Text 4596, 135 rader
Skriven 2005-01-06 13:24:00 av Michiel van der Vlist (2:280/5555)
   Kommentar till text 4580 av Robert Bashe (2:2448/44)
Ärende: Motorcycles
===================
 > RB>> That's a simple matter of physics, though...

 > MV> An area that you as a chemist better stay out... ;-)

 > Although my grasp of physics in this case appears to be
 > better than yours.

That's what you think...

 > RB>> four wheels have more contact to the pavement than two,

 > MV> No...

 > Are you serious or are you trying to make some kind of a joke?

I am dead serious. Read on...

 > MV> The contact area is equal to the weight of the vehicle
 > MV> divided by the tire pressure. (At first approximation).
 > MV> The number of wheels does not come into the equation.

 > Michiel, you're writing bullshit.

I already feared this was beyond the comprehension level of a chemist.

 > Surface area of contact per wheel times number of wheels equals
 > total surface area of contact. Period.

Your error lies in assuming the contact area per wheel is a constant. It isn't.

 > No interpretation. And the contact surface of a motorcycle
 > tire is even lower than that of a car tire because the
 > motorcycle tire is round, whereas the car tire has a flat tread.

Wrong again.

If the tire were infinitely rigid or the weight of the vehicle were zero, there
would be no /area/ of contact, there would be a /point/ of contact. And that
goes for a car too, as the tire is not an exact cylinder, but barrel shaped.

The tire however is not infinitly rigid and the vehicle's weight isn't zero. So
the tire deforms a little bit and we get a finite area of contact. How much
depends on how much load there is on the wheel and the rigidity of the tire.
And that is where the tire pressure comes in. if we consider the tire as a
flaccid bag that derives all its rigidity from the air pressure, (first
aproximation) one can easily see - if one is a physicist that is - that the
contact area is inversely proportional to tire pressure.

Also if the load on the tire decreases the area of contact also becomes
smaller. In fact, it is proportional.

So if you increase the number of wheel - while keeping the weight - you spread
the load over more wheels and the tire of each wheel indents less. The total
area of contact remains the same.

 > MV> Regarding contact area there is no reason why a motor cycle
 > MV> should have less brake capacity than a car.

 > You're nuts.

No, I am a physicist who understands these things.

 > MV> The problem with a motorcycle is that it is unstable at
 > MV> heavy breaking.

 > That may be an additional problem, but...

That is the main problem on most real life roads.

 > MV> It has a tendency to swing sideways.

 > ... swinging sideways isn't part of it. The main problem
 > with emergency braking of a motorcycle is not to fly over
 > the handlebars by braking the front wheel too vigorously.

Only on a dry clean road.

Over here roads are more often wet than not. On a wet road, the wheela start
slipping and a bike with a slipping front wheel is very hard to control. Of
course a bike with ABS does not have that problem. Not to the same extent
anyway.

 > MV> A car with properly aligned brakes DOES break more on the
 > MV> front wheels than on the rear. The reason that cars hardly
 > MV> ever somersault is that the centre of gravity is relatively
 > MV> lower than that of a motor cycle.

 > Cars don't "hardly ever" somersault merely by emergency
 > braking, Michiel. They don't at all.

You have obviously never taken a close look at these very small city cars.

 > They are heavier

That has nothing to do with it. It is the relative position of the centre of
mass that is the relevant factor. If the combined vector of horizontal
deceleration and gravity, when drawn fron the centre of mass crosses the road
in front of the front wheel(s) it will somersault, That cab happen to a car
too. Especially these relatively high and short city cars.

 > and have hydraulic brakes that act on _all_ wheels evenly,

You have obviously never taken the brake system of a car apart. If you had,
then you would know that the brakes do NOT apply evenly to front and back. The
discs opr drums are bigger on the front than on the rear. If they were t break
evenly, the rear wheels would start slipping before the fornt wheel do. That is
not an optimum situation.

 > not separately the front and back wheels as in a motorcycle.

Irrelevant. Breaking peower is diveded so that all wheels start slipping at the
same time. That is the idea anyway. That goes for a car as well as a
motorcycle.

The reason a motorcycle somersaults more easily is because its relative
position of the centre of mass is less favourable.

 > Hey, I thought you had some experience riding motorcycles,

I admit it has been a while. I was not a "nice weather" rider though. I have
plenty of experience on WET roads. Wet slippery roads. I guarantee you,
somersaulting is not a problem on wet roads....

 > but you write like someone who's never seen anything but a
 > picture of one.

I write as someone with riding experience AND the understanding of the physics
behind it.

Your experience may be more recent, but you obviously lack the understanding.

Cheers, Michiel

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