Discussion

1
KINETIC ANALYSIS OF FORCE DISSIPATION
CHARACTERISTICS IN MOUNTAIN BICYCLES Tyler Cluff,
Joel Roy D. Gordon E. Robertson, PhD,
FCSB School of Human Kinetics, University of
Ottawa, Ontario, Canada
Results In the single bump condition (Figure 2)
the FS bicycle recorded a 9.90 lower vertical
GRF when the rear tire struck the obstacle. This
can be interpreted by the higher average impact
speed of the DS bicycle (2.22 vs. 1.78 m/s).
However, a 39.9 reduction in the ground reaction
force was observed upon the rear wheels impact
with the ground when dropping off the bump. This
demonstrates that the rear suspension dissipated
a larger fraction of the force upon landing. The
results of the pothole trials (Figure 3)
displayed a 16.3 lower vertical GRF for the DS
bike when initially striking the bump. When
hitting the second bump a 17.5 decrease in the
vertical GRF was observed. Finally, the
attenuation of the force dropping off the second
bump was characterized by a 47.4 decrease in the
vertical GRF. In the platform drop condition
(Figure 4), the magnitude of the vertical GRFs
for the rear tire impacting the force platform
was found to be 31 lower in the DS bike.
Introduction Improvements in bicycle suspension
designs allow cyclists to ride rough terrains
with greater safety, confidence and comfort. For
example, dual- suspension (DS) bicycles have been
shown to offer the rider isolation from
vibrations and terrain-induced shocks by allowing
the wheels to move independently from the rest of
the bicycle (Delorenzo et al., 1994) The
suspension system attenuates vertical forces more
effectively and dissipates them over a greater
period of time when compared to an unsuspended
bicycle (Roy Robertson, 2000) but it must also
permits tires to remain in contact with the
ground for better control of the bike. This
project investigated changes in the vertical
ground reaction forces (GRF) between
front-suspension (FS) and dual-suspension (DS)
mountain bikes to provide understanding of the
benefits and limitations of these systems. It
also quantified the delays that occurs before the
rear tire regains contact with the ground
following its initial dropping off of a bump.
Figure 3. Vertical GRFs (N) for the double bump
of the DS (top) and FS (bottom) bike. Blue lines
are from first plate magenta are from second.
Summary The dual-suspension bicycle
significantly reduced the peak vertical ground
reaction forces for all three test conditions.
Furthermore, for both the single and double bump
conditions the dual-suspension bicycle permitted
the rear wheel to more rapidly regain contact
with the riding surface. Thus, the
dual-suspension mountain bike was demonstrably
better than the front-suspension bike for both
safety and rider comfort.
Figure 1. Dimensions of the single and double
bumps used in the experimental protocol.
Methods Two aluminum mountain bikes, one having a
front-suspension fork, the other having a
dual-suspension system, were used to collect the
data. Three tests were conducted with five trials
for each condition. In the first experimental
condition, the subject rode the bicycle over a
single bump measuring 8 cm in height and 10 cm
wide. In the second condition, the subject rode
over a simulated pothole consisting of two
successive bumps measuring 8 cm in height and
width and separated by 30.5 cm. In the third
condition, the subject rode onto a platform
measuring 35.6cm in height and dropped off the
edge onto a force platform. Each bump was mounted
onto a force platform with a second platform
imbedded adjacent so that contact was made after
leaving either bump. The force data were
processed using BioProc2 software (Robertson,
2006). The data were smoothed using a
second-order, critically-damped digital filter
set with a cutoff of 10 Hz. The peak forces for
each condition and the times for the rear wheel
to recontact the riding surface were determined
and averaged across all trials.
Figure 2. Vertical GRFs (N) for the single bump
of the DS (top) and FS (bottom) bike. Blue lines
are from first plate magenta are from second.
Discussion Greater control of a bike is
maintained when tires stay in contact with the
riding surface. This allows the riders input to
be transmitted to the trail surface more
effectively and with increased urgency, thus
making it safer and more efficient for the rider.
In the single bump condition, the rear wheel
returned to the surface 35 quicker in the DS
bike than the rear wheel of the FS. Furthermore,
in the double bump condition the rear wheel
contacted the ground 25 faster with the DS bike.
Figure 4. Vertical GRFs (N) for the platform drop
of the DS (top) and FS (bottom) bike.
References Delorenzo DS, Wang EL, Hull ML (1994)
Cycl Sci 312-26. Roy JP, Robertson DGE (2000)
Proceedings CSB XI p.125. Robertson DGE (2006)
http//www.health.uottawa.ca/biomech
/software/bioproc2.htm.