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New Math (Frame Geometry)

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New Math (Frame Geometry)

Old 06-30-12, 09:27 AM
  #51  
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Originally Posted by datlas
And this is the geometry of the Nashbar Aluminum frame. If you compare the 60cm, it's VERY close to the specs on my custom Ti frame.


My situation is similar to yours. I purchased the 60 cm Nashbar frame but have not assembled it yet. I have not had the "luxury" yet of buying a custom frame. I think you will find that some of the Giant frames, both aluminum and carbon fiber have geometries that come close to what you are looking for.
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Old 06-30-12, 12:03 PM
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Originally Posted by guadzilla
The purpose of Stack and Reach is to standardize bike dimensions.
Which is exactly my point... Reach is not independent of Stack. Stack consistently tells you what you'd have to do to get your bars at the correct height... but Reach isn't a consistent value that tells you what you need to do to get the bars in the correct lateral position. I'm talking about a number that is a property of the *frame* that represents its "reach". Reach doesn't do it.

As an example, I'm comparing two frames, because I need to decide which one is the best fit. One has a 500mm stack and 410mm Reach. The other has a 530mm stack and 410mm Reach. If a 110mm stem with 30mm of spacers gets me in the correct position on the first frame, shouldn't I expect the same stem with 0 spacers to put me in the same position on the second one? After all, the Reach is the same... but I need a 100mm stem to get the same fit. Wouldn't it make more sense if the Reach was independent of Stack? If you always measured Reach at a fixed distance above the bottom bracket (instead of the top of the headtube), then it *would* be consistent.
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Old 06-30-12, 12:22 PM
  #53  
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Originally Posted by rruff
Which is exactly my point... Reach is not independent of Stack. Stack consistently tells you what you'd have to do to get your bars at the correct height... but Reach isn't a consistent value that tells you what you need to do to get the bars in the correct lateral position. I'm talking about a number that is a property of the *frame* that represents its "reach". Reach doesn't do it.

As an example, I'm comparing two frames, because I need to decide which one is the best fit. One has a 500mm stack and 410mm Reach. The other has a 530mm stack and 410mm Reach. If a 110mm stem with 30mm of spacers gets me in the correct position on the first frame, shouldn't I expect the same stem with 0 spacers to put me in the same position on the second one? After all, the Reach is the same... but I need a 100mm stem to get the same fit. Wouldn't it make more sense if the Reach was independent of Stack? If you always measured Reach at a fixed distance above the bottom bracket (instead of the top of the headtube), then it *would* be consistent.
Ah, ok, I see your point, thanks for the explanation.

You are right, it won't tell you this info, as HTA will play a role here. But I don't see S/R numbers as being the definitive way of answering this question. Using your example, I know that if a 110mm stem fits the first bike, then a 100-120mm stem will get me the same fit on a second bike. It may not be precise enough for you, but at least for me, it is good enough, as it helps me pick the correct frame size.

I sea S/R as a way to pick the correct frame size, not ready-to-use measurements to get the bike set up correctly - and it does help with this very well. You are also correct - it won't necessarily help you get the exact fit right away.

The problem with using Reach at a fixed height above the BB is that if I have my bars at a substantially different height, then I will need to re-calculate the reach as applicable for me - and the same HTA-induced error margin applies here as well. So I may gain consistency, but the numbers become less intuitive.
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Old 06-30-12, 12:54 PM
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Originally Posted by guadzilla
The problem with using Reach at a fixed height above the BB is that if I have my bars at a substantially different height, then I will need to re-calculate the reach as applicable for me - and the same HTA-induced error margin applies here as well. So I may gain consistency, but the numbers become less intuitive.
The HTA has only a tiny effect... they are all ~72.5 +-1 degree. It's the HT *length* that has a large effect... *if* you are measuring at the top of the HT.

I think you still misunderstand. If you take the Reach at a fixed height above the bottom bracket, and you have determined that a frame with 400mm Reach gives you the correct extension with a -6 110mm stem, then this will be true of every 400mm Reach frame you look at. You won't need to screw around with adjusting for HT length (or Stack), to figure you what the proper stem length would be.
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Old 06-30-12, 06:54 PM
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Originally Posted by rruff
The HTA has only a tiny effect... they are all ~72.5 +-1 degree. It's the HT *length* that has a large effect... *if* you are measuring at the top of the HT.

