THE SMALL-AND-FAST sumo-bots presented in Part Two of Competitive MINDSTORMS can be built entirely out of the pieces available in the RIS. This actually has its advantages for the small-and-fast strategy, since fewer pieces help maintain a high speed. On the other hand, who says you can't add more mechanisms and pieces on a sumo-bot belonging to the small-and-fast strategy? A small sumo-bot can take advantage of additional pieces-maybe a third motor?

In this chapter, you'll add a third motor to enhance Zip-Bam-Bot's performance. Although this does stray from the RIS limits, the extra motor is the only piece in this sumo-bot not found in the RIS. What if you don't have a third motor? One way to obtain one is to buy the RoboSports or Ultimate Builders Expansion Pack, which both contain a motor. If you don't feel like buying an expansion pack just yet, you can do a search on eBay and easily find LEGO motors. And if you want to order motors from a more official source, you can buy them (and other LEGO motors) from the Pitsco LEGO Educational Division, at www.plestore.com.

What Does the Extra Motor Do?

What is the purpose of the extra motor? It is intended to power counter-rotating wheels (CRWs). These wheels are positioned "face-forward" on the robot and rely on the following tactic: rotating at a high speed, they attempt to reduce the effects of a front-end hit (by another sumo-bot) and also give XK3 the most effective hit upon impact. NOTE In Chapter 7 of Competitive MINDSTORMS, Brain-Bot Version ZR2 uses what can be considered "traditional" CRWs. The ones on XK3 are positioned in a different way and have a different purpose than the ones on ZR2.

Since small-and-fast sumo-bots can't push, they always need a means of detecting the opponent. For XK3, the CRWs are the means. You are going to be using XK3's CRWs as a type of bumper that can-in addition to everything else-detect the opponent. Using CRWs in this way produces some interesting issues on the building side, as well as on the programming side. You'll see what these problems are and one possible solution for overcoming them in this chapter.

As for XK3's substrategy, it is based around the same substrategies as XK1 and XK2 (built in Chapters 4 and 5 of Competitive MINDSTORMS), but it also adds one more factor to the equation: the CRWs. This means that XK3 will continually ram its opponent (as the other XKs do), but it will use the new wheels to make the approach more effective. XK3's strategy essentially falls in between the first and second approach to the repeated ramming method. It relies mainly on its momentum and repeated ramming to push the opponent off the arena, but it also has a mechanism to help defeat the opponent.

Right now, you probably have two burning questions in mind, something like these:

• The CRWs are always supposed to be spinning at a high speed, so how are they going to hit something, stop, and then start up again without breaking the motor?
• How in the world is XK3 going to know when its CRWs have hit something (stopped spinning)?

The answers come in the form of a well-known mechanical trick and some programming you might recognize from a chapter in Competitive MINDSTORMS, but we'll get to that later. We need to begin by building XK3 first!