Question regarding frame geometry. Do I have this right?
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  1. #1
    Vee
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    Question regarding frame geometry. Do I have this right?

    I have set out to learn all I can about bike fit, frame geometry, and sizing. After spending a couple weeks and countless hours crunching numbers, my conclusions are as follows:

    1. An XXcm frame size is just a quick reference number, and two separate brand bikes of a similar XXcm size may have completely different geometries, rendering the generic XXcm number of frame sizing useless across brands (other than using it as a basic starting point.)
    2. Frames across all brands can have drastically different geometries, so comparing one measurement to the next is not beneficial. Instead, the calculation of Stack and Reach, which are based off of a frames geometry, are used to compare across different frames.
    3. Using the frames geometry, calculating Reach of the frame, can then help you to calculate the length and angle stem you need to achieve your proper fit.
    4. At the end of the day, the measurements that end up truly mattering are almost all based off of your Bottom Bracket to Saddle height? For instance, no matter how big or small your frame is, you will (should) always keep the same Bottom Bracket to Saddle height, as that measurement is determined by leg length.
    5. Bottom Bracket to Saddle height seems to have a direct affect on the measurement of Saddle to Handlebar Drop.
    6. I am unsure of this one... Bottom Bracket to Saddle Height - (minus) Stack - (minus) Stem Height (including any spacers) will produce the measurement of Saddle to Handlebar Drop?
    7. With the above in mind, is it proper to size a frame by first determining your Bottom Bracket to Saddle height (taken from current bike), then calculating the Stack of new frame, then do the calculations for stem height (including any spacers), and finally finding a frame that gives you close to the same Saddle to Handlebar Drop that you are comfortable with?

      Take your Reach measurement and add your Stem Length (accounting for angle and any spacers) and be sure that your Reach falls within the Reach + Stem Length measurement of your current fit position.

      Once you find the above frame within the proper measurements, check to be sure the Stand Over Height of said frame is within the measurements of your In-Seam.

      If all checks out, couldn't you then be confident that this frame will fit you? Or is there more to it than this?

    Please help me to understand this better. I seem to be making sense of it all in my head, but I want to be sure this is the proper way of doing things.

    Edit: I left out one part about Reach, so this was added to the above post.

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    Seems overly complicated to me. But I guess it really depends on how precise you want to get.

    For me, I look at seat tube angle and top tube length (horizontal) and compare it to what I know already fits well both based on experience and on a professional fit I received a few years ago. I know that 1 degree of seat tube angle is roughly equal to a change of 1cm in top tube length. So if I know a 75 degree sta, 515 tt bike fits me, I know a bike with a 74 degree sta will more or less fit the same with a 525 tt. 73 degrees with a 535 tt, etc. This assumes the same length stem is being used. Once I know the bike will fit my required reach, then I start looking at head tube length and stack. Standover height is not something I really think about.

    Unless you are trying to become a fitter, I'm not sure you really need to understand too much more than the above. This process has worked for me on my last few bikes and the fit has been close enough that only minor adjustments were needed.

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    I think you have a pretty good handle on the basic difficulties of comparing frames, but I am against using Stack and Reach.

    Reach, as it has come to be defined, is a really slippery number. Because it is a horizontal measurement from a vertical line to a point on an angled line (head tube), the height of the head tube can change the reach number drastically. Two otherwise identical bikes will have different "Reach" if the head tubes are different lengths. This not only makes it harder to compare - it creates a false impression that does not actually reflect your distance from saddle to steerer. It also takes more calculation to normalize "Reach" numbers from frame to frame.

    A simpler method is to use the "Virtual Top Tube" length, and then correct it for seat tube angle. The correction is roughly 1cm per degree of seat tube angle difference. So a 74 degree STA bike with a 54cm top tube would gain a centimeter to be very comparable to a 73 degree STA bike with a 55cm TT. The reason for this is that your seat set back adjustment is made independant of actual STA, so you are physically correcting for STA when you set up your bike. You end up with a horizontal distance between two roughly parallel lines (head tube and seat tube) that is true whether the HT is long or short. Stem length is also roughly horizontal, so comparing it to Virtual TT length makes sense.

    Stack or HT height is primarily important in determining if you'll be able to get your bars high or low enough with a reasonably number of spacers and an ordinary stem. If you are going for a fairly traditional drop to the bars, most frames of appropriate size will allow you to mount the bars without any real screwing around. The difficulties arise when people with very long legs or very short legs are trying to get fit, or the frame is designed for a very upright posture.

