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  1. #1
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    Effective Down Tube length

    Reach and stack are useful measurements when it comes to understanding how a bike really fits. But manufacturers don't consistently list these measurements "at sag," nor is there a consensus on how much sag to consider. Reach increases as a fork goes through it's travel at an inverse relationship to stack's decrease.

    The common denominator is the distance between the bottom bracket and the top of the head tube. This gives you a much better idea of where your hands will be relative to your feet. That's what counts when you're off the saddle, and saddle-bar position is useless when handling really counts, especially with droppers. I have been tracking reach and stack of bikes I have ridden and the numbers are all over the map. But when I started using the hypotenuse of the triangle formed by reach and stack, I found that an effective down tube of 740-750mm fits me perfectly every time.

    Mark my words: some manufacturer is going to come to the same conclusion any day now and start sizing bikes this way. There's more to bike fit and even more to handling, of course. You need the saddle where you need it and seat tube angle plays into that . Reach and stack are confusing measurements in isolation, so check out what the effective down tube on bikes you've ridden that fit well, given similar stem and handlebar setups, and you may find a similar pattern.

  2. #2
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    The problem is that's kind of a secondary measurement. Reach and stack still need to be right. Would a reach of 700mm and stack of 250mm (hyp=743mm) fit you? Even a less extreme example where we increased your reach 50mm and adjusted your stack accordingly would result in a bike you probably won't want to ride.

    I need my bars at a certain vertical and horizontal location from the BB for balance and leverage. I also need my bars to be a certain height and distance from my saddle. Simply raising the bars to account for a reach that's too short does not help (I've tried). On full suspension bike front and rear sag tend to be in relatively consistent percentages and minor cockpit adjustments should make up for the slight variations.

    Also, it's obviously a moot point if you make large adjustments to the cockpit to achieve fit. A 460mm reach bike with a 90mm slammed stem with the same hypotenuse of my current bike will "fit" me in the technical sense (because I effectively compensated for reach and stack).

  3. #3
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    FWIW:

    SQRT(stack^2+reach^2) = bb to top of head tube.
    Do the math.

  4. #4
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    Isnít this what Lee McCormack refers to as RAD? Heís been using that for a measurement for a while.
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  5. #5
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    Quote Originally Posted by stripes View Post
    Isnít this what Lee McCormack refers to as RAD? Heís been using that for a measurement for a while.
    Similar. RAD refers to distance between the BB and hands (not simply the hypotenuse of reach and stack). He also talks about RAAD (Rider Area Angle in Degrees). It's basically polar coordinates vs cartesian. RAD and RAAD are a more convoluted method of expressing effective reach and stack. Lee's great but RAD/RAAD is not some revolutionary measurement system.

  6. #6
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    To clarify, McCormack's RAD measurement is the distance from the BB to the grips. This takes into account frame reach and stack, plus HTA, headset and stem stack height, handlebar dimensions, and stem angle. In other words, the whole front of the bike. That's important in the end. I am talking about comparing one frame to another, knowing that you can fine-tune the hand position with adjustable bits like handlebar.

    EDT works in the real world. Sure, a bike with a 750mm EDT could have a r&s of 700/250, but no one would build a real bike with those dimensions. I think it's useful to compare actual bikes.

    For example, bikes I have had EDT measurements from 430 to 470. These had similar ETTs. When I stand and wrangle these bikes over terrain, te smaller bikes were cramped and twitchy and the longer ones were stretched out and unruly. Reach and stack only make sense in each other's context, EDT is the common denominator.

  7. #7
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    So what you're trying to do (and Lee, too, somewhat) is distill multiple measurements into a sort of an index number. A single number that can be used to compare frames (rank them, so to speak).

    I don't think such a system will really work very well that way. I don't argue that Lee's method is one useful way to examine bike fit to get your riding position dialed. But I think you're trying to do something that won't translate exactly into pre-purchase decisions. A bike's dimensions exist in a continuum in three-dimensional space and you can achieve the same hypotenuse/EDT with a lot of different reach/stack combinations, few of which will feel right.

    If you want to develop some sort of useful index, it's going to have to be a much more complicated system than the Pythagorean Theorem. You're going to have to weight specific ranges of reach and stack and throw out others. And I have a strong inkling that each person's preferences for fit and handling are going to differ slightly, and so those ranges will need to be achieved experimentally for each rider.

    What all this comes down to, is that there's simply no magic bullet for something like this. The way I handle it is that the geometry numbers might intrigue me, but the only thing that will tell me for certain is whether I ride the bike. That's my index - all of the dimensions of the bicycle distilled into my own personal weighted equation that equals fun & comfort. So long as it's close, then I know I can make small adjustments to get it where I like it. I'm flexible enough that I can be quite happy with some range (a range where I haven't really been able to set absolute limits in all honesty - all I've been able to establish is that some bikes are within that range, and others are not). If it's a bike that's only available offline, then there's always going to be some level of risk in making a purchase because attempting to put my own personal index into mathematical terms is more complicated than my brain can process.

  8. #8
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    Quote Originally Posted by Lone Rager View Post
    FWIW:

    SQRT(stack^2+reach^2) = bb to top of head tube.
    This. Easy to calculate.

    Quote Originally Posted by mack_turtle View Post
    Reach and stack are useful measurements when it comes to understanding how a bike really fits. But manufacturers don't consistently list these measurements "at sag," nor is there a consensus on how much sag to consider. Reach increases as a fork goes through it's travel at an inverse relationship to stack's decrease.

    The common denominator is the distance between the bottom bracket and the top of the head tube. This gives you a much better idea of where your hands will be relative to your feet. That's what counts when you're off the saddle, and saddle-bar position is useless when handling really counts, especially with droppers. I have been tracking reach and stack of bikes I have ridden and the numbers are all over the map. But when I started using the hypotenuse of the triangle formed by reach and stack, I found that an effective down tube of 740-750mm fits me perfectly every time.

    Mark my words: some manufacturer is going to come to the same conclusion any day now and start sizing bikes this way. There's more to bike fit and even more to handling, of course. You need the saddle where you need it and seat tube angle plays into that . Reach and stack are confusing measurements in isolation, so check out what the effective down tube on bikes you've ridden that fit well, given similar stem and handlebar setups, and you may find a similar pattern.
    I agree! This is a very important measurement and I use it to normalize reach and stack measurements. I too wish that it was included on geometry charts.

    Handle bar, stem and spacers can all be adjusted to personal preference, so they shouldn't be included.
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  9. #9
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    Quote Originally Posted by mack_turtle View Post
    EDT works in the real world. Sure, a bike with a 750mm EDT could have a r&s of 700/250, but no one would build a real bike with those dimensions. I think it's useful to compare actual bikes.

    For example, bikes I have had EDT measurements from 430 to 470. These had similar ETTs. When I stand and wrangle these bikes over terrain, te smaller bikes were cramped and twitchy and the longer ones were stretched out and unruly.

    That's why you think your measurement system works, because you're comparing bikes you've owned. Your system doesn't work in theory, it seems to work in reality because EDT is a side effect. Of course the bikes you like have a similar "EDT". That doesn't mean all bikes with a similar EDT will fit well. If a bike is too long (reach), you could lower the stack to compensate to make the EDT to whatever number you like... that doesn't mean the bike will fit well.

    Reach and stack only make sense in each other's context
    Reach and stack make sense independently and that's why they are used. EDT only makes sense in the context of the stack or reach being correct because obviously you're not going to buy a that's too long or short regardless of the EDT.

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