左起KHS USA總裁、Zinn、校長
2010年年中，KHS美國總裁建議老朋友Zinn在九月的InterBike其間召開會議，建議各廠家統一車架的標示方式。會議雖未如期舉辦，但Zinn仍採納老朋友的建議，於2011新年到來之際，在VeloNews（http://velonews.competitor.com）上發表了他的幾個看法。下文是原文關於車架標示部分的節錄。
Zinn recommends "Stack and reach" to denote frame size
Technical FAQ with Lennard Zinn: LZ suggests some New Year’s resolutions for the bike industry
By Lennard Zinn
Published Jan 4th 2011 3:00 PM EST — Updated Jan 4th 2011 3:46 PM EST
Editor’s Note: Tech writer Lennard Zinn made a few resolutions this New Year’s Day — for the bike industry. Below he shares three suggestions, starting with the way bikes are measured:
Resolution for Frame Manufacturers: Use Stack and Reach to Denote Frame Size (and measure it to the top of the headset upper bearing cover).
Frame sizing was simple when steel lugged frames with level top tubes were king. There was variation in where the frame size (i.e., seat tube length) was measured to, but once that was sorted out, you could almost compare apples to apples by simply listing the seat tube length and the top tube length, both of which could be measured directly along the tubes.
Now, however, due to sloping top tubes, extra head tube and seat tube extensions above the top tube, integrated headsets (heck, even thread less headsets alone threw off frame sizing), and wide variations in seat angles on bikes designed for aero bars, frame size is very hard to define. A rider who fit well on a 56cm lugged frame with a 56.5cm top tube now might have no idea of how to determine whether a given frame would fit the same. The bike would need to be built up and standing on its wheels in order to measure the top tube length, which is an “effective” top tube length, measured horizontally from the center of the top of the head tube to the center of the seatpost. And the up-sloping top tube ensures that all of the possible measurements from the bottom bracket center to the top of top tube, to the top of seat tube, or to the center of top tube would be less than 56cm .
Enter “stack and reach,” the system pioneered by Dan Empfield, founder of slowtwitch.com and Quintana Roo, to effectively characterize frames so that they can be compared with the purpose of determining if they would fit a given rider. To use this system, you only need two points on the frame: (1) the center of the bottom bracket and (2) the head tube centerline intersect with the top of the head tube (and I think this should be changed to the head tube centerline intersect with the top of the upper headset bearing cover to correct for internal vs. external headsets).
The vertical distance between those two points is the frame’s stack. The horizontal distance is its reach. The frame size is then listed in the format “580/390,” which would mean a frame with 580mm of stack and 390mm of reach.
Why would you care about stack and reach? Well, consider four different 56cm road frames, all of which have a 57cm top tube. Frame A has a level top tube, frame B has a sloping top tube, frame C has “comfort geometry” with extra head tube extension above the sloping top tube, and frame D is a time trial bike with a 78-degree seat angle whose level top tube intersects the seat tube a few inches below the top of the seat tube in order to lower the front end to account for the height of the elbow pads. Assume that all four frames have the same bottom bracket height and are built for 700C wheels and the same headset type.
Stack and Reach chart 原大圖：http://velonews.competitor.com/2011/01/bikes-tech/technical-faq-with-lennard-zinn-lz-suggest-some-new-years-resolutions-for-the-bike-industry_154565/attachment/11-01-04-stack-and-reach-ch
It should be obvious that the head tube length of frame A will be shorter than that of frame B, which will in turn be shorter than that of frame C, and the head tube of frame D will be shorter than any of them. This means that the handlebar, if set up with the same headset, spacers, and stem, will be at different heights for all of them. So, the saddle-to-handlebar drop will be different for all of them. This will create the same kind of fit issue as if the seat tubes were all different lengths.
Now consider the seat tube angles. We said that frame D has a 78-degree seat angle, and let’s give frames A, B, and C seat angles of 73 degrees, 73.5 degrees, and 74 degrees, respectively. Obviously, to position the saddle the same relative to the bottom bracket on all four, the saddle will be pushed further back on the seatpost on frame D than on frame C, which will have its saddle in turn pushed further back on the seatpost on than on frame B, which will in turn have its saddle pushed further back on the seatpost on than on frame A. This means that the distance from the saddle to the bars is greater on frame D than on frame C, which in turn is greater than on frame B, which in turn is greater than on frame A. If you think of the effective top tube length as being something related to the distance from the handlebar to the saddle, it is obvious that the effective top tube lengths of these frames are all different, even though their measured top tube lengths are the same.
Now that you see its importance, how do you actually measure the stack and reach of a given frame? First off, you have to do it when it is built up with its fork, wheels and components.
When the bike is standing on the floor, measure from the floor to the center of the bottom bracket and from the floor to the top of the head tube. The difference between these two values is the frame’s stack.
Have a friend push the bike head-on against a wall, perpendicular to it, with the tire touching the wall. Measure horizontally from said wall to the top center of the head tube, and measure horizontally from the wall to the center of the bottom bracket. The frame’s reach is the difference between the two measurements. (See illustration to the left.)
You also can measure all of your body’s touch points on the complete bike in the same manner relative to the bottom bracket: “Seat Stack,” “Seat Reach,” “Bar Stack,” and “Bar Reach” (or, for bikes with aero bars, “Elbow Pad Stack,” “Elbow Pad Reach,” “Shifter Stack” and “Shifter Reach” — to the center of the shifter pivot). Stack and reach makes it easy to transfer measurements from one bike to another to set them up identically. But there’s more.
The reason I’m suggesting accessible stack and reach data for the new year is that if you had it for all frames on the market, you could easily determine which frames will “fit” you based on the stack and reach dimensions of your touch points.
The biggest drawback of current stack and reach frame sizing is that the effective head tube height varies due to headset type. If the frame has an integrated headset, it will have effectively about 2cm less stack than a frame with the same stack measurement with a standard headset. This is why I think that stack (and reach) would be measured to the headset upper bearing cover, rather than to the top of the head tube. As long as we’re resolved to go to the stack and reach system, why don’t we make this change, too? That way, we can start with a new, clean frame measurement system without a fudge factor in it.
Admittedly, I can’t tell somebody that I ride a 636/439 road frame, a 630/441 cyclocross frame, a 655/434 track frame for our steep, short indoor track here in Boulder , and a 653/449 mountain bike frame without feeling compelled to launch into a long explanation. But since we all got used to saying things like, “I ride a 56cm frame,” or even, “I ride a 56cm frame with a 56.5cm top tube,” and understanding conceptually what it means, perspicacity with this measurement system could come with familiarity, too. It will take consistency on the part of bike manufacturers in specifying it on every bike so that it becomes part of the cycling vernacular, though. And let’s include the headset while we’re at it.
店長簡單說明：
1. Zinn 建議用「Stack 與 Reach」作為車架的統一標示，取代現行多頭馬車式的車架標示方式，以利消費者辨識車架是否適合自己。
2. 以「五通中心點」、與「頭管中心線與上頭管培林蓋的交叉點」，作為兩個量測參考點。
3. Stack：兩個參考點的垂直距離。
4. Reach：兩個參考點的水平距離。
5. 建議看原文大圖示，比較容易瞭解。
沒有留言:
張貼留言