I think you still misunderstand. If you take the Reach at a fixed height above the bottom bracket, and you have determined that a frame with 400mm Reach gives you the correct extension with a -6 110mm stem, then this will be true of every 400mm Reach frame you look at. You won't need to screw around with adjusting for HT length (or Stack), to figure you what the proper stem length would be.
What you are doing rruff is speaking into a long vacuous tunnel with no light at the end. They aren't going to understand it any more than they did the third time you and I repeated ourselves. Time to give it up bro. They don't get the importance of sta aka the horizontal relationship of saddle to vertical BB centerline. Have a look below. Below is Cervelo's feeble attempt to pacify all the simple minded guys who can't digest the importance of frame dimensions in totality...enter stack and reach. Reach is a mis-nomer as you have tried to explain. Cervelo's definition of reach is from BB vertical centerline to steerer center. That of course is ridiculous. Reach is from the saddle to steerer or handlebar center. The most grevous oversight of Cervelo's effort to placate the 'top tube club' is complete lack of factoring in the critical relationship of saddle fore/aft position relative to BB vertical center...not being part of the equation of reach relative to seat tube centerline. This generally is the starting point of any decent fitter. Stack is just a metric for keeping score of the vertical height of the head tube. I personally believe it to be more meaningless than head tube length...it is simply a method of tracking the height of the head tube taking out HTA which is a factor when looking at head tube length for net height of the bars.


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Old 07-01-12, 06:32 AM
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Originally Posted by blargman
In my short experience though, I have found few manufacturer's that list effective top tube.
You shouldn't find any brand that does not list this dimension. The term "effective" is a misnomer and not always used. If a drawing is also supplied, it will be the horizontal TT length, which intersects the seat post, rather than the seat tube.

The TT length alone is NOT enough to define the reach of the frame. Both the TT length and the seat tube angle are needed to define the reach, since each degree that the STA is steeper, increases the reach by about 1cm (with the saddle in the same position, relative to the BB).

The stack concept eliminates errors due to fork length differences between brands. Unfortunately, it references the top-center of the head tube, so you still have to know the minimum headset stack height to have all the necessary fit information.

The manufacturer's listed reach also has a problem. It can only be compared accurately to other frames with the same stack height. If the stack heights are not the same, then a correction of 3mm for each 10mm of stack height difference needs to be applied. For example, you may see two frames from the same manufacturer, where a 54cm and 56cm have the same reach, but the stack height on the 56cm frame is 2cm taller. The reach is not really the same. Subtract 6mm from the reach of the smaller frame and that's the reach at the same stack height as the larger frame.
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Old 07-01-12, 07:33 AM
  #57  
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For those in here who do not like defined and easy to identify reference points for stack and reach. What would you rather have that works on different frame sizes, can be measured, and can be derived from other known dimensions?
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Old 07-01-12, 09:52 AM
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Originally Posted by mmmdonuts
For those in here who do not like defined and easy to identify reference points for stack and reach. What would you rather have that works on different frame sizes, can be measured, and can be derived from other known dimensions?
Dave above, rruff and I explained the problem with stack and reach. Stack and reach came about due to the conundrum of frameset mfr's when compact geometries which btw are a good thing...were developed. When this occured, seat tube and head tube lost their proportionality. This is key. The reason why seat tube length was used for years and that 99% of riders can still fit on frames with 'virtual' seat tube length = .667 X inseam is seat tube length was always proportional to head tube length with horizontal top tubes. But frames rarely have either straight or horizontal top tubes...nor even 'virtual' proportionality between seat tube length and head tube length with the advent of endurance frames with longer head tubes. This is when virtual and effective sizing became available. At least with the latter, there is some sense of saddle position unlike Cervelo's stack and reach. With virtual or effective top tube sizing...effective top tube length is the imaginary line from the seat post centerline to the steerer center along the top edge of the head tube. This is still better than stack and reach. So you want to know who does it right? Specialized does. Make no mistake, frame mfr's have been conflicted about frame sizing for some time. This includes Look and a few other mfr's who sized their framesets center to center which confuses so many.

