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Triban 3 says "57" on a 50cm frame?

Manc33Manc33 Posts: 2,157
edited December 2013 in Road beginners
I have had a Triban 3 for about a year now, it doesn't feel too small for me (the next size up felt like I was reaching too far forwards on the handlebars when trying in the shop).

I only actually measured the frame today and it is 50cm from the middle of the bottom bracket to the top of the seat tube! If I include the clamp in the measurement I get 51cm, but to the top of the frame itself it is 50cm.

Why are they putting "57" on a bike if it is actually 50cm? :x

An older bike I have is a 56cm, but that feels a bit big. Something like a 54cm would probably be about right, I guess. I am 5'10".

Posts

  • bucklesbuckles Posts: 694
    It probably has a sloping toptube, so the virtual size is 57cm.

    EvoProCarbon_Geometry.jpg

    Pic above is of a frame with a sloping toptube. As you can see from the diagram, the seat tube is shorter than that of a traditional frame with horizontal toptube.
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  • Manc33Manc33 Posts: 2,157
    edited December 2013
    Yes, the 56cm frame I have is a 1960s Pennine frame with a straight top tube.

    Triban 3 (50cm) top tube slopes. :roll:

    Does pretty much every road frame made today have a sloping top tube?

    The Triban 3 would be 55cm if the seat tube met up with the top tube (pretending the top tube is straight).

    I still have no clue what size frame I should be on.

    I guess if the top tube slopes, about 52cm (from middle of BB to top of seat tube) and if the top tube doesn't slope I need something like a 55cm.

    From Evans road sizing:

    "Please note:
    The sloping geometry found on Colnago road bikes are 4cm smaller than traditional geometry. For example a 56cm Traditional frame will be equal to a 52cm sloping frame"


    Thats about what I guessed at, its a 4cm discrepancy.
  • You are measuring seat post length like an MTB, they are quoting effective top tube length like a lot of road bikes nowadays.
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  • Manc33Manc33 Posts: 2,157
    So when they call a frame a "54cm" they are pretending its top tube is straight regardless of whether it slopes?

    This Triban 3 being called a "57" still doesn't make sense, it would be 55cm if the top tube was straight and it is 50cm in actuality.

    Putting flat bars on my bike confuses matters more, I have my hands about 6 inches forwards on hoods compared to flat bars on the same bike. Hoods are huge these days.
  • AnonymousAnonymous Posts: 28,799
    Why does it matter what size it says on the frame?
  • Carbonator wrote:
    Why does it matter what size it says on the frame?


    Errrrrrrrr................... so you can identify the frame size??
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  • Manc33Manc33 Posts: 2,157
    Carbonator wrote:
    Why does it matter what size it says on the frame?

    Errrrrrrrr................... so you can identify the frame size??

    It doesn't work if it says "57" on a 55cm bike. :P

    OK let me get a spirit level and do it exactly...

    EDIT: It is about 56.2cm I estimate, using a spirit level.
  • bucklesbuckles Posts: 694
    Manc33 wrote:
    So when they call a frame a "54cm" they are pretending its top tube is straight regardless of whether it slopes?
    Not really pretending... it just means you make up the seat tube length difference with your seatpost and (in theory) the bike fits you the same as a traditional 57, i.e. it should have the same reach, the same seat tube and head tube angles, the same wheelbase etc. There are supposedly benefits to frames with sloping toptubes i.e. increased stiffness and weight.
    Manc33 wrote:
    This Triban 3 being called a "57" still doesn't make sense, it would be 55cm if the top tube was straight and it is 50cm in actuality.
    The size it would be if the top tube was horizontal depends where you're measuring from.... centre to centre or centre to top
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  • AnonymousAnonymous Posts: 28,799
    Carbonator wrote:
    Why does it matter what size it says on the frame?


    Errrrrrrrr................... so you can identify the frame size??

    Errrrrrr................... you can identify the frame size by what it says on the frame. My question was why does it matter what the numbers are once you know which one in the range you have.
    It may as well say S,M,L,XL (as some do) for all the help the numbers give sometimes.

    My point is that its a bit irrelevant what the actual number is apart from that its bigger than the one below and smaller than the one above (unless they put the wrong number on I guess).

    You would have to sit on/ride the bike to see if it fitted you and if you had had a bike 3 years you should have some idea if it fitted you
  • amaferangaamaferanga Posts: 6,789
    It's (virtual) top tube length that's the more important measurement anyway.
    More problems but still living....
  • Errrrrrrrr................... so you can identify the frame size??

    I like to see you try and find a geo sheet for the B'Twin bicycles which would be much more useful.
  • As above - without the whole geo then the frame size as a number is meaningless anyway. If the seat tube angle differs then the top tube length on 2 bikes with the 'same' frame size is different, so one will fit and one won't - despite being the 'same' size.

    Due to the hugely variable frame shapes on MTBs a lot of manufacturers gave up on numbers for frame sizes ages ago - perhaps it's time roadies went the same way to avoid too many badly fitted bikes as ''my last was a 54 so this 54 must be right for me too....'

    And as an aside - if you are measuring 56.2cm to a virtual point, I would have thought 3mm margin for error (taking it to 56.5, rounded to 57) is pretty good....
  • tribans do size up small, i had a 60 which measured up to 57ish.
  • Manc33Manc33 Posts: 2,157
    I still remember MTB's being measured in inches in the 1990s. :)

    Now you just get an "L". :roll:

    When I was about 14/15 I think my MTB was an 18". I think they used to measure the seat tube distance inside the down tube and top tube.

    Then we got full suspension and no seat tube to measure. :lol:
  • I thought that the centre of the BB to the top of the seat tube was the best measurement to take? After all you can effectively alter the reach through different stems, bars and spacers anway?
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