Road Bike, Cycling Forums banner
1 - 20 of 23 Posts

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #1 · (Edited)
Well, I did have some time over the weekend to get on the drawing board and play with the geometry a little bit in respond to the argument on this subject and here is what I have to say. Like I said before, the frame/bike can be constrained in various ways to obtain various results, but to simply state that the top tube gets longer or shorter by adjusting the seat tube angle is really meaningless from a bike design standpoint.

If you were to fix the top tube length and let the head tube flow, then you would get a longer reach by changing the seat tube angle from 72 degree to 74 degree. This is evident by the change in front center dimension. However, if you do that, then you will need to shorten the top tube length to compensate for the change to get the same reach as before, assuming you are using the same stem, handlebar, etc. In another word, your reach should be the driving dimension, not the top tube length. On the other hand, the top tube lenght would be shorter if you were to fix the head tube (or reach) relative to the bottle bracket before changing the seat tube angle, which is what I would do if I were to design a frame, since the reach must be established first. So it is just a moot point for this whole argument, unfortunately started by me.

Now one can argue that it makes a difference between a frame that is already designed and a frame that is being designed. That is not really valid since we do not have the luxury to get a same size frame (a stock frame) with different seat tube angles, so this argument cannot really applied to a stock frame.

:thumbsup:
 

·
Registered
Joined
·
518 Posts
Read the following article-

http://richardsachs.com/articles/rsachsdouble.html

It might help clear up some of the confusion that some of you seem to
have.

By the way, understanding frame geometry does not require CAD drawings, not that there's anything wrong with them.
 

·
Registered
Joined
·
44 Posts
The whole point of C-40's comment was in relation to evaluating different frame sizes with different specs (regardless of manufacturer) about existing stock sizes.
How to make sense of them (as relates to your needs/ fit) while comparing.

Nothing more nothing less.

sheesh!! :)
 

·
Steaming piles of opinion
Joined
·
10,520 Posts
And what caused the argument is the "longer, shorter, more, less" discussions without various posters adding the "compared to what" part of their personal point of view.

If you leave the top tube the same length and increase the seat tube angle, the bike's reach measurement will be decreased. Because the seat extends above the top tube, increasing the STA will put the saddle closer to the head tube. Hence, a bike with a steeper STA will "always" have a longer TT than a slacker one, to compensate for that loss of reach. That's not a realistic argument because it implies constraint of the reach variable - which is by definition not constrainable when comparing designs, unless one is such an egomanic that they believe that only bikes that fit them personally can be described as properly designed, or are naive enough to believe that all bike designers ascribe to the same standard person, or to the same concept of proper ergonomic positioning.

On the other hand, if the wheelbase (or front-center/HTA) is constrained and the STA is steepened, the TT and reach will both become shorter, as a simple function of geometry. That's not realistic either, since practically speaking wheelbase is a design variable bounded at the lower end by wheel size and overlap, and functionally unbounded at the upper end. As a result, it's indeed true that steeper STA's will tend to have longer top tubes. Unfortunately, there are some unable to discriminate between tendency and fact.

The practical constraint in design that tends to result in longer TT's at steeper STA's is the so-called 'open angle', or the theoretically best angle between the torso and leg for best power transmission. Moving the body flatter (more aero) increases the reach required of the bike design, and so the TT length. But lowering the torso also decreases the open angle, restricting breathing and muscle utilization, and often resulting in thighs hitting the chest. To compensate, the STA must be steepened to allow adequate power utilization. Hence, STA can be said to be indirectly dependent on TT length, rather than the other way around.

Practically speaking, both sides of the argument had it wrong, which is a nearly universal truth.

:p
 

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #7 ·
jtolleson is a trouble maker???

jtolleson said:
This second unnecessary thread started by someone who claimed that compact geometry bikes rode better and w/a lower center of gravity...
May I suggest that you try out a compact frame to see if you like it? Come on man, if this thread was unnecessary then you would not be replying to it, would you. Get used to it man!:D
 

·
You're Not the Boss of Me
Joined
·
7,749 Posts
I'm not a man. My custom Seven has some TT slope, which was done to accommodate my standover needs. But I can assure you that it has no different "center of gravity" than my Bianchi or my Litespeed.

Oh, and whether I respond to a thread has nothing to do with whether it is "necessary," but thanks for inflating my sense of self-importance! I just thought you were really being silly about this... kinda a "dog with a bone" thing. The more you try to explain why you were right and C40 was wrong, the worse it looks for you. Let it go.

