Why didn't tanks sit flush on the frame on 90s sport bikes?

[SirFallalot]

I appreciate older and naked bikes for their well rounded tank that sits on the hidden frame and swoops around the engine and carbs. As I was searching for something sporty to try within my bike+insurance budget, I noticed on a lot of 90s and some 00s sport bikes the tank looks like it doesn't really fit, there's a kink that leaves a gaping hole not quite big for a pet or a cock, but too large to ignore.
Oversight or what? Seems to be fixed by all throughout the 00s.

Examples
https://s3-eu-west-1.amazonaws.com/crash.net/visordown.com/styles/large/s3/Honda%20CBR900RR%2092.jpg?itok=nHO8pLSY
https://storage.kawasaki.eu/public/kawasaki.eu/en-EU/model/imported/DD000003400EFDD997.gif
https://www.rsfabrications.com/wp-content/uploads/2010/04/cimg5405.jpg
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Lexmoto Valiant 125; 94 CB400; 96 CB750F2; 81 CB750 (restoring, lol not gonna happen); 2001 ZX9R(It's about to go :c); 2012 R1200R; 2015 R1200RS

[MarJay]

To fit the carbs/inlets? You can even see the edge of one poking out of the Kwak!
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[stinkwheel]

Maybe to keep air circulating so they can breathe? Some of them have air-scoops on the fairing but not all of them. My VFR doesn't, the air inlet is up under the tank. Even on the ones that do have scoops, there are still other breathers and additional fiddly bits to do with the air inlet system up under there.

There are also loads of fuel overflows/breathers so it might be to prevent a potentially explosive build-up of fuel vapour?

Another potential explanation is so you can get at the float bowl drain screws with a long screwdriver. You can on a CBR6F at least because I did so last weekend.

Another thought is maybe it's the ameliorate venturi effects at high speed. It's curved so it'll act like a wing in an airflow, especially if the engine is busy sucking all the air out from underneath it. Even seen the top of a tank-bag at 120mph? Domed and solid. I think someone on here even burst one once. I'd imagine there will be a significant amount of force exerted in the bike purely by the airflow.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[xX-Alex-Xx]

Cos you’ve got a fuck-off air box sitting underneath em.
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DILLIGAF

[trevor saxe-coburg-gotha]

yeah the airboxes are ridonkulous
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"Life is a sexually transmitted disease and the mortality rate is one hundred percent."

Mobylette Type 50 ---> Raleigh Grifter ---> Neval Minsk 125

[Kawasaki Jimbo]

It's a warm air vent. When you rode a '90's sports bike you had to keep the inner thigh area warmed and ready for the inevitable consequences. The modern crop of insect-faced bikes won't cut it with the ladies so manufacturers have abandoned the gaping hole.

[stevo123]

Agree, let hot air out to assist cooling of engine and keep petrol in the tank cooler.