    Saddle height is a fixed number that you should apply to most any bike - road, MTB, commuter. When correct, it is a height that is good for your knees and efficiency, so it applies well to different bikes. Saddle height is measured at the angle of the seat tube, so it is another reason to avoid purely veritcal measures like "reach".


    From your questions, it sounds like you are trying to completely fit yourself - frame, stem, spacers - from a formula. Don't do it. If you have a bike who's fit is good, you can use the above tools to compare geometries. But I would caution you to avoid taking a stab at stem length before you've sat on the bike.


    When trying to get your head around frame sizing, I suggest starting with a traditional frame. On these, the head tube height is set by the frame size, rather than being independant of it. It is much easier to tell how a sloping top tube frame is going to fit if you understand how it departs from a level TT traditional frame. Traditional frames are what we road for all of the 20th century, and what our fit rules come from. Sloping TT frames and variable HT heights are helpful to fit more people, but they confuse the issue.


    BTW, the people who really like Stack and Reach will tell you that they correct for things like BB drop. But road BB drop almost always falls in a very narrow range and won't really affect much else.

    Here's a chart of "classic" geometry, designed by Tom Kellogg for Merlin.
    http://www.merlinbike.com/bikes/2006...light_geo.aspx
    Note the HT and ST heights grow by about the same amount. Also note that TT length grows proportionally shorter as the frame size goes up, with the crossover point around 56cm. When you look at the geometry chart for a sloping frame ask yourself "for a given corrected virtual TT length, what kind of HT length are they using?" (Keep in mind the roughly 16mm difference between traditional and integrated headsets.) I'd save this page and use it as a reference.

    I realize that was a lot, but you seem to understand things well enough wade through that. A good frame size is one that puts the bars in about the right place with a fairly normal looking stem in a middle length. That usually means that there is more than one frame size that can work for you, since stems offer more adjustability than frames. This is where a test ride helps you decide whether the smaller, tighter frame is better, or the larger one with the longer wheelbase.

  4. #4
    Vee
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    Quote Originally Posted by rx-79g
    I think you have a pretty good handle on the basic difficulties of comparing frames, but I am against using Stack and Reach.

    Reach, as it has come to be defined, is a really slippery number. Because it is a horizontal measurement from a vertical line to a point on an angled line (head tube), the height of the head tube can change the reach number drastically. Two otherwise identical bikes will have different "Reach" if the head tubes are different lengths. This not only makes it harder to compare - it creates a false impression that does not actually reflect your distance from saddle to steerer. It also takes more calculation to normalize "Reach" numbers from frame to frame.

    A simpler method is to use the "Virtual Top Tube" length, and then correct it for seat tube angle. The correction is roughly 1cm per degree of seat tube angle difference. So a 74 degree STA bike with a 54cm top tube would gain a centimeter to be very comparable to a 73 degree STA bike with a 55cm TT. The reason for this is that your seat set back adjustment is made independant of actual STA, so you are physically correcting for STA when you set up your bike. You end up with a horizontal distance between two roughly parallel lines (head tube and seat tube) that is true whether the HT is long or short. Stem length is also roughly horizontal, so comparing it to Virtual TT length makes sense.

    Stack or HT height is primarily important in determining if you'll be able to get your bars high or low enough with a reasonably number of spacers and an ordinary stem. If you are going for a fairly traditional drop to the bars, most frames of appropriate size will allow you to mount the bars without any real screwing around. The difficulties arise when people with very long legs or very short legs are trying to get fit, or the frame is designed for a very upright posture.

    Saddle height is a fixed number that you should apply to most any bike - road, MTB, commuter. When correct, it is a height that is good for your knees and efficiency, so it applies well to different bikes. Saddle height is measured at the angle of the seat tube, so it is another reason to avoid purely veritcal measures like "reach".


    From your questions, it sounds like you are trying to completely fit yourself - frame, stem, spacers - from a formula. Don't do it. If you have a bike who's fit is good, you can use the above tools to compare geometries. But I would caution you to avoid taking a stab at stem length before you've sat on the bike.


    When trying to get your head around frame sizing, I suggest starting with a traditional frame. On these, the head tube height is set by the frame size, rather than being independant of it. It is much easier to tell how a sloping top tube frame is going to fit if you understand how it departs from a level TT traditional frame. Traditional frames are what we road for all of the 20th century, and what our fit rules come from. Sloping TT frames and variable HT heights are helpful to fit more people, but they confuse the issue.


