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I used to take a simplistic view of choosing a frame size. It boiled down to choosing the largest frame that gives me sufficient top-bar clearance.

On both road and MTB bikes, I can ride long distances with just this method. Yet I'm also starting to see that fit is a highly personal choice, and, for me, that personal choice may be to select a frame one size, or even two sizes smaller, than the one that gives me sufficient clearance.

It is personal in the sense that two people with exactly the same body dimensions may choose different-sized frames.

The trouble is that I am at a loss identifying why I am going to a smaller frame. The term "ergonomics" is typically used for comfort when setting up a desk, a chair, and a monitor, and for bikes as well it may be more appropriate to seek the best ergonomics when deciding whether to choose a frame smaller than the one that gives me sufficient clearance.

Can I think of bike fit purely as bike ergonomics, or is there a distinct or an additional aspect to the choice?

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    Note that even two people the same "size" may not have the same proportions. I have a longer torso (and shorter legs) than my dad who is the same height. So I need more reach and less stack than he does; I'd either pick a bike with a more relaxed geometry - longer wheelbase - or for the same model I'd pick a size smaller and add a longer stem.
    – DavidW
    Nov 11, 2022 at 15:32

3 Answers 3

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Bike fit should be 99% considered as ergonomics, but there are some other aspects that riders might consider - particularly when they are between sizes, or taking part in certain racing disciplines.

  • Smaller frames will have a shorter wheel base and will feel more lively to ride
  • Smaller frames will have a lower stack height potentially allowing a more aerodynamic postion
  • Smaller frames may have narrower bars allowing a more aerodynamic position
  • Smaller frames will be lighter
  • Smaller frames may introduce toe overlap
  • In some cases a smaller frame may lose a bottle cage mount
  • In some cases a smaller frame may come with a smaller wheel size
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    Some racing disciplines have limitations for bike fit too. For example for road racing disciplines the nose of the saddle must be a certain distance behind bottom bracket, which disqualifies many triathlon bikes if the race organiser decides to enforce UCI rules.
    – ojs
    Nov 11, 2022 at 15:36
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    I HATE an aerodynamic position - it's not just that it isn't important to me, it's that it is EXTREMELY uncomfortable to have my weight on my hands. I have very short legs for my height but I get an even smaller frame than my legs need (and a seat pole extender) so that I don't have to lean forward to reach the handle bars. Every rider is different and "a more aerodynamic position" is by no means everyone's goal. Nov 12, 2022 at 12:16
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At least in Mountainbiking the handling of the bike is for many people as or more important than ergonomic fit. For example, headtube angle, chainstay length, bottom bracket height and wheel size don't impact the ergonomics but are important for the handling characteristics. Top-bar clearance is not a thing anymore as it is low enough in all reasonable sizes on modern MTBs. Reach is important when standing and an indicator for how far you can move your body to change the weight balance.

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  • Important points, but you're answering a broader question; is that right? Might we think of it this way: 0- choose bike type; 1- choose wheel size; 2- choose chainstay/wheelbase; 3- select frame size. The question focuses on (3), whereas you're bundling 0, 1, 2 in the problem; did I get that right?
    – Sam7919
    Nov 11, 2022 at 22:42
  • @Sam Point 2 is entirely affected by frame size. To some extent point 1 is as well.
    – MaplePanda
    Nov 12, 2022 at 14:34
  • @MaplePanda Yes, it's obvious that wheel size is sometimes dropped (to 650c) on the smallest frames. From perusing geometry tables, it's also clear that the chainstay length doesn't change (much) for a given brand/model between different sizes. The wheelbase must change, of course, or else the bike would have to be taller and taller, and in the process changing its "type", until it might be closer to an upright/commuter bike.
    – Sam7919
    Nov 12, 2022 at 17:46
  • @MaplePanda The point is to look at the numbers relative to each other, not in the absolute. Yes, you may have memorized the specific numbers for you, for the frame size you usually use, and eventually I also will memorize the numbers for me and be able to reach conclusions about the type of a frame, but there must also be some conclusions to reach from one just line in the geometry table, for just one bike size.
    – Sam7919
    Nov 12, 2022 at 17:48
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We can also compare bicycle ergonomics with aeroplane ergonomics.

But continuing with the workstation ergonomics, bicycle ergonomics parallels workstation ergonomics in another way:

  • If a desk doesn't fit you, you can still use it for 30-60 minutes with no harm.
  • Likewise, you can ride a bike that doesn't fit you—but not for too long.

After many hours (on/at either) you will either:

  • know that the fit is not right, if you have a little pain (or a lot),
  • (and that's tougher) not know just how much better a good fit could be.

The main conclusion is this: unless you already have deep expertise in what a great fit is like, a test ride at the bike shop will not reveal which frame size to buy.

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