Influence of ST< on Top Tube length

[luaxwe]

People like talking about top tube length needs and requirements, regardless of seat tube angle. I would be interested to hear what role the ST< has on top tube length. I think most people give no regard to ST<, but i think if plays a big role in determining the #. I am not interested in ride characteristics with said <'s. Thanks :)
So far ,so good...thanks

Sorry, but I think I need to clarify myself. Assuming your position, which we are maintaining, is good, you come to buy an ots bike from me(I sell good fitting bikes). You tell me you need a 56 tt with a 120mm stem. In the meantime, your previous good fitting bike has a 74' seat tube<, but you are moving to a 73'< and 56 tt. Do you move to a 130mm stem? In other words, is a degree of ST< equivalent to a cm of tt at some point? Lets not talk about weight distribution on the bike just yet. I hope this makes more sense! Thanks

[e-RICHIE]

People like talking about top tube length needs and requirements, regardless of seat tube angle. I would be interested to hear what role the ST< has on top tube length. I think most people give no regard to ST<, but i think if plays a big role in determining the #. I am not interested in ride characteristics with said <'s. Thanks

they are not related atmo.
the lower body position is where all fitting starts (after cleats, that is...) and
it's thought of in terms of saddle height and saddle fore-aft. it's measured by
seat tube length and setback (or angle - yuck). once that is done, the upper
body (reach) can be looked at. "reach" is the distance from a rationally placed
saddle to the center of the handlebars, measured on the diagonal in most cases.
it's arrived at through 1) top tube length, 2) stem length, and 3) head tube
angle, all of which are chosen in harmony so that the bicycle both 1) fits your
hiney, and 2) works superbly swell atmo.

[taylorj]

they are not related atmo.
the lower body position is where all fitting starts (after cleats, that is...) and
it's thought of in terms of saddle height and saddle fore-aft. it's measured by
seat tube length and setback (or angle - yuck). once that is done, the upper
body (reach) can be looked at. "reach" is the distance from a rationally placed
saddle to the center of the handlebars, measured on the diagonal in most cases.
it's arrived at through 1) top tube length, 2) stem length, and 3) head tube
angle, all of which are chosen in harmony so that the bicycle both 1) fits your
hiney, and 2) works superbly swell atmo.

This why they call him "Master" as well as atmo.

[Too Tall]

"When you hear the chime, it's ATMO time"
"By Choice The Nation's Voice"*
"Three Chimes Mean Good Times on ATMO radio"
"Listen to the ATMO chimes, your signal for fine radio entertainment."**

[Dave Kirk]

I'll add a bit to Richard's post -

It seems that many feel that there needs to be a compensation in top tube length considering seat tube angle. Those that feel that way in most cases site KOPS as the reason why. It's true that if you are a strict KOPS guy that you would need to make adjustments to top tube length to keep the reach the same.

I personally see KOPS the same way I see the "inch of clearance between you and the top tube" and the "handlebars blocking the view of the front hub" fitting ideas. It's a coincidence at best and has zero basis in reality. IF you happen to feel best with your knee over the spindle AND this position puts your body in a balanced position over the wheels then you are golden.......... but it's only a coincidence that the knee is where it is and is a resultant, not a cause.

So in the end you'll find that many builders will take the same stand that ER and I take - that one does not need to consider seat angle (or setback for ER) when talking top tube length. Top tube length is top tube length and it is a major factor in determining reach and seat tube angle is used to position the body's center of mass properly over the wheels.

Time to make some sparks -

Dave

[luaxwe]

Please see the revision. This is what i am looking for! Bring it on!

[GrantM]

I think what you want to know is that Top tube "length" changes by approx 1 cm
for every degree of change to the seat tube angle.

The way I look at it, top tube simply connects your seat tube to your head tube.
If the frame is a sloper, you'll see how it's not even a straight line anymore.

Think "Effective reach". It's two halfs - The set back half + the "stack", which is the
section forward of the bb.

Drop to the handlebar changes how far your handlebars are, almost as much as the fore-aft.
Raising the bars shortens your reach, lowering the bars increases reach.

-g

[e-RICHIE]

Please see the revision. This is what i am looking for! Bring it on!

Sorry, but I think I need to clarify myself. Assuming your position, which we are maintaining, is good, you come to buy an ots bike from me(I sell good fitting bikes). You tell me you need a 56 tt with a 120mm stem. In the meantime, your previous good fitting bike has a 74' seat tube<, but you are moving to a 73'< and 56 tt. Do you move to a 130mm stem? In other words, is a degree of ST< equivalent to a cm of tt at some point? Lets not talk about weight distribution on the bike just yet. I hope this makes more sense! Thanks

if you are a seller of good fitting bikes, i would say caveat emptor. so here's
how to roll. lose the protractor. get a tape measure. assemble a surrogate
saddle/seatpost assembly (or several) that has the saddle placed in the center
of its rails. when you have frames of known sizes and seat tube angles (yikes
- i meant setback), insert the saddle/seatpost assembly and record the LINEAR
measurements with a tape measure. write them dowm in millimetres. after a few
dozen frames within the same size range, your intuition will tell you all you need
to know atmo.