    BTW, the people who really like Stack and Reach will tell you that they correct for things like BB drop. But road BB drop almost always falls in a very narrow range and won't really affect much else.

    Here's a chart of "classic" geometry, designed by Tom Kellogg for Merlin.
    http://www.merlinbike.com/bikes/2006...light_geo.aspx
    Note the HT and ST heights grow by about the same amount. Also note that TT length grows proportionally shorter as the frame size goes up, with the crossover point around 56cm. When you look at the geometry chart for a sloping frame ask yourself "for a given corrected virtual TT length, what kind of HT length are they using?" (Keep in mind the roughly 16mm difference between traditional and integrated headsets.) I'd save this page and use it as a reference.

    I realize that was a lot, but you seem to understand things well enough wade through that. A good frame size is one that puts the bars in about the right place with a fairly normal looking stem in a middle length. That usually means that there is more than one frame size that can work for you, since stems offer more adjustability than frames. This is where a test ride helps you decide whether the smaller, tighter frame is better, or the larger one with the longer wheelbase.
    rx-79g,

    You, my friend, pretty much made me day. Thank you for your in depth reply!

    I want to clarify one thing. I currently own a bike that I feel has been fit to me correctly. Naturally I want to try to keep things as close as possible to my current bike when choosing my next frame. My next idea is to build a bike, so I have been looking at framesets and frame geometry for this reason.

    In terms of using formulas, I am, in fact, trying to establish a list of formulas in search of a way to compare different framesets to my current frameset and fit. Basically, I am trying to learn as much as possible about it all, because I: A) want to be sure that I am fit properly on my current bike and B) want to be sure that I buy the proper size frame, since my first bike was purchased with me knowing little or nothing about frame sizing or geometry. I simply went with what I was suggested would fit me. This time around I want to KNOW what I buy will fit me and why.

    I understand that this might seem overcomplicated to some, and that some people would much rather just shrug and make a rough guess, but I want to learn about the process and the calculations behind fitting on a bike.

    I will take a look at the traditional sizing tonight. I figured this would be much simpler to work with, but alas I figured I could just jump in to the more complex sizing of today's sloping top tube frames.

    Thanks for your help so far, guys. Keep the replies coming.

  5. #5
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    Richard Hallett described a straightforward procedure for replicating your riding position on multiple bikes in this article.

    Using a FitStik is another good way to do it, but you need a FitStik.
    -Stan
    my bikes

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    You can push it through formulas all you want, but fit is as much an art as it is a science. Just keep that in mind. Even looking at the online fit calculators, you see there are ranges and different approaches. But you seem to be on the right track for sure.

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    Quote Originally Posted by Vee
    rx-79g,

    You, my friend, pretty much made me day. Thank you for your in depth reply!

    I want to clarify one thing. I currently own a bike that I feel has been fit to me correctly. Naturally I want to try to keep things as close as possible to my current bike when choosing my next frame. My next idea is to build a bike, so I have been looking at framesets and frame geometry for this reason.

    In terms of using formulas, I am, in fact, trying to establish a list of formulas in search of a way to compare different framesets to my current frameset and fit. Basically, I am trying to learn as much as possible about it all, because I: A) want to be sure that I am fit properly on my current bike and B) want to be sure that I buy the proper size frame, since my first bike was purchased with me knowing little or nothing about frame sizing or geometry. I simply went with what I was suggested would fit me. This time around I want to KNOW what I buy will fit me and why.

    I understand that this might seem overcomplicated to some, and that some people would much rather just shrug and make a rough guess, but I want to learn about the process and the calculations behind fitting on a bike.

    I will take a look at the traditional sizing tonight. I figured this would be much simpler to work with, but alas I figured I could just jump in to the more complex sizing of today's sloping top tube frames.

    Thanks for your help so far, guys. Keep the replies coming.
    I don't think it is really that hard, as long as you keep certain things separate.

    Understanding traditional frame sizing gives you a point of departure for looking at sloping tube frames. I say "sloping" rather than compact because many of them aren't compact - they are actually larger in most dimensions.

    Example:
    You are trying to find a bike like your 55cm traditional with a 55.5 top tube and a 125mm standard head tube, 73 degree seat tube (as are most every 55).

