What are advantages of bigger wheel?

Question

What's the advantages of bigger wheel? I feel that it's more forgiving on rough road, giving that the tyre width and pressure are the same. Is it just my imagination?

Since population height is improving, why don't we have even bigger wheel by now?

Answer 1

What's the advantages of bigger wheel? I feel that it's more forgiving on rough road, giving that the tyre width and pressure are the same.

The main advantage is, as you suspect, that a larger wheel does not drop as far into potholes, and generally suffers less from rough roads. That's purely geometry, as seen in this comparison of an ordinary wheel and a child's bike (1.8m high vs 500mm):

For similar reasons the larger wheel will have a lower rolling resistance, both because the casing flexes less at the road surface and because the wheel bearings rotate more slowly.

The disadvantages are significant, however. The larger wheel is less rigid laterally (for the same flange separation at the hub), heavier and has more wind resistance (larger frontal area). Those reasons prompted people to build small wheel bikes, but as is so often the case with good ideas, the UCI banned small wheels in response to Moulton's design.

As far as using larger wheels today, we do. Mountain bikes are increasingly using 622 wheels instead of 559 (the "29er" size), and it's becoming increasingly hard to find cheaper road bikes with anything other than 622 wheels (we had a question recently about where to find 650c/571mm wheels). Note that I'm only talking about safety bikes here, ordinaries use larger wheels because they have their pedals locked to the drive wheel so the only way to get higher gears is with a bigger wheel.

The wheel size is unlikely to increase as peoples average height does because everyone is small at some stage, and it's much easier to build a large bike with small wheels than the converse (Moulton's and folding bikes take that to the extreme, using 406 or even smaller wheels for adult bikes). So there will always be a market for smaller bikes, and economies of scale mean it's easier to build a ridiculously large frame with small wheels rather than building your own, unique, large wheels.

If you do want to build a bike that way, I suggest looking at unicycle parts because they have the same gearing problem as ordinary bikes, so they have rims and tyres up to about 36" (ISO 787mm). People do build bikes with those wheels:

But as you may guess, people wanting to go fast use small wheels. Almost every open speed record is held by a bike using small wheels, and only UCI records are set with larger ones.

Answer 2

Your first question is, relatively, easy to answer. Larger wheels have an easier time negotiating obstacles – roughness on the road and timbers and crevasses on the trail. The larger the wheel, the more it can span holes and the easier time it has climbing over stuff. The downside of large wheels is more rotating mass out at the rim.

Your second question is much harder to answer, but I would guess it has a lot to do the relative slowness of height growth and the costs of adding a new wheel size (since the old ones probably won't just go away).