I'm looking to buy a Trek DS 4 bike. In the store the sales person measured me and said I'm right in between sizes 19 and 21 and he wasn't sure which one I should pick. I test drove both sizes and didn't feel any difference, though I have only spent a few minutes on each and I suspect the differences would show themselves on a longer ride (like how tired my back and hangs get etc).
Any recommendations on which way (size up or size down) to go for more comfortable/efficient rides?
I would be cautious of any blanket advice suggesting that everyone should make the same decision. Whether to size up or size down depends on a number of factors:
Many will suggest that you size down when in doubt, this likely comes from observing that professionals (e.g., road riders) tend to use smaller frame sizes. This is done to get the rider in a more extreme aerodynamic position, these positions are generally not that comfortable and not advisable for most riders (remember the pros have access to daily physio sessions to constantly undo the effects of these positional contortions).
Ideally, you should be looking at your body proportions when making this decision. For example, I personally have longer legs, shorter torso and longer arms than average (BTW - no average person exists). While my legs are longer my femurs are relatively shorter (i.e., lower legs are relatively longer). This puts me a bit taller and forward on the bike as such a frame with more stack (i.e., size up) tends to work better as long as the frame's reach dimensions are reasonable. My slightly longer arms means I tend to use standard stems. For frames with longer reach geometries this may not work out however work out as well.
Below is a good summary of how short/long legs impacts your proportions and therefore whether you may want to size up or down.
Bike Sizing Guide - Road Bikes
Short legs
Short legs will also mean a long torso, so there is danger of the rider being too hunched up on a height suitable bike. The short legs will also mean a lower saddle height, so the distance between saddle and bars is reduced anyway. With a bit of luck, the short legs may also mean short arms which would balance out the long torso! The biggest danger here is selecting a bike based on top tube length i.e. larger than normal for overall height, but then suffering from cranks that are too long. A better compromise may be the use of a longer stem.
Long legs
Sorry for all these long legged people, but this is the worst case for achieving an optimum bike fit. The long legs will mean a high saddle, so the bars become relatively far away. Long legs imply a short torso, so the reach to the bars becomes excessive. One could consider a shorter top tube, but this would also tend to come with a shorter head tube which is counter-productive. What tends to happen with very long legged people is that stems have to be short and as high as possible, often achieved by flipping them over as well as using all the available spacers. In many cases this is still not enough and the saddle also has to move further forwards than is ideal. Many people do not like the aesthetics of this stem configuration, in which case the best option is to select a bike with Sportive or Touring geometry, i.e. one with relatively long head tube or Stack for its size.
Therefore if all else is equal, short legged people would want to size down and then select a long stem, while long legged people will want to size up then select a shorter stem. All of this gets further confounded by other issues you may have such as poor flexibility, back pain (you typically need to be more upright) and how you want the bike to handle (longer stems slow the handling a bit).
In short, there is no one size fits all advice as to whether you should size up or down.
I would absolutely agree with everything stated in the answer. While not excessively tall at 191cm, my legs are disproportionately long, which means I have a large drop to the handlebars - assuming that I want a good balance of power output and comfort on the pedals, which while potentially looking cool to some people, doesn't always fill me with confidence.
While my BMC Roadmachine (2022) is sold as an endurance frame with a very high stack, I sometimes feel that the 58cm frame is unsuitable for me, especially considering the tinkering I've been struggling with for the past 8 months. In an attempt to reduce the drop the the handlebars I considered the 60cm frame but the guy advised me that the top tube would be inordinately long, plus the stack was almost the same as the 58cm frame, which I agreed would be better in hindsight. After 8 months of messing on with the saddle and cleats I think I've finally dialed the position in but haven't done a ton of riding due to Beijing's minus 10 weather conditions.
There are so many issues to consider when buying a frame that even the most experienced fitter would struggle to find a single answer, and even something simple as crank length can have a huge impact, which I didn't initially consider. Having the longer crank might force you to alter your cleats, and when trying to balance power output, have to adjust your saddle height, which of course changes effective top tube length, thereby potentially needing to change the stem length...........when does it stop! I've come to the conclusion that us mere mortals have to accept something satisfactory rather than perfect! Being a roadie, we will never be satisfied and will spend four hours tinkering only to find that we've only changed the damn thing 1mm!!