    Bike A has a 46 cm seat tube, a 54.9 virtual top tube and a integrated headset headtube of 140mm. 73.5 STA.

    Bike B has a 49 cm seat tube, a 55.6 top tube and traditional headset headtube of 135. 73 STA.

    Which bike is closer to yours? Subtract 16mm off Bike A's headtube = 124. Add .5cm to its VTT = 55.4cm. So A is closer than B because of the HT difference. Could you ride B? Definitely - unless your stem is already as low as they can go on your current bike, you'll just use 10mm less spacers and get the same stem height.

    In this example, A is a true "compact"(same dimensions to contact points, but less tubing used), while I'd call B a "sloping tube" bike.

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    Quote Originally Posted by krisdrum
    Seems overly complicated to me. But I guess it really depends on how precise you want to get.

    For me, I look at seat tube angle and top tube length (horizontal) and compare it to what I know already fits well both based on experience and on a professional fit I received a few years ago. I know that 1 degree of seat tube angle is roughly equal to a change of 1cm in top tube length. So if I know a 75 degree sta, 515 tt bike fits me, I know a bike with a 74 degree sta will more or less fit the same with a 525 tt. 73 degrees with a 535 tt, etc. This assumes the same length stem is being used. Once I know the bike will fit my required reach, then I start looking at head tube length and stack. Standover height is not something I really think about.

    Unless you are trying to become a fitter, I'm not sure you really need to understand too much more than the above. This process has worked for me on my last few bikes and the fit has been close enough that only minor adjustments were needed.
    Really? That seems to indicate you don't care how you're positioned over the BB and pedals. Am I missing something?

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    Quote Originally Posted by Hank Stamper
    Really? That seems to indicate you don't care how you're positioned over the BB and pedals. Am I missing something?
    Actually, it means that you care so much to duplicate your position over the pedals that you are correcting for seat tube angle via seatpost set back.

    In that example, if the owner of a 74 degree 52.5cm TT bike were going to mount the same saddle and post on a 73 degree 53.5cm TT bike, he'd have the saddle slid 1cm+ forward on the rails to "correct" for the differing seat tube angle. That correction is no different than subtracting 1cm from the TT.

    In a perfect world, stock road frames would just use 73 degrees across the board, since short and tall people have the same leg proportions and need the same proportional set back. But the convention is to range from 74+ to 72 degrees, causing us to have to rectify that with basic math.

    I realize everyone isn't built the same, but average people of different heights do have proportionally built legs, which is mainly what determines seat setback. And almost no one selects bicycle brand and model because they want or need a particular seat tube angle. Instead we buy the bike that fits well overall, then correct the seat angle. Since I ride 51cm frames, I have a fair amount of setback on my saddles to make up for the 74 degree STAs these bikes (needlessly) come with.

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    Quote Originally Posted by Hank Stamper
    Really? That seems to indicate you don't care how you're positioned over the BB and pedals. Am I missing something?
    rx nailed it. I assumed, and we know what that leads to. So, no, I am concerned with how I am related to the BB and pedals for each of the given frame dimensions in my example, but I do correct for setback. For instance I usually run my saddle in the middle of a 20mm setback post on a frame with a 75 degree sta. I've compensated for that on a frame I am building with a 73 degree sta by mating it with a zero setback post.

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    Quote Originally Posted by krisdrum
    rx nailed it. I assumed, and we know what that leads to. So, no, I am concerned with how I am related to the BB and pedals for each of the given frame dimensions in my example, but I do correct for setback. For instance I usually run my saddle in the middle of a 20mm setback post on a frame with a 75 degree sta. I've compensated for that on a frame I am building with a 73 degree sta by mating it with a zero setback post.

    Thank you. Rx too. I had initially thought perhaps you meant to mention changing set back too but then decided, for reasons now unknown to me, you probably didn't. That and I'm easily confused on Friday afternoons at work. Anyway, I get it now. Thanks.

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    Quote Originally Posted by Hank Stamper
    Thank you. Rx too. I had initially thought perhaps you meant to mention changing set back too but then decided, for reasons now unknown to me, you probably didn't. That and I'm easily confused on Friday afternoons at work. Anyway, I get it now. Thanks.
    Today is Wednesday.

    My fault for not mentioning the adjustment to setback to compensate.

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    Quote Originally Posted by krisdrum
    Today is Wednesday.
    My fault for not mentioning the adjustment to setback to compensate.
    Wow. It's my last work day of the week. But still where to heck did I get Friday. I need a nap or something.