So is there a bottom line or summary in all of this which leads to so much confusion on the 41? Yes as I have stated all along. There is no silver bullet or single metric for frame sizing that works. A frame's fit has to be taken in totality. That means:
1. length of the top tube
2. height of the head tube
3. seat tube angle which determines the critical horizontal distance from sit bones to BB center for proper weight distribution on the bike. When you push the saddle back on the rails or opt for a different seat post with greater setback because of Cervelo's wacky fixed sta's, reach is affected which isn't reflected in their 'definition' of reach....unlike typical 'effective' sizing which is top tube measurement from center of seat post to center of steerer long the top of the head tube. This is how Specialized, Trek, Giant and several other mfr's do it.
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Old 07-01-12, 10:14 AM
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Stack isn't too bad really for defining a frame's vertical dimension, and Reach would be ok if it was calculated at a fixed height above the BB. There is a good reason for not bringing the STA into the Reach measurement because it would just confuse things again. And I guess for "setback" you can just look at the STA, and figure out the sort of post you would need to get the saddle in the right spot.

But like I said earlier, there is really no substitute for determining the xy positions of your saddle and bars relative to the bottom bracket, and then calculating exactly what it would take to connect the dots. In fact a fairly simple graphical program would be a nice thing for every shop and frame manufacturer to have on hand for sizing... although I guess it might encourage customers to be excessively anal about every mm.
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Old 07-01-12, 10:23 AM
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Originally Posted by rruff
Stack isn't too bad really for defining a frame's vertical dimension, and Reach would be ok if it was calculated at a fixed height above the BB. There is a good reason for not bringing the STA into the Reach measurement because it would just confuse things again. And I guess for "setback" you can just look at the STA, and figure out the sort of post you would need to get the saddle in the right spot.

But like I said earlier, there is really no substitute for determining the xy positions of your saddle and bars relative to the bottom bracket, and then calculating exactly what it would take to connect the dots. In fact a fairly simple graphical program would be a nice thing for every shop and frame manufacturer to have on hand for sizing... although I guess it might encourage customers to be excessively anal about every mm.
As you say...stack isn't so bad and exists for the reason that Dave stated...it takes fork length out of the equation...and implicitly includes HTA as well for net head tube height. It is however a metric few are conversant with...unlike most know if they are comfortable on a bike with a 125mm head tube length versus 225mm. Reach however is flawed the way Cervelo does it. But look no further that this thread to underscore what you also correctly stated about sta confusing people. It does as testiment by this thread.

I highlighted in bold above your most important comment which has been the whole contention of this thread. Setback which includes seat tube angle to achieve proper weight behind the BB is 'critical' and there is no substitute for considering a rider's position relative to the BB (which affects reach) which stack and reach do not account for.

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Old 07-01-12, 10:36 AM
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Changing the seatpost setback to compensate for a less-than-ideal STA does NOT affect the reach or the fit. If you know what reach and stack you need, then the only problem that can occur is finding out that you can't get a post that put's the saddle where you want it. Any other fit issue is a failure to properly apply the stack and reach concept. This is no different than failing to properly compensate when comparing two frames with the same TT length, but different STAs. Both frames might have the same 55cm TT, but if one has a 74 degree STA and the other a 73, then the reach on the first frame is really about 10mm longer, once both saddles are placed in the same position, relative to the BB.

One problem that stack takes care of is differences in the BB drop and fork length, which can cause fitting errors. If you go only by the head tube length and don't know or pay attention to these dimensions, you may find two frames with the exact same head tube length that differ in fit by 10mm or more. Many brands don't even list the length of their forks, which makes their geometry charts incomplete.

These days, the terms center to center and center to top are meaningless, since most frames no longer have round tubes to reference. Even in the days when c-c and c-t were meaningful, there was no reason to blame a fitting problem on the reference lines, unless you weren't smart enough to read the head tube length.
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Old 07-01-12, 10:53 AM
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Originally Posted by DaveSSS
Changing the seatpost setback to compensate for a less-than-ideal STA does NOT affect the reach or the fit.
Semantics comes in. Changing the seatpost setback to compensate for less than ideal sta DOES affect rider reach AND fit to the handlebar...common sense. If does not affect 'Cervelo's definition' of reach however...you are quite correct.

