People have started on ‘Busa’s. The fact that it is possible does not make it a good idea.
Reasons for a 650R:
-It is new ans stylish
-the 650 parallel engine produces a flat torque curve, so no matter where you are in the power band, the same twist of the throttle will result in the same increase in acceleration. (This is not the case in a rising torque curve like a I4, where the higher the rpm, the more pronounced the effect of the throttle will be).
-The suspension, brakes, and geometry are “good enough” by sporting standards; that is, they work and do their job, but do not do it so well as to be grabby or twitchy – aka the sport bike where a sudden fistful of brake results in flipping end over end.
The reasons against:
-Despite not reacting quickly and twitchy to all inputs (again, like a sport), nor being overly heavy, the Ninja 650 and SV650 both work in the realm of what was a super bike 20 years ago. They accelerate very quickly, they stop very quickly, and they turn in very quickly. In normal riding situations these traits are not too much for a confident rider IMO. The issue becomes when in stressful situations. The rider can still wheelie the bike with a clutch let out too fast, or stoppie the bike (or worse) with too tight a right hand.
The confidence the bike exudes can easily get a rider in over their head. Doing great, feeling great, you’re in the moment, then you realize you’re coming into the corner too hot. Most riders will try to loose as much speed as possible. The number of things they can screw up here is huge – target fixate on the shoulder, lock the wheels, etc. The new rider simply doesn’t have either the mental or physical skills yet to deal with it.
And everyone does it – mistakes are how we learn.
But on a lighter, slower, less stiff bike, you aren’t going as fast, your brain has more time to see what it is doing wrong and correct. Additionally, the “lesser” bikes are more forgiving to over-corrections (which all new drivers and new riders do, it is part of muscle learning).
Now, if you have dirt bike experience, and you think you’ve got what it takes to tame the 650, go for it. But don’t say it is fine for everyone, because it isn’t.
But also remember that smooth control of the bike and yourself is only half the equation. The other half is managing the road and other road users.
I know at least two guys who started on dirt bikes, raced them, and damn near made a mess in their pants the first time on a twisty road with traffic. Just because you have 75% of the muscle memory you need doesn’t give you the mental skills you need to hold your line around a corner with an 18 wheeler barreling down on you. It’s a different mind game than being in the dirt.
And that mind game is made easier when you aren’t worrying about your bike. As they say, each rider only has $10 worth of concentration. On a busy road you need at least $5 just for the other traffic and $2 spent on figuring out your immediate route. Now if you’re a new rider on a bike that requires careful smooth operation you’ll be spending another buck on keeping your throttle smooth, another worrying about shifting, and another one on the brakes. So what happens to your mental capacity when something else pops up? Something’s going to give.
It’s much nicer when you’re on machine that “just does what you want” without any surprises. It gives you more room for error.
I had about 1000 kms under my belt on a trio of bikes (GS450 making about 45hp, and a VF500F making 65hp, and a ZZR-250 making 35hp) when I test rode a BMW F650GS (making 75hp and 55 foot pounds of torque). Despite my experience, that bike made me very aware, the entire time, of how fast it could haul, how hard it could brake, and just how easy it was to turn. Frankly, it was too fast in every way. I spent a lot of energy making sure I didn’t bin it. When I drove home on my 250, my brain was free to enjoy the ride.
When I took my ERC, three of the guys there were on CBR125s. Tiny little “toy” bikes. They all had slightly less road time than I had. But when it came to controlling their bikes, they rode like the veterans on the 600s. They leaned those bikes over so far, braked so hard. Having seen that, there is no question in my mind that the best tool to learn on is an easy and forgiving one that you can “forget about” and just focus on your riding.
You are absolutely right about how everyone tries to simplify what makes a good bike down to displacement. Similarly everyone tries to link displacement directly with performance. And really, displacement is only a broad indicator of all the things that make a bike easy or hard, slow or fast. Just don’t make the mistake of discrediting over-simplification as lack of reason.