Two frames , different tube angles sizing query

Serious Cat
Serious Cat Posts: 489
edited January 2014 in Road general
Normally Im your typical frame size 54 medium, I was looking at two frames which are both medium but have quite differing angles and therefore im unsure about sizing.

Frame #1
Top Tube length 545mm
Head Tube angle 73 degrees
Seat Tube angle 73.5 degrees

Frame #2
Top Tube length 550mm
Head Tube angle 72.5 degrees
Seat Tube angle 74 degrees

Normally if a frame has a TT of 550mm I consider it too long, but the HT angle is slacker by 0.5 degrees, am I correct in thinking this would equate to 5mm less reach and yield the same 545mm as frame 1 ? The ST being 0.5 degree steeper on frame 2, would this lessen reach by a further 5mm and result in frame 2 having a reach which in real terms would be 10mm less that frame 1 ?

Im clueless concerning geometry and angles and id be grateful for any help you can give me with this question.
This serious internet site..............I serious cat


  • me-109
    me-109 Posts: 1,915
    I would say frame 2 has a longer wheelbase. The TT length is what it is; your reach cannot be shorter.
  • Monty Dog
    Monty Dog Posts: 20,614
    More complicated than suggested - the steeper seattube angle of frame 2 means than you'll need more saddle layback to achieve the same seat position as frame 1 so effectively the reach will be longer.
    Make mine an Italian, with Campagnolo on the side..
  • owenlars
    owenlars Posts: 719
    Silly question but are you measuring actual tt length or virtual tt length? If you are comparing two bikes and one has a sloping top tube you need to measure the horizontal distance from saddle to stem on each to make a meaningful comparison.
  • The 550mm measurement is a virtual one
    This serious internet site..............I serious cat
  • John.T
    John.T Posts: 3,698
    Head tube angle does not have much affect on TT length unless you have a lot of spacers under the stem or a flipped stem. As a rough (quite) rule of thumb each degree increase in seat angle shortens the TT by 1cm for the same saddle position in the seat clamp. IE. It brings the saddle forward.
    Frame 2 is about 1cm longer than frame 1 if you put the saddle the same distance behind the BB.