Stack & Reach definition

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Lightweenie
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by Lightweenie

Hello all,

I realise that stack&reach seems replace the classic seat-tube/top-tube length as a means for selecting the correct frame size. So I read the Cervelo paper on Stack&Reach (it seems that they came up with the notion?), and it seems to me that this is incomplete for determining the size. More specifically:

1. It seems to determine only the relative position of the top of the top-tube with respect to the bottom bracket. But what about the relative position of the saddle? Shouldn't this be determined by the seat-tube angle? I noticed that Cervelo has a constant seat-tube angle, but does this also make sense to have one angle for all sizes? What if one wants to compare models from different manufacturers that perhaps have different angles? Also I guess the seat-tube length should matter, I think that even if the distance from saddle to bottom bracket is the same it should make a difference in feel based on how much seat post is showing.

2. Another thing I think is not considered by just stack and reach, is the fork trail. Any thoughts on that?

So perhaps one should list stack&reach, fork trail, seat-tube angle and length to get a full information on frame measurement?

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ITTY
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by ITTY

I think people tend to overthink the geometry thing. To me, the most important dimensions are top tube length (determining reach) and head tube length (determining stack). Saddle height and fore/aft are not set by frame dimensions, only 'suggested'. Reach and stack can be changed with stem/spacer choices, but these affect handling.
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wingguy
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by wingguy

Saddle setback doesn't automatically follow seattube angle. A lot of people would want the saddle set to where it works best for them, not just where it sits centre rail on their bike. Usually differences are well within the range of rail adjustment (unless you're at or near the limit, in which case it can matter for fit). In the smallest sizes the difference can be more sizeable - for example a Cervelo 48 and a Colnago 42s with a theoretical 650mm saddle hight would have a 28mm setback difference - but then a 0mm post on the Cervelo would get you to about where a 25mm post would on the Colnago.

If you accept that line of thought, then stack and reach is the most precise way of determining the handlebar position between bikes because it's not affected by STA, or BB drop etc like TT and HT are.

Personally I don't really worry about fork trail because I think toe overlap is an imaginary problem.

Lastly most geometry charts, Cervelo included, give you all the standard info in addition to stack and reach anyway so this is a redundant coversation, isn't it?

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Lightweenie
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by Lightweenie

hm, I should clarify one thing. Say you have two frames with the same stack and reach but different seat-tube angles. Probably you can still place the saddle on the same relative position as in your example. But I think that this would result in a totally different bike feel, wouldn't it?

About your last point, the thing is that I my current frame is older and not so popular so I can't find a geometry chart for it. Say I am extremely comfortable with its geometry and am looking for a new frame with as similar geometry as possible (but it is not easy to measure angles - at least i have no idea how to do it accurately enough). Reading this Cervelo page it seemed to imply that stack&reach is all one needs, so I started wondering... Also even if you have detailed geometry charts of two frames, when you can do some basic trigonometry calculations and determine the exact difference, it would be interesting to know wether there is some simpler measure.

wingguy
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by wingguy

Ref feel... no, not if the saddle ends in the same place. Why would it? Regardless, that's not a sizing issue, and you'll never be able to know for sure how a bike subjectively feels by looking at a chart.

Ref calculating stack and reach... no simple way that I know of. Best grab a tape measure, plumb line and spirit level. Then you can check saddle setback at the same time as well. If you have it set where you want it there's nothing to lose by checking against the bikes you're looking at.

savechief
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by savechief

See this WW post for a Stack & Reach Calculator:
viewtopic.php?f=3&t=94384

Trail Calculator:
http://yojimg.net/bike/web_tools/trailcalc.php
Time VXRS Ulteam (7.16 kg)
viewtopic.php?f=10&t=120268

dunbar42
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by dunbar42

Here are a couple of posts by Leonard Zinn on the topic. He shows that two bikes with 56cm effective top tubes can have huge differences in reach based on the seat tube angle. I think stack is more useful than reach but both are an improvement over head tube length and ETT.

http://velonews.competitor.com/2011/02/ ... ape_159970

http://velonews.competitor.com/2011/02/ ... les_161205

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WheresWaldo
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by WheresWaldo