What you repeatedly mentioned however is pertinent...and that is STA. There is no escaping the importance of it which is perhaps the biggest misunderstanding in this thread. For those analytic types which includes you, me and rruff, best way to determine how well a frame fits is to plot it in X,Y coordinates as rruff stated...using a simple CAD program. Cervelo's attempt for the layperson is stack and reach. This basically sets the A or primary datum as the BB centerline.
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Old 07-01-12, 12:09 PM
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I agree with DaveSSS. What C4L went into is not what I asked. Dave is correct in that adjusting setback to compensate for STA does not affect reach BECAUSE BB and saddle location should be the fixed points. Once the reach is known the STA is irrelevant as a component of that dimension. STA comes into play to select a saddle and seatpost that can put the rider in the right place with setback, rail length, etc. I scanned the Cervelo frame sizing presentation, didn't study it, and did like what I saw. The proportional increase in stack and reach makes sense to me.
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Old 07-01-12, 01:04 PM
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Originally Posted by DaveSSS
If you know what reach and stack you need, then the only problem that can occur is finding out that you can't get a post that put's the saddle where you want it.
Quite true... but it isn't necessarily trivial. Sort of like not being able to get a stem that works decently because Reach or Stack are wrong. So another term which represents "setback" of the frame... which might as well be STA, or something similar for TT bikes... should also be considered an essential frame dimension.
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Old 07-01-12, 01:08 PM
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C4L yet again demonstrates that his homegrown "knowledge" is more correct to him than industry standard terminology.

Regarding his rant on page 2 about Cervelo's definition of stack and reach-- those are industry terms. They are right, you are wrong C4L. See here for one example-- many others exist (it's a standard term, after all ): https://velonews.competitor.com/2012/...tically_216035

There's a glaringly obvious problem with Campag4Life's insistence on using the STA and saddle as a component of reach: you *DON'T* adjust the saddle or seatpost setback for reach! Duh. You set seatpost setback and saddle position based on your knee position, and I'm not even getting into a KOPS debate here, I will just say that a given person's knee positioning is more critical and more inflexible than their reach.

Why? You can sit more or less upright based on reach. In fact this is normal, riding a time trial bike vs road bike vs XC race MTB vs downhill MTB vs beach cruiser.
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Old 07-01-12, 06:27 PM
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Originally Posted by rruff
Quite true... but it isn't necessarily trivial. Sort of like not being able to get a stem that works decently because Reach or Stack are wrong. So another term which represents "setback" of the frame... which might as well be STA, or something similar for TT bikes... should also be considered an essential frame dimension.
Dave makes several good points but honestly rruff, we have already been through this 5 times and what I marked in bold and what I have said repeatedly is the key deficit here for the guys who still don't get it and, it doesn't look like they are going to either.
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Old 07-01-12, 06:34 PM
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Originally Posted by ColinL
C4L yet again demonstrates that his homegrown "knowledge" is more correct to him than industry standard terminology.

Regarding his rant on page 2 about Cervelo's definition of stack and reach-- those are industry terms. They are right, you are wrong C4L. See here for one example-- many others exist (it's a standard term, after all ): https://velonews.competitor.com/2012/...tically_216035

There's a glaringly obvious problem with Campag4Life's insistence on using the STA and saddle as a component of reach: you *DON'T* adjust the saddle or seatpost setback for reach! Duh. You set seatpost setback and saddle position based on your knee position, and I'm not even getting into a KOPS debate here, I will just say that a given person's knee positioning is more critical and more inflexible than their reach.

Why? You can sit more or less upright based on reach. In fact this is normal, riding a time trial bike vs road bike vs XC race MTB vs downhill MTB vs beach cruiser.
No you don't...in bold. Duh as you say and no doubt a popular word in your vocabulary...lol.
However...reach is increased if increasing setback...Duh...in your parlance. This is why two size frames of the same model with different STA sometimes have the same net reach even though their top tubes are different length if setting the rider up in the same position relative to the BB.
No doubt this will go right over your head because you don't have a clue what is being discussed here.
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Old 07-02-12, 04:55 PM
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There is no method for measuring a frame that's perfect, but the stack and reach concept eliminates the BB drop, STA and fork length as possible sources of error. If you once know what stack and reach that you need, then comparisons will be more accurate. providing that you know how to compensate for different stack heights.

