTUBE and raising and lowering the seat.

3) If you want to know whether this bike fits you, why not just find a shop that has one and (gasp) try riding it? That will give you a lot more information than calculating the exact setback of the seat.

4) If you're still intent on doing the calculation, realize that different manufacturers (and people) have different definitions for just about every measurement on a bike. I have no idea what "low position" actually means, I'm just speculating on why they might list an effective vs actual seat tube angle. If you really want to know, I would contact Specialized directly and ask.


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## dirtrider7 (Oct 24, 2009)

rustola said:


> 1) If you're determined to calculate the exact setback, angle, or whatever, grab yourself a paper, pencil, protractor and straight edge. It's really not that hard
> 
> 2) Not totally sure why you want to calculate this anyway ... realize that there is quite a bit of adjustment of effective seat angle simple by moving your seat fore and aft on the seatpost, and even more with the choice of a post with an offset or non-offset clamp. Without actually doing any calculations, I would guess that this adjustment supercedes the effect of a slightly offset seatTUBE and raising and lowering the seat.
> 
> ...


Fair points.
1. One would need the frame to do that versus cribing off of geometry chart which if more suscint, it could be calculated.
2. I have long femurs and like to ride well behind KOPS. Therefore purchasing a frame with a slack tube angle is crtical to me and perhaps not to you. I don't want to run an excessive setback post but I have.
3. Gasp...test rides are an insignificant snap shot because a bike is never dialed in well enough for an accurate bellweather relative to fit.
4. A good suggestion. Provided I don't get a marketing guy to respond. 

Thanks again for your response. Your drawings were an excellent illustration.


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