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  1. #1
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    Correct Top Tube vs. Correct Frame Size

    I need a bike with a top tube of about 50.5 cm. To get a top tube of that length, I need to get a frame that is one or two sizes too small. That is, I would have to get a 49 cm. frame, whereas my inseam would indicate a 52 cm. frame. My LBS says that it doesn't matter that the frame is so small, but this strikes me as being counter-intuitive. Is the LBS right? Should I get the correct size frame and install a very short stem?

  2. #2
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    how tall are you, what is your inseam?

  3. #3
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    Look at bikes with compact geometry fames! The purpose of compact geometry is to allow you to stand over two or three different sizes so you can pick the one with the right top-tube length - not the only one with the proper stand-over height.

  4. #4
    Why Cars? myates1980's Avatar
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    The basic rule of thumb I follow when finding the correct size of a bike is to have a person hold it up while you sit on a properly fitted saddle, and keeping your back straight you should "just" be able to reach the grips with your fingertips. This is assuming you have the correct heigth of the frame for your inseam...one inch of clearance below your crotch is again "the rule of thumb" for this...but essentially it all comes down to comfort for you. The saddle heighth adjustment is more of a factor than many realize...you should have the seat fitted so you are getting an almost total extension of your legs, nothing more than a 5 degree bend of the knee at the end of the downward stroke.
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  5. #5
    I drink your MILKSHAKE Raiyn's Avatar
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    Quote Originally Posted by myates1980
    This is assuming you have the correct heigth of the frame for your inseam...one inch of clearance below your crotch is again "the rule of thumb" for this..
    Presuming a road bike

  6. #6
    "Uh-uh. Respek Knuckles." hypersnazz's Avatar
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    It also depends on what bikes you're looking at...some run a little long in the top tube, some shorter. I went with LeMond with sloped top tube 'cause I need a really long TT but I have a short inseam and LeMonds tend to run a little on the long side.

    Compact geometry works if you fit it, but it's not as versatile as its proponents like to say. The further you move off the bike by raising the seat and bars, the further away from the bike's center of mass you get and the worse it'll handle. Every bike has a sweet spot, that is, optimum position both over the bike and between the wheels.

    I would say talk to several shops and ride several bikes because they're all going to feel fractionally different, and every shop has its own way of doing things (and every shop insists their way is the 'right' way).

  7. #7
    Why Cars? myates1980's Avatar
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    Quote Originally Posted by Raiyn
    Presuming a road bike
    Actually thats just assuming it's a triangular frame...unless I'm mistaken about MTB and Hybrid sizing.
    Last edited by myates1980; 07-28-05 at 12:07 AM. Reason: typo
    ==========================
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  8. #8
    I drink your MILKSHAKE Raiyn's Avatar
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    Quote Originally Posted by myates1980
    Actually thats just assuming it's a triangular frame...unless I'm mistaken about MTB and Hybrid sizing.
    You are. You want a bit more clearance when riding offroad to protect "the boys"

  9. #9
    Why Cars? myates1980's Avatar
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    Quote Originally Posted by Raiyn
    You are. You want a bit more clearance when riding offroad to protect "the boys"
    This is true. What sort of riding are you doing Richardpu? Cycling? Trail? Commute? The style of your riding is a factor in sizing, you may end up wanting a shorter frame for cycling because you're going to be setting the seat higher for that hunched over cycle style riding. I DO size my mountain bikes with a little more crotch clearance and a longer frame so my toes don't hit the front wheels when making sharp wheel turns. Again...sizing is not really a science, it just comes down to what most comfortable for YOU. The height of the seat is the most important part of fitting your bike IMHO, everything else should be sized after the seat is adjusted for comfort and proper positioning of your body for your style of riding.
    ==========================
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  10. #10
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    If you've got enough ducats, you could always go custom. That way you could conceivably get both the top tube length and standover height you want. Not all customs are terribly expensive, but they ain't cheap either.

    Just a thought.
    One thing about living in Santa Carla I never could stomach -- all the damn vampires.

  11. #11
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    I have very long legs, and a normal upper torso. I need a 59cm frame for the correct seat height, but I need a shorter top tube due to my shorter top.

    Are there frames out there known for a shorter top tube? I know most road frames have long top tubes to give you that sleek, low to the handlebars racing possition in the saddle. Not good for me!

    Any ideas?

  12. #12
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    Quote Originally Posted by islenska
    how tall are you, what is your inseam?
    I am 5' 6.5" and my inseam is 78.5 cm.

  13. #13
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    Quote Originally Posted by richardpu
    I need a bike with a top tube of about 50.5 cm. To get a top tube of that length, I need to get a frame that is one or two sizes too small. That is, I would have to get a 49 cm. frame, whereas my inseam would indicate a 52 cm. frame. My LBS says that it doesn't matter that the frame is so small, but this strikes me as being counter-intuitive. Is the LBS right? Should I get the correct size frame and install a very short stem?
    50.5 cm seems small for a TT length for you. are you talking actual, or effective TT? I would think that you would need an *effective* TT length of somewhere between 52 and 53.5. On a bike of this size, i doubt you will have to use a very short stem, you'll probably use a 100 mm and be fine. but, this all depends on what you are using the bike for, what type of riding position you want and are comfortable with, etc. have you ridden both the 49 and the 52? which one feels better?

    personally, i'd rather be right on the border standover-wise and have a bike that feels right, instead of having a bike that is too small.

    what bike(s) do you have now? if you have a bike now, you should do some measurements and see how they compare with what you are looking at. if you like your current bike...try to get something similar geometry-wise...if you dont like it...well, then you've identified a problem that you can try to correct in your new bike.

  14. #14
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    I put "short top tube" into Google and found a message in a cycling forum that identified three bikes that supposedly have shorter top tubes. They were Giant OCR, Specialized and one other. But as far as I could tell, the top tubes on these bikes were no different from those on most bikes.

  15. #15
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    A longer top tube will mean you are reaching over further. I don't like to reach over far.

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