Robot Building for Beginners, Third Edition

CHAPTER 19 ■ WHEELS

By using 22 cm circumference wheels instead of 16 cm circumference wheels, the linear speed increases
proportionally.

22 cm / 16 cm = 137.5%

To counteract the 37.5% increase in speed, the 9 V NiMH rechargeable battery can be replaced with a
6 V pack of four AAA-size cells. The reduction in voltage from 9 V to 6 V decreases speed proportionally.

6 V rechargeable alkaline rated voltage / 8.4 V actual NiMH rated voltage = 71.4%

Overall, the speed is about the same:

(22 cm / 16 cm) × (6 V / 8.4 V) = 98%

Not only does this give the robot more appropriate wheels, but also the AAA pack lasts seven times
longer!. The difference in mass between the two configurations is only about 5 grams more for the battery
pack, or about +2.5 %.
If you choose 6 V (AAA pack) instead of 9 V (9 V battery) to permanently run your line-following robot,
be sure to recalculate resistor values for the circuits. Otherwise the LEDs are going to be pretty dim.

Obtaining New Alternative Wheels

LEGO currently manufacturers another wheel size that is an excellent alternative (see Figure 19-14). The rim
is 30.4 × 20 (#56145) and the tire is 43.2 × 22 ZR (#44309). The narrower diameter improves line-following
accuracy and the narrower width reduces friction.

Figure 19-14. Out-of-production 49.6 × 28 VR (left) compared to current-production 43.2 × 22 ZR (right)