  14. #14
    cmg
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    so what are the frames involved in question? so we can play arm chair fitter.

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    cmg
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    Quote Originally Posted by Scooper
    Richard Hallett described a straightforward procedure for replicating your riding position on multiple bikes in this article.

    Using a FitStik is another good way to do it, but you need a FitStik.

    good info. i'll give it a try on my next frame switch.

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    You've got a good handle on frame specs.

    My recommendation would be what some framebuilders actually do: Draw out your existing frame on a piece of drafting or meat wrapping paper. If you can't find something large enough, then reduce it to 1/2 scale or such. Use a permanent ink marker. Then do the same for the frame you're considering, but in pencil right on top of the existing frame, with the BB as the starting point. That should give you a good idea where the two frames differ.

    In essence, consider your top tube length first, then seat tube length. Some people reverse that order and it's not a bad idea to do so as a double check. Then check the headtube length. Truly, sloping top tube designs complicate things!

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    Vee
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    Thanks guys! Good discussions going on here.

    I thought long and hard about what rx-79g said about reach, specifically, "it creates a false impression that does not actually reflect your distance from saddle to steerer." And after thinking about it a bit I do not see how this is true. Since reach uses the bottom bracket as a point of measure, and the bottom bracket is also used as a point of measure when setting your saddle fore and aft, couldn't you then deduce that reach can fairly accurately provide the distance from saddle to steerer? Since saddle is setup based on bottom bracket, and reach picks up at the bottom bracket, two frames with identical reach values should have the same measurements from Saddle to Steerer.

    I am having trouble understanding Stack height. I understand what it represents, but I do not understand how two frames, say Frame A, with a stack height of 568, and Frame B, with a stack height of 607, can have drastically different stack heights, the same Saddle to Bottom Bracket measurement, and somehow end up with nearly identical Saddle to Handlebar measurements. Shouldn't Frame B, with its larger stack height, end up with a lower Saddle to Bottom Bracket measurement than Frame A? I understand that I am leaving out the information about any spacers or stem lengths/angles, but I guess what I am getting at is, does Stack height really even matter if you can correct it completely using spacers and stems?

    On my current bike I run about 35 mm of spacers (including 20mm of headset). I was hoping to reduce this down to 0mm of spacers (read: none) if possible. All of my calculations, thus far, have gotten me as low as using 10mm of spacers, but I am nowhere near my goal of 0mm. Perhaps I have an unrealistic goal?

    Lastly, I am happy to hear that some of you are interested in playing arm chair fitter. I will try to post my current frame and fit measurements first thing tomorrow morning for those interested, as well as some matching measurements for some frames I have been considering.

    Thanks again for all of you guys help!

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    Quote Originally Posted by Vee
    Thanks guys! Good discussions going on here.

    I thought long and hard about what rx-79g said about reach, specifically, "it creates a false impression that does not actually reflect your distance from saddle to steerer." And after thinking about it a bit I do not see how this is true. Since reach uses the bottom bracket as a point of measure, and the bottom bracket is also used as a point of measure when setting your saddle fore and aft, couldn't you then deduce that reach can fairly accurately provide the distance from saddle to steerer? Since saddle is setup based on bottom bracket, and reach picks up at the bottom bracket, two frames with identical reach values should have the same measurements from Saddle to Steerer.

    I am having trouble understanding Stack height. I understand what it represents, but I do not understand how two frames, say Frame A, with a stack height of 568, and Frame B, with a stack height of 607, can have drastically different stack heights, the same Saddle to Bottom Bracket measurement, and somehow end up with nearly identical Saddle to Handlebar measurements. Shouldn't Frame B, with its larger stack height, end up with a lower Saddle to Bottom Bracket measurement than Frame A? I understand that I am leaving out the information about any spacers or stem lengths/angles, but I guess what I am getting at is, does Stack height really even matter if you can correct it completely using spacers and stems?

    On my current bike I run about 35 mm of spacers (including 20mm of headset). I was hoping to reduce this down to 0mm of spacers (read: none) if possible. All of my calculations, thus far, have gotten me as low as using 10mm of spacers, but I am nowhere near my goal of 0mm. Perhaps I have an unrealistic goal?

    Lastly, I am happy to hear that some of you are interested in playing arm chair fitter. I will try to post my current frame and fit measurements first thing tomorrow morning for those interested, as well as some matching measurements for some frames I have been considering.

