Friction

Pushcarts - Friction

How has it been done before?

Engineering Facts

Friction is about two surfaces rubbing together. Friction is a force that will slow down movement and doing so will produce heat. Friction is important to understand when designing a pushcart. Sometimes you will need lots of friction (e.g. for your brakes) and sometimes you will need as little friction as possible (e.g. between your wheel and axle).

The more friction you have between the road and the wheels ('road-holding' capacity) the better you will go around corners but the slower you will go in a straight line. So you will want wheels that hold the road well when turning but allow your pushcart to move fast in a straight line.

Your wheel surface, your wheel diameter and your wheel 'profile' (cross section) will all play a part in controlling friction.

Wheel surface

Materials differ in the friction they produce when rubbed with other materials. This is called the 'coefficient of friction'. For example, felt mats can be used on a slippery dip to minimise friction when you slide down and make the ride faster. Rubber mats will slow you down or even stop you sliding at all.

Places where you will need lots of friction for your wheel surface will be when:

turning corners
braking

Places where you will need very little friction will be:

when moving in a straight line (between the wheel and road)
between the wheel and its axle - with bearings and bushes and even lubrication
in the pivot joint where the front axle joins the frame

Wheels can have pneumatic tyres, solid tyres or hard plastic surfaces. Each of these has a different coefficient of friction. Pneumatic tyres have a better road holding capacity.

Wheel diameter

Bigger wheels are better for:

handling bumps
going faster

Smaller wheels are better because they are often cheaper.

Wheel profile

The area of the surface of the wheel that actually touches the road determines how much friction is produced.

When you are going in a straight line you can have very little surface touching the road. One example of this are the wheels of racing bicycles. They are narrow with very little surface touching the road.

When you are turning or on soft ground you need more friction (grip or road holding). An example of this are the wide soft wheels of racing cars and dune buggies. These allow more control of the vehicle. When you are turning, the outside tyres do all the work and need to hold the road well.

Learn more about friction!

Suggestions and Choices

You can eliminate even more friction by lubricating your movable joints e.g. wheel bushes and steering pivot joint and other moving parts.

Vaseline, grease or graphite dust work well.

Try This

Slippery Dip Science

Make a ramp about 1 metre long using a piece of strong cardboard or wood (or even use a slippery dip!)
Cover one surface of a series of similar blocks with a variety of materials
Allow the blocks to slide down the ramp on their covered surface
Which material provides good 'ramp-holding capacity'
Experiment with changing the slop of the ramp.

You could use Slippery Dip Science to test your wheels as well.

For Teachers

Support Materials developed by engineeringLinks at UTS

Pneumatic tyre (left) and solid tyre (right).

Compare the surface area of each tyre that touches the road. Which one is better for road holding and turning? Which one is better for straight fast travel?

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