On Wed, 09 Apr 2003 09:36:38 GMT, CandT <
[email protected]> wrote:
.> .>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.
How it works is irrelevant. The fact that it ADDS WEIGHT implies that it increases the force (of
gravity) on the ground. DUH!
.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.
BS. Unless you have eliminated gravity, the force is greater than that of a lighter bike. The TIME
of action may be delayed, but eventually, the full force of the bike's total weight gets applied 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 !)
BS. Gravity doesn't change.
.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 you still feel the full weight (force) of the ball, eventually.
.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,
Pressure is irrelevant, since the AREA of contact (between suspension & non-suspension bikes) is the
same. The only relevant factor is the ADDED WEIGHT of the suspension, which adds weight (force &
pressure) to the ground.
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.
BS. At some point, the spring stops moving (at its extreme extent or compression), and the full
force of the bike's weight (including the added weight of the suspension) is applied to the ground!
At that point, the suspension bike is applying a greater force to the ground. QED
.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.
BS. Air bags don't protect one from gravity. Eventually, the air bag stops moving, & then only
gravity is working.
These fundamental properties in question here may .also be referred to as impulse. .</CONTRIBUTION>
. . . . .CandT
===
I am working on creating wildlife habitat that is off-limits to humans ("pure habitat"). Want to
help? (I spent the previous 8 years fighting auto dependence and road construction.)
http://home.pacbell.net/mjvande
It is all tuckered out. No, PETERED out.