    Thanks again for all of you guys help!
    The trouble you're having is the reason I'm against using reach.

    In essense, the trouble is that reach is measured horizontally between a vertical line (drawn from the BB) and an angled line - about 73 degrees on average, coming from the steerer or head tube. Those two lines are eventually going to converge. Because of that, the higher you measure the headtube reach, the shorter it is. If you had a stack that was six feet tall, the Reach would be zero, or even negative. But if your saddle was also up six feet, you'd still have the same reach to the bars.

    For this reason, even if your TT length was identical, the bike with the taller stack will always have the shorter reach. Does that tell us anything useful when two bikes with identical TT lengths have diffferent "reach", just because one has a longer head tube?

    A vertical line does not represent anything real on a bicycle, and isn't useful for comparing distances between lines that are running at very different angles from vertical. It is either a pure marketing device, or the work of people who are so wrapped up in what they are doing that they forget what these numbers are for - comparing bicycle geometries.

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    wim
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    Huh?

    Quote Originally Posted by rx-79g
    A vertical line does not represent anything real on a bicycle, and isn't useful for comparing distances between lines that are running at very different angles from vertical. It is either a pure marketing device, or the work of people who are so wrapped up in what they are doing that they forget what these numbers are for - comparing bicycle geometries.
    I wouldn't be so hard on the vertical line. Should you ever need to define or measure trail, you'll need it in the worst way.. Also comes in handy when establishing seat tube setback, saddle setback and knee position over the pedal axle.

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    Quote Originally Posted by wim
    I wouldn't be so hard on the vertical line. Should you ever need to define or measure trail, you'll need it in the worst way.. Also comes in handy when establishing seat tube setback.
    Not to mention stand over height, KOPS, BB height and low bridge height. However, none of those things are important to the problem of comparing the dimensions of two bicycles when it comes to how a rider fits on them. A vertical line from the BB is arbitrary and, as illustrated, confusing.

    I'd vote for "front center", another BB centered measurement, to be equally useless.

    It seems that once people gave up on seat tube length as largely pointless, they had to come up with some new pointless numbers to talk about.

    I nominate two new terms and measures! From the tops of the hoods to the rear hub: It will be called "Wheelie", and be an important measure of the leverarm used to get the front wheel off the ground by brute strength. People can ask what the Wheelie is of your bike, we can try to calculate Wheelie from a bare frame using a special chart, and complain about certain brifters having poor Wheelie. Awesome.

    Also, ask me about the calculations for gears and BB drop to get good "Chain Levelness" - an important new factor in training and racing. "I can't get good Chain Level in a 52x16." "Should I shift on inclines to preserve Chain Levelness?"

    Hey, it's no dumber than people asking about a frame's chainline.

  21. #21
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    Quote Originally Posted by rx-79g
    It will be called "Wheelie", and be an important measure of the leverarm used to get the front wheel off the ground by brute strength.
    Very good idea, but you need help with the marketing. If you'd call it "Front Wheel Vertical Displacement Force Factor" (the "factor" is important, sounds engineery), you'd have something there.

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    the simple answer...

    If you want to reduce the amount of spacer you're using, measure your head tube length, including the headset. Look for a frame with a head tube and headset total that is taller by the amount of your spacers and you'll have ABOUT zero spacer. I say "about" because this comparison does not take into account the fork length (that's rarely listed) or the BB drop. Both of these are needed to get an exact height of the top of the head tube, relative to the BB.

    This is exactly why stack is a valuable dimension, if the manufacturer lists it. If not, you may be off by any difference in the unknown fork length. If the stack is not listed, you can at least account for BB drop difference, if significant. Most are very close to 70mm. A frame with an 80mm drop, like a Serotta, could have a 10mm shorter head tube and use the same amount of spacer.

    If you'd post the frames under comparison, I can tell you how they compare.

    I basically agree with rx-79g regarding the reach value. It can be very confusing, unless you know how to use it. That's also true of the TT length, however. As correctly noted, you can't directly compare the TT length of frames with different STAs. The correction is simple. Just add 8-10mm to the TT length of the frame with the steeper STA and then compare the lengths to predict the stem length difference. The exact correction is (cosA-cosB) times the c-c frame size.

    To use the reach value properly, also requires a correction, if the stack heights are not the same. The correction is no more difficult than the one used to correct the TT lengths. All you do is subtract 3mm from the reach for each 10mm that the stack height is shorter and then compare the two reach values. What that does is give you the reach at the same stack height on both frames.

