be found at most any well-stocked R/C hobby store. The
tires are available in a variety of diameters, with the 2”, 2-
1/2”, and 3” diameters the best for small bots. They do go
up to 6” should you need the larger size.
Large LEGO “balloon” tires have a recessed hub that
exactly fits the small round servo horn included with Hitec
and many other servos. You can simply glue the horn into
the rim of the tire.
A gear glued or screwed into the servo horn can be
used as a substitute wheel or as a gear that drives a wheel
mounted on another shaft.
Urethane skateboard/inline roller skate wheels
make for incredibly “grippy” wheels for robots. The trick —
if it can be called that — is to find metal or plastic discs that
just fit into the hub of the wheel. Many skate wheels use
metric dimensions; for those in the US, our hardware is in
If you have access to a 3D printer, lathe, or CNC router,
you can easily make inserts that fit the skate wheels
perfectly. Figure 9 shows the construction of the wheel,
and Figure 10 shows a wheel attached to an R/C servo
(mounted on a robot).
No matter what method you use to create custom
wheels for servos, be sure not to block the screw hole. If
necessary, insert the screw into the servo horn first, then
attach the tire.
Using Couplers to
Couplers are used to connect two drive shafts. A
common application is to use a coupler to connect the drive
shaft of a motor with the axle of a wheel. Couplers can be
rigid or flexible.
Rigid couplers are best used when the torque of the
motor is low, as it would be in a small tabletop robot.
Conversely, flexible couplers are advised for higher torque
applications as they are more forgiving of errors in
There are many types of commercially-available rigid
and flexible couplers, and cost varies from a few dollars to
well over US$50 depending on materials and sizes.
Common flexible couplers include helical, universal joint
(similar to the U-joint in the drive shafts in older cars), and
three-piece jaws. Figure 11 shows a commercially-made
coupler that has a flexible middle. These are available from
online sources such as Small Parts (see the Sources box).
For low torque applications, you can make your own
couplers using brass, steel, or aluminum tubing available at
hobby stores. Tubing is typically sold by its inner diameter —
1/8” tubing is 1/8” inside, for example. The thickness of
the tubing determines its outside diameter. Get tubing that
42 SERVO 04.2014
detail of a
wheel using a
uses a servo
wheel to an
shafts of a