[swoop]

i think these guys are telling you the absolute golden truth and you should look no further... but i think you're looking for the gross oversimplified thing that says for every degree you add or subtract a cm of tt.
the thing is that this is only incidentally true within a certain range of sizes and measurements... so in fact, its kind of useless info. its like a broken clock being right twice a day.

the truth is what theses guys are saying (they are master buddha-esque light emanating rose smelling farts level dudes at this)... that its not about those numbers, that its driven by setback which is driven by things happening at your feet, bb, and cranks too.

[zank]

I personally see KOPS the same way I see the "inch of clearance between you and the top tube" and the "handlebars blocking the view of the front hub" fitting ideas. It's a coincidence at best and has zero basis in reality. IF you happen to feel best with your knee over the spindle AND this position puts your body in a balanced position over the wheels then you are golden.......... but it's only a coincidence that the knee is where it is and is a resultant, not a cause.

Dave

:girl_hug: from me for Dave Kirk.

[luaxwe]

i think these guys are telling you the absolute golden truth and you should look no further... but i think you're looking for the gross oversimplified thing that says for every degree you add or subtract a cm of tt.
the thing is that this is only incidentally true within a certain range of sizes and measurements... so in fact, its kind of useless info. its like a broken clock being right twice a day.

the truth is what theses guys are saying (they are master buddha-esque light emanating rose smelling farts level dudes at this)... that its not about those numbers, that its driven by setback which is driven by things happening at your feet, bb, and cranks too.

i do agree with you completely. i am just being driven by curiosity and a desire to broaden my knowledge base.

[GrantM]

anyone interested can play here:

http://www.bikeforest.com/CAD/bikeCAD.php



-g

[mschol17]

Sorry, but I think I need to clarify myself. Assuming your position, which we are maintaining, is good, you come to buy an ots bike from me(I sell good fitting bikes). You tell me you need a 56 tt with a 120mm stem. In the meantime, your previous good fitting bike has a 74' seat tube<, but you are moving to a 73'< and 56 tt. Do you move to a 130mm stem? In other words, is a degree of ST< equivalent to a cm of tt at some point? Lets not talk about weight distribution on the bike just yet. I hope this makes more sense! Thanks

I think the simple answer in your test case is that you would need to move from a setback seatpost to a straight seatpost to keep the saddle in the same spot wrt the bottom bracket. Then, to keep the reach the same you would also need to have a longer stem.

[GrantM]

I think the simple answer in your test case is that you would need to move from a setback seatpost to a straight seatpost to keep the saddle in the same spot wrt the bottom bracket.

Most set back posts have 20-25mm more set back than a Zero offset post.
That's not the same as 1 degree of seat angle change.

-g

[luaxwe]

I'll add a bit to Richard's post -

I personally see KOPS the same way I see the "inch of clearance between you and the top tube" and the "handlebars blocking the view of the front hub" fitting ideas. It's a coincidence at best and has zero basis in reality.

Dave

But what about the 'extended forearm to end of middle finger from the nose of the saddle' reach method.

[mschol17]

Most set back posts have 20-25mm more set back than a Zero offset post.
That's not the same as 1 degree of seat angle change.

-g

That's what saddle rails are for, right? :thumbs_up:

[e-RICHIE]

But what about the 'extended forearm to end of middle finger from the nose of the saddle' reach method.

lose it atmo.
it's so 1970's bicycling magazine.

[GrantM]

That's what saddle rails are for, right? :thumbs_up:

Sure.
No need to change the seat post, just slide the saddle 1 cm forward.
If you're already all the way forward on the saddle rails, then a post
change takes care of it if needed.

The $64 million dollar question isn't how to replicate 1 position on 2 bikes,
it's finding the proper position for the rider in the first place... the rest is easy-peasy
from there....

-g

[Shawn G]

For whatever reason, the concept of achieving proper setback/seat tube angle confuses the crap out of me. I would think (and maybe I'm way off here) that proper steback would be achieved when you are able to place the saddle in the center of the rails and and that spot is the point at which the combination of "comfort" and efficiency of pedal stroke are maximized. That being said, I would think that choice of seatpost amongst any of a number of other things could have a significant effect on this.
I don't know why I have such a hard time wrapping my head around this concept, but I do. I am further mystified about how one designs a frame around setback without seeing the individual the frame is designed for in person, because I would think that femur length, etc. would/could have an effect on this. I would think for every indivdual there would be some differences in where the "best" setback would be. Finally, am I completely off base in understanding that the slacker STAs associated with Eddy Merckx's (and also some Pegoretti frames) will automatically provide a greater setback? If so, what is the intended benefit?
I apologize if my lack of understanding is too complete for a reasonable answer. Any insight would be greatly apprectiated! Please PM if that would be easier.

Shawn G

[Shawn G]

it's finding the proper position for the rider in the first place... the rest is easy-peasy
from there....

-g

I think this is the part that I am having trouble with...

Shawn G