I've never seen any brand fail to list the STA, which is necessary to figure the seatpost setback needed. Most people don't realize this, but Cervelo fibs about the STA. When the R3 first came out with it's ridiculously short 399mm chainstays, they still listed the STA as 73 degrees, It's easy to prove that the tire would hit the seat tube, if it was angled as 73 degrees, along a line through the center of the BB, but it is not. The seat tube is moved forward of the BB centerline and actually angled somehwere in the 71-72 degree range. The net result probably places the saddle somehwere near the position of a bike with a normal STA of 73 degrees. I owned one of these frames, but only long enough to ride it for 200 miles. It sucked, IMO.

After a few years of production, Cervelo came to their senses, lengthened the chainstays to a normal 405mm and changed the fork angle on the 51cm from 73 to 72.2 degrees, which pretty well duplicates brands like LOOK, who know what they are doing.
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Old 07-02-12, 08:49 PM
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Wow, this repartee is hillarious. All "stack" and "reach" are doing is referencing one part of the bike (the top of the headtube) relative to another (the bottom bracket). For most people, if two bikes have identical stack and reach, putting the identical bars and stem and adjusting an identical saddle so the tip is the same for-aft distance from the bottom bracket will result in bikes of identical fit. Yes, you still have to do some work figuring out the saddle for-aft position, but for most people, this is within the range of the saddle adjustment.

The trick is fitting two different bikes of different angles. Again, stack and reach can be compared. A bike with identical reach and, say, 20mm higher stack will require 20mm less spacers under the stem. Put the saddle in the correct place relative to the bottom bracket and the fit will be identical.

Seat tube angle, then, has nothing at all to do with fit. It is merely a way to change the chainstay length; steeper seat tubes lead to shorter wheelbases. You never position your saddle relative to the saddle clamp as a measure of fit. You position the saddle relative to the bottom bracket. In fact, all the most important measures of fit are the measures referenced to the bottom bracket. When I was fit to my bike, I was given four numbers: saddle height (along a line from the saddle contact point through the bottom bracket), saddle setback (relative to bottom bracket), bar reach (relative to the saddle tip, which is referenced to the bottom bracket), and bar drop (relative to the saddle height, which is referenced to the bottom bracket). Notice where the reference of all these four measurements are? The bottom bracket.

This is what Stack and Reach are meant to correct. In a traditional geometry chart, every measurement is ultimately referenced to a horizontal line between the two dropouts and a vertical line through the bottom bracket. This is a carryover from how framebuilders build jigs and might give a person an indication for how the bike will ultimately handle. Stack and Reach just moves the horizontal reference line down to intersect the bottom bracket for the purpose of sizing.

Effective Top Tube and Head Tube Height are the poor man's stack and reach. Given that all road bikes have nearly identical head tube angles, these two parameters work almost identically to Reach and Stack. The only issue is some bikes have significantly lower bottom brackets than others, which affects bar height. Unless you are pushing at the edge of bar height and saddle for-aft adjustment, a frame with roughly the same effective top tube and head tube height can be fit identically.
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Old 07-03-12, 06:51 AM
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Originally Posted by Brian Ratliff
Wow, this repartee is hillarious. All "stack" and "reach" are doing is referencing one part of the bike (the top of the headtube) relative to another (the bottom bracket). For most people, if two bikes have identical stack and reach, putting the identical bars and stem and adjusting an identical saddle so the tip is the same for-aft distance from the bottom bracket will result in bikes of identical fit. Yes, you still have to do some work figuring out the saddle for-aft position, but for most people, this is within the range of the saddle adjustment.

The trick is fitting two different bikes of different angles. Again, stack and reach can be compared. A bike with identical reach and, say, 20mm higher stack will require 20mm less spacers under the stem. Put the saddle in the correct place relative to the bottom bracket and the fit will be identical.

Seat tube angle, then, has nothing at all to do with fit. It is merely a way to change the chainstay length; steeper seat tubes lead to shorter wheelbases. You never position your saddle relative to the saddle clamp as a measure of fit. You position the saddle relative to the bottom bracket. In fact, all the most important measures of fit are the measures referenced to the bottom bracket. When I was fit to my bike, I was given four numbers: saddle height (along a line from the saddle contact point through the bottom bracket), saddle setback (relative to bottom bracket), bar reach (relative to the saddle tip, which is referenced to the bottom bracket), and bar drop (relative to the saddle height, which is referenced to the bottom bracket). Notice where the reference of all these four measurements are? The bottom bracket.

