PeteHski

I chose the Endurace over the Ultimate. It adds a little long haul comfort and compliance for no perceptible loss of performance. I suspect it’s actually a little faster on mixed road centuries, carrying my 55 year old bones! The slightly wider 22 mm internal rims, 30 mm GP5000S TR tyres and slightly more comfortable cockpit all make a positive difference for me.

SoSmellyAir

I understand that this is how you (and maybe others) evaluate a frame. That is fine. But it does not explain the contradiction between your two original statements (that I had highlighted above), which is all the more surprising given your apparent grasp of the trigonometry. To me, we seem to be essentially doing the same thing by looking at a third measurement in addition to stack and reach.

DaveSSS 

Effective TT length minus seat tube setback, calculated with trigonometry, is not the same as reach, which is always measured at the top-center of the upper headset bearing. The idea is to eliminate calculations. I can look at a geometry chart and tell if a frame fits in a matter of seconds.

RChung

It's odd that stack and reach depend on a component that's not part of the frame: it depends on the fork. Different fork legs will change a frame's stack and reach.

SoSmellyAir

Huh? Effective top tube and reach are different distances along the same line, and the difference (which you are calling seat tube setback) is determined by the seat tube angle.

Understanding Bike Geometry (geometrygeeks.bike)

Now you have gone full circle. If the idea is to avoid calculation, then effective top tube is a number that more readily conveys the horizontal distance. Just because you are used to looking for a certain range of seat tube angles does not make the effective top tube number useless.

DaveSSS 

You're really confused. The BB center is the starting point for reach and stack. Stack is the vertical distance from that point to the top-center of the headset bearing. The reach is the horizontal distance from a vertical line through the BB to the same point on the tip headset bearing. There is no fork involved.

RChung

Dan Empfield, who posts as Slowman on Slowtwitch, is the originator of stack and reach. He agrees with me, not you.

DaveSSS 

There's no use discussing this any further. I've bought my last 6 frames using only stack and reach, since they all had the same 74.5 degree STA. With the stack and reach numbers, I know exactly what stem length I need and the stem angle and spacer setup, before I buy the frame. I got no surprises. I never look at effective TT.

Italian builders often never used seat tube angle, back in the days of horizontal TTs. They used setback of the seat tube, which was different for each frame size. A geometry chart from the 90's listed the seat tube length, along the centerline of the tube, the TT length and the "seat back" that is the distance from a vertical line through the BB center to the seat tube center line. My 55cm frame had a 55cm TT and 15.2mm "seat back". The STA cosine would be 15.2÷55 =.2764. The STA would be 73.96 degrees.

SoSmellyAir

You are really on a roll (of errors) in this thread.

The fork potentially affects both reach and stack because a longer or shorter fork effectively rotates the frame around the BB, moving the top of the head tube relative to the ground. This is how those "flip chips" work.

The fact that this is how you shop for bikes does not make the effective top tube number useless. Otherwise, no one would report it.

RChung

Yep. Without the fork, you wouldn't be able to identify horizontal or vertical lines through the BB, nor seat tube angle, nor head tube angle. (Even with a fork, you'd get different horizontal and vertical lines if you put two different sized tires/wheels on front and rear). Frame stack and reach (and STA, and HTA) depend on things that aren't part of the frame; parts of the frame that affect fit don't depend on stack and reach. Dan Empfield agreed when I pointed that out. Other frame builders have also agreed.

That stack and reach work as well as they do considering these points is actually kinda nice.

PeteHski

Maybe it is odd that I consider the fork as part of the frame set when making use of stack and reach values.

DaveSSS 

Most road bike frames come with a matching fork. Since stack and reach are measured from the center of the bottom bracket, a fork change does nothing to alter those values. If a different fork length were used it would change the steering geometry and slightly change the BB height, but that does not change stack and reach values. Stack and reach are not relative to the ground.

tomato coupe

They sure are -- stack is measured perpendicular to the ground and reach is measured parallel to the ground. As several people have pointed out, anything that changes the frame's relationship to the ground, such as changing the fork length, changes the stack and reach.

RChung

Have you seen photos of Moser's 1994 Hour Record bike? If he had used the same diameter front and rear wheels, even with the same fork, stack and reach would have changed.

RChung

You didn't click the link in post #57 above, did you?

Dan Empfield, the originator of stack and reach, agrees with me, not you.

PeteHski

When I see a manufacturer quote Stack and Reach I take it for granted they will be quoted with the original fork that comes with the bike. Same goes for all other fitting parameters which are dependent on the orientation of the frame relative to the ground plane.

Fork length only becomes relevant if you decide to change it.

tomato coupe

Yeah, that was part of the discussion that RChung linked. He describes a (rare) situation where it was relevant.

PeteHski

I’m not disagreeing. I realise that Stack and Reach will change with fork length, wheel sizes, tyre profile etc. But so what? They are fitting parameters relating to the whole bike in whatever configuration they are quoted.

So if my bike is quoted with a Stack and Reach of Y and X I would expect those numbers to relate to the designed fork length and wheel sizes. I don’t see what is odd about that?

RChung

I'm sure what I consider odd is more a reflection of me than it is of you: but I think it odd that a measurement of a frame depends on things outside of the frame of reference.

I tell the students that we teach undergrads how to do simple things, we teach the graduates how to do the complete detailed obsessive things, and we spend the rest of our careers figuring out when we have to do the complete detailed thing and when we can get away with the simple thing -- but that being able to get away with the simple thing doesn't mean that there's no difference between the simple and the detailed. I tell the grad students that they have to learn the details both because of edge cases where the simple thing fails, and also because we learn more if we understand why the simple thing failed.

Dan used cartesian coordinates based on the BB to formalize stack and reach. He could've used polar coordinates to describe those same two points in space -- but that would have made it clearer that he needed one more reference line, and Dan is more a cartesian guy than a polar guy.

wheelreason

Not in a significant way due to the limited range possible, but as the fork gets longer, both the top of the head tube AND the bb will get higher, but the former at a greater rate, so the stack will increase, but not enough to make a difference...

DaveSSS 

I thought that we were talking about real bikes that are sold with a matching fork and how to compare the fit of different models. There's no reason to make some huge change to fork length.

wheelreason

LOL. Doesn't change the facts. Two pedantic for you to?... #drumstick...

wheelreason

Correct, except that it rotates around the rear axle (more or less) not the bb...

DaveSSS 

Too obscure and irrelevant to real life situations.

tomato coupe

You're not looking at effective top tube length only because you've effectively fixed it by selecting the stack, reach, and seat tube angle. One could just as easily ignore seat tube angle by selecting the stack, reach, and effective top tube length. Both methods will get you to the same end point.
The discussion would end if you stopped making incorrect statements.