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Frame sizes

Discussion in 'Bicycle Mechanics and Repairs' started by trustysteed, 5 Mar 2008.

  1. trustysteed

    trustysteed Guest

    I'm a bit confused by frame sizes. there seems to be two ways of measuring as in either a S45 (5'5 to 5'8) or a M48 (5'8 to 6'0) which makes sense to me, i'm 5'7 but I also see frame sizes listed as 19", 21" etc.

    where does the 19" etc get measured?

  2. mr Mag00

    mr Mag00 rising member

    Deepest Dorset
    i believe that road bikes seem to be cm measurements from the centre of the bottom bracket to the top of the seat tube. where as mtb use imperial, inches, but again the measurement is from bb to top of seat tube.

    the right frame for you on road bikes is to measure from floor to top of crotch standing upright -10cm, am i right?, and with mtb its more about stand over height giving you a couple of inches between frame and your undercarriage.

    Thats the way i see it, i might be wrong.
  3. trustysteed

    trustysteed Guest

    i'm a 29.5 inch inside leg (that 0.5 in is important, mind!) so i guess a 19in frame is about right. which works out to about 49.5cm so i guess a M48 on the other measurement.

    ta, nice one.
  4. Tynan

    Tynan Veteran

    I'd advise sitting on the bike you want, the relaxed, semi relaxed and trad frames are all a bit different, never mind the reach
  5. stevenb

    stevenb New Member

    South Beds.
    Road bikes are different from another too.
    My new bike ordered turns out to have a top tube length that you'd find on the next size up.
    Subsequently I'm getting a 51cm frame with a top tube of 540mm.
    My previous Felt F75 had a top tube of 545mm on a 54cm frame. But after being profesionally measured I would have needed a cross between the 53cm and 51cm...so its better for me to have the 51cm using a longer stem 100mm or 110mm and have the seatpost up a bit. My legs were long enough for a 53cm but trying that meant the reach of my arms was too stretched.

    All very confusing....but so glad I did get measured.;)
  6. Monty Dog

    Monty Dog New Member

    If you don't know what toptube length you require, it's worth getting a 'bike fit' done. TT length for a 48cm frame could vary from 50cm to 54cm - depending on make - too short and your knees will hit the bars and too long will give you neck ache. FWIW traditional measurement in the UK is centre to top in inches whereas most european frames are centre-to-centre in cm - but some Italian frames are measured from the top of the BB shell. Obviously, many compact frames give 'virtual' measurements i.e. using an imaginary horizontal toptube - that's why toptube length is generally more important to determine fit.
  7. 02GF74

    02GF74 Über Member

    you are right to be confused.

    traditionally road frames were measure from centre of bttom bracket to centre of top tube; in those days the top tubes were horizontal- current trend is for them to be sloping like on mountain bikes but not as extreme.

    The other method is to top of seat tube.

    The spanners that are thrown into the works are that the seat tube usullay extends past the top tube and the top tube slopes (as mentioned).

    rule of thumb sizing for toad bike frames, measured in traditional form (you would needto look at the spec. sheet to figure out the euqivalent) is 10 inch subtracted from trouser length of 1/3 of you height.

    For mountain bike subtract another 6 inches.

    Which is why, as ^^^ says, the virtual (i.e. horiztontal ) top tube length is the better measure. there is a formula that you can use inolving upper body length, arm length etc. that I forget.

    Unless you are planning to compete and need to optimise your performance, I would say that getting measured up for a frame is worth while - frame buiulders will do this but will charge you.
  8. simonali

    simonali Über Member

    You also get the 'effective top tube' measurement on compact frames, which is how long the TT would be were it horizontal!
  9. Paul_Smith SRCC

    Paul_Smith SRCC Über Member

    You need to find out how that particular bike is measured/listed, what you really need to know is the effective size as bike shapes can vary how the manufacturer has lists the size. On that chart linked to 'A', 'B' and 'C' may all be used, the majority will list the effective size though which is 'A' on that chart, as that will give the clearest indication.


  10. trustysteed

    trustysteed Guest

    thanks Paul!
  11. Danny

    Danny Legendary Member

    But your chart seems to show A as being from the the middle of the bottom bracket to the top of the saddle post (as opposed to the seat tube). As this can go up and down it does not seem to be very useful measure to me.

    Or am I missing something?
  12. Paul_Smith SRCC

    Paul_Smith SRCC Über Member

    The frame shown has a sloping top tube, so they have drawn a red one on top to show what it would be if horizontal, 'A' is the measurement of the effective as apposed to actual seat tube


  13. Danny

    Danny Legendary Member

    OK - I understand the diagram now.