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Briefly

One way of asking

In the video accompanying Michael Jackson's "Black or White", people of various ethnicities are morphed to one another. Suppose you are looking at someone riding a 26″ bike. Now the bike is morphing to the shape of a 29″ bike. As an observer of this morphing operation, can you identify two or three characteristics that have changed in the rider's position?

Another way...

Think of an Amsterdam cyclist, riding in a very comfortable posture. Even if I take the video and edit out the bike, by just looking at the cyclist you immediately recognize the bike they're on. Think now of a cyclist 500 m before the end of a TdF stage (but not 100 m, when everyone's off the saddle). Just the video of the cyclist, without the bike, tells you they're aggressively riding a road bike. What would you do to morph the Amsterdam cyclist to the TdF one? I'll take a stab: 1- the back goes from upright to near horizontal, 2- the saddle is raised to near the highest that it can go without the cyclist rocking, 3- the elbows are bent from near straight to almost perpendicular, etc. What are the signature changes in posture for a rider from a 26″ to a 29″ bike?

(Note: we ignore here 27.5″ bikes and 27.5″ bike postures. Looking at just going from 26″ to 29″ accentuates the observations.)

An observer can identify a good fit

When you see a cyclist riding, you can immediately identify a good fit:

  • The leg is almost completely extended at the bottom of the stroke.
  • The knee never reaches, let alone goes over, the level of the pelvis (the thigh never points up). At the limit, the thigh may become horizontal during the stroke.

The cyclist knows a good fit by feel

While cycling, an experienced rider can also identify whether the fit is correct:

  • Grabbing the handlebar (whether straight or dropbars; and the latter either on the hoods or on the drops) feels like "grabbing the bull by the horns". The hold is secure and comfortable. There is no excessive tension in any arm muscle, nor, certainly, in any back muscle.
  • As long as the rider cycles semi-regularly, the cyclist can depend on returning from a multiple-hour tour with zero muscle or joint pain. At most the rider is exhausted and hungry.

Now assume that all of the above is good for one cyclist on a 26″ bike, with either straight or drop handlebars.

What doesn't change

Imagine that you are stretching the 26″ bike to the dimensions of the 29″ bike. Some things do not change:

  • The ground clearance underneath the bottom bracket is about the same.
  • The cyclist most definitely does not stretch forward. The hold remains comfortable. Hence the "bike stretch" to 29″ means that something in the frame needs to change drastically (what?).

What has to change

Some things will have to change:

  • Even though the front wheel is bigger, it cannot touch the cyclist's foot while turning. Hence the front wheel must be moved a bit forward, similar to the characteristic look of Harley-Davidson motorcycles.

Question

How does a cyclist's profile change from 26″ to 29″ wheels? For example: you see a seasoned cyclist riding a 26″ bike. They go home, change to a 29″ bike, and ride again. What looks now different, if anything?

A note about height

29″ bikes are certainly a better match for cyclists who are particularly tall (say above 190cm), and 26″—or 27.5″—bikes are more suitable for smaller riders (say below 160cm). Still, the average cyclist (say 160-190cm) is served by the two sizes. It also appears that it is not the case that we simply divide the heights and say that cyclists below 175 cm (5′ 9″) are better served by 26″ (or 27.5″) bikes, whereas cyclists above 5′ 9″ are better served by 29″ bikes.

I am not soliciting ideas for determining whether a 26, 27.5, or 29 bike is more suitable for a given rider. Consider a cyclist (who's neither above 190 nor under 160) who identified that a 29″ makes it easier to go over small rocks and tree trunks. That cyclist knows well the feeling of a good 26″ bike fit. What is different, if anything, in a 29″ bike fit?

To state the obvious: even though I wrote everything above as fact, you are welcome to refute any of it.

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    I’m not exactly sure what you are asking, and the question is too vague to give a concrete answer. For starters, the massive geometry differences between bikes of the 26” era and those of the 29” era will have much more of an effect than the change in wheel size.
    – MaplePanda
    Nov 26 '21 at 23:31
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    Is the cyclist pedaling at near the speed of light? Nov 27 '21 at 0:59
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    I think you're missing the point still. The cyclist's posture changes, yes, but it has more to do with the geometry updates than the wheel size difference, and it all fluctuates so wildly that nobody can really answer properly. To summarize: reach has grown by like 80mm in all sizes, stack has increased, BB height has increased, seat tube angles have steepened by 3-4*, bars have widened by 50mm, cranks have shortened, bars are now higher than saddles...but I don't see the point of the question either. What knowledge are you gaining?
    – MaplePanda
    Nov 27 '21 at 3:08
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    @whatsisname for realistic cycling situations, I believe the first questions are whether we can assume that the cyclist is spherical and in vacuum
    – ojs
    Nov 27 '21 at 12:04
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    So, is the real question "How do I read frame geometry tables so that I won't order wrong size of frame again?"
    – ojs
    Nov 28 '21 at 19:17
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Wheel size can sometimes have repercussions on fit due to the bike design considerations where wheel size is a factor. But, those considerations largely deal with design choices rather than absolutes. There are no categorically true statements that can be made about the fit of all bikes of a given wheel size.

