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how do you measure mtb frame size?

scooter15scooter15 Posts: 41
edited January 2019 in MTB workshop & tech
do you measure from center of bottom bracket to top tube, OR, from bb to the top of seat post clamp?, i did this and i have either 17in or 20 in and im not 100% sure as im looking to buy a new bike this month. many thanks

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  • from bb to the top of seat post clamp?

    That. though it matters less now than it did. Arguably the reach is a better measure (BB to top of HT) as it takes into account the length of the bike too
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  • The RookieThe Rookie Posts: 27,748
    Centre of bottom bracket to top of seat tube (not top of clamp although that may be close, some clamps can stick up 1/2").

    DO NOT use an existing frame size to buy a new bike, manufacturers other (much more important) dimensions are more critical to getting the frame size right. Even different bikes from the same manufacturer can vary a lot in sizing, between manufacturers is a pointless comparison.

    You need to compare
    Effective top tube length - from the centre of the top of the head tube horizontally back to the centre of the seatpost/seat tube. In addition add or subtract if one bike has an offset seatpost and the other inline as appropriate.
    The next two are harder to measure
    Reach - Horizontal distance from the centre of the bottom bracket to the centre of the bar mounting in the stem
    Stack - Vertical distance between the same points
    In addition you then need to allow for whether one has wider bars which needs a shorter stem.

    Its reach and stack that define how well an MTB rides off roads when you're out the saddle (as its not a contact point) plus you then need to make sure the saddle is in the right place for pedally bits (which comes from ETT) - all assuming your current bike is actually the right size of course!

    The height of the seat tubet is meaningless as your seatpost will give in excess of 150mm of adjustment, what is harder to adjust is the pedal to bars relationship by more than a few MM (20mm max on stack, very little on reach without impacting the steering) which is why they are more important.

    Of course a bike with longer travel at the front/bigger wheels will tend towards a higher stack as the base of the headtube is higher and also the geometry (longer travel) will be more gravity orientated, if that's what you want you may then need a small reduction in reach to put your shoulders forward again.
  • steve_sordysteve_sordy Posts: 2,126
    In other words you need to ride the bike!
  • your all right, im measuring off an 8 year old gt bike lol. will go and try in the shop , cheers all!
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