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?? about Seat Tube Angle(10 posts)

?? about Seat Tube Anglesjomi
Nov 20, 2002 6:30 PM
Do I understand correctly:

1. a 55 top tube with a 73 STA will create a longer reach than a 55 top tube with a 74 STA

2. I have read some people state that they stay away from bikes that have a 74STA. Is this because they need a longer reach??

3. I am 68.5" and my inseam is 32.5". My usually concern is the top tubes are too long based on my height requirements. I ended up with a 4" drop between seat & bars. Wouldn't a 73STA in MOST cases make it even worse for me?

thanks
--Stephen--
backwards....C-40
Nov 20, 2002 7:49 PM
For any given TT length, a 74 degree STA will produce a longer reach to the bars than a 73 degree STA.

On a frame with a 74 degree seat tube angle, the saddle must be moved further back than a frame with a 73 degree seat tube angle to achieve the same position relative to the bottom bracket. This movement of the saddle must be accounted for. Since the TT length is the standard reference value for comparing the reach to the handle bars, the difference in SADDLE POSITION is added or subtracted from the actual TT length to produce an "effective" TT length. The effective TT length will accurately predict the difference in reach to the handlebars between any two frames and allow you to determine the difference in stem length that would be needed to maintain the same reach. When comparing frames with different STA, add TT length to the frame with the steeper (74) angle OR subtract length from the frame with the shallower (73) angle.

The exact formula for the difference in TT length is: saddle height x (cosA-cosB). Saddle height is measured from the center of the BB to the top of the saddle, parallel to the seat tube. An average amount is 1.2cm per degree for a midsize frame. This would make the effective top tube length of the 74 degree frame in your example 56.2cm, compared to the 55cm length for the 73 degree frame.

If the inseam that you have posted is an accurate cycling inseam (floor to hard crotch contact in bare feet), you have about the same inseam as I do, but you are nearly two inches taller. You should not have an excessive top tube length problem with a stock frame. If anything, your proportions would tend toward the opposite problem.

The ideal vertical frame size for your inseam is a 54cm, measured c-t. In some brands you can only get odd sizes. A 55cm would best suit your height.

Top tube length has nothing to do with vertical drop between the bars and the stem. It's the head tube length that you must pay attention to. For example, my 54cm Colnago has a 135mm head tube length. I have 9cm or 3-5/8" of vertical drop from the top of the saddle to the top of the bars, using an 84 degree stem with no spacers. To reduce this distance, spacers can be used to raise the stem (2cm tops) or the stem rise can be increased.

Here's an interesting example of two frames that would fit the same. A 55cm Colnago with a 54.3cm TT and 74 degree STA will have the same reach to the bars as a 55cm Litespeed with a 73 STA and a 55.5cm TT. The saddle on the Colnago would be positioned 1.2cm further back on the seatpost than the Litespeed, but both would have the same saddle position relative to the BB.
Pay attention to c-40's post..........Len J
Nov 21, 2002 4:47 AM
it's a hard concept to grasp, and in fact many posters on this board get it wrong (See responses below), but c-40 is spot on.

While KOP may be a starting point, there is a position of the Butt & Knee)relative to the BB that is ideal for each rider. This "ideal" position doesn't change because of seat tube angle, what changes is the amount of setback of the seat. As the STA gets steeper, you must move the seat back further to maintain the same position realitive to the BB.

Good job c-40

Len
Yeah, that C-40 knows his stuff and is good at explaining! (nm)jtferraro
Nov 21, 2002 11:57 AM
ST angle does NOT effect reach!timfire
Nov 20, 2002 9:22 PM
Seat tube angle does not effect reach.** Let me say that I do not believe in KOPS or the idea that there is a "ideal" relationship beetween seat and BB. If you want to move the seat to achieve a desired position that's one thing, but it has nothing to do with the bike, that's all you. If you take the two bikes that the original poster mention, both with 55cm TT's but one with a 73sta and one with a 74sta, both will have the same reach: 55cm +/- a few cm's for the seat rails (plus the length of the stem).

I believe seat tube angle (theorectically) really only serves one purpose: to regulate the hip angle. And hip angle relates to riding style. Track and Crit racers have steeper angles because they ride more aggressively and need to open up their hips. Tourers need slacker angles so that they can have a more upright position with a proper tighter hip angle.

[**: At least, STA will not effect reach significantly. A slacker STA will lengthen reach by a couple mm's, but that would be easily adjusted.]

--Tim Kleinert
sorry, but you are wrong...C-40
Nov 21, 2002 6:25 AM
You have contradicted yourself. First you state that there is no ideal relationship between the saddle and BB and then you say the the STA regulates the hip angle. The hip angle is directly related to the saddle positon relative to the BB. You can't have it both ways.

If there is some ideal hip angle, you need an appropriate STA and seatpost sytle to to position the saddle as required to achive this angle. When the saddle is moved forward or backward the reach to the bars is significantly affected.
re: ?? about Seat Tube AngleFez
Nov 20, 2002 9:32 PM
1. a 55 top tube with a 73 STA will create a longer reach than a 55 top tube with a 74 STA

Probably the opposite is true, but the most important thing to understand about the seat tube angle is that it does not change the saddle fore/aft position relative to the bottom bracket.

All other things being equal, a steeper seat tube angle would require the saddle position a little further back on the rails. A slacker seat tube angle would require the saddle a little further forward. Sometimes a change to a straight or setback seatpost may be required to compensate for the different angles.

2. I have read some people state that they stay away from bikes that have a 74STA. Is this because they need a longer reach??

Different strokes for different folks. Longer legged folks could try a setback seatpost with a 74 STA

3. I am 68.5" and my inseam is 32.5". My usually concern is the top tubes are too long based on my height requirements. I ended up with a 4" drop between seat & bars. Wouldn't a 73STA in MOST cases make it even worse for me?

Depends. May want to try a straight post. May want to experiment with different rise stems. But try not to go with a stem less than 90mm. May want to try a different brand of frame.
Seat Tube Angle affects everything!cyclequip
Nov 21, 2002 4:55 AM
It changes your effective reach, drop, setback and bike handling. While there are enough adjustments available on seat rails, post, stem and bar spacing to 'make up' the differences in STA's, for any one rider there most definitely is an optimum setup - tri riders prefer the more upright (74) seat tubes (not because of some mythical hip opening or hip angle regulation) but because of a belief that the more forward position over the BB uses less hamstring muscle (needed for the run) while crit riders prefer the upright angles because steeper tubes (also head tube) promote quicker steering and greater ground clearance. Stage riders prefer more setback because the rider can use greater natural ankling to promote a longer effective lever and a more resilient pedal action and also get a greater level of comfort with relaxed tube angles.
qualifications....Steve_0
Nov 21, 2002 9:17 AM
most triathletes prefer angles nearing 78 degrees. Though there is certainly belief that the steep angles reserves hamstrings for the run (and this was the basis of the original 'tri' bike), the primary reason for a steep angle is for aerodynamic benefit.

Tri racing is essentially a time-trial; no drafting. Therefore every aerodymamic benefit is sought. The steep angles allow for more aerodynamic posturing.

To expand, many time-trialists also use steep-geometry bikes. These athletes are certainly not saving the hams for a brisk run after their race; like the triathlete, theyre seeking aerodynamic advantage.
Thanks All! Finally Got It!sjomi
Nov 21, 2002 8:45 AM
I had to reread the posts about a dozen times but now I can picture it.

thanks!