Hi Jeeter,
so did the 2 Vs (Vincent, Velocette) back in the day.
There may have been sensible reasons for it all those years ago.
In these days of computer aided frame & suspension design where 12" wheel super-scooters can safely do the ton,
I reckon a 21" front wheel on a road bike is just a fashion statement.
Hmmm ... so if it's a fashion statement what makes Speedway Class A bikes stick to them? I was just curious. I think maybe the reason Class A bikes use larger/more narrow front wheel/tire combos is due to the loamy/loose tracks that Speedway typically has. I mean, I can't think of any other reason to run such aggressive knobs on their tires as well. So perhaps it has to do with the loose tracks and the loamy surface that the taller/more narrow front wheel/tire can get a good bite in the cushion.
Which may explain grass track and short Speedway track bikes, but does nothing for providing a good reason why they still run that combination on the half mile and even the mile tracks on Class A bikes.
I know why MX bikes and desert bikes use 21 inch fronts, the 21x18 combo has proven all but perfect on those types of bikes. As was pointed out they roll over obstacles better (and other riders' feet in corners or their legs when we're running "backwards night" at our local MX track and the guy that fell in front of you simply will not move his legs out of the way, so you pop the clutch and ride right over them with your wonderful 21 inch front wheel! I raced MX for many years, 250cc class on a very VERY hot 175cc, I was no more than 4 foot 10 inches and beating grown men on 250cc Elsinors on a weekly basis. Man I loved crushing their egos! Haahaa!
).
So yea, I get the reason dirt bikes use larger fronts (the larger front also has a slower rotational speed making the front end less responsive and less "jittery" in 5th gear sections of whoops). But it has me a bit lost on why Class A bikes still use larger wheel sets even after 60+ years of using the same wheel combination. I figured since the Class A bikes are supposed to be prototypes that they were using something good that the Class C and GNC guys were unable to use due to rules (like the way the AMA outlawed rigid frames in 1968 for dirt track racing, the AMA wanted very defined lines between the Calss A bikes and the "flat track" bikes. They wanted the flat track bikes to resemble the production counterparts more clearly.)
Perhaps the larger front wheel/tire may have caused the flat track bikes to use a different racing line than the 19 inch equipped bikes. Y'know, kinda like why the AMA (and anyone sane) was spooked about how the triple/multi cylinder flat track bikes used a totally different racing line than the twin cylinder bikes. Those two different racing lines intersected (very dangerously, I may add) right at the exit of each turn, creating the potential for serious contact between bikes. The twin cylinder bikes all cut into the low bottom of each corner to hit the apex, then swung out wide to exit the corner (in an attempt to make the track into as much of a circle as possible). The multi-cylinder bikes used a very wide racing line along the outside cushion of the corners. Those two racing lines crossed paths at the exit of each corner producing the opportunity for some spectacular crashes involving colliding 100mph bikes.
(NOTE: Marco Simoncelli used that same type of very wide corner entry and wide exit which effectively block-passed the rider he was overtaking. He'd been protested many times for his "unique" racing line.)
Perhaps the larger front wheels produced the same type of issues when raced with bikes that used 19 inchers?
Just sayin'.
