bike fit calculators, top tube length, etc...

Butch Coolidge
Butch Coolidge Posts: 8
edited July 2014 in Road buying advice

I'm looking at possibly changing my frame and I've used a couple of bike fit calculators, like the competitive cyclist version

I'm near enough 5'6 with a leg length of 30" and the bike fit tool tells me that, dependant on fit type, my top tube should be anywhere from 50.5 - 52cm.

This is all well and good but, frame manufacturers show my dimensions (based simply on height, sometimes inseam) have me on frames with a TT of 53cm. Some manufactures have a smaller model with a TT of around 51cm, which would seem correct. Some manufacturers do not make a frame small enough to fit my body, according to the fit calculators.

I know that I could use a bike with a TT of 53 with a shorter stem or a TT of 50 with a longer stem, but what is best?

Of course most bikes use compact or sloping geometry, so you have to use the virtual TT. Quite often the manufacturers name is on the seat post and I think it's designed so you can read it, rather than getting a frame too big and half the letters in the seat tube........

I think there must be some truth in the calculators because I did have a Wilier with a TT of 49, I had to increase the stem to 110 which seemed OK. I now have a bike with a TT of 52, it has a stem of 100 which seems a little long (as it should I suppose).


  • StillGoing
    StillGoing Posts: 5,211
    Depends on the geometry of the frame as you say. It's a theoretical top tube length measured as an imaginary horizontal line from head tube to seat tube. A compact frame has a sloping top tube while others can have a flat top tube. If the seat tube and head tube are the same distance apart, there's bound to be some difference in length of the actual top tube whilst the theoretical length should be very similar.

    I'd say it all depends what you intend to use the bike for and your flexibility and riding style. I'm similar height and use a 48 sloping for my Basso which works out as a theorectical top tube length of 51.3cm. The Basso is a race orientated bike designed to have a noticeable saddle and bar height gap with the bars slammed down onto the head tube and I'll ride it for 100 miles at a time without problem. A bigger frame would see the saddle almost on the top tube and barely any difference between the bar and saddle height. My winter bike is a different geometry more akin to a sportive and has a longer top tube of 53cm. The saddle and bar height difference isn't as big as on the Basso with the ride feeling more upright.

    So you need to be sure what you're going to use the bike for, but be mindful that some compact frames for us shorties often don't have the room for 2 water bottles if you're using 750ml ones.
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  • diamonddog
    diamonddog Posts: 3,426
    I am slightly shorter than you but same inseam. I ride a PX Pro Carbon with 530 vtt and a Canyon with 525 vtt both size 50/XS with sloping geometry and both feel fine, I have never felt stretched.

    As mentioned by philthy3 some smaller compact frames (Pro Carbon) only take one bottle whereas the Canyon takes two but I have never found this to be problem.

    Go and try a few different ones and make sure they are right for you.
  • drlodge
    drlodge Posts: 4,826
    You don't need any calculators, work out your "ideal" top tube length from your current bike:
    - measure (or obtain from geometry information) the effective top tube length, seat tube angle and stem length

    From this you can work out if you ideally need a longer or shorter eTT if your stem is a bit long or short. e.g. my eTT is 54.5cm and 110mm stem - so stem is not too long or short, just right.

    Then when looking at a different bike, you already know what eTT you want, but adjust if the seat tube angle is different, 1 cm shorter per 1 degree steeper angle. My STA is 73.5 so if new bike is 73.0 I need eTT of 55cm (5mm longer)
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