Serious question

[BarBender]

Now, imagine a little circle. A narrow bike tyre. And draw a straight line tangentially to it. The road. Now imagine a big circle. A big bike tyre. Draw a tangential line. Notice something about the contact areas of the circle and the line? They're the same. Real world, not quite because the tyre deflects a bit and squashes flat. A fat tyre will squash a bit more than a little tyre . But not much.

And that squashing actually makes handling worse. because it makes the tyre into a flattish section tyre. Like a car tyre.

Imagine having a flat section tyre on your bike . (Sidecar tyres are like that). Now lean the bike over. Hm. What happens to that tyre. Oh, its either resisting leaning over and bulging a bit (you ain't gonna go round NO corners like that !). or it's tipped over onto the sidewall. Oh. big expensive painful bang.
.

Ixion - I agree in part...but what if we started from scratch, turned it on its head and based the design of a motorcycle around a 190 on the front and on the rear? What would be the finished product and what would that look like?

[terbang]

In 'a twist of the wrist' there is reference to ideal weight distribution during a corner, can't remember the actual figures, but its something like 60/40 to the rear wheel that therefore needs a larger tyre contact. Thats why we turn better with the gas on. Interesting enough, if you increase the diameter of the wheel, while keeping the same width tyre, you will increase your tyre contact as well. However you will add a whole host of other issues with a larger wheel.
Cars and motorcycles turn the same way when the bike's speed is below around 15 kays. Turn wheels left, vehicle turns left and vice versa, but when the bike gathers speed there you turn bars left and the bike leans right (countersteer) and the lean angle presents the tyre to the road as a segment of a cone thus providing a turning force. Greater lean, greater turning force. I often wonder if a lot of our modern wide tyres have some sort of aesthetic value thrown in there as well. They look so cool.

[Ixion]

No reason you couldn't do it. But it wouldn't handle as well as it would with narrower tyres. .Some scooters have had fat tyres front and rear. So did the old Sunbeam S8s. Reason was for comfort in the days of crude suspensions. Big fat soft squishy tyres as shock absorbers.

The main point, in response to the oruiginal question, is that on a car, a wider tyre means more rubber contacting the road. The tyre is flat on the road, double the width and you double the contact area (more or less) .

On a bike , a wider tyre doesn't mean much more if any more rubber contacting the road (the actual amount will depend on the amount of "squish" in the tyre's shape, and thus on the weight of the bike etc). All the extra width is just out there touching nothing but air (think of those circles again)

EDIT: a larger DIAMETER wheel is another matter.

[Motu]

Big fat front tyres work well.....in the right conditions.Flattrackers run the same size tyres front and rear - when you first ride one you can feel this huge heavy front tyre,it's like there is a tractor tyre on the front.But when you get to use that tyre it's just so fantastic,you feel like a unicycle and feel you don't need a rear wheel.Sideways and nearly falling off the seat with the lean angle....and flapping this monster front tyre around,good stuff.I've run 4.00x18 and 3.75x19 front tyres on the dirt track.

[BarBender]

The main point, in response to the oruiginal question, is that on a car, a wider tyre means more rubber contacting the road. The tyre is flat on the road, double the width and you double the contact area (more or less) .
out there touching nothing but air (think of those circles again)
.

Just being hypothetical here - Would a fat front and rear combo be as agile if the the motorcycle were two wheel drive?
Sure it wouldnt have as much contact on the road as say a car....but could possibly guarantee both contact and - as Terbang mentioned - the constant 60/40 rear to front weight ratio?

[The_Dover]

who the fuck cares?

knob.

[Ixion]

Just being hypothetical here - Would a fat front and rear combo be as agile if the the motorcycle were two wheel drive?
Sure it wouldnt have as much contact on the road as say a car....but could possibly guarantee both contact and - as Terbang mentioned - the constant 60/40 rear to front weight ratio?

Well two wheel drive is going to allow greater cornering force. Because the rear tyre would only be passing half the amount of torque (assuming a 50.50 split). And a tyre can pass a certain set limited amount of torque, either sideway (for cornering) or forwards for traction. Less needed for traction = more for cornering.

But the fat front would make for fairly dodgy steering (outside special cases such as Mr Motu's flattrackers).

Which may be why the only 2WD bikes I've heard of are off roaders.

[Finn]

The same forces act on any wheel.
Problem with a car is it isn't going to lean because of the second track.

Thanks but that was me not being serious.

[James Deuce]

Car suspension is also much better at separating and isolating steering, braking , and acceleration forces than a bike. Except front wheel drive cars. That's a silly idea designed to make cars small and cheap to mass manufacture. Except Honda front wheel drive cars. The clever buggers do some interesting things with front wheel/hub offsets to all but eliminate torque steer.

Interestingly, thanks to about geological epoch of development on telescopic forks, they still work better than for instance, BMW's duo-lever front end. Separating braking forces from the steering on a bike has the effect of not providing much braking feel when you really need to know what's going on.

[Big Dave]

Because it would look like a Van-Van

[James Deuce]

Van-Van's are cool! Put big panniers on them and you can go on adventure rides AND go white water rafting.

[motoGP]

Actually got a very good article in one of the latest BIKE magazines. R6 vs. a Lotus (?) both with very similar horsepower output. The bike accelerated much faster, and thus had to brake earlier at corners because of its higher speed. Ultimately the R6 won by a few seconds, both machines use their power in very different ways, but a very interesting article. Compare prices, well that’s a whole other ball game…

[Mr. Peanut]

Would be hard to counter-steer a large front wheel at speed. Also increases unsprung weight.

[The Stranger:Me]

Thanks but that was me not being serious.

I suspected you weren't, that's why I kept it short.

[86GSXR]

Great thread guys. I see mention of "a twist of the wrist". I assume this is a book, would someone be kind enough to post the author's name and where I could get a copy? Cheers.