looking to get into this and after a fair bit of looking I've found this bike. Photos here: https://www.gumtree.com.au/s-view-details.html?adId=1245748386

It says 51cm frame and Medium, but everything I can find online suggests that a medium mens should be 54cm ish? Just wondering if anyone has had experience with the brand or could suggest whether it may be suitable.Thanks!

  • Product recommendations are off topic here, but Fluid is definitely not a mainstream brand. There are zero details given in the ad, which is not a good sign. Apr 17, 2020 at 14:17
  • Go for a test ride. That's the only way to know for sure.
    – Criggie
    Apr 18, 2020 at 15:48

2 Answers 2


Most answers to this question will advise you to test the bike, read about fitment, etc. But let’s cut through that - are your body proportions roughly average? If yes, a 51cm is too small for you. 54 is a good target.


Be aware that these numbers may not mean what they mean normally. I am 5 feet 8½ (174 cm) and I use small size frames. BUT, the numbers are sometimes the horizontal length and sometimes something else. In these days of compact sizing it is not as coherent as it used to be in the horizontal top tube days.

MY main bike I use is size small and numerically 490 mm. Why is that? Because it is actually the actual seat tube length. So, the effective top tube length is actually 536 mm on that bike. I saw at least one e-shop to recommend the sizes to a wrong size category of customers (much smaller than I am). The classical numeric size for this frame would probably be frame size 52 cm (which corresponds to the virtual seat tube length).

So, because you are almost the same size as me you will have a similar (not necessarily exactly same) frame, around those 54 cm (53.6 cm in my case), maybe a bit more.

Also note that in compact sizing this size could still well correspond to the small size. Both my gravel bike and my road bike are small. My road bike is size 52(S), recommended to persons of 170-180cm. The larger 54(M) is recommended to persons 175-185cm. The former has the effective top tube length 535 mm, the latter 547 mm.

Be sure to check the actual geometry of the bike you are ordering.

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