Figure 1: Direct Drive Transmission	Figure 2: Friction Drive Transmission
Figure 3: Belt Drive Transmission	Figure 4: Gear Drive Transmission

1. Speed vs. Force
   

   The most simple type of transmission is direct drive, which means the motor is connected directly to the axle of the driven wheel. Direct drives are not common in vehicles; one of the few vehicles that uses direct drive is a unicycle. Every time your feet make one revolution, the wheel makes one revelution.

   Figure 5: Unicycle

2. Speed
   

   Imagine two of your neighbors have a unicycle race. Bruce's unicycle has a regular wheel, and Karen's has a very large wheel. If they both pedal the same rate (number of revolutions per minute), which one of them will win? In both cases, each revolution of the pedal means one revolution of the wheel. BUT, one revolution of Karen's wheel will roll twice as far as bruce's. So Karen would win if they pedaled at the same rate. If Bruce wanted to win, he would have to pedal twice as fast as Karen.

   Figure 6: Different wheel sizes

   
   

   Have you ever seen pictures of very old bicycles that have huge front wheels? These bicycles allowed the rider to go faster without pedaling like a maniac!

   Figure 7: Old time bicycle

   
   As mentioned before, most vehicles are not direct drive, so let's look at another type of vehicle: a 3-speed bicycle. A bicycle uses a chain drive. It allows you to move the pedal, and the chain transfers the energy from the pedals to the rear wheel (see Figure 8).

   Figure 8: Forces on a Bicycle

   The chain glides over different sized sprockets, depending on the speed of the rider. Which sprocket combination will make the rider go the fastest, given the same pedaling rate, or "cadence"? (Hint: how many times will the back sprocket [and therefore the back wheel] turn with each rotation of the front sprocket?)

   Figure 9: 3-Speed bicycle sprocket combinations

   Each rotation of the front sprocket will make the back wheel rotate once in Combo 1, twice in Combo 2, and four times in Combo 3. So, combination 3 will go the fastest. (These sprocket combinations can also be called "gear ratios", because the new speed is calculated as the ratio of the driven (front) sprocket over the driven (back) sprocket.)
   So how does this affect the way the biker would use the bicycle? Well, when the rider starts out, he uses first gear (Combo 1). As he pedals faster, the bike starts going faster. After a while, his legs are moving very fast, so he switches to second gear (Combo 2). No his legs only go half as fast as a second ago, but the bike is still going fast. He can increase his cadence again and make the bike go even faster. Once his cadence is very high again, he can shift up to third gear (Combo 3).
   If the rider is going 5 mph in first gear, how fast is he going in third gear with the same pedaling rate or cadence?
   Well, the jump from first to second gear doubles the speed, and the step from second to third gear doubles it again. So, the rider is going four times as fast as in the first gear. He is going 20 mph, but his legs are moving at the same rate as at the very begining!
   The term "3-speed" bike is not entirely correct, because a biker can go more than just three different speeds. As we saw in the previous example, our bike rider was able to continuously speed up from 5 mph to 20 mph. But the name comes from the fact that given one cadence, the three gear ratios will give you three different speeds. Of course, your legs can pedal at many different rates, but "3-speed" bike sounds better than "3-gear-ratio" bike.
   

3. Force
   You may ask, then, why isn't it the best to go for the highest speed possible? Well, you can't get something for nothing! So what are you giving up when you gain speed? Let's investigate.
   Imagine two bikers approaching a very steep hill. Jeff and Dave are both in third gear, because they are going very fast. Dave downshifts into second gear. But Jeff decides to stay in third gear, because he knows that third gear is for going fast, and he wants to go up this hill very fast.

   Figure 10: Dave downshifting at a hill

   Dave is going half the speed now, because he just downshifted. Jeff smirks as he blows by Dave. But Jeff hits the hill, and he suddenly realizes that his legs can't go very fast anymore -- it becomes very hard to pedal! He gets slower and slower, and finally stops pedaling because it's too hard. Dave passes, slowly but surely, and makes it to the top of the hill while Jeff stops part way up.

   Figure 11: Jeff stops and Dave makes it

   What happened? If only Jeff could have kept pedaling at the same rate, he would have beat Dave by a mile! Let's look at each pedal stroke. Each time Dave and Jeff pedal once, Dave's back wheel goes around once (let us say it travels 10 ft), but Jeff's back wheel goes around twice (20 ft).

   Figure 12: Distance Traveled per Stroke Comparison

   Dave realizes that he only has to expend half the energy per pedal revolution than Jeff does, because Jeff goes twice as far each time. That is why Jeff started getting very tired, because his pedals were difficult to push. In other words, his pedals required more force than Dave's did.
   So does Dave expend less energy going up the same hill?
   Dave expends have the energy per pedal revolution, but this is only because he goes half the distance per revolution. Dave has to bedal twice as many time sto get up the hill. So, the energy expended by both Dave and Jeff going up the entire hill would be the SAME in either case.

   Figure 13: Energy units comparison

   So, the bottom line is, when we gain a speed advantage, we are losing the force advantage. The pedals are more difficult to turn. You can gain either speed or force advantage, but not at the same time.

4. Selecting Proper Gear Ratios
   So, how can you choose the best gear ratio? Experimentation is probably the easiest way to find out. Try some of the Investigations below or build a test bed to try different combinations and note your findings.
   The ideas is that your motor, like your legs when you ride a bike, will like to go a certain speed. They also have a limit as to how much force they can exert. First you must find the speed at which the motor gives the most power (this is usually half the speed at which the motor will rotate if there is no load, or force, exerted on the motor shaft). Try to keep the motor turning at approximately that speed as you experiment with different gear ratios.
   It helps if you build your car in such a way that you can change the gear ratios easily as you experiment. Remember, the ideal gear ratio may change some if you change different characteristics of your car (size, weight, wheel size, etc.). Just remember, if your car is not going very fast it can either be that the wheel speed is too slow, or (like Jeff riding uphill) the force required to turn the wheel is too high. Try a different gear ratio!

• Slide of inner tube
• Rubber band
• O-Ring
• Spools
• Drawer pulls
• Videocassette reels
• Reclaimed pulleys (Electronics parts)
• Washers

• Parts out of model cars
• Rubber bands
• Circular wood or plastic cutouts
• Spools

• Gears out of electronics
• RC model car gears
• RC model car gearboxes

• See Investigation 5-1
• See Investigation 5-2
• See Investigation 5-3