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Better frame measurement: "Front End Length"

Discussion in 'BMX & Dirt Jumping' started by cmc, Dec 10, 2007.

  1. cmc

    cmc Turbo Monkey

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    "Front End Length":

    This is the theoretical horizontal distance (parallel to the ground) from between two vertical lines (perpendicular to the ground), one intersecting your BB and the other the top of your head tube.



    WHY ? Because it takes out the variables of both seat tube height and seat tube angle. Road bikers care a lot where their seat is. BMX and DJ don't because you're standing all the time.

    Before you say who cares, this is bull**** geek stuff, read the following description from the empirebmx website:

    Example from empirebmx.com : "Fly Bikes Tierra frame 19.8", 20.6", and 21" TT, 13.25" CS, 75 HA, 71 SA, 4.65 lbs; the seat tube is offset 0.4" towards the back, so the actual TT length is 0.4" longer than how the bike will truly feel (21" rides like 20.6", etc.) "

    A 0.4" longer when your choices between different $350 frames are based in 0.5 increments . . . .

    What you want to know is where are your feet/knees are in relation to your hands. (Knee gonna hit your bars?) A frame with a laid back seat angle will have a longer top tube than one with a more vertical seat angle--but the bikes will feel exactly the same to ride.

    Companies are telling you numbers that are easy for THEM to measure because the bike is in their hands. But it doesn't help us compare 2 different frames we haven't ridden unless you're going to do some Trig. (calculating the seat angle factor).

    The distinction between "actual" versus "effective" TT lengths is also irrelevant to BMX and DJ. This comes from road biking where old school road bikes originally had top tubes that were parallel to the ground. When they started slanting them down (and intersecting lower on the seat tube), they got shorter, even though your seat is still in the same place. For example see here: http://www.somafab.com/geometry03.html

    Heh heh. Now maybe I'll get back to work. No wait, it's lunch time.
     

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  2. Castle

    Castle Turbo Monkey

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    it would really help if companies just added in center to center downtube measurements in their geo charts...

    not only in the dj/bmx world but in the dh world where both you hardly ever sit down on the bike..... you are more interested in the relationship (distance wise) between your feet and hands...
     
  3. kidwoo

    kidwoo Celebrating No-Pants Day

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    I'll make sure to carry two plumb lines with me next time I go buy a bike.
     
  4. WestCoastHucker

    WestCoastHucker Turbo Monkey

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    lots of companies already measure this. it's called "front center"...
     
  5. Cru Jones

    Cru Jones Turbo Monkey

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    who cares, this is bull**** geek stuff



    :biggrin:
     
  6. cmc

    cmc Turbo Monkey

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    Never heard a BMX company use that term. But you're right.


    This post proves that roadies will out-geek any of us:

    http://www.serotta.com/forum/archive/index.php?t-5648.html



    A relatively accurate formula for calculating the difference in the "effective" TT length is: saddle height x (cosA-cosB), where A and B are the two seat tube angles. In your case, I'll assume that you saddle height is 85cm. If you plug 72 and 73 degrees into the formula, the result is a difference of 1.4cm per degree. To compare several frames, one must be selected as the "nominal" STA. For example, if your current frame has a 72 degree STA and the others you're looking at have 73 degree STAs, then you should ADD 1.4cm to the advertised TT length of these frames to make an accurate comparison.

     
  7. cmc

    cmc Turbo Monkey

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    How can "front center" be longer than "actual top tube" ?? If it is, then it's something different than what I'm talking about.


    http://www.ridemonkey.com/forums/showthread.php?t=147956&page=2

    CreeP
    03-23-2006, 06:11 PM

    well, the most important measurements to me -as a bit of a bike designer myself- are: front center, cs length, bb-centertop of headtube, and the angle of that line, and of course head angle and bb height. Those weird ones i just mentioned really tell you how the bike will fit. TT length really is bs, especially since there are so many variations in HA and STA. If you have a very upright style and want some cockpit room forward you could easily end up with the wrong feel, and a seat so far from your knees, if you go solely by toptube length simply because the seattube angle has such an effect on tt length.

    So for my Peyto: 420mm fork
    front center: 24"
    bb-ctht 650mm
    400mm cs 388 slammed
    actual tt 21.5
    bb height 320mm
    HA: 72
    SA: 73
    (a similar Peyto these days will cost you between 900 and 1400 depending on joining method and tube selection and extra features)
     
  8. Landon

    Landon Monkey

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    For what it's worth, the method you described for measuring frames is the same one I've been using for years.

    Not really practical to do without a drawing, but I think it's a good way to size bikes like this.

