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If the road bikes already had "almost" 29 inch wheels, why wouldn't we simply use those stock wheels instead of inventing a new standard?

Also, if a pro rider from the 90's rode bigger wheels in those old bikes, would this rider have any advantage?

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    Interesting question. Mtb originated as a cruiser modification, not a road bike modification, but I'm not sure those cruisers were 26". In any case: those guys probably figured out quickly that 29", while readily available, wouldn't fit the frames they wanted to ride nor would be as strong. – stijn Aug 28 '17 at 12:16
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Wheel & tire sizes are a source of unending confusion. The more you learn, the more confusing they get.

29" mountain-bike wheels have the same bead-seat diameter as modern road bikes: 622 mm (which are sometimes referred to as 28" wheels in Europe, which is confusing, because they're slightly smaller at the rim than 27" wheels…see what I mean?).

When mountain biking as we know it today was being pioneered on the repack ride on Mt Tamalpais, riders weren't using road bikes, and they weren't using mountain bikes because mountain bikes per se did not exist. They were using "cruiser bikes," which had fatter tires and slacker angles, making them better suited to bombing down fire trails. This is a case of people using what they had to hand and adapting it to circumstances, rather than designing something for an intended purpose.

The question of why cruisers have a different (and otherwise non-standard) BSD is another matter. I will note that a rim with a 571-mm BSD mounted with a 1¾" tire (which passed for "fat" back in the day) has a very similar outer tire diameter to a road bike using a 622-mm or 630-mm rim with a skinny tire, and sizing the rim to the intended outer tire diameter is common in bike history. Also, Schwinn, which created the cruiser, dominated the US bike market and could make de-facto standards whenever they wanted.

I can't answer whether 29ers back in the day would have conferred an advantage, although you might not have been able to squeeze those wheels into those old cruisers.

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  • so basically they didn't use the available bigger wheels when they built their own bikes because there was no tire choice for it? – Emílio Dolgener Cantú Aug 28 '17 at 15:42
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    I'm pretty sure that's at least partly correct. Or tires may have been available, but were not easily accessible to them. I think 622-mm Hakkapeliitta tires (for example) were around back then, but you couldn't find them at a bike shop in the USA. Using a bigger wheel may not have been regarded as a benefit even if it were an option. – Adam Rice Aug 28 '17 at 16:20
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    I think availability was a huge factor - This was in the tailing-off of the bike boom and common bikes would have been more accessible and therefore cheaper. If you're busting bikes regularly, then cheap is good. – Criggie Aug 28 '17 at 22:34

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