Wheels and Tyres Wheels and Tyres

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The tyre size is denoted by a series The tyre size is denoted by a series of numbers on the sidewall of a tyre. of numbers on the sidewall of a tyre.

When tyres were in the early When tyres were in the early days of design, a tyres height days of design, a tyres height was similar in size to its width. was similar in size to its width. That is to say if a tyre was 175 That is to say if a tyre was 175

millimeters wide, it was 175 millimeters wide, it was 175 millimeters high. millimeters high.

Today it is common for a tyres Today it is common for a tyres width and height to be different. width and height to be different. The way manufacturers display The way manufacturers display

this size differentiation is to this size differentiation is to express it as a ratio. express it as a ratio.

What is the name given to this What is the name given to this ratio?ratio?

• It is called the aspect ratio

The term aspect ratio is the ratio The term aspect ratio is the ratio of the tyres height when of the tyres height when

compared to its width; this is compared to its width; this is expressed as a percentage. expressed as a percentage.

Label the tyres below:Label the tyres below:What aspect ratios fit the tyres below?

Tyre with 100% aspect ratio

Tyre with 30% aspect ratio

What is meant by the following tyre What is meant by the following tyre size, 175/ 70 R 14 80 S size, 175/ 70 R 14 80 S

• In pairs have a go at working out what each number means

175175

• The first number – the 175

• Is the sectional width of the tyre measured in millimeters

The 2The 2ndnd number the 70 number the 70

• Is the aspect ratio

• This means the height is 70% of the width

The R means… The R means…

• The tyre is a radial tyre

• This means the plies run at 90 degrees to the bead

The 14 is… The 14 is…

• The rim diameter measured in inches

The 80 is… The 80 is…

• The load index

• This means this tyre is able to cope with loads up to 450 KG (almost half a tonne, which is 500KG)

The S is…The S is…

• The speed rating

• An S speed rating means that the tyre is capable of speeds up to 180 KPH or 113 MPH

• Plenty fast!!!!

Some tyres have a size which is Some tyres have a size which is displayed like this … 175 R 14, displayed like this … 175 R 14, what does this mean about the what does this mean about the

aspect ratio? aspect ratio?

• It means the aspect ratio is said to be ‘normal’ which is said to be approx 82%

List the faults which has caused the List the faults which has caused the following wear problemsfollowing wear problems

Under inflation Over inflation Excess camber

Questions Questions

1. What is meant by the term aspect ratio?2. What unit of measurement is the wheel

rim usually measured in?3. What unit of measurement is the tyre

width measured in?4. A tyre with excess wear in the centre

would indicate what?5. A tyre with excess wear on both outer

edges would indicate what?

Answers Answers

1. The aspect ratio is the percentage the height of the tyre is compared to the width, expressed as a ratio

2. Inches

3. Millimeters

4. Over inflation

5. Under inflation