"CandT" <
[email protected]> wrote in message
news:[email protected]...
| >
| >That defies the laws of physics. Both are equal.
| >
| ...
| >
| >It adds greater weight, hence greater force on the ground (vertically & horizontally, since they
| >are also ridden faster).
| >
|
| Hmmm, this thread does bring a smile to my face. I'm can't recall where Mr Vandeman's got his
| physics qualification, somewhere well respected I
assume, but
| I seemed to have taken in more from my Tertiary course here in the UK
(aged
| 16-17).
|
| OK - this is how a shock absorber works.
|
| A bike travelling forward hits a bump with the front wheel.
|
| On a non-suspended bike, the whole force of that contact is transferred
through
| the tyre (which has a slight damping effect, but for the sake of argument,
lets
| assume you are running wooden wheels!), forks, and riders arms, and indeed
there
| is an equal and opposite force transferred into the ground. (The ground it attempting to lift the
| entire weight of rider+bike)
|
| Now, on a suspended fork, the force of the contact is actually transferred
into
| a deforming force of the spring initially, and then is partially
disappated as
| heat, the compression of a gas, and sound in the shock 'absorbing' parts
of the
| fork. Only a portion of the force is returned to the ground. (Now, I know
not
| all forks have springs, sometimes its air, or elastomer, but the effect is
the
| same)
|
| Effectively, the rider+frame are treated as a seperate body to the
wheel+fork.
| They are both travelling forward with the same speed, and when a bump is contacted, the wheel and
| fork obtain a vertical accelleration which is
protected
| from the rider. The rider and frame still only have the forward motion.
(or
| actually a tiny downward motion with gravity, but ironically enough - the
faster
| the rider is going, and the better the shock absorbing technology, the
less that
| force will be !)
|
| Use this as an analogy. You are standing still holding a spring which is
50%
| compressed by the weight of a bowling ball on top. In the other hand you
have a
| stick with a bowling ball on top. If you quickly jerk upwards with the
stick
| holding hand, you have to move everything up with a force equal and
opposite to
| the weight of the stick and ball, and your hand hurts... If you jerk
upwards
| with the spring holding hand, initially, to spring will compress further,
and
| the ball will remain unmoved. The point being that that initial jerk
upwards its
| translated not into the kinetic energy of the bowling ball, but into a
deforming
| force of the spring, and your hand doesnt hurt.
|
| But then again - I'm not a physicist, though a good friend of mine has a
Masters
| Degree in Physics, and I'm sure he would like to get in on this
discussion.
|
| ...
|
| In fact - this is what he has to say about the subject. This should really
be
| the last word - please...
|
| <CONTRIBUTION BY Marmite (BSc MSc Physics Hons)> Damage to the ground is directly related to
| pressure, which can be
calculated by
| dividing the Force by the Area of contact. A simple analogy is try walking through a flower bed
| with boots and stilletoes, how much impact does each
of
| them do? The stilletoes will most likely sink straight in. In the case of
the
| boots the force will be spread over a larger area causeing the pressure
reduce
| and stops you sinking into the ground (an extreme example are snow boots).
|
| In the case of a mountain bike, the area of contact is fixed (i.e the area
of
| contact of the bike wheels), although the effective area of contact is
larger,
| which is due to the fact that the bike isn't stationary, and any extra
forces
| acting on the ground are spread over the area of distance travelled for
the
| duration the force is applied. When talking about damage to the underlying ground you also need to
| talk about the Force. The larger the force on the ground, the more the damage. Now looking at
| Newtons laws you can calculate
the
| generalistic difference in the force between using shock absorbers and
without.
|
| N1 - A body will remain at constant velocity unless acted upon by an
external
| force. (i.e. the acceleration of the bike relative to the ground is caused
by
| bumps etc.). N2 - The force on the body is directly proportional to the rate of change
of
| momentum (i.e. mass * acceleration. The force on the bike is related to
the
| magnitude of the acceleration caused by the impact of bumps). N3 - Every force applied to a body
| is matched by an equal and opposite
force
| (i.e. the Force on the bike = the force imposed on the ground).
|
| If you don't have shock absorbers, the mass of the whole bike is
accelerated
| sharply by any bumps in the ground, hence the forces experienced by the
bike due
| to that change in momentum (N2) and the equal reaction force to the ground
(N3)
| are high.
|
| In the case of shock absorbers, the change in velocity is applied over a
longer
| period, hence the acceleration experienced by the wheels is less
(acceleration =
| change in velocity / time), also for the most part, it is only the mass of
the
| wheels that accelerate up and down, and not the whole mass of the bike. Therefore the actual
| change in momentum (N2) is less (as both the the mass
and
| the acceleration is less), the comparative force applied on the bike and
rider
| (N2) are therefore lower (giving a smoother ride) and hence reaction force
and
| damage applied to the ground (N3) is equally as low.
|
| These fundamental laws are used in many other situations, for example
crumple
| zones and air bags lower the damage to the driver and the other body
involved,
| by spreading the impact over a longer time and hence lowering the force experienced by the driver.
| These fundamental properties in question here
may
| also be referred to as impulse. </CONTRIBUTION>
|
|
|
|
| CandT
Very well put and totally above that of MV I am sure. He is too scared to pass on his dissertation
to me to read, so I shall assume it does not exist.
Simon ..........I am sure HE will have some childlike retort.