This is what Stack and Reach are meant to correct. In a traditional geometry chart, every measurement is ultimately referenced to a horizontal line between the two dropouts and a vertical line through the bottom bracket. This is a carryover from how framebuilders build jigs and might give a person an indication for how the bike will ultimately handle. Stack and Reach just moves the horizontal reference line down to intersect the bottom bracket for the purpose of sizing.

Effective Top Tube and Head Tube Height are the poor man's stack and reach. Given that all road bikes have nearly identical head tube angles, these two parameters work almost identically to Reach and Stack. The only issue is some bikes have significantly lower bottom brackets than others, which affects bar height. Unless you are pushing at the edge of bar height and saddle for-aft adjustment, a frame with roughly the same effective top tube and head tube height can be fit identically.
Brian...do you have the four metrics you mention above available?...in particular the dimension you mention about saddle setback to BB? What point on the saddle defines its relationship to the BB?...the term you call setback.

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Old 07-03-12, 07:47 AM
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Originally Posted by Campag4life
Brian...do you have the four metrics you mention above available?...in particular the dimension you mention about saddle setback to BB? What point on the saddle defines its relationship to the BB?...the term you call setback.
Setback is referenced from the saddle tip. You take a plumb bob, drop it down from the saddle tip and measure the horizontal distance between the plumb bob and the bottom bracket. On my bike, I believe it is somewhere in the 60-65mm range. If you duplicate the saddle height, setback, reach and stack, you will get identical fits with identical components. The great thing about reach and stack for frame sizing is, once you get the saddle placed right relative to the bottom bracket, you can use the difference in reach and stack between two frames to determine changes you need to make to the stem and/or spacers between the two bikes to make an identical fit.

The saddle is tough because the are lots of different types and they are all different lengths and shapes and there is no established reference point. If you change saddles, typically you make your best guess and then you adjust it slightly one way or another until it feels right. I use one saddle now for everything; if you do that, it isn't that bad. I'd be really mad if Selle Italia ever stops making the Flite.
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Old 07-03-12, 08:19 AM
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rruff...I will leave it to you explain why two out of four metrics that Brian believes are important...both relating to saddle tip position have nothing to do with stack and reach...a complete contradiction. rruff, you could bring up sta again, but that would make 6 times and of course 5 should suffice.
Have fun boys.
PS: rruff, I wouldn't bother.

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Old 07-03-12, 08:36 AM
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I've gotten into flamewars before (very, very few here-- it is not that kind of place) and the only proper thing to do when cooled down is apologize. But he's still piling it on. Maybe he thinks that's ok because this is 'just the internet', but I would call that trolling.

I would hope he's not like that in real life.
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Old 07-03-12, 09:01 AM
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Seriously Campag, what bug is up your butt? And what it your point in all this? Do you honestly believe that frame geometry has anything at all to do with fit? There are three contact points the human body makes on a bike: hands, feet, and ass. Put these three points in the correct position relative to each other, you get identical fits however the frame is shaped. Frame geometry is simply the measurements of pieces of the frame.
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Old 07-03-12, 09:20 AM
  #75  
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Originally Posted by Campag4life
...

rruff...I will leave it to you explain why two out of four metrics that Brian believes are important...both relating to saddle tip position have nothing to do with stack and reach...a complete contradiction. rruff, you could bring up sta again, but that would make 6 times and of course 5 should suffice.
Have fun boys.
PS: rruff, I wouldn't bother.
What is it with you and seat tube angle?

There are three contact points on a bike. One is fixed: that be the bottom bracket, i.e. feet. As you well know, you need two coordinates to define a point in space relative to another point. There are two contact points that need defining relative to the feet: that be the hands and the ass. Two points require four measurements to fix in space. That be the four measurements I stated in the above post.

Now then, where does seat tube angle come into the picture other than to tell you what seatpost you need to buy? Or is this your point after you unravel all the superiority complex? That you need the seatpost angle in addition to stack and reach to figure out what components you need to buy? If that's your point, then why don't you just come out and say so?
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Last edited by Brian Ratliff; 07-03-12 at 09:24 AM.
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