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?
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.
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.