The 26" MTB era largely overlapped with low, stetched, XC type postures as the design and cultural norm in the mountain biking world. Front travel numbers were getting larger at the same time of the rise of 29", and so the result is of nominally same-sized 26" and 29" frames, yes the 29" frame would put the top face of the head tube higher up and be more naturally conducive to a more upright position. But none of that has anything to do with what's possible for bikes designed around either wheel size, just what you get with the average off the shelf positions on average extant bikes. People can and do get back to 90s type positions on 29" XC bikes by flipping the stem negative. You have to want to get pretty crazy low before you start making categorical statements about what's possible on the 29-to-26 direction. In the other direction you can run into spacer stack height limits in some cases that would make a given 26" bike with an aluminum or carbon steerer be unable to reasonably match the positioning on a 29" bike, but that's again not categorical to 26" bikes because you could also make a frame with a taller headtube.

There are a lot of smaller-size bikes in the world where to avoid toe overlap, the head tube angle is too slack and the top tube is too long, so you have a bike that handles and fits bad, and it's all to accommodate a front wheel that's standard for the genre but too large for the frame size. (It's also to accommodate people's overblown ideas of the relationship between wheel size and speed/efficiency, but that's another topic).

A rider can have a fit done, a target posture arrived at for a certain type of riding, and functional bikes designed to give identical spatial arrangements of contact points in a number of wheel sizes. If you built all those bikes with all reasonable combinations of design choices that affect handling but not fit (HT and ST angle, fork offset, and chainstay length being major ones), and geared them identically, you would likely find some differences or patterns in the actual posture the rider employed on a given wheel size. You would probably find some differences in the cadence they landed on, when and how much body english was required to clear obstacles, and other things that are affected by wheel size characteristics. But the just-riding-along postures would be largely the same.

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  • Also 29er suspension forks were often 80mm travel where the 26" version would be 100mm in the early days of mass availability of 29ers, probably to maintain a familiar ride position and geometry despite the bigger wheels
    – JoeK
    Nov 27 '21 at 10:13
  • @JoeK We see the same thing nowadays for 27.5 vs 29. My guess is that that allows manufacturers to make only one front triangle for both wheelsizes though, not to facilitate rider familiarity.
    – MaplePanda
    Nov 28 '21 at 5:40
  • Re: "There are no categorically true statements that can be made about the fit of all bikes of a given wheel size." But does that not then mean that we could fit a cyclist to a bike of some size, then suddenly change to one size down and adjust posture up (as well as cranks and stem lengths), or switch to one size up and adjust posture down (+cranks+stem)? (Or is it indeed the case that fitting up or down is not absurd, and we can in fact change the fit to adapt to a size up or down?)
    – Sam
    Nov 28 '21 at 8:38
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    Yes in some cases you could get the same bars-pointing-forward spatial arrangement of contact points on two different size bikes in a size run. There would be other differences in handling and position/ergonomics when turning. The questions you're asking suggest confusion between what is dictated solely by wheel size versus what is dictated by design choices in a frame or size run of frames. Nov 28 '21 at 16:55
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    @Sam Sizing down is somewhat more common nowadays because of the huge reach numbers, yes. However, as Nathan mentions, the handling changes too (shorter wheelbase, different weight distribution, etc). You can't change just the contact points independently like that. If you're asking "can I swap the stem on my too-large bike so I can keep it without dealing with the return process?", the answer is "yes".
    – MaplePanda
    Nov 28 '21 at 20:02
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If the three contact points of Hands, Feet, and Backside are in the same relative places, then the rider's static posture will be identical regardless of the bike's shape or wheel size.

But nothing is the same forever - we've now got a trend toward wider bars, and the stems are much shorter so when moving the hands will pivot around a different centre.

I suspect this question is not answerable directly in text form.

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