Every year the kingdom of Mechanasia hold a Soap Box Derby. Apprentices from the Guild of Engineers make little buggies. They race these down a hill. The buggy which travels the farthest from the bottom of the hill wins. The winner gets a big trophy and a pat on the back. Hilary wanted to be the youngest apprentice to win the derby. Her robot, Oilyrag, was helping her make one.

"Friction makes the buggy go slow." said Oilyrag. "The faster it goes down the hill, the farther it will travel." he added.

"That's not all," said Hilary, "Friction makes things stop. So lots of it will make our buggy stop quickly".

"Get rid of friction, that's what I say." Said Oilyrag. "You get friction when things rub together. So how do we get rid of it?" he asked.

"You can't," replied Hilary, "Youjust make it as small as possible."

"How," mumbled the robot as he crawled under a bench to look for a screwdriver.

"Well," said Hilary, "We could fix the axle to the frame and let the wheels spin on it. Or we could join the wheel and axle and make them both spin."

"Which is the best way?" asked the robot.

Hilary looked thoughtful. "The wheels will spin on the axle easily, but too much spin and they will wobble and wobbly wheels add friction."

"If the axles don't line up you get lots of friction." said Oilyrag, "We could use bearings to get them in line."

Hilary nodded. "That will mean we'll use more parts - but if they work it'll be worth it." Then she said, "I don't know which is the best way. We'll have to do some tests."

So Hilary and Oilyrag tested different designs. Which do you think is the best?