But since you won't, you are ignoring the adjustment to saddle fore-aft that must be main when STA is steepened (assuming the rider wants a similar position vis-a-vis pedal spindle).
 

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #9 ·
jtolleson is something!

jtolleson said:
I'm not a man. My custom Seven has some TT slope, which was done to accommodate my standover needs. But I can assure you that it has no different "center of gravity" than my Bianchi or my Litespeed.

Oh, and whether I respond to a thread has nothing to do with whether it is "necessary," but thanks for inflating my sense of self-importance! I just thought you were really being silly about this... kinda a "dog with a bone" thing. The more you try to explain why you were right and C40 was wrong, the worse it looks for you. Let it go.

But since you won't, you are ignoring the adjustment to saddle fore-aft that must be main when STA is steepened (assuming the rider wants a similar position vis-a-vis pedal spindle).
Saddle position once set relative to the bottom bracket should not change when rotating the seat tube. :cool:
 

·
You're Not the Boss of Me
Joined
·
7,749 Posts
Someone needs a little reading comprehension 101. There's no shame in not understanding this, but what's starting to get a little annoying is your resistance to hearing multiple explanations (most far more detailed than anything I could offer) of the interplay between effective TT length and STA.

And starting a second thread to continue it... sheesh.

PS, for viewers just now joining the debate, this horse was beaten in http://forums.roadbikereview.com/showthread.php?t=63124
 

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #13 ·
what???

jtolleson said:
Someone needs a little reading comprehension 101. There's no shame in not understanding this, but what's starting to get a little annoying is your resistance to hearing multiple explanations (most far more detailed than anything I could offer) of the interplay between effective TT length and STA.

And starting a second thread to continue it... sheesh.
Are you referring to jtolleson??? :confused: :mad2:
 

·
eminence grease
Joined
·
18,559 Posts
AJL said:
uh jtolleson was the poster that you quoted - are you on some sort of mind altering medication?
School's out and the nurses are not seeing to the daily Ritalin doses. I bet we're in for more and more of this as the summer wears on.
 

·
Not Banned
Joined
·
49,249 Posts
wait I'll send it on a total digression

but, but a compact frame is stiffer. the smaller triangle makes the frame stiffer
(only when you are out of the saddle)
Go to Giants website, see how they turn the 'long seatpost flex' lemon
into 'dampens ride' lemonade
 

·
Registered
Joined
·
3,497 Posts
jtolleson: Someone needs a little reading comprehension 101

Frankly I think you need some reading comprehension. It seems to escape you that C40 originally talked about changing effective TT instead of changing reach. And try not assume that everyone has trouble with these concepts the way you did. OK?
 

·
You're Not the Boss of Me
Joined
·
7,749 Posts
No need to flame, Serious. My recollection is that it was originally your error that C40 corrected, but I'm not jumping on your ad hominem bandwagon.

I think we can all stand by the exchange that was in the other thread, and I'll also stand by what I've posted here. Tigoat started this second thread on the same topic with the claim that "If you were to fix the top tube length and let the head tube flow, then you would get a longer reach by changing the seat tube angle from 72 degree to 74 degree." I said that was incorrect because it disregards the adjustment to saddle fore-aft that is made to keep the same position relative to the bottom bracket.

Seriously, although the issue creates confusion, it isn't debatable. Many cycling related websites cover it in detail.
 

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #19 ·
terry b said:
School's out and the nurses are not seeing to the daily Ritalin doses. I bet we're in for more and more of this as the summer wears on.
Glad to see you guys having a little fun with my post, pleasure is all mine!

Cheers!:thumbsup:
 

·
titanium junkie
Joined
·
667 Posts
Discussion Starter · #20 ·
jtolleson is a girl?

jtolleson said:
No need to flame, Serious. My recollection is that it was originally your error that C40 corrected, but I'm not jumping on your ad hominem bandwagon.

I think we can all stand by the exchange that was in the other thread, and I'll also stand by what I've posted here. Tigoat started this second thread on the same topic with the claim that "If you were to fix the top tube length and let the head tube flow, then you would get a longer reach by changing the seat tube angle from 72 degree to 74 degree." I said that was incorrect because it disregards the adjustment to saddle fore-aft that is made to keep the same position relative to the bottom bracket.

Seriously, although the issue creates confusion, it isn't debatable. Many cycling related websites cover it in detail.
Oh please do not bring C40 into this thread, C40 and tigoat had settled previously as stated, so it will just be jtolleson and tigoat from now on. How romantic!!! Did you learn your cycling stuff from the web? How nice! :cool:
 
1 - 20 of 23 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top