    -L
     
  9. cmc

    cmc Turbo Monkey

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    cool...

    as far as ease of measurement, Castle is right: center-to-center downtube would be a more useful number than TT".

    the small BB height variation between frames would skew it only a tiny bit, basically negligible.



    The less-than-an-inch vertical variation you see in BB height would not be enough to throw off the downtube measurement very much.
     
  10. sittingduck

    sittingduck Turbo Monkey

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    Sort of like "effective top tube", but measured from the BB instead. I like it. I like it a lot. I have always thought top tube and seat tube measurements were misleading and somewhat worthless. Fork lengths would change the equation slightly, but not nearly as much as variables in the seat tube change the TT measurements.
     
  11. sittingduck

    sittingduck Turbo Monkey

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    Use one, and make marks on the floor. :D Or bring a 4' level.
     
  12. Leethal

    Leethal Turbo Monkey

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    Front Center is this...

     
  13. bbrz4

    bbrz4 Chimp

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    BikeCAD is sweet!
     
  14. cmc

    cmc Turbo Monkey

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    old thread.... but, kind of amusing cuz maybe every couple of years someone will suggest a "new" & better frame measurement (like i did), but that everyone has already known by different names, but mainly "cockpit". . . . yet it will not ever catch on as an industry spec.

    seems like TRANSITION has been one of the best at providing information on their bikes. e.g. http://www.transitionbikes.com/Bikes_TOP.cfm




    http://bicykel.com/?p=887

    Proposed new Mountain Bike fit standard
    (Dec 2008)

    Transition Bikes

    In a joint effort between Turner and Transition bikes, a new standard for mountain bike frame sizing has been introduced. This new standard is based around reach and stack measurements, and is intended to clarify bike fit across bike models and brands.

    The evolution of mountain bikes has made the old system of top tube and seat tube based sizing irrelevant. The wide range in seat tube angles and bottom bracket heights can create a situation where one bike could feel longer or taller than another frame with the same measurements. Once a rider is in a standing position, the fit and feel of their bike’s cockpit is determined exclusively by the handlebar and pedals. Finding the specific geometric relationship between those two points is the only way to accurately compare the fit and feel of different bicycle frames.

    “I was really stoked to get the call from Sam at Transition Bikes regarding a new method of measuring mountain bikes, especially those used for technical riding like our DHR. For years I have not listed a top tube measurement for the DHR as I knew it was irrelevant in comparing the fit of our bike with other brands,” says David Turner. “The great thing about this sizing method is that anyone with a piece of string with a weight on it and a measuring tape can accurately measure their current bikes. The rider can then compare their current bike to any brand using Reach and Stack and know exactly how it will fit without even riding it.”

    The horizontal distance from the bottom bracket center to the top of the headtube centerline is referred to as reach. The vertical distance between these two points is known as stack. By comparing the reach and stack on different frame models, the rider is able to identify exactly how their bike will fit and feel on the trail. This system eliminates any uncertainty created by the seat tube angle, and forever drops the need for “actual” and “effective” top tube measurements. The reach and stack of a frame is the most important sizing information for a freeride or downhill rider but it is extremely valuable for trail riders and XC racers as well.



    “I was aware that Turner was using the reach number to size their DHR, but under the name cockpit; so I approached David Turner with the idea that this should be an industry standard measurement,” said Sam Burkhardt from Transition Bikes. “Dave expressed interest in adding a vertical component and going forward with an industry standard. We chose the terms reach and stack after Kris Wehage from E.13 Components pointed out the use of the same measurements in the Time Trial and Triathlon communities. Rather than create a new name, we felt it would be easier for manufacturers and riders to adopt the system using existing names.”

    Turner and Transition bikes encourage all mountain bike manufacturers to include these numbers on their specification charts for consistency in the industry, and to aid rider’s looking for the best possible fit for their body type and riding style.
     
    #14 -   May 24, 2011
    Last edited: May 24, 2011
  15. cdavis

    cdavis Chimp

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    ha, cmc that's awesome.
     
  16. sittingduck

    sittingduck Turbo Monkey

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    Nice. Just like you wanted but even better, since it also gives the height of the HT relative to the BB.
    Let's hope this catches on.
     
  17. TortugaTonta

    TortugaTonta Monkey

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    Merckx have been giving those mesurements on their road bikes for a while. . . http://www.eddymerckx.be/EMX-7

    Thats how I draw up a frame I plan to build. I start with A to C and ht angle, then drop a plumb line and locate the bb in relation to axle centerline and reach or cockpit. Thats part of the reason I like to use bb+ or - rather than bb height.
     
  18. cmc

    cmc Turbo Monkey

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    Bump!