  • Cervelo did not invent or even first mention Stack and Reach. One of the first mentions of using Stack and Reach was on SlowTwitch.com, I think it was sometime around 2004, but I might be mistaken.
  • Seat tube angle has nothing to do with Reach, reach is the measurement from the center of the bottom bracket to the center of the head tube aligned with the top of the head tube. Saddle position is determined primarily by physiology not what the seat tube angle is. Your saddle will likely be in the same relative position offset rearward from the bottom bracket because of femur and tibia length regardless of seat tube angle, or at least it should be.
  • Fork offset, which determines trail along with wheel size and head tube angle does not affect bike fit specifically. It does have everything to do with handling.

Practical value of Stack and Reach is that taken nearly by itself you can compare two similar frames to determine general fit requirements. for example, if you require a certain amount of handlebar height looking at the specs of two similar bikes with a substantial stack difference should be a red flag that one of those bikes will not be a good fit. Reach should be thought of the same way, your ass is where your ass is, period. Just because you have a seat tube that might differ by one or more degrees does not mean your body geometry changes, your ass is still in the same place, so your saddle should be in the same relative position with regard to the bottom bracket. Pretty simple, isn't it. Please note that we are speaking about similar bikes here, road vs. road. Tri bikes are a totally different animal with regard to fit, although Stack and Reach are just as important.

Is Stack and Reach, the final word on bike fit? No, but it's close. There are a few things that it does not address, such as stem thickness and angle, which will affect the final position of the handlebars. It won't tell you a single thing about how a bike will handle, will it be quick or slow steering, will it ride comfortably, etc. It is a quick and easy way to determine at a glance if two frames will be compatible with your specific physiological requirements. That's all it was ever meant to be, nothing more, nothing less.

Concerning Mr. Zinn, although he is likely a much smarter man than I, his conclusion about seat tube angle is not entirely correct. Of course if you believe that a saddle must be centered over the center of the seat tube, then seat tube angle has a profound affect on overall saddle to handlebar length (notice I didn't say Reach here). But again our darn bodies does not permit this to be 100% true! You will have a very small range of rearward offset from bottom bracket to sit bones, simply because your body will dictate such. Your saddle will end up in the same rearward position whether your seat tube angle is 73° or 72°, there is no other way to force your body to do something it isn't built to do. I am sure Mr. Zinn knows this but since he must answer each question in a paragraph or less, he chose to oversimplify the Reach discussion with that particular questioner.

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Calnago
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by Calnago

@WheresWaldo: Perfect post :thumbup:
This thread could be locked now.
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fogman
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by fogman

I always thought that "reach" takes seat tube angle out of the equation.
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Lightweenie
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by Lightweenie

WheresWaldo, indeed great post! I think I have all my questions answered now and understand the concept of stack&reach much better!

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Lightweenie
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by Lightweenie

WheresWaldo, indeed great post! I think I have all my questions answered now and understand the concept of stack&reach much better! Thanks a lot!

Krull

by Krull

fogman wrote:I always thought that "reach" takes seat tube angle out of the equation.


just read waldò` post.

for example: assumed, a cervelo and an Isaac have the same reach, cervelo has a s.a. of 73°, Isaac 75°. for identical seat-position, use a zero-seatback-post with the cervelo and a ca 2cm-setback post with the Isaac (1° seat-angle=ca 1cm setback)
regards

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WheresWaldo
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by WheresWaldo

I'm blushing :D .
I am also a very big believer in Stack and Reach, so when I had my BLOG a few years ago I did all the research I could on the subject. I wrote several lengthy posts about STACK and Reach including formulas to calculate it from frame dimensions, because not all manufacturers were onboard at the time. I wish I had known about Stack and Reach 30 years ago, it would have saved me from making a lot of sizing mistakes. Since Stack and Reach have become ubiquitous over the past decade, I can honestly say I haven't since bought a bike that I did not know in advance was going to work for my intended purpose or physiology.

With such foreknowledge that Stack and Reach provides, it should be easy to at least compare two manufacturers frames to see if it physically fits. This is very different than seeing if it "fits" your particular purpose or riding style.

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by CP

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