Asymmetric weapons have become quite popular in combat as robots get more
competitive. This weapon design uses
only one hitting feature (a "tooth") to
contact the opponent instead of two
This concept can be applied to
any spinning weapon design such as a
disk, bar, or drum. New builders often
have questions about how to design
an asymmetric weapon. When
designed incorrectly, the vibration
from the unbalanced weapon can
impair your driving ability and even
damage your own robot.
The first step is to decide whether
or not this is the right weapon design
for your bot. Like any design, its
advantages come with some
drawbacks. As long as you know the
limitations, you can make sure your
robot design addresses these factors.
Let's take a look at some of the
drawbacks of an asymmetric design:
• An asymmetric weapon may
have to be replaced more often. Its
one tooth has to take every hit, so
wear becomes more of an issue
compared to a two-tooth design.
• An asymmetric weapon will be
slightly heavier than a similar
symmetric weapon. Since the
counterweight is closer to the center
of the weapon, it requires more mass
to balance the opposing tooth and
achieve a balanced weapon.
• This design can be more difficult
• It is more complex to design.
The main advantage of an
asymmetric design is that it allows you
to spin your weapon twice as fast
without sacrificing the depth of
contact between your weapon tooth
and the opponent during a hit (also
referred to as "bite;" see Figure 2).
The more bite you can achieve,
the more energy you will be able to
transfer into the opponent because
you will have a larger area of contact.
When deciding on teeth quantity in
your design, this is the main factor to
Bite is defined by both drive speed
and weapon speed. You can calculate
how much bite your weapon can get
in just a few steps as described below.
• Find out how fast your robot
drives. You need to know the speed of
your drive gearmotors and the
diameter of your wheels. Using that
information: Drive speed =
(circumference of the wheel) *
(revolutions per minute).
A plug-and-play version of the
same equation is: Drive speed =
(wheel diameter in inches) (π) *
(drive gearmotor RPM).
For example, if your robot uses
1.5 inch diameter wheels and your
gearmotors spin at 400 RPM: Drive
speed = 1.5 in 3. 14 400 RPM =
1,884 inches per minute.
• Find out the time it takes your
weapon to spin from one tooth to the
next. A traditional weapon takes half a
revolution from tooth to tooth, while
an asymmetric weapon takes one full
revolution: Time between hits = 1 /
[(weapon RPM)* (number of teeth)].
For example, if your robot has
two teeth and your weapon is
spinning at 3,500 RPM:
• Now, we can calculate bite by
finding how far your robot travels in
the time you calculated above: Bite =
(drive speed) *(time between hits).
In our example: Bite = 1,884
inches per minute 0.0005 minutes =
0.27 inches of bite.
Now, you know that the most bite
your robot can get is 0.27 inches —
assuming you are driving at full speed,
your weapon is on max, and the
motor RPM specifications reflect
reality. You will always get slightly less
bite than this, but it's safe to design
your weapon tooth assuming the bite
you calculated as a maximum. A
bigger weapon tooth may look more
intimidating, but it won't make a
difference to the performance of your
weapon beyond the increase in
diameter. You can increase your bite
by increasing your drive speed, but this
may make it difficult to drive your
robot controllably. You can also
increase bite by decreasing your
Don’t Bite Off More Than
You Can Chew
● by Andrea Suarez
Part 1: Understanding
26 SERVO 02.2014
Time = 1 = 0.000143 minutes
3500 RPM × 2 teeth