    Here's an example. If one frame has as a reach of 380mm with a stack height of 540mm and a smaller one has a reach of 370mm with a stack height of 520mm, then subtract 6mm from the reach of the shorter frame, to give 364mm. The true difference in reach is 16mm, not 10mm.

    The front-center value is not worthless either. It can tell you how much toe overlap to expect and it can be used to predict differences in the front/rear weight balance. Some people hate to deal with toe overlap and paying attention to the F-C can avoid that problem.
    Last edited by C-40; 12-23-2010 at 07:20 AM.

  23. #23
    Vee
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    Quote Originally Posted by C-40
    If you want to reduce the amount of spacer you're using, measure your head tube length, including the headset. Look for a frame with a head tube and headset total taller, by the amount of your spacers and you'll have ABOUT zero spacer. I say "about" because this comparison does not take into account the fork length (that's rarely listed) or the BB drop. Both of these are needed to get an exact height of the top of the head tube, relative to the BB.

    This is exactly why stack is a valuable dimension, if the manufacturer lists it. If not, you may be off by any difference in the unknown fork length. If the stack is not listed, you can at least account for BB drop difference, if significant. Most are very close to 70mm. A frame with an 80mm drop, like a Serotta, could have a 10mm shorter head tube and use the same amount of spacer.

    If you'd post the frames under comparison, I can tell you how they compare.

    I basically agree with rx-79g regarding the reach value. It can be very confusing, unless you know how to use it. That's also true of the TT length, however. As correctly noted, you can't directly compare the TT length of frames with different STAs. The correction is simple. Just add 8-10mm to the TT length of the frame with the steeper STA and then compare the lengths to predict the stem length difference. The exact correction is (cosA-cosB) times the c-c frame size.

    To use the reach value properly, also requires a correction, if the stack heights are not the same. The correction is no more difficult than the one used to correct the TT lengths. All you do is subtract 3mm from the reach for each 10mm that the stack height is shorter and then compare the two reach values. What that does is give you the reach at the same stack height on both frames.

    Here's an example. If one frame has as a reach of 380mm with a stack height of 540mm and a smaller one has a reach of 370mm with a stack height of 520mm, then subtract 6mm from the reach of the shorter frame, to give 364mm. The true difference in reach is 16mm, not 10mm.

    The front-center value is not worthless either. It can tell you how much toe overlap to expect and it can be used to predict differences in the front/rear weight balance. Some people hate to deal with toe overlap and paying attention to the F-C can avoid that problem.
    I had no idea of this. Scary how I completely overlooked this fact.

    Edit: Actually, I seem to have adjusted for this in regards to spacer height where stack is increased due to spacers. Guess I just forgot that this would also affect the difference on the total stack.
    Last edited by Vee; 12-23-2010 at 07:55 AM.

  24. #24
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    yes...

    Quote Originally Posted by Vee
    I had no idea of this. Scary how I completely overlooked this fact.

    That's why reach values can be misused. Some people got the idea that reach was some sort of magic number that allowed any two frames to be compared. Not the case at all. You can only compare reach values at ONE stack height. If done properly, the comparison will produce the same result as a TT length comparison (with correction for any difference in the STA).

    There is also a small difference in fit from differing HTAs, but it only amounts to 1-2mm per degree. Occasionally, you might find a 2 degree difference in a HTA, combined with a stem with a lot of spacer under it, that would create a 4-5mm change in the fit, but not often.

  25. #25
    Vee
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    Quote Originally Posted by C-40
    That's why reach values can be misused. Some people got the idea that reach was some sort of magic number that allowed any two frames to be compared. Not the case at all. You can only compare reach values at ONE stack height. If done properly, the comparison will produce the same result as a TT length comparison (with correction for any difference in the STA).

    There is also a small difference in fit from differing HTAs, but it only amounts to 1-2mm per degree. Occasionally, you might find a 2 degree difference in a HTA, combined with a stem with a lot of spacer under it, that would create a 4-5mm change in the fit, but not often.
    I think I understand THIS part completely now. I think the reason you mention small differences because of different HTAs is because the 3mm of reach reduction per 10mm of stack is actually just a ballpark figure. The true figure is Cos(HTA), where Cos(HTA) * 10 is the value of reach reduction per 10 mm of stack difference. Is that correct?

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